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Doctoral Thesis

Mechanisms of hydrodenitrogenation of amines over sulfidedNiMo, CoMo, and Mo supported on Al₂O₃

Author(s): Zhao, Yonggang

Publication Date: 2004

Permanent Link: https://doi.org/10.3929/ethz-a-004779056

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Diss. ETH No. 15555

Mechanisms of Hydrodenitrogenation of Amines Over

Sulfided NiMo, CoMo, and Mo Supported on Al2O3

A dissertation submitted to the

SWISS FEDERAL INSTITUTE OF TECHNOLOGY ZURICH

for the degree of

DOCTOR OF CHEMICAL ENGINEERING

Presented by

Yonggang Zhao

Bachelor and Master of Chemical Engineering, Fushun Petroleum Institute Born on 21 September 1974

in Jiangsu, China

Accepted on the recommendation of

Prof. Dr. Roel Prins, examiner Prof. Dr. Michèle Breysse, co-examiner

Zürich, 2004

I

CONTENTS

Abstract................................................................................................................................ V

Zusammenfassung ........................................................................................................... VII

1 Introduction.......................................................................................................................... 1

1.1 Hydrodenitrogenation .................................................................................................. 1

1.2 Structure of the catalysts.............................................................................................. 3

1.3 The role of H2, H2/H2S and active sites ....................................................................... 8

1.4 Reaction mechanism.................................................................................................. 12

1.4.1 Aromatic C-N bond breaking............................................................................ 12

1.4.2 C-N bond cleavage of aliphatic amines ............................................................ 15

1.4.3 C-N bond cleavage of hetero cyclic amines ..................................................... 18

1.5 Experimental.............................................................................................................. 21

1.5.1 Unit ................................................................................................................... 21

1.5.2 Product analysis ................................................................................................ 24

1.5.3 Catalysts............................................................................................................ 24

1.5.4 Weight time....................................................................................................... 25

1.5.5 Reactants and reaction conditions used in every chapter ................................. 25

1.6 References.................................................................................................................. 28

2 On the role of β-hydrogen atoms in the hydrodenitrogenation of 2-methylpyridine

and 2-methylpiperidine ..................................................................................................... 35

2.1 Abstract...................................................................................................................... 35

2.2 Introduction................................................................................................................ 35

2.3 Results........................................................................................................................ 37

2.3.1 HDN of 2-methylpyridine................................................................................. 37

2.3.2 HDN of 2-methylpiperidine.............................................................................. 39

2.3.3 Comparison of piperidine, 2-methylpyridine, and 2,6-dimethylpiperidine ... 40

2.3.4 HDN of 1-aminohexane.................................................................................... 43

II

2.3.5 HDN of 2-aminohexane ....................................................................................45

2.4 Discussion ..................................................................................................................47

2.5 References ..................................................................................................................51

3 Investigation of the mechanism of the hydrodenitrogenation of n-hexylamine over

sulfided NiMo/γ-Al2O3 .......................................................................................................53

3.1 Abstract ......................................................................................................................53

3.2 Introduction ................................................................................................................53

3.3 Results ........................................................................................................................55

3.3.1 HDS of pentanethiol and hydrogenation of hexene ..........................................55

3.3.2 HDN of hexylamine ..........................................................................................58

3.3.3 HDN of dihexylamine .......................................................................................63

3.3.4 HDN of trihexylamine.......................................................................................66

3.4 Discussion ..................................................................................................................69

3.4.1 Hexylamine .......................................................................................................69

3.4.2 Dihexylamine ....................................................................................................71

3.4.3 Trihexylamine ...................................................................................................74

3.4.4 General discussion.............................................................................................75

3.5 Conclusions ................................................................................................................78

3.6 References ..................................................................................................................79

4 Mechanisms of the hydrodenitrogenation of alkylamines with secondary and tertiary

α-carbon atoms over sulfided NiMo/γ-Al2O3 ...................................................................81

4.1 Abstract ......................................................................................................................81

4.2 Introduction ................................................................................................................81

4.3 Results ........................................................................................................................82

4.3.1 2-Pentylamine and 2-pentanethiol.....................................................................82

4.3.2 3-Methyl-2-butylamine and 3-methyl-2-butanethiol ........................................85

4.3.3 3,3-Dimethyl-2-butylamine...............................................................................88

4.3.4 2-Methylcyclohexylamine.................................................................................90

4.3.5 2-Methyl-2-butylamine and 2-methyl-2-butanethiol ........................................91

4.4 Discussion ..................................................................................................................94

III

4.4.1 HDN and HDS mechanism............................................................................... 94

4.4.1.1 Acid-base mechanism........................................................................... 94

4.4.1.2 Metal-like mechanism........................................................................... 96

4.4.2 HDN of amines with secondary α-carbon atoms .............................................. 99

4.4.3 HDN of 2-methyl-2-butylamine and benzylamine ......................................... 102

4.5 Conclusions.............................................................................................................. 103

4.6 References................................................................................................................ 105

5 Mechanisms of HDN of Alkylamines and HDS of alkanethiol on NiMo/Al2O3,

CoMo/Al2O3, and Mo/Al2O3 ......................................................................................... 107

5.1 Abstract.................................................................................................................... 107

5.2 Introduction.............................................................................................................. 107

5.3 Results...................................................................................................................... 109

5.3.1 Simultaneous reaction of pentylamine and hexanethiol ................................. 111

5.3.2 Simultaneous reaction of 2-hexylamine and 2-pentanethiol........................... 116

5.3.3 2-Methyl-2-butylamine and 2-methyl-2-butanethiol ...................................... 122

5.4 Discussion................................................................................................................ 124

5.4.1 Pentylamine..................................................................................................... 124

5.4.2 2-Hexylamine.................................................................................................. 126

5.4.3 2-Methyl-2-butylamine................................................................................... 128

5.5 Conclusion ............................................................................................................... 130

5.6 References................................................................................................................ 131

6 Mechanism of the hydrodenitrogenation of adamantylamine and neopentylamine

over sulfided NiMo/γ-Al2O3............................................................................................. 133

6.1 Abstract.................................................................................................................... 133

5.2 Introduction.............................................................................................................. 133

6.3 Results...................................................................................................................... 134

6.3.1 Neopentylamine .............................................................................................. 134

6.3.2 Adamantylamines and adamantanethiol ......................................................... 136

6.4 Discussion................................................................................................................ 138

6.4.1 HDN of neopentylamine ................................................................................. 138

IV

6.4.2 HDN of AdNH2 and HDS of AdSH................................................................140

6.5 Conclusion................................................................................................................143

6.6 References ................................................................................................................144

7 Mechanism of the direct hydrodenitrogenation of naphthylamine over sulfided

NiMo/γ-Al2O3 ....................................................................................................................147

7.1 Abstract ....................................................................................................................147

7.2 Introduction ..............................................................................................................147

7.3 Results ......................................................................................................................149

7.4 Discussion ................................................................................................................154

7.4.1 Direct denitrogenation.....................................................................................154

7.4.2 Direct desulfurization......................................................................................160

7.5 Conclusions ..............................................................................................................162

7.6 References ................................................................................................................162

8 Concluding remarks.........................................................................................................165

8.1 Conclusion................................................................................................................165

8.2 Outlook.....................................................................................................................167

8.3 References ................................................................................................................170

Acknowledgements

Publications

Curriculum Vitae

V

Abstract

The hydrodenitrogenation (HDN) of alkylamines has been studied over sulfided NiMo,

CoMo, and Mo catalysts supported on γ-Al2O3 at reaction conditions in the range of 3-5 MPa,

270-350 °C, and 10-100 kPa H2S. The heterocyclic amines 2-methylpyridine and 2,6-

dimethylpyridine as well as the alkylamines of n-hexylamine, 2-pentylamine, and 2-methyl-2-

butylamine were chosen as HDN models. The corresponding alkanethiols were studied as

well.

In the HDN of 2-methylpiperidine, the products 2-methylpyridine, 3,4,5,6-

tetrahydropyridine, and 2-hexylamine as well as hydrocarbons were formed. Only a trace of

hexylamine was observed. As the reactivity of 2-hexylamine was much higher than that of

hexylamine at the same reaction conditions, the ring opening of 2-methylpiperidine occurred

preferentially between the nitrogen atom and the methylene group, instead of between the

nitrogen atom and the carbon atom bearing the methyl group. It demonstrates that β-hydrogen

atoms are not involved in the HDN of 2-methylpiperidine. This was confirmed with the much

higher HDN conversion of piperidine than 2-methylpiperidine and 2,6-dimethylpiperidine.

The HDN of hexylamine, dihexylamine, and trihexylamine was studied between 300 and

340 °C, 3 and 5 MPa total pressure, 5 and 20 kPa amine pressure, and 10 and 150 kPa H2S

pressure over a sulfided Ni-Mo/γ-Al2O3 catalyst. The conversion of hexylamine and

dihexylamine decreased slightly with H2S pressure, but that of trihexylamine increased

substantially. The conversion increased with the H2 pressure and decreased with increasing

partial pressure of the hexylamines. In the HDN of n-hexylamine, a substantial amount of

hexenes and hexanethiol was formed by elimination and nucleophilic substitution. Two

methods were used to distinguish between elimination and nucleophilic substitution. One is to

test the initial product selectivities at short weight time. The initial alkene selectivities were

low and accounted for only a minor part of the n-alkylamine conversion. Furthermore, the

simultaneous reactions of hexylamine and pentanethiol show that the hexenes/hexane ratio in

the HDN of the hexylamine was almost equal to the pentenes/pentane ratio in the

hydrodesulfurization (HDS) of pentanethiol. Therefore, it was concluded that the majority of

the hexene in the HDN of hexylamine originates from hexanethiol formed by nucleophilic

substitution of hexylamine with H2S.

VI

The HDN of alkylamines with secondary and tertiary α-carbon atoms and benzylamine

as well as the HDS of the corresponding alkanethiols was studied over sulfided NiMo/Al2O3

CoMo/Al2O3 and Mo/Al2O3 catalysts. The similar alkenes/alkane ratios in the HDN of the

alkylamines and HDS of the corresponding alkanethiols confirm that nucleophilic substitution

is the predominant reaction in the HDN of the amine group attached to secondary carbon

atoms. 2-Methyl-2-butylamine and benzylamine reacted much faster than the amines with

secondary α-carbon atoms. In the HDN of 2-methyl-2-butylamine and benzylamine,

hydrocarbons were formed. Only a trace of the relevant thiol was formed and this amount did

not increase much with increasing H2S pressure. The much higher

methylbutenes/methylbutane ratios in the HDN of 2-methyl-2-butylamine as in the HDS of 2-

methyl-2-butanethiol demonstrate that 2-methyl-2-butylamine and the activated benzylamine

react by means of an E1 mechanism.

The HDN of 1-adamantylamine, 2-adamantylamine and neopentylamine and the HDS of

1-adamantanethiol were studied. The adamantanethiols and neopentanethiol were formed as

the primary products of the adamantylamines and neopentylamine by substitution of the NH2

group with H2S. Adamantane and neopentane were secondary products, demonstrating that

hydrogenolysis can hardly take place. Furthermore, elimination cannot take place and a

classic SN2 substitution is not possible for the adamantylamines either. It is proposed that the

NH2-SH substitution in adamantylamine takes place by adsorption of the amine group at the

metal sulfide surface and by shifting the adamantyl group to a neighbouring sulfur atom.

1-Naphthylamine was studied to test the direct hydrogenolysis in hydrodenitrogenation.

Tetralin, naphthalene, 1,2-dihydronaphthalene and 5,6,7,8-tetrahydro-1-naphthylamine were

formed in the HDN of naphthylamine. The reactions of the intermediates 1,2,3,4-tetrahydro-

1-naphthylamine, 1,2-dihydronaphthalene and 5,6,7,8-tetrahydro-1-naphthylamine were

studied as well. 1-Naphthylamine reacts to 1,2,3,4-tetrahydro-1-naphthylamine by

hydrogenation, and then it reacts by NH3 elimination to 1,2-dihydronaphthalene. 1,2-

dihydronaphthalene subsequently reacts to tetralin as well as naphthalene. This direct

denitrogenation of naphthylamine to naphthalene could take place by hydrogenation of 1-

naphthylamine to 1,2-dihydro-1-naphthylamine, followed by NH3 elimination or followed by

a Bucherer-type NH2-SH exchange, dehydrogenation and C-S bond hydrogenolysis.

VII

Zusammenfassung

Die Hydrodenitrogenierung (HDN) von Alkylaminen wurde über sulfidierten, auf γ-

Al2O3 geträgerten NiMo, CoMo, und Mo Katalysatoren unter Reaktionsbedingungen von 3-5

MPa, 270-350 °C und 10-100 kPa H2S untersucht. Als Modellverbindungen für die HDN

wurden sowohl die heterocyclischen Amine 2-Methylpyridin und 2,6-Dimethylpyridin als

auch die Alkylamine n-Hexylamin, 2-Pentylamin, und 2-Methyl-2-butylamin gewählt.

Ebenfalls untersucht wurden die entsprechenden Alkylthiole.

Während der HDN von 2-Methylpiperidin wurden die Produkte 2-Methylpyridin, 3,4,5,6-

Tetrahydropyridin und 2-Hexylamin sowie Kohlenwasserstoffe gebildet. Hexylamin ist nur in

Spuren beobachtet worden. Da die Reaktivität von 2-Hexylamin unter denselben

Reaktionsbedingungen viel höher war als die von Hexylamin, fand die Ringöffnung von 2-

Methylpiperidin bevorzugt zwischen dem Stickstoffatom und der Methylengruppe statt,

anstatt zwischen dem Stickstoffatom und dem methylsubstituierten Kohlenstoff. Dies zeigt,

dass β-Wasserstoffatome in der HDN von 2-Methylpiperidin nicht involviert sind. Der viel

höhere HDN Umsatz von Piperidin verglichen mit dem Umsatz von 2-Methylpiperidin

bestätigt dies.

Die HDN von Hexylamin, Dihexylamin und Trihexylamin wurde über sulfidiertem Ni-

Mo/γ-Al2O3 Katalysator zwischen 300-340 °C, 3-5 MPa Totaldruck, 5-20 kPa

Aminpartialdruck und 10-150 kPa Schwefelwasserstoffpartialdruck untersucht. Während der

Umsatz von Hexylamin und Dihexylamin mit zunehmendem H2S Druck leicht abnahm,

nahm derjenige von Trihexylamin deutlich zu. Der Umsatz nahm mit zunehmendem H2 Druck

zu und mit zunehmendem Partialdruck der Hexylamine ab. Während der HDN von n-

Hexylamin bildete sich durch Eliminierung und nucleophile Substitution eine erhebliche

Menge an Hexenen und Hexanthiol. Für die Unterscheidung zwischen Eliminierung und

nucleophiler Substitution wurden zwei Methoden angewendet. Die eine besteht darin, die

anfänglichen Produktselektivitäten nach kurzer Kontaktzeit zu untersuchen. Die anfänglichen

Alkenselektivitäten waren klein und trugen nur wenig zum Umsatz von n-Alkylamin bei.

Weiter zeigte die gleichzeitige Reaktion von Hexylamin und Pentanthiol, dass das

Hexene/Hexan Verhältnis in der HDN von Hexylamin beinahe gleich war wie das Verhältnis

Pentene/Pentan in der Hydrodesulfurisierung (HDS) von Pentanthiol. Daraus wurde

VIII

geschlossen, dass die Mehrheit des in der HDN von Hexylamin auftretenden Hexens von

Hexanthiol stammen muss, welches durch nucleophile Substitution von Hexylamin mit H2S

entsteht.

Sowohl die HDN von Alkylaminen mit sekundären und tertiären α-Kohlenstoffatomen

und von Benzylamin, als auch die HDS der entsprechenden Alkanthiolen, wurde über

sulfidierten NiMo/Al2O3, CoMo/Al2O3 und Mo/Al2O3 Katalysatoren untersucht. Die

ähnlichen Alkene/Alkan Verhältnisse während der HDN der Alkylamine und während der

HDS der entsprechenden Thiole bestätigen, dass nucleophile Substitution der bevorzugte

Reaktionsmechanismus der HDN der Aminogruppe am sekundären Kohlenstoffatom ist. 2-

Methyl-2-butylamin und Benzylamin reagierten viel schneller als die Amine mit sekundären

α-Kohlenstoffatomen. Während der HDN von 2-Methyl-2-butylamin und Benzylamin wurde

die Bildung von Kohlenwasserstoffen beobachtet. Das relevante Thiol wurde nur in Spuren

gebildet, und dessen Menge nahm mit steigendem H2S Druck nur wenig zu. Das viel höhere

Methylbutene/methylbutan Verhältnis während der HDN von 2-Methyl-2-butylamin als

während der HDS von 2-Methyl-2-butanthiol zeigt, dass 2-Methyl-2-butylamin und das

aktivierte Benzylamin nach einem E1 Mechanismus reagieren.

Die HDN von 1-Adamantylamin, 2-Adamantylamin und Neopentylamin und die HDS

von 1-Adamantanthiol wurden ebenfalls untersucht. Durch Substitution der NH2 Gruppe mit

H2S wurden als primäre Produkte die Adamantanthiole und Neopentanthiol gebildet.

Adamantan und Neopentan waren sekundäre Reaktionsprodukte, was zeigt, dass

Hydrogenolyse praktisch nicht stattfinden kann. Im weiteren ist für die Adamantylamine

weder eine Elimination noch eine klassische SN2 Substitution möglich. Es ist vorgeschlagen,

dass die NH2-SH Substitution an Adamantylamin über Adsorption der Aminogruppe auf der

Metallsulfidoberfläche und anschliessende Verschiebung der Adamantylgruppe zu einem

benachbahrten Schwefelatom abläuft.

Um die direkte Hydrogenolyse in der HDN zu testen, wurde 1-Naphthylamin untersucht.

Während der HDN von Naphthylamin wurden Tetralin, Naphthalin, 1,2-Dihydronaphthalin

und 5,6,7,8-Tetrahydro-1-naphthylamin gebildet. Die Reaktionen der Zwischenprodukte

1,2,3,4-Tetrahydro-1-naphthylamin, 1,2-Dihydronaphthalin und 5,6,7,8-Tetrahydro-1-

naphthylamin wurden ebenfalls untersucht. 1-Naphthylamin reagiert zuerst über

Hydrogenolyse zu 1,2,3,4-Tetrahydro-1-naphthylamin und dann über NH3 Eliminierung zu

1,2-Dihydronaphthalin. 1,2-Dihydronaphthalin reagiert dann zu Tetralin und Naphthalin.

IX

Diese direkte Entstickung von Naphthylamin zu Naphthalin könnte über Hydrierung von 1-

Naphthylamin zu 1,2-Dihydro-1-naphthylamin und anschliessende NH3 Eliminierung oder

über anschliessenden Austausch von NH2-SH nach Bucherer, Hydrierung und Hydrogenolyse

der C-S Bindung ablaufen.

Introduction Chapter 1 1

1. Introduction

1.1 Hydrodenitrogenation

In an oil refinery, hydrotreating is a very important process to diminish the contents of

sulfur, nitrogen, and metals in the oil fraction so that fewer air-polluting emission of sulfur

and nitrogen oxides are formed when these oil fractions are burned in cars and trucks.

Furthermore, most catalysts used in a refinery for the processing of oil fractions cannot

tolerate sulfur and metals. The main reactions in hydrotreating are hydrodesulfurization

(HDS), hydrodenitrogenation (HDN), hydrodeoxygenation (HDO) and hydrodemetalization

(HDM). It has long been recognized that HDN is more difficult than HDS. People did not pay

much attention to HDN in the old days as only comparatively small quantities of nitrogen

compounds present in conventional petroleum feedstocks. However, this situation has

changed because of the growing need to process heavy and low-quality feed stocks, which are

rich in highly refractory nitrogen compounds. Furthermore, environmental legislation requires

a deep reduction of the sulfur content of gasoline and diesel fuel to 50 ppm by the year 2005.

Nitrogen-containing compounds inhibit the hydrodesulfurization (HDS) of sulfur-containing

compounds through competitive adsorption. At the low level of sulfur that must be reached in

deep HDS, however, nitrogen compounds will be harmful. Therefore, it is important to

understand HDN. In recent years, there has been considerable interest in the development of

more effective HDN catalysts [1], as witnessed by the rapidly expanding literature in this area

[2-10].

Industrial hydrotreating catalysts contain molybdenum and cobalt or nickel, supported

on γ-Al2O3. Since oil fractions always contain sulfur, a metal or metal oxide that would be

introduced as the catalyst would quickly become sulfided by the H2S that is produced during

hydrotreating. In practice, one therefore sulfides supported metal oxides under controlled

conditions before starting the hydrotreating process. When supported alone on alumina, Mo

sulfide has a much higher activity for the removal of S, N and O atoms than Co and Ni

sulfide. Therefore, molybdenum sulfide is traditionally considered to be the catalyst. Sulfided

Co-Mo/Al2O3 and Ni-Mo/Al2O3 catalysts, on the other hand, have substantially higher

catalytic activity than Mo/Al2O3. As a consequence, cobalt and nickel are referred to as the


promoters of the Mo activity [4,7,10,11]. Cobalt is used mainly as a promoter for sulfided

Mo/Al2O3 in HDS, while nickel is the choice for HDN. In addition to molybdenum and cobalt

or nickel, hydrotreating catalysts often contain additives such as phosphorus, boron, fluorine

or chlorine, which may influence the catalytic as well as the mechanical properties of the

catalyst [12-15]. The chemical properties of tungsten are similar to those of Mo; tungsten is,

however, more expensive and its industrial use is, therefore, limited. Especially when

hydrogenation is carried out under severe conditions, such as in hydrocracking, sulfided Ni-

W/Al2O3 catalysts have advantages over sulfided Ni-Mo/Al2O3 catalysts. The concentration of

the metals is usually 1-4 wt% for Co and Ni, 8-16 wt% for Mo and 12-25 wt% for W. The

NiW catalysts have the highest activity for aromatic hydrogenation at low hydrogen sulfide

partial pressure of the three catalyst combinations mentioned above [16].

Hydrotreating catalysts originated in the 1920s when German researchers developed

unsupported metal sulfide catalysts to liquefy coal. However, it was not until the 1970s that

the structures of these catalysts and the mechanisms of their catalytic action began to be

understood. It was established that under catalytic reaction conditions, most of the

molybdenum in industrial hydrotreating catalysts is present as small MoS2 particles in the

pores of the γ-Al2O3. It was not until the 1980s that the location of the cobalt and the nickel

promoter ions in the hydrotreating catalysts was more or less determined. The role of

phosphate and fluorine additives is still under investigation. Supports other than γ-Al2O3 like

amorphous silica-alumina, are also used in commercial units and their functions are topics of

academic and industrial research.

Typical hydrotreating catalysts need to be sulfided before achieving their active state.

The sulfiding is traditionally done during the start-up phase by exposing the catalyst to the

sulfur-containing feed or by adding H2S to the hydrogen. The sulfiding procedures have

significant influence on the catalyst activity and stability. After a period of use, the catalysts

will lose activity. There are several reasons for this. One is sintering of the active phase; the

second is decomposition of the active phase; the third is the covering of the active sites by

reactants and products; the fourth is coking, and the fifth is the deposition of metal sulfides.

The fourth and the fifth are the most important causes of activity loss. Regeneration of the

used catalysts is possible, however. It is important that the burn-off is carefully controlled to

avoid catalyst overheating, which could irreversibly change the active phase of the catalyst.


1.2 Structure of the catalysts

The preparation of hydrotreating catalysts can be done by a two-step pore volume

impregnation procedure or by co-impregnation. In the sequential pore volume impregnation,

γ-Al2O3 is impregnated with an aqueous solution of (NH4)6Mo7O24, followed by drying and

calcination (heating in air). In a second step, the resulting material is impregnated with an

aqueous solution of Co(NO3)2 or Ni(NO3)2 and dried and calcined. Alternatively, and

preferentially used in the industry, all inorganic materials are co-impregnated, and the

resulting catalyst precursor is dried and calcined. Several studies indicated that there are

interactions between Mo and Ni or Co in the oxidic state. Thus, it is known that the order of

impregnation and calcination - first Mo and then Co or Ni or vice versa - plays an important

role in the activity of the final sulfided catalyst. Catalysts in which the support is impregnated

first with a solution containing Mo invariably have a higher activity. It has been suggested

that the nickel or cobalt cations interact with the polymolybdate phase by forming a metal

heteropolymolybdate [17,18]. Several publications deal with this project: For instance, the

infrared absorption bands of NO adsorbed on Co-Mo/Al2O3 are shifted from those of NO on

Co/Al2O3 [19], and Raman bands indicating polymeric molybdenum oxide species decrease in

intensity with increasing the cobalt loading in an oxidic Co-Mo/Al2O3 catalyst [20]. These

results suggest that nickel or cobalt cations interact especially with the most polymeric

molybdenum oxide species to form species in which nickel or cobalt and molybdenum

interact. In this way the promoter cations stay at the surface and close to the molybdenum

cations and are positioned to form the active Ni-Mo-S structure during sulfidation.

Furthermore, the promoter ions interact to a lesser extent with the support and thus can be

used more efficiently after sulfidation.

After the impregnation, drying, and calcination steps, the oxidic hydrotreating catalyst is

formed. The real hydrotreating catalysts will be sulfided in a mixture of H2 with one or more

sulfur-containing compounds such as H2S, CS2, dimethyl disulfide, thiophene, and even

elemental sulfur. The oil fraction to be treated can be used for the sulfidation as well. The

properties of the final sulfidic catalyst depend to a great extent on the calcination and

sulfidation conditions. High-temperature calcination induces a strong interaction between

molybdenum and nickel or cobalt cations and the alumina support. Consequently, it is

difficult to transform the oxidic species into sulfides. Mössbauer spectroscopy of Co-


Mo/Al2O3 catalysts showed that at increasingly high calcination temperatures, increasingly

more of Co2+ ions are incorporated into the bulk of the alumina, forming a spinel [21]. The

higher the calcination temperature, the higher the sulfidation temperature needed to bring

these cations back to the surface to provide a high catalytic activity for hydrotreating. When

the sulfidation temperature is too high, the metal sulfides sinter or do not form the

catalytically active Co-Mo-S phase. Optimum calcination and sulfidation temperatures are in

the range 673-773 K for Al2O3-supported catalysts [22].

The sulfidation mechanism was investigated by temperature-programmed sulfidation, in

which the oxidic catalyst was heated in a flow of H2S and H2, and the consumption of H2S

and H2 and the evolution of H2O were measured continuously [23]. It was found that H2S is

taken up and H2O is formed, even at room temperature, indicating a sulfur-oxygen exchange

reaction. This conclusion was confirmed by Cattaneo and Prins [24] with quick extended X-

ray absorption fine structure (QEXAFS) studies (Figure 1.1, phase 2), which also

demonstrated that the Mo (VI) species containing both oxygen and sulfur transform into

intermediate MoS3-type species at temperatures between 520 and 570 K (Figure 1.1, phase 3).

At higher temperatures, the MoS3 is reduced to MoS2 (Figure 1.1, Phase 4) with concomitant

H2 consumption and H2S evolution. [23].

During sulfidation as well as during actual hydrotreating, the conditions are highly

reducing with H2S always present; thermodynamics predict that molybdenum should be in the

MoS2 form. Nevertheless, XPS has shown that complete sulfidation of Mo, and especially of

W, is difficult to attain. Apparently, some of the Mo(VI) ions interact so strongly with the

Al2O3 support that they can only be sulfided above 500°C. Mo and W can be sulfided more

easily, however, in the presence of Ni or Co ions. EXAFS studies of Mo K-edge absorption

spectra demonstrated that, in well-sulfided Mo/Al2O3 catalysts, the Mo-S and Mo-Mo

distances are the same as in MoS2 [25,26], the only difference being that, in the catalyst, each

Mo ion is surrounded, on average, by fewer than six Mo atoms, as in the case of pure MoS2.

EXAFS is a bulk technique by means of which the environment of surface Mo ions as well as

Mo ions in the interior of the MoS2 particles (Fig. 1.2) is determined. As a consequence, the

co-ordination number lower than six indicates that the proportion of surface Mo ions is

substantial and that the MoS2 particles on the surface of the support typically contain about 60

Mo atoms [26,27] as shown in Figure 1.2.


0 1 2 3 4 5 6

fresh

328

363

398

428

468

498

533

568

603

638

673

673

0

3

6

9

R [Å]

|FT[χ(

k) •

k3 ]|

O Mo

S

Mo

Sulfidation T [K]

Phase 1: Oxidic state

Phase 2: Coexistence of O and S

Phase 3: Intermediate Mo-Mo 2.5 Å

Phase 4: Formation of MoS2

Fig. 1.1. Quick Extended X-ray Absorption Fine Structure of a Mo catalyst.

Fig. 1.2. MoS2 particles, where the small balls are Mo and the big ones are sulfur.

MoS2 has a layer lattice and the sulfur-sulfur interaction between successive MoS2

sandwiches (Fig. 1.3) is weak. Nowadays people pay a lot of attention to the edges of MoS2

[28-30]. Byskov et al. used a model consisting of a periodic single layer of a S-Mo-S slab

with two molybdenum atoms in cross-section (2-Mo model) [28,29]. This model exposes the

Mo-edge at one side and the S-edge at the opposite side (Fig. 1.4). They added two sulfur

atoms per molybdenum atom on the Mo-edge of the stoichiometric model, and thus, the two

edges are fully covered by sulfur atoms. Raybaud et al. used a model consisting of two layers


of S-Mo-S sheets as shown in Figure 1.5 [30]. The Mo24S48 unit is periodically repeated along

the directions parallel to the edge surfaces, while perpendicular to the edge surface a vacuum

layer of 12.8 Å separates the neighboring units.

Fig. 1.3. Surface structure of the bulk MoS2 lattice for the (100) plane.

Fig. 1.4. Stoichiometric MoS2 including two rows of MoS2 units (A) top view; (B) side

view.

SMo S

Mo

(A) (B)

Fig. 1.5. Perspective view of optimised MoS2 edge surfaces: (A) with bare Mo-edge and fully

saturated S-edge; (B) with fully sulfided Mo- and S-edge


The incorporation of cobalt or nickel into the MoS2 structure can significantly increase

catalyst activity for hydrotreating reactions. Nickel atoms may be present in three forms after

sulfidation: as Ni3S2 crystallites on the support, as nickel atoms adsorbed on the edges of

MoS2 crystallites (the so-called Ni─Mo─S phase), and as nickel cations at octahedral or

tetrahedral sites in the γ-Al2O3 lattice (Fig. 1.6). Depending on the relative concentrations of

nickel or cobalt and molybdenum and on the pretreatment conditions, a sulfided catalyst may

contain a relatively large amount of either Ni3S2 or Co9S8 or the Ni─Mo─S (or Co─Mo─S)

phase.

Ni NiAl2O3

Ni3S2SMoNi

Fig. 1.6. Three forms of nickel present in a sulfided Ni-Mo/Al2O3 catalyst: as active sites on

the MoS2 edges (the so-called Ni─Mo─S phase), as segregated Ni3S2, and as Ni2+

ions in the support lattice.

It is well accepted that promoter atoms are located at MoS2 edges, but the exact location

of the promoter atoms relative to molybdenum and sulfur, and the mechanism of their effect is

still under debate. Using the 2-Mo model, Byskov et al. [29] found that the configuration with

Mo atoms substituted by cobalt atoms at the S-edge (Fig 1.7A) is more stable than the

structure with Mo atoms substituted by cobalt atoms at the Mo-edge (Fig 1.7B). However,

Raybaud et al. [31] concluded that the substitution of Mo atoms by Co atom on the Mo-edge

(Fig 1.7C) is energetically preferred on the fully sulfided S-edge (Fig 1.7D).


Co

S Mo

Co

(A) (B)

(C) (D)

Co

Fig. 1.7. Co-Mo-S models with cobalt atoms at different locations: (A) two Mo atoms are

substituted by Co atoms at the fully sulfided S-edge; (B) two Mo atoms are

substituted by Co atoms at the fully sulfided Mo-edge; (C) three Mo atoms are

substituted by Co atoms at the bare Mo-edge; (D) three Mo atoms are substituted by

Co atoms at the fully sulfided S-edge.

1.3 The role of H2, H2/H2S and Active sites

A review on the chemistry of catalytic hydrotreating was published by Topsøe et al.

[32]. Several recent reviews deal specifically with the individual reactions, such as HDS [33-

35], HDN [36-37], HDO [38], and HYD [39]. These studies predominantly focused on

reactants such as hetero ring model compounds contaning sulfur, nitrogen and oxygen. Real

feedstocks have also been evaluated. The first attempt to review the information on hydrogen

activation was made in 1988 by Moyes [40]. The activity of the commercial hydrotreating

catalysts results from the ability of Mo(W)S2 to adsorb and activate hydrogen. The presence

of active surface hydrogen is critical for hydroprocessing reactions to proceed at desirable


rates. In addition, active surface hydrogen extends the catalyst life by slowing down

deactivation.

The adsorption and desorption of hydrogen on MoS2 was reported by Badger et al. [41].

The reaction of H2 with Mo(W)S2 involves several processes occurring in parallel. It results in

a gradual removal of sulfur as H2S leading to a decrease in the stoichiometric S/Mo(W) ratio.

At the same time, H2 is adsorbed on the metal sulfide phase from where it can be transferred

to take part in various reactions. This is indicated by the study on the hydrogen adsorption by

MoS2 prepared by different methods from ammonium tetrathiomolybdate (ATTM) published

by Kalthod and Weller [42]. Polz et al. [43] proposed a heterolytic splitting of H2 on Mo-S

pairs giving a hydride and SH group,

H2 + □- S2- = □ – H + - SH-

as well as homolytic H2 dissociation with the aid of the S2-2 species giving two SH groups.

H2 + S22- = 2 SH-

The H2S dissociation can occur on the same vacancy as follows

H2S + □- S2- = □ – SH + - SH-

The study of Barbour and Campbell [44] supports the involvement of the reactions.

Theoretical and spectroscopic studies [45-56] provide some evidence for the occurrence of

several forms of Mo-SH and Mo-H groups. The results of Maternova [57,58] obtained by

AgNO3 titration and of Li et al. [59] obtained by TPR and TPD support the presence of at

least two kinds of SH groups, one processing reversible hydrogen and the other irreversible

hydrogen. It was suggested that the former is a highly labile hydrogen that can migrate from

one sulfur ion to another. Such hydrogen can be desorbed by purging in N2 in 723 K. Some

experimental evidence for the presence of entities other than SH, e.g., Mo-H was presented by

Jalowiecki et al. [60]. These authors confirmed the presence of another form of reactive

hydrogen (H*) on MoS2 previously reduced in H2 between 373 and 973 K.


Sundberg et al. [45] reported that promoters increased the amount of adsorbed hydrogen.

However, increasing the H2 pressure by a factor of 20 resulted only in a threehold increase in

the hydrogen retention. The bond strength between hydrogen and Co(Ni) promoted Mo(W)S2

was greater than that for the unpromoted sulfides. In this regard, the direct involvement of

Co(Ni) in hydrogen bonding cannot be ruled out. Beneficial effects of promoters on the

hydrogen activation by Mo(W)S2 may be attributed to the increased rate of hydrogen

activation although this issue has not yet been studied experimentally.

Several studies indicated the importance of the H2S/H2 ratio and temperature on the

distribution of vacancies and SH groups [61-67]. This was further advanced, most notably by

Kasztelan et al. [30, 68-70] who recognized the presence of several types of sulfur and metal

ions, each of them playing a specific role during hydrogen activation. For example, the role of

corner and edge S ions differs significantly from that of S ions in the basal plane. It is

generally accepted that the latter cannot dissociate molecular hydrogen; however, they are

capable of storing active hydrogen after dissociation of the molecular hydrogen occurring on

other sites.

It was commonly assumed that the catalytically active sites in a hydrotreating catalyst

are the Mo cations at the surface of the MoS2 crystallites with at least one sulfur vacancy so

that the reacting molecule can chemically bind to the Mo cation [4,7,31]. Since sulfur anions

in the basal planes of MoS2 are much more difficult to remove than anions at edges and

corners, exposed Mo ions are predominantly present at edges and corners. Catalysis therefore

occurs at MoS2 edges and corners rather than on the basal plane, as verified by a surface

science study. A MoS2 single crystal, with a high basal plane to edge surface area ratio, had a

low HDS activity. Its activity increased after the sulfur atoms were sputtered from the basal

plane and after exposure of the Mo ions [71].

The HDS and HDN activities of a MoS2/Al2O3 catalyst increase substantially upon

addition of Co or Ni. Several explanations for the promoter function of Co and Ni have been

proposed [4,7,31]. The most famous model is that in which the promotion effect is ascribed to

cobalt present in the Co-Mo-S phase, with cobalt ions located at the MoS2 surface; a

significant contribution of separate Co9S8 was excluded [72]. This so-called Co-Mo-S model

(or Ni-Mo-S model for Ni-Mo catalysts) is currently the most widely accepted model. The

Co-Mo-S model itself does not indicate whether the catalytic activity comes from Mo

promoted by the presence of Co or from the Co sites themselves. Both cobalt and nickel


sulfide, supported on carbon, have a higher HDS activity than MoS2/C [73]. Therefore it has

been suggested that the cobalt in the Co-Mo-S phase and the nickel in the Ni-Mo-S phase

might be the catalysts and not the promoters. In the past, the idea that Co and Ni might be the

catalyst in sulfided Co-Mo and Ni-Mo systems was rejected, because sulfided Co/Al2O3 and

Ni/Al2O3 catalysts have a very low HDS activity. However, during the usual catalyst pre-

treatment of Co/Al2O3 or Ni/Al2O3 catalysts and in the absence of Mo, cobalt and nickel ions

interact strongly with Al2O3. Therefore, during subsequent sulfidation, the metal ions are not

sulfided at all and do not contribute to the HDS activity. Alternatively, severe sulfidation

brings the metal ions back to the surface but lowers their dispersion and activity.

Carbon-supported cobalt and nickel sulfide catalysts, when carefully prepared, are

indeed highly active. The activity of a sulfided Co-Mo/C catalyst, based on the number of Co

atoms, compared much better with the number of estimated surface Co atoms in a sulfided

Co/C catalyst than with the number of estimated edge Mo atoms in a MoS2/C catalyst [74].

The observation that the hydrogenation pattern of Co-Mo and Ni-Mo catalysts resembles that

of sulfided Co respectively Ni catalysts and is different from that of supported MoS2 is further

evidence that Co and Ni are the catalytic sites rather than Mo. IR [75] and Mo EXAFS

investigations [26,76] showed that Mo is fully coordinated and is not accessible to substrate

molecules in Co-Mo and Ni-Mo catalysts. The Mo cannot, therefore, be catalytically active.

An EXAFS study by Startsev et al. of the adsorption of selenophene (the Se analogue of

thiophene) on a sulfided Ni-Mo/Al2O3 catalyst seemed to confirm this conclusion. They

claimed that selenophene adsorption changed the Ni but not the Mo EXAFS spectrum,

indicating that selenophene coordinated to Ni and not to Mo [77]. This would prove that Mo

is not accessible to selenophene. Medici et al. observed that the Ni EXAFS measured by

Startsev et al. cannot be simulated by selenophene adsorbed on Ni [78]. Leliveld et al. have

repeated the selenophene EXAFS studies of Startsev et al. [79]. They showed that after

reaction of selenophene and hydrogen with a Co-Mo catalyst at 200°C Se was exclusively

coordinated to the Co atoms. At 400°C, on the other hand, the Se atoms were found in bridge

positions between Co and Mo. The authors interpreted this as proof for two different vacancy

sites in which the Se atom can adsorb. One site has a S vacancy associated with Co, while the

other site has a vacancy between Co and Mo. In subsequent work, the authors admitted,

however, that an alternative explanation is possible [80]. It may be that at higher temperature

redistribution takes place of the Se atoms that are originally bonded to Co in a terminal


position and S atoms in bridging positions between Co and Mo. Because of the stronger M-Se

bonds, this seems a very likely possibility. Nevertheless, it seems clear that the first reaction

takes place on Co and not on Mo.

Recent discussions on the catalytic sites have concentrated on a combined action of Ni

(or Co) and Mo [29-31,81-82]. In HDS, a sulfur-containing molecule is supposed to adsorb on

a site with a sulfur vacancy and react to a hydrocarbon molecule and a sulfur atom. This

sulfur atom occupies the vacancy and must be removed by hydrogenation before the catalytic

cycle can start all over again. It has been pointed out that a sulfur atom between a Ni (Co) and

Mo atom is less strongly bonded than a sulfur atom between two Mo atoms. Therefore it can

be more easily removed. This would explain the promoter action of Ni and Co on Mo in HDS.

If HDN were to occur analogously, a nitrogen atom should be taken up by the metal sulfide

catalyst particles and later be removed by hydrogenation. This seems less likely than the

equivalent sulfur uptake and removal in HDS, and suggests that in HDN different sites are

used as in HDS.

1.4 Reaction Mechanism

Environmental legislation requires a deep reduction of the sulfur content of gasoline and

diesel fuel to 50 ppm by the year 2005. Nitrogen-containing molecules inhibit the

hydrodesulfurization (HDS) of sulfur-containing compounds through competitive adsorption.

In the past, this was not a severe problem, as the amount of these nitrogen-containing

compounds in petroleum is much lower than the amount of sulfur-containg molecules.

However, a low level of sulfur has to be reached in deep HDS. In that case, nitrogen

compounds will be harmful. Therefore, it is very important to understand the mechanism of

the removal of nitrogen. A deep understanding of these mechanisms will be essential to

develop the necessary catalyst.

1.4.1 Aromatic C-N bond breaking


C-N bonds in aromatic rings are much stronger than those in aliphatic rings.

Consequently, C-N bonds in rings, as in pyridine and pyrrole, can be broken only after

hydrogenation of the ring to give piperidine and pyrrolidine, respectively. Direct cleavage of a

C-X bond external to an aromatic ring, as in C6H5-X, to give a benzene and HX is the rule for

X = Cl and SH. For X = OH and NH2, direct cleavage occurs only to a limited extent and only

at high H2/H2S ratios and high temperatures (above 400 °C). The strength of the C-O bond in

phenol and that of the C-N bond in aniline are increased through conjugation with the

aromatic ring. Under normal hydrotreating conditions, the C-N bond can therefore be broken

only when it is aliphatic. Hydrogenation of the N-containing heterocycle or of the aromatic

ring to which the amine group is attached is necessary in order to obtain a substantial degree

of nitrogen removal. This difference is a consequence of the lower energies of the C6H5-Cl

and C6H5-SH bonds relative to the C6H5-OH and C6H5-NH2 bonds.

How aniline reacts to benzene is not clear. This reaction is often referred to as

hydrogenolysis, because it behaves as a concerted reaction in which the C-N bond is split and

the fragments are simultaneously hydrogenated. Such hydrogenolysis reactions are well

known for hydrocarbons over metal catalysts, and are believed to occur on an ensemble of

metal atoms. The hydrocarbon molecule adsorbs on the metal surface, and neighbouring

carbon atoms bind to neighbouring metal atoms. In this way, the C-C bond is weakened and

broken, and H atoms on nearby metal atoms are used to hydrogenate the fragments. It is

difficult to believe that such a hydrogenolysis reaction can occur on the surface of metal

sulfides. First, the distance between the metal atoms in a metal sulfide is much longer than in

a metal, thus making it unlikely that there is a geometrical fit for the four-center M-(C)-M-(N)

reaction intermediate. Second, if hydrogenolysis of C-N bonds occurs, then hydrogenolysis of

C-C bonds should also occur, because these bonds are not much stronger than the aliphatic C-

N bonds. Over sulfided catalysts, in the presence of H2S, only very little C-C bond breaking

occurs, however.

Thus, how is benzene formed directly from aniline and, in the same way, how is

naphthalene formed directly from naphthylamine? Although a definite answer has not been

given yet, some hypothesis can be suggested. One explanation for the direct denitrogenation

would be to assume that aniline is hydrogenated to tetrahydroaniline, which undergoes

elimination to cyclohexadiene. Cyclohexadiene then quickly reacts to cyclohexene or


benzene. Since tetrahydroaniline is not flat, the elimination of ammonia is possible in the anti

conformation.

Another explanation is the partial hydrogenation of aniline to dihydroaniline, followed

by elimination of ammonia. At first glance, this explanation seems flawed because, owing to

the planar structure of the cyclohexadience molecule, the NH2 group on the C1 atom and the H

atom on the neighbouring C2 atom are in the eclipsed conformation. This would mean that the

subsequent elimination (e.g. of 1,2-dihydro-aniline to benzene and ammonia) must occur in

the anti-periplanar rather than in the syn-antiplanar conformation. A closer look at 1,2-

dihydro-aniline suggests, however, that the elimination will not occur by an E2 mechanism,

but by an E1 mechanism. The cyclohexadienyl carbocation resulting from scission of the C-N

bond will be strongly stabilized by conjugation with butadiene fragment, as shown in Figure

1.8.

NH2 NH2 NH3

H2 H+ -H+

Fig. 1.8 C-N hydrogenolysis via partial hydrogenation, protonation and syn-elimination.

The third possibility is hydrogenolysis on a single Mo or Ni (Co) atom on the metal

sulfide surface. The drawback of the single-atom hydrogenolysis mechanism is that no

examples for single-atom hydrogenolysis over metal surfaces have been provided. The fourth

explanation is that aniline might have reacted to thiophenol through enol-keto tautomerism

and NH2-SH exchange by addition of H2S and elimination of NH3 analogous to reactions

described in [83-85]. The resulting thiophenol could then have reacted to benzene [6] as

shown in Figure 1.9.

NH2+NH2

HH

+NH3SH +SH SH

+H+ +H2S -NH3 -H+

Fig. 1.9 NH2-SH exchange by addition of H2S and elimination of NH3.


To determine whether the hydrogenolysis of an aryl C-N bond is real or apparent, we

studied the HDN of 1-naphthylamine and the reactions of the possible intermediates 1,2,3,4-

tetrahydro-1-naphthylamine, 1,2-dihydronaphthalene and 5,6,7,8-tetrahydro-1-naphthylamine

in chapter 7.

1.4.2 C-N bond cleavage of aliphatic amines

The removal of nitrogen is easier from aliphatic amines than from aromatic amines. It is

generally accepted that the first step in the HDN of nitrogen-containing aromatic molecules is

the hydrogenation of the heterocyclic ring. Only after the breaking of the aromaticity can C-N

bond cleavage in the resulting saturated molecules take place. Several mechanisms of the C-N

bond scission and nitrogen removal have been proposed [5,86-88]. Nelson and Levy [86]

were the first to suggest Hofmann-type elimination and nucleophilic substitution as

mechanisms for C-N bond scission of aliphatic nitrogen-containing molecules. The initial step

in C-N bond scission is the addition of a proton to a nitrogen lone pair with the formation of a

quarternary ammonium compound, which provides a better leaving group than the amine

group. C-N bond scission can then occur via elimination of a β−hydrogen atom with the

formation of an alkene (Fig. 1.10) or via nucleophilic substitution of the amine group at the

α−carbon atom by a sulfhydryl group to form an alkanethiol (Fig. 1. 11).

H C N HH

HC C C NH C NC ++

BH

+ B+

Fig. 1.10. Hofmann elimination mechanism.

C N+H2S

HS- + C N S C

H

+ NH

S C

H

+ H2 H2S H C+

+

Fig. 1.11. Nucleophilic substitution of the amine group by an SH group, followed by

hydrogenolysis of the C-S bond.


Laine proposed a different type of HDN mechanism where the C-N bond scission in

saturated heterocycles requires metal atoms or ions rather than acidic sites. Piperidine is

activated by a metal site and the intermediate formed is attacked by H2S to form a C-S bond

and leads to the C-N bond scission (Fig. 1.12).

NH

MHN

MH-

H2SHN

M H-SH2

+

MH2NH

SHH2N SMH2

H2N SH H2 H2N

H2

-NH3

+ +

++

-H2S/fast

Fig. 1.12. Reaction pathways for piperidine HDN

An alternative explanation of C-N bond breaking in molecules that lack β-H atoms is

direct hydrogenolysis of the alkylamine to a hydrocarbon and NH3 (Fig. 1.13).

C N + H2 C H HN+

Fig. 1.13. Direct hydrogenolysis of the alkylamine.

Proof of real hydrogenolysis of an aliphatic C-N bond, that is a concerted reaction in

which a C-N bond is broken and C-H and N-H bonds are formed, has not yet been presented

for metal sulfide catalysts. Therefore, the term hydrogenolysis should not be used to describe

C-N bond breaking until hydrogenolysis has been proven mechanistically.

Another alternative mechanism for the HDN of alkylamines would be

dehydrogenation to an imine (Fig. 1.14) followed by the addition of H2S, the removal of NH3,

as well as hydrogenation and hydrogenolysis.


C

H

NH

-H2C N

H2SC

SH

NH

C S HN+

C SH2

C

H

SHH2

H2S + C

H

H

Fig. 1.14 Mechanism of hydrogenolysis of an alkylamine by means of an imine.

Concurrent with the β-hydrogen elimination and nucleophilic substitution reactions, a

disproportionation reaction can occur between two alkylamine molecules, and this

complicates the study of the HDN reaction mechanism. This disproportionation results in the

formation of a dialkylamine and ammonia in case of an alkylamine (Fig. 1.15) and in the

formation of a trialkylamine and alkylamine in case of a dialkylamine; it takes place even on

alumina at low hydrogen pressure [89,90]. Substantial amounts of the disproportionation

products N-pentylpiperidine, dicyclohexylamine, and dipentylamine were observed in the

HDN of piperidine [90], cyclohexylamine [91], and pentylamine [6,92] respectively. An

imine mechanism can explain the formation of dialkylamine as well (Fig. 1.16). For example,

secondary ethylimine will be formed in HDN of ethylamine. Then the imine will react with an

ethylamine to form an intermediate, followed by elimination and hydrogenation to form

secondary ethylamine.

C

+ C N+

H

HH

N:H H

C

C N+

H

HH

NH H

C

C :N

H

HH

N+

H H

+

Fig. 1.15. Disproportionation mechanism.

NH2

NH N NHNH2

NH

Fig. 1.16. Imine mechanism to form a secondary amine in the HDN of ethylamine.


Several studies [92-98] have dealt with the HDN of aliphatic amines over different

catalysts. Portefaix et al. showed in HDN studies at 2 MPa over sulfided NiMo/Al2O3 that an

increase in the rate of formation of denitrogenation products occurred when going from

neopentylamine to amylamine and tert-amylamine; this increase corresponds to the increase in

the number of β−hydrogen atoms in the neopentylamine, amylamine and tert-amylamine. This

was taken as proof that aliphatic C-N bond cleavage takes place by Hofmann elimination.

Vivier et al. were the first to prove that C-N bond cleavage by nucleophilic substitution can

take place in the HDN of amines [99]. They observed that benzylamine and α,α-

diphenylmethylamine, which do not have β−hydrogen atoms and thus cannot react by

elimination, react fast to toluene and diphenylmethane respectively. Benzylamine and α,α-

diphenylmethylamine react most probably by nucleophilic substitution of the amine group by

an SH group followed by rapid hydrogenolysis of the intermediate thiol.

Cattenot et al. showed that both elimination and nucleophilic substitution play a role in

the C-N bond scission of pentylamines on unsupported transition-metal sulfides at

atmospheric pressure [92]. The ratio of the two mechanisms depended on the type of metal

sulfide catalyst and the type of amine. Over MoS2, n-pentylamine reacted by nucleophilic

substitution with H2S to pentanethiol as well as with another n-pentylamine molecule to

dipentylamine. Pentenes were observed as secondary products and supposed to be formed by

elimination from dipentylamine. These findings suggest that the molecular structure is one of

the most important factors in HDN and that different molecules may undergo nitrogen

removal by different mechanisms.

A detailed investigation of the HDN of alkylamines will be carried out over sulfided

NiMo/Al2O3. The aim of the work was to determine which of the different mechanisms

(elimination, substitution, and disproportionation) plays the major role in the HDN of

aliphatic amines. In Chapter 3 we will present our results of the HDN of the linear n-

alkylamines hexylamine, dihexylamine, and trihexylamine, while in chapter 5 we will present

our results of the HDN of alkylamines with the amine group attached to secondary and

tertiary carbon atoms.

1.4.3 C-N bond cleavage of hetero cyclic amines


Heterogeneous compounds like pyridine, quinoline, and acridine are the main nitrogen-

containing compounds in oil. Pyridine is often considered to be the simplest heterocyclic

nitrogen compound and is often used as a model compound for comparing the HDN activity

of catalysts and studying the HDN mechanism [100-104]. The supposed reaction scheme is

shown in Figure 1.16.

N N NH2 NH2

NH3 C5H10 C5H12

H

-

Figure 1.16 HDN network of pyridine over sulfided Ni-Mo/Al2O3 catalysts

The HDN of pyridine proceeds via the hydrogenation of the pyridine ring to piperidine,

followed by denitrogenation [6,105-108]. The hydrogenation of pyridine to piperidine is

inferred to take place on sulfur-deficient sites of the metal sulfide surface since a negative

effect of H2S was observed on this step. This reaction is favored by hydrogen as well

[6,107,109]. The second step, ring-opening of piperidine and the following nitrogen-removal

reaction, might occur by elimination and lead to alkene intermediates. This reaction is favored

by H2S [6,110,112]. At high H2S partial pressure, 2-methyl-thiacyclopentane and

thiacyclohexene were found as products [110,111]. The high H2S concentration induced a

nucleophilic substitution of the amino group of 5-aminopentene-1 by the SH group, and the

resulting 5-thiopentene-1 reacted intramolecularly to give a five- and six-membered

thiacyclo-alkane. The fact that these two molecules were observed in the HDN of pyridine,

piperidine, and 5-aminopentene-1, but not in that of 1-pentylamine, strongly supports the

mechanism shown in Figure 1.17. That ring opening and removal of nitrogen are promoted by

H2S indicates that these reactions take place on relatively sulfur-rich sites on the metal sulfide

surface.


+

N NH2

SSH

N NH2

S

Fig. 1.17. Possible HDN mechanism of pyridine via SH nucleophilic substitution

Thermodynamics may play an important role in the HDN of aromatic heterocycles. In

general, at higher temperatures the equilibrium between ring hydrogenation and

dehydrogenation shifts to the dehydrogenation side leading to a decrease in the equilibrium

concentration of the hydrogenated compound, and thus a decrease in nitrogen removal rate.

Satterfield and Cocchetto [101] observed that under such conditions, the overall rate of

pyridine HDN showed a maximum with increasing temperature. Therefore, the rate-

determining step in the HDN of pyridine is changing with reaction conditions. At a

temperature of 573 K and pressure of 3 MPa, the rate of hydrogenation of pyridine is about

the same as that of the ring-opening of piperidine over NiMo, CoMo, and NiW catalysts

[6,62,109,112]. H2S may have a promoting as well as a poisoning effect on the HDN of

pyridine under these conditions. At a low H2S/H2 ratio, pyridine hydrogenation to piperidine

is not inhibited, but ring opening of piperidine is occurring slowly, thus leading to a low

overall HDN conversion to C5 hydrocarbons. Under such conditions, the effect of adding

nickel to molybdenum is not very important for the total HDN to give C5 products [112]. This

observation has led some authors to conclude that nickel does not promote molybdenum in

HDN as it does in HDS [113,114]. However, at higher H2S/H2 ratio, hydrogenation of

pyridine to piperidine is retarded by H2S, but ring opening is accelerated. Thus, even at a

lower pyridine conversion, the yield of hydrocarbons increases [109]. At an even higher

H2S/H2 ratio, the negative effect of H2S is responsible for low conversions to piperidine and

low HDN conversions. At lower hydrogen pressures and higher reaction temperatures,

pyridine hydrogenation becomes rate determining, and H2S becomes increasingly toxic. At

higher hydrogen pressures and lower temperatures, H2S acts as a promoter because the ring-


opening reaction becomes rate determining. Under such conditions, nickel clearly promotes

the HDN of pyridine.

During the HDN of pyridine and piperidine, a higher molecular weight product, N-

pentylpiperidine is formed readily, especially when the partial pressure of intermediates

(piperidine and pentylamine) is high. It is formed by the disproportionation of piperidine and

pentylamine [6, 100, 114]. Even though N-pentylpiperidine can be denitrogenated at high

conversion of the reactant, this makes the HDN netwok of pyridine quite complicated to

study. It is well known that a nucleophilic attack is hindered by substitution on the α carbon

atom [115]. Therefore, 2-methylpyridine could be the right molecule to study the HDN

mechanism.

Portefaix et al. observed that the HDN reaction of 2,6-dimethylpiperidine was faster

than that of piperidine [93]. Their result suggests that the presence of a methyl group leads to

faster ring opening. Thereofore, it confirms that β−hydrogen has a very important influence

on the C-N bond cleavage.

Cerny and Trka performed their investigations in an autoclave at 15.5 MPa and 250 °C.

They concluded that the 2-methylpiperidine ring opens preferentially on the side that does not

contain the methyl group and that the HDN reactions of more substituted pyridine derivatives

are slower [116], namely, ring opening of 2-methylpiperidine by C-N bond cleavage occurred

preferentially on the CH2-N side and not on the CH(CH3)-N side. In the other words,

β−hydrogen does not show positive influence on the C-N bond cleavage. Further study will be

shown in Chapter 2.

1.5 Experimental

1.5.1 Unit

The hydrodenitrogenation reactions were performed in a continuous flow fixed-bed

reactor, where the solid catalyst is situated in the middle of a tubular reactor heated with an

oven. A simplified scheme (Fig. 1.18) shows the main parts of the unit.


Fig. 1.19. Simplified scheme of the unit.

The reactants (amine and sulfur compounds) were solved in cyclohexane, which was

used as a solvent, and in heptane, which is used as reference for the chromatograph. The feed

mixture is pumped with a syringe pump (Isco, Modell 500D). H2S was fed to maintain the

properties of the sulfided catalyst constant. H2 was fed to keep the high pressure. The gas

flows of hydrogen (H2) and of the hydrogen-hydrogensulfide mixture (90% H2/10% H2S)

were controlled by two mass flow controllers (Brooks, Series 5850E). Gases and liquids are

mixed before being heated, in order to avoid plugs in the liquid inlet. The liquid reactants

were fed co-current to the gases at the top of the reactor inlet. Gases and liquids are then

heated at 240°C in the pre-heater to achieve a homogeneous gas phase. Then, the gas feed

flows to the reactor, where the temperature was monitored and controlled inside the catalytic

bed. Above the catalytic bed, 8 g SiC was added to the reactor to achieve plug-flow

conditions and the desired temperature. The active catalyst (in general 0.05 g) was diluted

with 8 g SiC to obtain good heat transfer. The fixed-bed reactor was held in place using glass

wool and a metal support. A more detailed description of the reactor is given in the

dissertation of M. Flechsenhar [117]. After the reaction, the product flows to a 6-port valve

heated at 300°C and at the same pressure as the reactor, to maintain all products in the gas

phase (Fig. 1.19).


612

3

4 5

Reactor

Condenser inlet

He inlet

He outlet

Column

Injector

Sample

Fig. 1.19. Sampling system with the 6-port-valve.

Helium, which was the carrier gas for the column, was continuously flowing in the

valve (inlet port 4, outlet port 3). Reaction products were continuously flowing through the

sample loop (50 µl, port 5-2) and then to the condenser. When the sample was taken, the

valve turned 60°, to connect the helium inlet to the sample loop, so that the contents of the

sample loop was purged in the injector. After 12 seconds the valve returned to the original

position. In this period the reactor flow was led directly to the condenser. This sampling

system shows no mass balance problem if the operation temperature is above 240°C

(depending on the boiling point of the reactant used). The reaction products were injected

from the valve into the gas chromatograph equipped with a 30 m PTA-5 fused-silica capillary

column for quantitative analysis. Detection was made with a flame ionisation detector (FID)

as well as with a pulsed flame photometric detector (PFPD), which is especially sensitive to

nitrogen and sulfur-containing compounds. The detection method is discussed more

extensively in the dissertation of F. Rota [118]. After the sampling valve, the products were

condensed in the condenser (40 °C). The heavy products were collected as a fluid in the

condenser and the light gases (H2, H2S, and NH3) were purged outside the back pressure

regulator. The mixed gas was purified in a NaOH water solution. The total pressure was

maintained constant using a back pressure regulator. A safety system was integrated with the


unit. In case of over- and under-pressure (± 0.3 MPa) and in case of over- and under-heating

(± 20 °C) the inlet gases of H2 and H2/H2S mixture and liquid were interrupted.

1.5.2 Product analysis

The product analysis was performed online with a Varian gas chromatograph (Modell

3800) equipped with a flame ionization detector (FID) and a pulsed flame photometric

detector (PFPD). The FID detector was used to determinate the concentration of the carbon-

containing species. The signal of the FID is proportional to the number of carbon atoms

present in the molecule. All products were calibrated in the presence of heptane as an internal

standard to calculate the amount of each species. The response factors (Rf) of the compounds

were determined by injecting a known amount together with a known amount of heptane.

Then, with the relationship ni/nHeptane=Rf·Ai/AHeptane, it was possible to calculate the number of

moles of the compound i (ni) during the reaction.

1.5.3 Catalysts

The Ni(Co)Mo/γ-Al2O3 (Condea, BET surface area 210 m2/g, total pore volume 0.44

cm3/g) catalyst used in this work contained 8 wt% Mo and 3 wt% Ni or Co and was prepared

by successive pore-volume impregnation of γ-Al2O3 (CONDEA, pore volume: 0.5 cm3·g-1,

specific area: 230 m2·g-1) with an aqueous solution of (NH4)6Mo7O24·4H2O (Aldrich) and then

with an aqueous solution of Ni(Co)(NO3)2·6H2O (Aldrich). The catalysts were dried in air at

ambient temperature for 4 hours and then dried in an oven at 120°C for 15 hours after each

impregnation step. Then, they were finally calcined at 500°C for 4 hours. The catalysts were

crushed and sieved to 230 mesh (0.067 mm). The two catalysts were prepared in large amount

(20 g) so that all catalyst used in the catalytic experiments were from the same batch.

A sample of 0.050 g of catalyst was diluted with 8 g SiC to achieve plug-flow

conditions in the continuous flow fixed bed reactor. The oxidized form of the catalyst was

sulfided in situ with a mixture of 10 % H2S in H2 (25 ml/min) at 370°C and 1.0 MPa for 4

hours. The oxidized form of the catalyst is assumed to be mostly NiO and MoO3, whose

activities are practically zero. During sulfidation they transform to NiS, MoS2, and H2O.


1.5.4 Weight time

Weight time was defined as τ = wcat / nfeed, where wcat denotes the catalyst weight and

nfeed the total molar flow fed to the reactor. The weight time (τ) was changed by varying the

liquid and gaseous reactant flow rates, while their relative ratio was kept constant. In the

calculation of the weight time equations (1), (2) and the weight time definition are combined

to equation (3), and an ideal gas mixture behavior is assumed.

total total

reactant reactant

n pn p

= (1)

totaltotal feed reactant

reactant

( % ) pn F wtp

ρ•

= ⋅ ⋅ ⋅ (2)

cat cat cat

feed,totaltotal

feed reactantreactant

( % )

w wmoln pF wt timep

τ

ρ•

= = =

⋅ ⋅ ⋅

g (3)

ρfeed denotes the density of the liquid mixture, which contains 70% cyclohexane. Therefore

the density was always around 0.72 g/ml. •

F is the flow rate (ml/min) pumped with the

syringe pump. The unit of weight time is g·min/mol (1 g.min/mol = 0.68 · 10-3 g.h/l).

1.5.5 Reactants and reaction conditions used in every chapter

The NiMo/γ-Al2O3 catalyst was used in Chapter 2. After sulfidation of the catalysts, the

pressure was increased to 5.0 MPa, and the liquid reactant was fed to the reactor by means of

a high-pressure syringe pump (ISCO 500D). Blank experiments with and without SiC were

carried out at 573 and 623 K. The composition of the gas-phase feed in most experiments

consisted of 5 kPa amine reactant, 140 kPa decane (as solvent for the amine), 20 kPa heptane

(as reference for GC analysis), 20 kPa H2S, and 4.8 MPa H2 (unless indicated otherwise).

Mass spectrometry and NMR spectroscopy were used to identify the reaction products. The

MS analysis was performed with an Agilent 6890 gas chromatograph equipped with a HP-

5MS capillary column (crosslinked 5% PH ME siloxane, 30 m × 0.25 mm × 0.25 µm) and


with an Agilent 5973 mass selective detector. The temperature of the injector was 270 °C, the

initial temperature of the column oven was 80 °C, and heating to 300 °C started after 2 min at

20 °C /min. 1H and 13C NMR spectra of isolated compounds were recorded on a Bruker DPX-

300 instrument at 300 and 75 MHz, respectively, at room temperature using CDCl3 as a

solvent.

The NiMo/γ-Al2O3 catalyst was used in Chapter 3 as well. After sulfidation, the pressure

was increased to 3 or 5 MPa, and the liquid reactant was fed to the reactor by means of a high-

pressure syringe pump (ISCO 500D). Cyclohexane, decane, and octane were used as solvents

and heptane as an internal standard for GC analysis. The hydrogen pressure was varied from

2.8 to 4.8 MPa. Three types of molecules were used as reactants to study the HDS, HDN, and

hydrogenation reactions simultaneously. We used pentanethiol and hexanethiol as thiols,

hexylamine and cyclohexylamine as amines, and 1-hexene and cyclohexene as alkenes. The

choice of the alkanethiol, alkylamine, and alkene in a simultaneous HDS, HDN, and

hydrogenation experiment was primarily made so as to obtain separate peaks in the GC

analysis. All the chemicals were purchased as commercial standards from Aldrich and Fluka.

The partial pressure of the alkanethiols and alkenes was kept at 5 kPa, while the partial

pressure of the amines was 5, 10, or 20 kPa. The H2S pressure was varied between 10 and 150

kPa and the experiments were carried out at 300, 320, and 340 °C. When changing the partial

pressure of the reactant, the solvent flow was adapted to keep the partial pressure of hydrogen

constant. The product selectivity (S) was defined as the number of molecules converted to a

certain product (nP) divided by the number of converted reactant molecules (nR), both

multiplied by their number of carbon atoms, CnP and CnR respectively: S = (nP*CnP) /

(nR*CnR). With this definition, the mass balance of the carbon atoms is preserved. For

instance, in the reaction the selectivity of DHA

is 66.7% and the selectivity of hexanethiol is 33.3%. The original feed was usually re-entered

after the set of HDN experiments to check whether the activity of the catalyst had remained

constant. Then the whole reactor set-up was cleaned for another series of experiments. The

gases used were hydrogen (PanGas 4.0) and a mixture of 10% H2S in H2 (Linde).

6 13 3 2 6 13 2 6 13( ) ( )C H N H S C H NH C H SH+ → + ,

In Chapter 4, the same catalyst as in Chapter 2 and 3 was used. The total pressure in the

HDN and HDS experiments was 3 MPa and the partial pressure of the alkylamines was 5 kPa.

The experiments were carried out at 270, 300, and 340 °C with a partial pressure of H2S of 10

or 100 kPa. When changing the partial pressure of the reactant, the solvent flow was adapted


to keep the partial pressure of hydrogen constant. The accuracy in the measured conversion

was 2% (relative). 2-Pentylamine (Lancaster), 2-pentanethiol (Aldrich), 3-methyl-2-

butylamine (Aldrich, 98%), 3-methyl-2-butanethiol (ABCR, 100%), 3,3-dimethyl-2-

butylamine (ABCR, 100%), 2-methylcyclohexylamine (Fluka), 2-methyl-2-butylamine

(Aldrich, 98%), 2-methyl-2-butanethiol (TCI, Japan, 99%), benzylamine (Fluka, purum), α-

toluenethiol (Fluka, purum), and cyclohexane (Fluka, puriss.) were all used as purchased in

the HDN tests. 2-Methyl-3-pentylamine was synthesized from 2-methylpentanone-3 by

reaction with hydroxylamine and reduction of the resulting oxime with LiAlH4 in ether. The

product was purified by destillation.

In chapter 5, HDN and HDS experiments were carried out over NiMo/Al2O3,

CoMo/Al2O3 and Mo/Al2O3 catalysts. The total pressure was 3 MPa and the partial pressure

of the alkylamines was 5 kPa. The HDS of alkanethiols was studied at 5 kPa in the

simultaneous reaction with 5 kPa of the relevant alkylamine. The experiments were carried

out at 270, 300, 320 and 370 °C with a partial pressure of H2S of 10 or 100 kPa. When

changing the partial pressure of H2S, the hydrogen pressure was adapted to keep the total

pressure constant. Pentylamine (Aldrich), hexanethiol (Fluka), 2-hexylamine (Lancaster), 2-

pentanethiol (Aldrich), 2-methyl-2-butylamine (Aldrich, 98%), 2-methyl-2-butanethiol

(ABCR), and cyclohexane (Fluka, puriss.) were all used as purchased.

In chapter 6, a NiMo/Al2O3 catalyst was used in the HDN and HDS reactions. The total

pressure was 3 MPa and the partial pressure of neopentylamine was 5 kPa in all experiments.

The partial pressures of the adamantylamines and 1-adamantanethiol were kept at 1 kPa to

avoid condensation. The experiments were carried out at 300 and 340 °C with a partial

pressure of H2S of 10 and 100 kPa.1-Adamantylamine (Aldrich, pract.), 2-adamantylamine

(Fluka, purum), 1-adamantanethiol (Apin Chemicals Limited), adamantane (Fluka, purum),

neopentylamine (TCI, purum) and cyclohexane (Fluka, puriss.) were all used as received. For

the purpose of solubility, 1-adamantylamine, 2-adamantylamine, 1-adamantanethiol and

neopentylamine were dissolved in cyclohexane.

In chapter 7, the NiMo/Al2O3 and CoMo/Al2O3 catalysts were in HDN reactions. For the

purpose of solubility, 1-naphthylamine (Aldrich, 98%), 1,2,3,4-tetrahydro-1-naphthylamine

(Acros, 98%), 1,2-dihydronaphthalene (Fluka, 98%), 5,6,7,8-tetrahydro-1-naphthylamine

(Aldrich, 99%), tetralin (ABCR) and naphthalene (ABCR) were dissolved in benzene or

toluene (Fluka, puriss. p.a.). The catalytic experiments were performed in a stirred 17-ml


autoclave as well as in a microflow reactor. The NiMo/Al2O3 and CoMo/Al2O3 catalysts that

were used in the autoclave were sulfided in a glass flow reactor by heating at 5 °C/min and

then sulfiding at 400 °C for 4 h in a mixture of 10% H2S in H2. Thereafter, nitrogen was

passed through the reactor at the same temperature for 0.5 h and subsequently the catalysts

were cooled to room temperature. The reactor was opened to air at room temperature. An

amount of 5 to 10 mg of catalyst was transferred to the autoclave and resulfided for 1 h in a

mixture of 10% H2S in H2 at 400 °C (heating rate 5 °C/min) and 0.35 MPa. After

resulfidation, the catalyst was cooled to room temperature, the autoclave was opened and the

reaction mixture (0.6 or 1.0 ml) was added quickly. After closing the autoclave, hydrogen was

added up to 0.6 MPa at room temperature and the temperature was increased to the reaction

temperature (between 300 and 350 °C). Liquid samples of 0.05 to 0.1 ml were collected at

different times and analyzed off-line by gas chromatography (Shimadzu GC-14 B), using an

HP1 (cross-linked methyl siloxane) or a DB-5ms (5%-phenyl methylpolysiloxane) column

and a flame ionization detector. The experiments in the microflow reactor were carried out

with the NiMo/Al2O3 catalyst only. A sample of 0.02 g NiMo/Al2O3 diluted with 8 g SiC was

first dried for 2 h at 400 °C and then sulfided for 4 h in situ with a mixture of 10% H2S in H2

at 1 MPa. After sulfidation, the pressure was increased to reaction pressure and the solution of

the reactand in toluene was fed to the reactor with a high-pressure syringe pump.

1.6 References [1] M.J. Girgis, B.C. Gates, Ind. Eng. Chem. Res 30 (1991) 2021.

[2] J.K. Minderhoud, J.A.R. van Veen, Fuel Proc. Tech. 35 (1993) 87.

[3] R.A. Sanchez-Delgado, Organometallic Modeling of the HDS and HDN Reactions,

Kluwer Academic, 2002.

[4] T.C. Ho, Catal. Rev. Sci. Eng. 30 (1988) 117.

[5] R.M. Laine, Catal. Rev. Sci. Eng. 25 (1983) 459.

[6] H. Schulz, M. Schon, N.M. Rahman, Stud. Surf. Sci. Catal. 27 (1986) 201.

[7] R. Prins R., V.H.J. de Beer, G.A. Somorjau, Catal. Rev. Sci. Eng. 31 (1989) 1.

[8] A.N. Startsev, Catal. Rev. Sci. Eng. 37 (1995) 353.


[9] B. Delmon, G.F. Froment, Catal. Rev. Sci. Eng. 38 (1996) 69.

[10] R. Prins, Handbook of Heterogeous Catalysis 4, Ed. Ertl G., Knözinger H., Weitkamp

J., Wiley-VCH Wienheim (1997) 1908.

[11] H. Knözinger, in Proc. 9th Int. Congress Catal. (Eds. M.J. Phillips, M. Ternan), The

Chemical Institute of Canada, Ottawa, 5 (1988) 20.

[12] M. Lewandowski, Z. Sarbak, Fuel 79 (2000) 487.

[13] M. Jian, R. Prins, J. Catal. 179 (1998) 18.

[14] M.Y. Sun, D. Nicosia, R. Prins, Catal. Today 86 (2003) 173.

[15] F. Gioia, F. Murena, J. Hazardous Materials 57 (1998) 177.

[16] A. Stanislaus, B.H. Cooper, Catal. Rev-Sci. Eng. 36/1 (1994) 75.

[17] M. Adachi, C. Contescu, J.A. Schwarz, J. Catal. 162 (1996) 66.

[18] S. Kasztelan, J. Grimblot, J.P. Bonnelle, J. Phys. Chem. 91 (1987) 1503.

[19] N. Topsøe, H. Topsøe, J. Catal. 75 (1982) 354.

[20] X. Gao, Q. Xin, Catal. Lett. 18 (1993) 409.

[21] C. Wivel, B.S. Clausen, R. Candia, S. Morup, H. Topsøe, J. Catal. 87 (1984) 497.

[22] R. Prada Silvy, P. Grange, B. Delmon, Stud. Surf. Sci. Catal. 52 (1990) 233.

[23] P. Arnoldy, J.A.M. Van den Heijkant, G.D. de Bok, J.A. Moulijn, J. Catal. 92 (1985)

35.

[24] R. Cattaneo, T. Weber, T. Shido, R. Prins, J. Catal. 191 (2000) 225.

[25] B.S. Clausen, H. Topsøe, R. Candia, J. Villadsen, B. Lengeler, J. Als-Nielsen, F.

Christensen, J. Phys. Chem. 85 (1981) 3868.

[26] S.M.A.M Bouwens, R. Prins, V.H.J. de Beer, D.C. Koningsberger, J. Phys. Chem. 94

(1990) 3711.

[27] T. Shido, R. Prins, J. Phys. Chem. B 102 (1998) 8426.

[28] L.S. Byskov, B. Hammer, J.K. Norskov, B.S. Clausen, H. Topsøe, Catal. Lett. 47

(1997) 177.

[29] L.S. Byskov, J.K. Norskøv, B.S. Clausen, H. Topsoe, J. Catal. 187 (1999) 109.

[30] P. Raybaud, J. Hafner, G. Kresse, S. Kasztelan, H. Toulhoat, J. Catal. 189 (2000) 129.

[31] P. Raybaud, J. Hafner, G. Kresse, S. Kasztelan, H. Toulboat, J. Catal. 190 (2000) 128.

[32] H. Topsøe, B.C.Clausen, F.E. Massoth, Hydrotreating Catalysis. In Catalysis, Science

and Technology (Eds. J. Anderson, M. Boudart, Springer Berlin, 1996 Vol. 11.


[33] T. Kabe, A. Ishikawa, W. Qian, Hydrodesulfurization and Hydrodenitrogenation,

Wiley–VCH 1999.

[34] F.E. Massoth, Adv. Catal. 27 (1978) 265.

[35] R.R. Chianelli, Catal. Rev. Sci. Eng. 26 (1984) 361.

[36] S. Ren, Z. Wang, Y. Hu, Ranliao Huaxue Xuebo 15 (1987) 255.

[37] G. Perot, Catal. Today 10 (1991) 447.

[38] E. Furimsky, F.E. Massoth, Catal. Today 52 (1999) 381.

[39] L. Qu, R. Prins, J. Catal. 207 (2002) 286.

[40] R.B. Moyes, in Hydrogen Effects in Catalysis (Eds. Z. Paal, P.G. Menon) Marcel

Dekker, 1988, 583.

[41] E.M. Badger, R.H. Griffith, W.B.S. Newling, Proc. Soc. (Lond.) 1949, A 197, 184.

[42] D.G. Kalthod, S.W. Weller, J. Catal. 95 (1985) 455.

[43] J. Polz, H. Zeilinger, B. Muller, H. Knözinger, J. Catal. 120 (1989) 22.

[44] J. Barbour, K.C. Campbell, Chem. Commun. (1982) 1371.

[45] P. Sundberg, R.B. Moyes, J. Tomkinson, Bull. Soc. Chim. Belg. 100 (1991) 967.

[46] P. Ratnasamy, J. Fripiat, Trans. Faraday Soc. 66 (1970) 2897.


[48] S. Vasudevan, J.M. Thomas, C.J. Wright, C. Sampson, J. Chem. Soc., Chem.

Commun. 1982, 418.

[49] C.J. Wright, D. Fraser, C. Sampson, R.B. Moyes, P.B.J. Wells, J. Chem. Soc. Faraday

Trans. 176 (1980) 1585.

[50] C. Vogdt, T. Butz, A. Lerf, H. Knözinger, Polyhedron 5 (1986) 95.

[51] C.J. Wright, D. Fraser, R.B. Moyes, P.B. Wells, Appl. Catal. 1 (1981) 49.

[52] C. Sampson, J.M. Thomas, S. Vasudevan, C.J. Wright, Bull. Soc. Chim. Belg. 90

(1981) 1215.

[53] P.N. Jones, E. Knözinger, W. Langel, R.B. Moyes, Tomkinson, Surf. Sci. Catal.

207(1988) 159.

[54] M. Lacroix, C. Dumonteil, M. Breysse, Am. Chem. Soc. Div. Petr. Chem. Prepr. 1998

35.

[55] C.G. Wiegenstein, K.H. Schultz, Surf. Sci. 396 (1998) 284.

[56] P.C.H. Mitchel, D.A. Green, E. Payen, A.C. Evans, J. Chem. Soc. Faraday Trans. 91

(1995) 4467.


[57] J. Maternova, Appl. Catal. 3 (1982) 3.

[58] J. Maternova, Appl. Catal. 6 (1983) 61.

[59] X.S. Li, Q. Xin, X.X. Guo, P. Grange, B. Delmon, J. Catal. 137 (1992) 385.

[60] L. Jalowiecki, A. Aboulaz, S. Kasztelan, J. Grimblot, J.P. Bonelle, J. Catal. 120 (1989)

108.

[61] L. Qu, R. Prins, Appl. Catal. A 250 (2003) 105.

[62] C. Moreau, C. Aubert, R. Durand, N. Zimita, P. Geneste, Catal. Today 4 (1988) 117.

[63] L. Vivier, S. Kasztelan, G. Perot, Bull. Soc. Chim. Belg. 100 (1991) 801.

[64] L. Vivier, P. d’Araujo, S. Kasztelan, G. Perot, Bull. Soc. Chim. Belg. 100 (1991) 806.

[65] M.S. Rana, B.N. Srinivas, S.K. Maity, G. Murali Dhar, T.S.R. Prasado Rao, J. Catal.

195 (2000) 31.

[66] M.L. Occelli, R. Chianelli, Hydrotreating Technology for Pollution Control, Marcel

Dekker 1996.

[67] H. Topsøe, B.S. Clausen, N.Y. Topsøe, P. Zeuthen, Stud. Surf. Sci. Catal. 53 (1990)

77.

[68] S. Kasztelan, Langmuir 6 (1990) 590.

[69] A. Wambeke, L. Jalowiecki, S. Kasztelan, J. Grimblot, J.P. Bonnelle, J. Catal. 109

(1988) 320.

[70] S. Kasztelan, L. Jalowiecki, A. Wambeke, J. Grimblot, J.P. Bonnelle, Bull. Soc. Chim.

Belg. 96 (1987) 1003.

[71] M.H. Farias, A.J. Gellman, G.A. Somorjai, R.R. Chianelli, K.S. Liang, Surf. Sci. 140

(1984) 181.

[72] C. Wivel, R. Candia, B.S. Clausen, S. Mørup, H. Topsøe, J. Catal. 68 (1981) 453.

[73] J.C. Duchet, E.M. van Oers, V.H.J. de Beer, R. Prins, J. Catal. 80 (1983) 386.

[74] J.P.R Vissers, V.H.J. de Beer, R. Prins, J. Chem. Soc. Faraday Trans. 83 (1987) 2145.


[76] S.P.A. Louwers, R. Prins, J. Catal. 133 (1992) 94.

[77] A.N. Startsev, S.A. Shkuropat, V.V. Kriventsov, D.I. Kochubey,

K.I. Zamaraev, Mendeleev Commun. (1991) 6.

[78] L. Medici, M. Harada, R. Prins, Mendeleev Commun. (1996) 121.

[79] R.G. Leliveld, A.J. van Dillen, J.W. Geus, D.C. Koningsberger, J. Phys. Chem. B 101

(1998) 11160.


[80] R.G. Leliveld, A.J. van Dillen, J.W. Geus, D.C. Koningsberger, J. Catal. 175 (1998)

108.

[81] J.K. Nørskov, B.S. Clausen, H. Topsøe, Catal. Lett. 13 (1992) 1.

[82] H. Toulhoat, P. Raybaud, S. Kasztelan, G. Kresse, J. Hafner, Catal. Today 50 (1999)

629.

[83] C.D. Chang, W.H. Lang, US Pat. 4380669 (1980), to Mobil Comp.; C.D. Chang, P.D.

Perkins, Eur. Pat. 0082613 (1983), to Mobil Comp.

[84] T. Stamm, H.W. Kouwenhoven, D. Seebach, R. Prins, J. Catal. 155, (1995) 268.

[85] A. Tschumper, R. Prins, Appl. Catal. 172 (1998) 285; 174 (1998) 129.

[86] N. Nelson, R.B. Levy, J. Catal. 58 (1979) 485.

[87] M. Zdrazil, J. Catal. 141 (1993) 316.

[88] S. Rajagopal, R. Miranda, J. Catal. 141 (1993) 318.

[89] M. Fikry Ebeid, J. Pasek, Coll. Czech. Chem. Commun. 35 (1970) 2166.

[90] P. Hogen, J. Pasek, Coll. Czech. Chem. Commun. 39 (1974) 3696.

[91] S. Eijsbouts, C. Sudhakar, V.H.J. de Beer, R. Prins, J. Catal. 127 (1991) 605.

[92] M. Cattenot, J.L. Portefaix, J. Afonso, M. Breysse, M. Lacroix, G. Perot, J. Catal. 173

(1998) 366.

[93] J.L. Portefaix, M. Cattenot, M. Guerriche, J. Thivolle-Cazat, M. Breysse, Catal. Today

10 (1991) 473.

[94] J.L. Portefaix, M. Cattenot, M. Guerriche, M. Breysse, Catal. Lett. 9 (1991) 127.

[95] M. Egorova, Y. Zhao, P. Kukula, R. Prins, J. Catal. 206 (2002) 263; and chapter 2

[96] M. Breysse, J. Afonso, M. Lacroix, J.L. Portefaix, M. Vrinat, Bull. Soc. Chim. Belg.

100 (1991) 923.

[97] J.H. Lee, C.E. Hamrin, B.H. Davis, Appl. Catal. A 111 (1994) 11.

[98] P. Clark, X. Wang, P. Deck, S.T. Oyama, J. Catal. 210 (2002) 116.

[99] L. Vivier, L. Dominguez, G. Perot, S. Kasztelan, J. Mol. Catal. 67 (1991) 267.

[100] M. J. Ledoux, A. Bouassida, R. Benazouz, Appl. Catal. 9 (1984) 41.

[101] C. N. Satterfield, J. F. Cocchetto, AIChE J. 21 (1975) 1107.

[102] A. Benitez, J. Ramirez, A. Vazquez, D. Acosta, A. Lopez Agudo, J. Catal. 133 (1995)

103.

[103] H. S. Joo, J. A. Guin, Fuel Process. Techn. 49 (1996) 137.

[104] J. Cinibulk, Z. Vit, Appl. Catal. A 204 (2000) 107.


[105] P. Betancourt, A. Rives, C. E. Scott, R. Hubaut, Catal. Today 57 (2000) 201.

[106] C. N. Satterfield, M. Modell, J.A. Wilkens, Ind. Eng. Chem. Proc. Des. Dev. 19

(1980) 154.

[107] R. T. Hanlon, Energy Fuels 1 (1987) 424.

[108] G. C. Hadjiloizou, J. B. Butt, J. S. Dranoff, Ind. Eng. Chem. Res. 31 (1992) 2503.

[109] M. Jian, J.L. Rico Cerda, R. Prins, Bull. Soc. Chim. Belg. 104 (1995) 225.

[110] M. Cerny, Coll. Czech. Chem. Commun. 47 (1982) 928.

[111] M. Cerny, A. Trka, Coll. Czech. Chem. Commun. 48 (1983) 1749.

[112] Z. Vit, M. Zdrazil, J. Catal. 119 (1989) 1.

[113] M. J. Ledoux, G. Agostini, R. Benazouz, O. Michaux, Bull. Soc. Chim. Belg. 93

(1984) 635.

[114] N.Y. Topsøe, H. Topsøe, F.E. Massoth, J. Catal. 119 (1989) 252.

[115] J. March, Advanced Organic Chemistry, 3rd ed., Chap. 10. Wiley, New York, 1985.


[117] M. Flechsenhar, Dissertation 12471, ETH Zürich, 1997.

[118] F. Rota, Dissertation 14085, ETH Zurich, 2001.

HDN of 2-Methylpiperidine Chapter 2 35

2. On the role of β-hydrogen atoms in the hydrodenitrogenation of 2-

methylpyridine and 2-methylpiperidine

2.1 Abstract

The hydrodenitrogenation (HDN) of 2-methylpyridine and its intermediate products 2-

methylpiperidine, 1-aminohexane, and 2-aminohexane was studied. The presence of most

intermediates could be explained by a combination of pyridine ring hydrogenation, piperidine

ring opening by elimination, and nitrogen removal by elimination, as well as by nucleophilic

substitution of the amino group by a sulfhydryl group, followed by elimination of H2S or

hydrogenolysis of the C–S bond. Aminoalkenes, which are expected to be the primary

products of the ring opening of alkylpiperidine, were not observed, probably because of fast

hydrogenation to the corresponding amines. The ring opening of 2-methylpiperidine occurred

preferentially between the nitrogen atom and the methylene group, rather than between the

nitrogen atom and the carbon atom bearing the methyl group. This was confirmed by

comparative HDN experiments of piperidine, 2-methylpiperidine, and 2,6-dimethylpiperidine.

Although the methyl groups offer extra β hydrogen atoms, these primary hydrogen atoms are

not used for elimination. Instead, the methyl groups hinder the adsorption leading to the

elimination of the β hydrogen atoms on the side of the molecule bearing the methyl group.

2.2 Introduction

Heterocyclic compounds like pyridine, quinoline, and acridine are the main nitrogen-

containing compounds in oil. They are removed by hydrodenitrogenation (HDN) in a

hydrotreating process in which gasoline or gas oil is treated with hydrogen over a metal

sulfide catalyst like nickel promoted molybdenum sulfide (Ni–MoS2) supported on alumina

[1]. Several groups have studied the HDN of pyridine [2–7], because, as the smallest nitrogen-

containing heterocyclic molecule, pyridine was believed to be the simplest model molecule to


study HDN. Although the network of reactions taking place in the HDN of pyridine is now

well understood, the study of the kinetics of the HDN of pyridine proved to be extremely

difficult. The reason for this difficulty is the occurrence of a side reaction of piperidine, the

first intermediate in the HDN of pyridine. Two piperidine molecules disproportionate to N-

pentylpiperidine and ammonia [1–5]. Opening of the piperidine ring and removal of ammonia

can take place from piperidine as well as from N-pentylpiperidine. Consequently the network

of the HDN becomes very complicated and a trustworthy kinetic analysis of the separate

reactions is almost impossible.

The disproportionation of piperidine to N-pentylpiperidine takes place by nucleophilic

substitution at the carbon atom in the α position to the nitrogen atom in the piperidine ring

(Fig. 2.1) [3, 8]. It is well known that a nucleophilic attack is hindered by substitution on the α

carbon atom [9]. Substitution of a hydrogen atom by a methyl group on the α carbon atom

might therefore hinder the disproportionation so much, that it is strongly suppressed and that

it hardly interferes with the other reactions taking place during the HDN of pyridine and

piperidine. Therefore we decided to study the HDN of 2-methylpyridine and 2-

methylpiperidine.

NN

+H H H

NH H

NH N

H HN

NH H

N N + NH3

-H

+ H2

Fig. 2.1. Mechanism of the disproportionation of piperidine.

2-Methylpyridine and 2-methylpiperidine were studied before by Cerny and Trka [10,

11] and Ren et al. [12]. Ren et al. studied the Langmuir–Hinshelwood–Hougen–Watson

kinetics of the HDN of 2-methylpyridine in a continuous-flow reactor at 4.9 MPa and 240–

280 °C [12]. They observed 2-methylpiperidine as primary product and hexane and

cyclohexane as final products. No intermediates between 2-methylpiperidine and hexane were

reported. Cerny and Trka performed their investigations in an autoclave at 15.5 MPa and 250

°C. Because of the high H2 pressure, low temperature, and absence of H2S in their


experiments, mainly ring hydrogenation and only a small amount of products due to nitrogen

removal were observed. They concluded that the 2-methylpiperidine ring opens preferentially

on the side that does not contain the methyl group and that the HDN reactions of more

substituted pyridine derivatives are slower [10]. This is in disagreement with the results of

Portefaix et al., who observed that the HDN reaction of 2,6-dimethylpiperidine was faster

than that of piperidine [13]. Their result suggests that the presence of a methyl group leads to

faster ring opening. Portefaix et al. performed their HDN work at the much lower H2 pressure

of 2 MPa and relatively high H2S pressure of 33.3 kPa; this may explain the different results.

Further study is clearly called for.

Another reason for studying the HDN of 2-methylpiperidine is the presence of three

additional hydrogen atoms on the methyl carbon atom in β position relative to the nitrogen

atom. HDN occurs (partly) via Hofmann elimination in which, on the one hand, the bond

between the α carbon atom and the nitrogen atom is broken and, on the other hand, the bond

between a hydrogen atom and the β carbon atom is broken. Portefaix et al. compared the

HDNs of piperidine, 3,5-dimethylpiperidine, and 2,6-dimethylpiperidine and concluded that

Hofmann elimination is quicker when more β hydrogen atoms are present [13]. This

implicates elimination of a β H atom from the methyl groups in 2,6-dimethylpiperidine as an

important step in the HDN of this molecule. Portefaix et al. reported only the conversion of

the reactant and nothing about the resulting products. Therefore, it seemed of interest to

investigate if the elimination reaction of 2-methylpiperidine takes place by removal of a

hydrogen atom from the methyl group and leads preferentially to 1-aminohexane.

2.3 Results

2.3.1. HDN of 2-Methylpyridine

The results of the HDN of 2-methylpyridine at 340 °C, 4.8 MPa H2, and 20 kPa H2S are

shown in Figure 2.2. No products with mass higher than that of the reactant (such as

condensation products) were observed and the mass balance was always better than 95%. The

product selectivities show (Fig. 2.2b) that 2-methylpiperidine is the only primary product, as

expected, since the HDN of heterocyclic N-containing aromatic molecules can occur only


after ring hydrogenation [1, 3, 14, 15]. The maximum yield of 34% 2-methylpiperidine and its

selectivity against 2-methylpyridine conversion indicate that the ratio of the effective rate

constants of formation and further reaction of 2-methylpiperidine is about 0.8 [16].

Fig. 2.2. Relative concentrations (a) and selectivities (b) of the products of the HDN of 2-

methylpyridine as a function of weight time.

2-Hexene (cis and trans), 1-hexene, and hexane were observed as the main secondary

products (Fig. 2.2b). These products are actually expected to be tertiary products, because

HDN of aliphatic amines is generally considered to occur by Hofmann elimination or by

nucleophilic substitution of the NH2 group by an SH group followed by elimination or

hydrogenolysis [1, 14]. In either case, the nitrogen atom of 2-methylpiperidine is removed in

two steps. The first step is a ring opening by C–N bond breaking and the second step is the

removal of the nitrogen atom in the form of ammonia by breaking the other C–N bond. Of the

products that are possible after the first C–N bond breakage, only traces of 1-aminohexane

and 2-aminohexane were observed. The reason is that their rates of further reaction are much

higher than their rates of formation, as discussed in Sections 4 and 5. These amines have a

high basicity and thus larger equilibrium adsorption constants than 2-methylpyridine. Even at

low concentration they may therefore have an important (inhibiting) influence on the HDN

kinetics [15]. For that reason, the HDN of 2-methylpiperidine, the primary product of the

HDN of 2-methylpyridine, and of 1-aminohexane and 2-aminohexane, the expected

secondary (or tertiary, see below) products, were studied in detail as well.


2.3.2. HDN of 2-Methylpiperidine

The HDN of 2-methylpiperidine was carried out at 340 °C, 4.8 MPa H2, and 20 kPa

H2S. Figure 2.3 shows that at least four compounds have nonzero selectivity at zero

conversion of 2-methylpiperidine and thus might be considered primary products. Three of

these products were identified by their GC retention times and mass spectra as 1-

aminohexane, 2-aminohexane, and 2-methylpyridine. The yield of 2-aminohexane was much

higher than that of 1-aminohexane. This confirms the results of Cerny [10], although they

were obtained under quite different conditions, and suggests that the bond between the N

atom and the methylene group is more easily broken than that between the N atom and the

CH(CH3) group. This is also perfectly in line with the results of Cattenot et al. [8] and Vivier

et al. [17], indicating that the amino group bonded to a methylene group cleaves very easily

by nucleophilic substitution (SN2).

GC–MS showed that the fourth compound had a molecular weight of 97, but no

commercially available compound could be found that had the same retention time and a

matching mass spectrum. Therefore, the product of the HDN reaction was collected and a

fraction that contained the basic nitrogen-containing molecules was separated from a

hydrocarbon fraction. Since pulsed flame photometric detection had shown that the fourth

compound contains a nitrogen atom, it was extracted from the HDN product with an aqueous

HCl solution. Neutralization of this aqueous extract and subsequent extraction with

chloroform gave a chloroform solution of all primary products as well as the remaining 2-

methylpiperidine. After evaporation of the chloroform, the mixture of nitrogen-containing

compounds was separated by column chromatography using silicagel and a 50 : 50 : 1

solution of CH3OH: CHCl3 :NH4OH (25% aqua solution of NH3) as a mobile phase. The

fraction containing the fourth unknown product was evaporated and the raw material obtained

with a purity of 90% was analyzed by NMR spectroscopy. The 1H NMR spectrum of the

fourth compound in CDCl3 showed peaks at δ 3.46–3.52 (m, 2H, CH2N), 2.13 (t of t, 3 J =6.5

Hz, 5 J =1.8 Hz, 2H, CH2C=), 1.91 (t, 5 J =1.8 Hz, 3H, CH3), 1.62–1.71 (m, 2H, CH2CH2C=),

and 1.51–1.59 ppm (m, 2H, CH2CH2N), while its 13C NMR spectrum in CDCl3 showed peaks

at δ 168.45 (s, CN), 48.98 (t, CN), 30.26 (t, CH2C=), 27.33 (q, CH3), 21.57 (t, CH2CH2N),

and 19.52 ppm (t, CH2CH2C==). The NMR spectra together with the mass spectrum obtained

(MS (EI, 70 eV) m/z 97 (M+, 63), 96 (11), 69 (61), 68 (21), 56 (26), 55 (17), 54 (15), 42 (100),


41 (65), 39 (33), 28 (41), 27 (26)) enabled us to identify the fourth primary product as 2-

methyl-3,4,5,6-tetrahydropyridine. Both NMR and mass spectra were in accord with the

spectra assigned to this molecule in the literature [18, 19].

Fig. 2.3 Relative concentration (a) and selectivities (b) of the products of the HDN of 2-

methylpiperidine as a function of weight time.

HDN experiments with 2-methylpiperidine under conditions other than 340 °C and 5

MPa suggested that 2-methyl-3,4,5,6-tetrahydropyridine is formed by a catalytic as well as a

thermal reaction. Experiments in the empty steel reactor and in the reactor filled with SiC

only, without catalyst, showed that the 2-methyl-3,4,5,6-tetrahydropyridine yield increased by

increasing the temperature from 300 to 350 °C, as expected for a simple dehydrogenation

reaction. Over the NiMo/Al2O3 catalyst diluted with SiC, however, the yield decreased

substantially from 300 to 350 °C. The 2-methyl-3,4,5,6-tetrahydropyridine yield was also

lower over the catalyst than over the SiC or in the empty reactor. This suggests that in the

presence of the catalyst, there occurs not only dehydrogenation but also reactions to other

products, which lower the yield of 2-methyl-3,4,5,6-tetrahydropyridine.As expected, in the

presence of the catalyst, decreasing the H2 pressure from 5 to 3 MPa raised the 2-methyl-

3,4,5,6-tetrahydropyridine yield.

2.3.3. Comparison of Piperidine, 2-Methylpiperidine, and 2,6-Dimethylpiperidine

As indicated in the Introduction, Portefaix et al. reported that the amount of HDN

product was larger for 2,6-dimethylpiperidine than for piperidine under the following reaction

conditions: 275 °C, 2 MPa H2, and 33.3 kPa H2S [13]. They related this to the presence of


more β H atoms in 2,6-dimethylpiperidine, which would facilitate Hofmann elimination.

These results seem to contradict those of Cerny [10] and our results for 2-methylpiperidine

described in the previous section, which suggested that the ring opening occurs preferentially

between the nitrogen atom and the methylene group and not between the nitrogen atom and

the carbon atom bearing the methyl group. We therefore decided to repeat the measurements

of Portefaix et al. under their conditions.

From the results presented in Figure 2.4 it is clear that the conversion of piperidine is

very slow and hardly reaches 2% at a weight time of 10 g.min/mol, whereas the conversion of

2-methylpiperidine is almost 20% and that of cis-2,6-dimethylpiperidine is more than 50% at

the same weight time. The conversions at a weight time of 2.4 g.min/mol are in good

agreement with those of Portefaix et al. [13]. Analyzing the resulting products, however, we

found that for piperidine the main product was not that of HDN but pyridine. For 2-

methylpiperidine the main products were 2-methylpyridine (17%) and 2-methyl-3,4,5,6-

tetrahydropyridine (64%), and for cis-2,6-dimethylpiperidine the main products were 2,6-

dimethylpyridine (2%), 2,6-dimethyl-3,4,5,6-tetrahydropyridine (66%), and trans-2,6-

dimethylpiperidine (32%), with the selectivities in parentheses.

Fig. 2.4 Total conversion in the HDN of piperidine, 2-methylpiperidine, and cis-2,6-

dimethylpiperidine as a function of weight time at 275 °C, 2 MPa, and 33.3 kPa

H2S.

The observed high selectivities to fully dehydrogenated pyridine molecules at low

conversions are not in contradiction with thermodynamics, which indicates that the

pyridine/piperidine ratio cannot be higher than 0.01 at 275 °C and 2 MPa H2 [20]. The

tetrahydropyridine/piperidine ratio can be much higher, however. Portefaix et al. apparently


underestimated the latter ratio and the isomerization of cis-2,6-dimethylpiperidine to trans-

2,6-dimethylpiperidine, when assuming, without any product analysis, that most of the

piperidine-type molecules would convert to HDN products. At high weight time and high

conversion, thermodynamic controls and HDN products will indeed dominate. At low

conversion, however, kinetics may dominate the product distribution and it is in this regime

that mechanistic results should be obtained.

All the compounds mentioned above are products of dehydrogenation and

isomerization, and not of HDN or C–N bond cleavage. The selectivities for ring opening and

HDN were calculated from the sum of the observed amines and saturated and unsaturated

hydrocarbons and amounted to 9% for piperidine, 5% for 2-methylpiperidine, and 0.7% for

cis-2,6-dimethylpiperidine at 2.4 g.min/mol. These selectivities are small; dehydrogenation

and isomerization (for cis-2,6-dimethylpiperidine) dominate at the low H2 pressure of 1.8

MPa. The yield (selectivity times conversion) of these ring opening and HDN products was

indeed higher for cis-2,6-dimethylpiperidine than for piperidine, as reported by Portefaix et al.

[13].

We also studied these three piperidine molecules under conditions in which elimination

is the dominating reaction, so that a fair comparison of the HDN rates of the three molecules

could be made. At 340 °C, 5 MPa, and 20 kPa H2S the 2-methylpiperidine conversion was

20% lower than that of piperidine, while the conversion of cis-2,6-dimethylpiperidine was

higher than that of piperidine below a weight time of τ = 5.5 g.min/mol and lower above this

value. The reason for the high initial reaction rate of cis-2,6-dimethylpiperidine is fast

isomerization of cis to trans-2,6-dimethylpiperidine. For τ >5.5 g.min/mol, the equilibrium

between cis- and trans-2,6-dimethylpiperidine is established, and other, slower reactions

determine the reaction rate of both isomers of 2,6-dimethylpiperidine. Even at 340 °C, 5 MPa,

and 20 kPa H2S, conversion to products other than obtained by HDN is not negligible for

these three molecules. For piperidine the total selectivity for dehydrogenation to pyridine and

disproportionation to N-pentylpiperidine was always below 10%. For 2-methylpiperidine and

cis-2,6-dimethylpiperidine the selectivities to the dehydrogenation products (substituted

pyridine and tetrahydropyridine) were 22 and 24% respectively, at the lowest weight time

measured (1.4 g.min/mol). Taking into account only the products of hydrodenitrogenation, we

found that piperidine undergoes HDN 30% faster than 2-methylpiperidine and 50% faster

than 2,6-dimethylpiperidine (Fig. 2.5).


Fig. 2.5 HDN conversions in the HDN of piperidine, 2-methylpiperidine, and cis-2,6-

dimethylpiperidine as a function of weight time at 340 °C, 5 MPa, and 20 kPa H2S.

2.3.4. HDN of 1-Aminohexane

The HDN of 1-aminohexane becomes fast above 300 °C (Fig. 2.6) and 2-hexene and

hexane are the main products. A plot of the product selectivities versus weight time (Fig. 2.7)

shows that 1-hexene and trans- and cis-2-hexene are primary products. According to the

Hofmann elimination mechanism only 1-hexene can be formed from 1-aminohexane.

However, the isomerization of 1-hexene to 2-hexene is so fast above 300 °C that it is difficult

to distinguish if 2-hexene is a primary or secondary product. Addition of 1-pentene to the feed

indeed showed that the isomerization to cis- and trans-2-pentene was fast. This means that the

ratio of 1-hexene to 2-hexene above 300 °C is determined mainly by thermodynamics and

hardly by the kinetics of the formation of these alkenes. Consequently, the ratio of 1-hexene

to 2-hexene cannot be used to distinguish between the two ways of C–N bond breakage in 2-

methylpiperidine either. At 260 °C the conversion of 1-aminohexane is less than 5%, even at

high weight time (20 g.min/mol), versus 50% at 300 °C. Comparison of the selectivity plots at

300 °C (Fig. 2.7) and 260 °C (Fig. 2.8) confirms that trans- and cis-2-hexene are secondary

products, because the selectivities decrease at decreasing temperature and conversion.


Fig. 2.6. Conversion and relative product concentration in the HDN of hexylamine between

280 and 340 °C and at τ = 5 g.min/mol

Fig. 2.7. Product selectivities of the HDN of hexylamine at 300 °C.

Fig. 2. 8 Product selectivities in the HDN of 1-aminohexane at 260 °C


At the higher H2S pressure of 80 kPa, the selectivity to hexane was higher than at 16

kPa. HDN activity was hardly influenced by this change in H2S partial pressure (at a constant

H2 pressure of 3.8 MPa), but selectivity did change. Not only was hexane selectivity higher,

but 2-hexene selectivity was substantially lower and 1-hexene selectivity higher at 80 kPa

H2S. Apparently, isomerization of 1-hexene to cis- and trans-2-hexene requires vacancies at

the metal sulfide surface. The higher selectivity toward hexane formation indicates that

nucleophilic attack of H2S on 1-aminohexane must have led to hexanethiol, which very

quickly reacted to hexane by hydrogenolysis and 1-hexene by elimination [1].

2.3.5. HDN of 2-Aminohexane

The HDN of 2-aminohexane was complicated by the formation of di-2-hexylamine, a

disproportionation product of the reaction of two 2-aminohexane molecules (Fig. 2.1). As

expected for this molecule with two chiral atoms (2-aminohexane itself has one chiral atom),

the gas chromatogram showed two peaks of equal intensity, equal mass spectra, and only a

small difference in retention time. One peak belongs to the (R,R)- and (S,S)-isomers, the other

to the meso (R,S)-isomer.

Experiments between 220 and 350 °C showed that not only di-2-hexylamine but also 1-

hexene, and cis- and trans-2-hexene behave as a primary product (Fig. 2.9). Hofmann

elimination explains why 1-hexene as well as 2-hexene is formed. The activation energy for

elimination is higher than that of nucleophilic substitution because the hexene selectivity

increased with temperature. The selectivity of di-2-hexylamine is very high at low

temperature. At the lowest temperature studied (220 °C), it was higher than 90% when

extrapolated to zero 2-aminohexane conversion. In this case, the only other product was

hexane. The formation of hexane is explained by nucleophilic substitution of the NH2 group

by an SH group, followed by hydrogenolysis of the C–S bond.

Increasing the H2S pressure from 16 to 80 kPa, at the same H2 pressure of 3.8 MPa, led

to faster conversion of 2-aminohexane to hydrocarbons, while production of the

disproportionation product di-2-hexylamine decreased (Fig. 2.10). Whereas at 16 kPa H2S it

reached a maximum yield of 25%, at 80 kPa H2S the maximum yield was only 10%. At the

higher H2S partial pressure, a new intermediate was observed. It behaved as a primary product

and was analyzed to be 2-hexanethiol. This intermediate is formed by an SN2 reaction


between 2-aminohexane and H2S. At higher H2S partial pressure, it will be formed faster and

will hydrogenolyze less rapidly to hexane because of fewer vacancies on the metal sulfide

surface. Therefore, it is easier to observe 2-hexanethiol at higher H2S pressure.

Fig. 2.9. Relative concentrations (a) and selectivities (b) of the products of the HDN of 2-

hexylamine as a function of weight time at 20 kPa H2S.

Fig. 2.10. Product selectivities of the HDN of 2-hexylamine as a function of weight time at 80

kPa H2S.


2.4 Discussion

Combining the results of the HDN of 2-methylpyridine, 2-methylpiperidine, 1-

aminohexane, and 2-aminohexane, we arrive at the reaction scheme presented in Fig. 2.11.

For all intermediates, except two, direct relationships between parent and daughter molecules

could be established by measuring the product selectivities as a function of weight time and

extrapolating to zero weight time. Thus, 2-methylpiperidine proved to be the primary product

of 2-methylpyridine, while 2-methylpyridine as well as 2-methyl-3,4,5,6-tetrahydropyridine

behaved as primary products of 2-methylpiperidine.

The other two apparent primary products in the HDN of 2-methylpiperidine, 1-

aminohexane and 2-aminohexane, should actually be secondary rather than primary products.

If opening of the piperidine ring would occur by Hofmann elimination, it would lead to 5-

amino-1-hexene when the C–N bond with the methylene group is broken, and to 6-amino-1-

hexene and 6-amino-2-hexene when the C–N bond with the CH(CH3) group is broken (Fig.

2.11). These products were not detected in the HDN of 2-methylpiperidine. The equivalent of

5-amino-1-hexene has never been observed in the HDN of pyridine either [6]. The reason is

most probably that these aminoalkenes adsorb strongly on the catalyst surface because of the

presence of a nitrogen atom in the molecule and are very quickly hydrogenated to the

corresponding saturated amines before they desorb from the catalytic site. Alternatively, if

opening of the pyridine ring would occur by nucleophilic attack by H2S, then 5-

aminohexanethiol, 6-aminohexanethiol, and 6-amino-2-hexanethiol would be primary

products. Thiols react very quickly by elimination to alkenes and by hydrogenolysis to

alkanes. In the first and most important case, aminoalkenes should be formed; in the latter

case, amines. Again, because of strong adsorption and fast hydrogenation, the aminoalkenes

have not been detected. As a result, only 1-aminohexane and 2-aminohexane occur in the

product, their selectivities do not go to zero at low conversion, and they behave as (quasi)

primary products in the HDN of 2-methylpiperidine.


NH

N

N

NH2

H2N

NH2

H2N

Fig. 2.11. Scheme of the reaction network of the HDN of 2-methylpiperidine and 2-

methylpyridine.

2-Methylpyridine and 2-methyl-3,4,5,6-tetrahydropyridine both behaved as primary

products in the HDN of 2-methylpiperidine. One might expect 2-methyl-3,4,5,6-

tetrahydropyridine to be the dehydrogenation intermediate between 2-methylpiperidine and 2-

methylpyridine, in which case it is surprising that 2-methylpyridine behaves as a primary

product too. If the rate of dehydrogenation of 2-methyl-3,4,5,6-tetrahydropyridine to 2-

methylpyridine is of the same order of magnitude as its rate of desorption from the catalytic

site, both molecules might have nonzero selectivities at zero 2-methylpiperidine conversion.

Another explanation could be that 2-methyl-3,4,5,6-tetrahydropyridine is (partially) produced

by a thermal dehydrogenation reaction, while 2-methylpyridine is directly, without desorption

of intermediates, produced by a catalytic reaction. We have not studied this question any

further, because it is only a side effect in our study of the HDN of 2-methylpyridine and 2-

methylpiperidine.

An investigation of the HDN of 1-aminohexane and 2-aminohexane is important not

only to gain a better understanding of the kinetics of the HDN of 2-methylpyridine and 2-

methylpiperidine, but also to understand how the ring opening of the piperidine ring takes

place. Because of the methyl group in α position to the nitrogen atom, C–N bond breakage in

2-methylpiperidine can take place in two ways: between the nitrogen atom and the carbon

atom of the methylene group, or between the nitrogen atom and the carbon atom carrying the

methyl group. The latter possibility should prevail if, as suggested by Portefaix et al. [13, 21]

and Cattenot et al. [8], the number of β H atoms determines the course of the Hofmann

elimination reaction. Unfortunately, the ratio of the concentrations of 1-aminohexane and 2-

aminohexane cannot be used as a direct measure of the ratio of the N–CH2 and N–CH(CH3)

bond breaks. The reason is that the concentrations of these amines depend not only on their


rates of formation, but also on their rates of reaction to hexenes and hexane. Thus, the very

small amount of 1-aminohexane that is produced in the HDN of 2-methylpiperidine (Fig.

2.3b) can be due to either slow breaking of the N–CH(CH3) bond, or rapid disappearance of

1-aminohexane by HDN, or both. For that reason, it was necessary to investigate the HDNs of

1-aminohexane and 2-aminohexane separately.

Comparison of the conversions of 2-aminohexane (Fig. 2.9a) and 1-aminohexane (not

shown) showed that the reactivity of 2-aminohexane is higher than that of 1-aminohexane.

Despite a higher reactivity, much more 2-aminohexane than 1-aminohexane was detected in

the HDN of 2-methylpiperidine (Fig. 2.3b). This proves that the first C–N bond break in 2-

methylpiperidine occurs predominantly between the nitrogen atom and the carbon atom of the

methylene group. If only the number of β H atoms plays a role, as suggested by Portefaix et

al. [13], then 2.5 times more 1-aminohexane than 2-aminohexane should have been formed.

Actually, 3 to 4 times more 2-aminohexane was formed! It is clear that the number of β H

atoms is not the most important factor in Hofmann elimination. The same conclusion was

reached in the HDN of 2-methylcyclohexylamine, in which the type of β H atom proved to be

the most important factor [22]. Thus, the β H atom at the tertiary carbon atom was removed

much faster than the β H atom at the secondary carbon atom, leading to more 1-

methylcyclohexene than 3-methylcyclohexene. Analogously, the results of the HDN of 2-

aminohexane described in Section 2.3.5 demonstrated that 3 to 4 times more 2-hexene was

produced than 1-hexene, although there are 1.5 times fewer H atoms on the CH2 group in β

position to the nitrogen atom than on the CH3 group in 2-aminohexane. It is clear that the ease

with which the C–H bond breaks plays an important role in the elimination. A hydrogen atom

on a tertiary carbon atom is more easily abstracted by a base than β H atoms on secondary or

primary carbon atoms. This is the basis of the Zaytzev rule, which states that elimination

preferentially leads to more substituted alkenes [9].

The fact that much more 2-aminohexane than 1-aminohexane is formed in the HDN of

2-methylpiperidine further indicates that the methyl group actually has a negative rather than

a positive influence on the elimination. All β H atoms on the two carbon atoms in β position

to the nitrogen atom belong to methylene groups. Thus, they should have the same tendency

to be eliminated. If the methyl group played no role in elimination, neither positive nor

negative, then, on the basis of the number and type (methylene) of H atoms, equal amounts of

2-aminohexane and 1-aminohexane should have been formed. The fact that the rate of


breakage of the N–CH(CH3) bond is lower than that of the N–CH2 bond indicates that the

methyl group hinders the adsorption of 2-methylpiperidine in a conformation in which the

nitrogen atom and the β H atom of the methylene group next to the CH(CH3) group approach

the metal sulfide surface. Such a steric hindrance does not exist for the adsorption of the other

side of the 2-methylpiperidine molecule on the metal sulfide surface. Our results are in good

agreement with the rule that nucleophilic substitution is favored at low temperature, while

elimination is favored at high temperature. The H2S pressure may also steer the reaction in

different directions. At low H2S pressure, nucleophilic substitution is dominated by the

reaction of an amine reactant with another amine molecule, leading to disproportionation

products such as di-2-hexylamine (Fig. 2.9). In this sense, the metal sulfide surface that is

depleted of sulfur behaves similarly to a metal surface, on which disproportionation of amines

is important as well [23]. At high H2S pressure, H2S becomes the dominant nucleophile that

reacts with the amine, transforming the amine into a thiol molecule that reacts relatively

quickly to an alkene by elimination and to an alkane by hydrogenolysis. Hydrogenolysis

requires sulfur vacancies at the metal sulfide surface. Consequently, an increase in H2S

pressure has a positive effect on hexane formation than lower H2S pressures, because more

thiol is formed. At higher H2S pressures, however, hardly any vacancies are available

anymore and the thiol can undergo only elimination to an alkene.

From the higher rate of HDN conversion of cis-2,6-dimethylpiperidine than of

piperidine, Portefaix et al. concluded that the rate of elimination of ammonia from an amine is

larger when more β Hatoms are present [13]. Our analysis of all the products of the HDN

reactions of piperidine, 2-methylpiperidine, and cis-2,6-dimethylpiperidine shows that this

conclusion is not correct. Indeed, the rate of disappearance of cis-2,6-dimethylpiperidine is

higher than that of piperidine (Fig. 4) at 275 °C, 2 MPa H2, and 33.3 kPa H2S. However, the

majority of the product at 2 MPa H2 is not formed by elimination, but rather by

dehydrogenation and isomerization. On the other hand, at 340 °C and 5 MPa,

dehydrogenation is much less important and the main products were formed by ring opening

and HDN. Under such conditions, the rate of elimination decreases from piperidine to 2-

methylpiperidine to cis-2,6-dimethylpiperidine. This is then in agreement with our

observation that the 2-methylpiperidine ring is preferentially opened between the N atom and

the methylene group. Thus, it is clear that, contrary to the proposal of Portefaix et al. [13], the

addition of a methyl group in α position to the nitrogen atom in piperidine does not increase


the HDN rate. On the contrary, the methyl group constitutes a strong steric hindrance for the

right adsorption conformation of the nitrogen atom and the β H atom.

2.5 References

[1] R. Prins, Adv. Catal. 46 (2001) 399.

[2] J. Sonnemans, W.J. Neyens, P. Mars, J. Catal. 34 (1974) 230.


[4] R.T. Hanlon, Energy Fuels 1 (1987) 424.

[5] G.C. Hadjiloizou, J.B. Butt, J.S. Dranoff, Ind. Eng. Chem. Res. 31 (1992) 2503.

[6] M. Jian, R. Prins, Catal. Lett. 35 (1995) 193.

[7] R. Pille, G. Froment, Stud. Surf. Sci. Catal. 106 (1997) 403.

[8] M. Cattenot, J.L. Portefaix, A. Afonso, M. Breysse, M. Lacroix, G. Perot, J. Catal.

173 (1998) 366.

[9] J. March, “Advanced Organic Chemistry,” 3rd ed., Chap. 10. Wiley, New York, 1985.


[11] M. Cerny, A. Trka, Czech. Chem. Commun. 48 (1983) 3413.

[12] S. Ren, Z. Wang, Y. Hu, Ranliao Huaxue Xuebo 15 (1987) 255.

[13] J.L. Portefaix, M. Cattenot, M. Gerriche, J. Thivolle-Cazat, M. Breysse, Catal. Today

10 (1991) 473.


[15] G. Perot, Catal. Today 10 (1991) 447.

[16] O. Levenspiel, Chemical Reaction Engineering, 3rd ed., Chap. 8. Wiley, New York,

1999.

[17] L. Vivier, V. Dominguez, G. Perot, S. Kasztelan, J. Mol. Catal. 67 (1991) 267.

[18] G. Asensio, M.E. Gonzales-Nunez, C.B. Bernardini, R. Mello, W. Adam, J. Am.

Chem. Soc. 105 (1983) 6877.

[19] D.H. Hua, W.M. Shou, S. Narasimha Bharathi, T. Katsuhira, A.A. Bravo, J. Org.

Chem. 55 (1990) 3682.

[20] J.F. Cocchetto, C.N. Satterfield, Ind. Eng. Chem. Process Des. Dev. 15 (1976) 272.


[21] J.F. Portefaix, M. Cattenot, M. Guerriche, M. Breysse, Catal. Lett. 9 (1991) 127.

[22] F. Rota, V. Ranade, R. Prins, J. Catal. 201 (2001) 389.

[23] G. Meitzner, W.J. Mykytka, J.H. Sinfelt, J. Catal. 98 (1986) 513.

HDN of n-Hexylamines Chapter 3 53

3. Investigation of the mechanism of the hydrodenitrogenation of n-

hexylamines over sulfided NiMo/γ-Al2O3

3.1 Abstract

The hydrodenitrogenation (HDN) of n-hexylamine, dihexylamine, and trihexylamine

was studied between 300 and 340 °C, 3 and 5 MPa total pressure, 5 and 20 kPa amine

pressure, and 10 and 150 kPa H2S pressure over a sulfided Ni-Mo/γ-Al2O3 catalyst. The

conversion increased with the H2 pressure and decreased with increasing partial pressure of

the hexylamines. The conversion of hexylamine and dihexylamine decreased slightly with

H2S pressure, but that of trihexylamine increased substantially. The contributions of

elimination and nucleophilic substitution to the HDN were determined by the initial product

selectivities at short weight time. The initial alkene selectivities were low and accounted for

only a minor part of the n-alkylamine conversion. Since the hexene/hexane branching ratio in

the HDN of the alkylamines was almost equal to that in the hydrodesulfurization of

pentanethiol in the presence of an alkylamine, it was concluded that the majority of hexene in

the HDN of the hexylamines originates from hexanethiol. Nucleophilic substitution of the

hexylamines with H2S to give an alkanethiol is the predominant HDN reaction of all three n-

hexylamines.

3.2 Introduction

Reactions such as hydrodenitrogenation (HDN) and hydrodesulfurization (HDS) take

place in the hydrotreating of oil fractions, one of the most important catalytic processes in the

petroleum industry. A considerable number of studies have led to a better understanding of

the mechanisms involved in these reactions. It is generally accepted [1-9] that the first step in

the HDN of nitrogen-containing aromatic molecules is the hydrogenation of the heterocyclic

ring. Only after the breaking of the aromaticity can C-N bond cleavage in the resulting


saturated molecules take place. Several mechanisms of the C-N bond scission and nitrogen

removal have been proposed [1,10-12]. Nelson and Levy [1] were the first to suggest

Hofmann-type elimination and nucleophilic substitution as mechanisms for C-N bond scission

of aliphatic nitrogen-containing molecules. The initial step in C-N bond scission is the

addition of a proton to a nitrogen lone pair with the formation of a quarternary ammonium

compound, which provides a better leaving group than the amine group. C-N bond scission

can then occur via elimination of a β−hydrogen atom with the formation of an alkene or via

nucleophilic substitution of the amine group at the α−carbon atom by a sulfhydryl group to

form an alkanethiol.

Several studies [6,13-18] have dealt with the HDN of aliphatic amines over different

catalysts. Portefaix et al. showed in HDN studies at 2 MPa over sulfided NiMo/Al2O3 that an

increase in the number of β−hydrogen atoms in pentylamines and piperidines increased the

conversion of these molecules [6,13]. This was taken as proof that aliphatic C-N bond

cleavage takes place by Hofmann elimination. However, they did not measure the reaction

products and ascribed the total conversion of the amines to the rate of HDN. We showed that

a substantial part of the conversion went to dehydrogenated molecules rather than HDN

products [14]. If only the HDN products were taken into account, then the introduction of a

methyl group onto the α−carbon atom of piperidine and, thus an increase in the number of

β−hydrogen atoms, actually decreased the rate of nitrogen removal. Furthermore, ring

opening of 2-methylpiperidine by C-N bond cleavage occurred preferentially on the CH2-N

side and not on the CH(CH3)-N side. This and the observation of thiol intermediates in the

HDN of 2-methylpiperidine [14] and methylcyclohexylamine [19] suggests that nucleophilic

substitution is even important in the HDN of aliphatic amines that contain β-hydrogen atoms.

Vivier et al. were the first to prove that C-N bond cleavage by nucleophilic substitution

can take place in the HDN of amines [7]. They observed that benzylamine and α,α-

diphenylmethylamine, which do not have β−hydrogen atoms and thus cannot react by

elimination, react fast to toluene and diphenylmethane respectively. Benzylamine and α,α-

diphenylmethylamine react most probably by nucleophilic substitution of the amine group by

an SH group followed by rapid hydrogenolysis of the intermediate thiol. The C-N bond

cleavage in these molecules may be of the SN1 and not SN2 type, because of the stabilizing

influence of the phenyl groups on the intermediate carbenium ion that results from removal of

the amine group. Cattenot et al. showed that both elimination and nucleophilic substitution


play a role in the C-N bond scission of pentylamines on unsupported transition-metal sulfides

at atmospheric pressure [15]. The ratio of the two mechanisms depended on the type of metal

sulfide catalyst and the type of amine. Over MoS2, n-pentylamine reacted by nucleophilic

substitution with H2S to pentanethiol as well as with another n-pentylamine molecule to

dipentylamine. Pentenes were observed as secondary products and supposed to be formed by

elimination from dipentylamine. These findings suggest that the molecular structure is one of

the most important factors in HDN and that different molecules may undergo nitrogen

removal by different mechanisms.

Concurrent with the β-hydrogen elimination and nucleophilic substitution reactions, a

disproportionation reaction can occur between two alkylamine molecules, and this

complicates the study of the HDN reaction mechanism. This disproportionation results in the

formation of a dialkylamine and ammonia in case of an alkylamine and in the formation of a

trialkylamine and alkylamine in case of a dialkylamine; it takes place even on alumina at low

hydrogen pressure [20,21]. Substantial amounts of the disproportionation products N-

pentylpiperidine, dicyclohexylamine, and dipentylamine were observed in the HDN of

piperidine [22], cyclohexylamine [23], and pentylamine [15,24] respectively.

We decided to carry out a detailed investigation of the HDN of alkylamines over

sulfided NiMo/Al2O3. The aim of our work was to determine which of the different

mechanisms (elimination, substitution, and disproportionation) plays the major role in the

HDN of aliphatic amines. To elucidate the mechanism it is necessary to determine the

primary HDN products and to compare the influence of different reaction conditions on

product distribution. Accordingly, we carried out experiments at low weight times and under

different reaction conditions to check the formation of primary products and, thus, to prove

the mechanism of the HDN of aliphatic amines. In this work we will present our results of the

HDN of the linear n-alkylamines hexylamine, dihexylamine, and trihexylamine, while in

subsequent work we will publish our results of the HDN of alkylamines with the amine group

attached to secondary and tertiary carbon atoms.

3.3. Results

3.3.1. HDS of pentanethiol and hydrogenation of hexene


To compare the relative rates of the HDS of alkanethiols, the HDN of alkylamines, and

the hydrogenation of alkenes, we performed the simultaneous conversion of octanethiol,

hexylamine, and 1-pentene at 300 °C and 3 MPa in the presence of 10 kPa H2S. It is clear

from the results presented in Figure 3.1 that an alkanethiol reacts very much faster than an

alkylamine and an alkene.

0 2 4 6 8 100

20

40

60

80

100

HA

1-pentene

octanethiolC

onve

rsio

n, %

Weight time, g.min/mol

Fig. 3.1. Conversions of octanethiol and hexylamine, and yield of pentane in the simultaneous

HDS of 5 kPa octanethiol, HDN of 5 kPa hexylamine (HA), and hydrogenation of

5 kPa 1-pentene at 300 °C, 3 MPa, and 10 kPa H2S.

Figure 3.2 shows the HDS conversion of pentanethiol at 300 °C and 3 MPa in the presence of

different pressures of hexylamine and H2S; it was equally fast as that of octanethiol (Fig. 3.1).

0 2 4 6 8 100

20

40

60

80

100

10 kPa HA, 50 kPa H2S




Pen

tane

thio

l con

vers

ion,

%


Fig. 3.2. Conversion of 5 kPa pentanethiol at 300 0C and 3 MPa in the presence of 5, 10 and

20 kPa hexylamine (HA), and 10 and 50 kPa H2S.

The pentanethiol conversion decreased strongly with increasing H2S pressure from 10 to 50

kPa and less strongly with increasing hexylamine pressure from 5 to 20 kPa. The non-zero

selectivities at short weight time (Fig. 3.3) demonstrate that 1-pentene and pentane are


primary products, while the low initial selectivity of 2-pentene indicates that 2-pentene is a

secondary product. The pentene selectivity decreased with increasing hexylamine pressure as

well as with increasing H2S partial pressure, while the opposite dependence was observed for

the complementary pentane. As a consequence, the molar pentenes/pentane ratio (pentenes

stands for the sum of all the pentenes) decreased with increasing partial pressure of

hexylamine and H2S (Fig. 3.4).

0 1 2 3 4 50

10

20

30

40

50

cis-2-C5=

trans-2-C5=

1-C5=

C5

Sele

ctiv

ity, %


Fig. 3.3. Product selectivities to pentane, 1-pentene, trans-2-pentene, and cis-2-pentene in the

HDS of 5 kPa pentanethiol at 300 °C and 3 MPa in the presence of 20 kPa

hexylamine, 5 kPa cyclohexene, and 10 kPa H2S.

0 2 4 6 8 100.0

0.2

0.4

0.6

0.8

1.0

Hex

enes

/hex

ane

Weight time, g.min/mol0 2 4 6 8 10

0.0

0.3

0.6

0.9

1.2

1.5

Pen

tene

s/pe

ntan

e


Fig. 3.4. Pentenes/pentane ratio and hexenes/hexane ratio in the simultaneous HDS of 5 kPa

pentanethiol and HDN of 5 or 20 kPa hexylamine at 300 °C and 3 MPa, and 10 or 50

kPa H2S.

5 kPa HA and 10 kPa H2S 20 kPa HA and 10 kPa H2S

20 kPa HA and 50 kPa H2S

The conversion of 1-hexene to hexane was high in the absence of alkylamine; at 300 °C

in the presence of 10 kPa H2S it was already 40% at τ = 0.8 g.min/mol (Fig. 3.5).


0 3 6 9 120

20

40

60

80

100

20 kPa CHA, 300°C

5 kPa CHA, 300°C

20 kPa CHA, 340°C

300°C

C6 Y

ield

, %


Fig. 3.5 Conversion of 5 kPa 1-hexene at 300 and 340 °C and 3 MPa in the presence or

absence of 5 or 20 kPa cyclohexylamine (CHA), 5 kPa pentanethiol (PT), and 10

kPa H2S.

300 °C 5 kPa PT, 20 kPa CHA, and 340 °C

5 kPa PT, 5 kPa CHA, and 300 °C 5 kPa PT, 20 kPa CHA, and 300 °C

In the presence of 5 kPa cyclohexylamine and 10 kPa H2S, however, it was only 8% at τ = 0.8

g.min/mol. Like at 300 °C (Fig. 1), the alkene conversion at 340 °C in the presence of an

alkylamine (Fig. 3.5) was much lower than the conversion of pentanethiol (98%). The

conversion of 1-hexene to hexane decreased substantially with increasing hexylamine and

H2S partial pressure, while in the presence of alkylamine the influence of H2S was only

moderate. This indicates that the alkylamine is more strongly adsorbed than H2S.

3.3.2. HDN of Hexylamine

The conversion of 5 kPa hexylamine at 300 °C and 5 MPa in the presence of 50 kPa of

H2S was 6% at low weight time (0.9 g.min/mol) and reached 37% at high weight time (8.7

g.min/mol). It almost doubled when the reaction temperature was increased from 300 to 320

°C (Fig. 3.6). Figure 3.7 shows the corresponding product distributions at 300 °C (Fig. 3.7A)

and 320 °C (Fig. 3.7B). The main product of the HDN of hexylamine was hexane, which

behaved as a primary product because the selectivity extrapolates to a non-zero value at τ = 0.

The selectivity of the hexenes (the sum of 1-hexene, 2-hexene, and 3-hexene) decreased with

decreasing contact time at 300 °C, suggesting that hexene may be a secondary product or a

secondary as well as a primary product. At 340 °C, hexene behaved as a primary product. The


selectivity of 1-hexanethiol increased with decreasing weight time, showing that it is a

primary product.

0 2 4 6 8 100

20

40

60

80

20 kPa HA, 300 °C, 3 MPa

5 kPa HA, 300 °C, 3 MPa

5 kPa HA, 300 °C, 5 MPa

5 kPa HA, 320 °C, 5 MPa

Con

vers

ion,

%


Fig. 3.6. Influence of the partial pressure of hexylamine (HA) on its conversion as a function

of weight time at 300 and 320 °C, 3 and 5 MPa, and 50 kPa H2S.

5 kPa HA at 320 °C and 5 MPa 5 kPa HA at 300 °C and 5 MPa

5 kPa HA at 300 °C and 3 MPa 20 kPa HA at 300 °C and 3 MPa

0 2 4 6 8 100

20

40

60

80

C6-SH1-C6

=

C6=

C6

A

Sele

ctiv

ity, %


0 2 4 6 8 100

20

40

60

80

C6-SH1-C6

=

C6=

C6

B

Sele

ctiv

ity, %


Fig. 3.7. Product selectivities in the HDN of 5 kPa hexylamine at 5 MPa, 50 kPa H2S, and 300 0C (A), and 320 0C (B).

hexane, hexenes, 1-hexene, hexanethiol

As 1-hexene is too easily hydrogenated at 5 MPa total pressure, it does not give us much

information about the HDN mechanism. Therefore, the total pressure was decreased from 5 to

3 MPa to obtain less severe hydrogenation conditions. This change resulted in a decrease in

the hexylamine conversion that was about proportional to the change in H2 pressure (Figs. 3.6

and 3.8). Figure 3.9 shows that, at 50 kPa H2S and short contact time (1.4 g.min/mol), the


main product was hexanethiol (45% selectivity), followed by hexane (39% selectivity) and

hexenes (12%). At 3 MPa, dihexylamine was observed as well (4% at short contact time). The

selectivities of 2-hexene and 3-hexene were zero. This shows that initially no isomerization of

1-hexene takes place. At τ = 14.2 g.min/mol, the hexanethiol selectivity decreased to 6% and

the hexane selectivity increased to 62%. Isomerization had become important and the sum of

the selectivities of 1-hexene, 2-hexene, and 3-hexene reached 30%. The hexanethiol

selectivity was much higher at 3 than at 5 MPa (cf. Figs. 3.7A and 3.9). Some dihexylimine

was observed as well. It was also observed in a blank experiment carried out without the

catalyst in the empty Inconel 718 reactor. In the product mixture of the HDN of hexylamine,

the yield of dihexylimine reached only 0.4% at τ = 1.4 g.min/mol, while during the HDN of

dihexylamine and trihexylamine it amounted to 1.2% at τ = 1.0 g.min/mol. These small

amounts of dihexylimine were ignored.

0 2 4 6 8 100

20

40

60

80

50 kPa H2S100 kPa H2S

150 kPa H2S

300 °C, 3 MPa10 kPa H2S

320 °C, 5 MPa

50 kPa H2S

Con

vers

ion,

%


Fig. 3.8. Influence of H2S on the conversion of 5 kPa hexylamine at

320 °C, 5 MPa, and 50 kPa H2S 320 °C, 5 MPa, and 150 kPa H2S

300 °C, 3 MPa, and 10 kPa H2S 300 °C, 3 MPa, and 50 kPa H2S

300 °C, 3 MPa, and 100 kPa H2S

At 3 MPa and 300 °C, the hexane selectivity extrapolated to a non-zero value at τ = 0

and the hexene selectivity to a zero or very low value (Fig. 3.9), indicating that hexane

behaves like a primary and hexene like a secondary product. This ratio decreased with

increasing hexylamine and H2S partial pressure. The hexenes/hexane ratio, obtained from the

HDN of hexylamine, was smaller than the pentenes/pentane ratio, obtained from the HDS of

pentanethiol (Fig. 3.4), at small weight time, but approached the pentenes/pentane ratio at

high weight time.


0 4 8 12 160

10

20

30

40

50

Hex

enes

, %

w e ight tim e, g .m in/m ol0 4 8 1 2 1 6

0

2 0

4 0

6 0

8 0

Hex

ane,

%

W e ig h t tim e , g .m in /m o l

0 4 8 1 2 1 60

2 0

4 0

6 0

Hex

anet

hiol

, %

W e ig h t t im e , g .m in /m o l0 4 8 12 16

0

5

10

15

20

25

Dih

exyl

amin

e, %

W e ig h t tim e , g .m in /m o l

Fig. 3.9. Product selectivities in the HDN of 5 kPa hexylamine at 300 °C, 3 MPa, and

10 kPa H2S ( ), 50 kPa H2S ( ), and 100 kPa H2S ( ).

The conversion of hexylamine decreased by about a factor of three at τ = 5.8 g.min/mol

when increasing the hexylamine partial pressure from 5 to 20 kPa while keeping the other

parameters constant at 3 MPa, 300 °C, and 50 kPa H2S (Fig. 3.6). The selectivities of

hexanethiol, hexane, and hexene did not change much with the change of the hexylamine

partial pressure; only the initial selectivity of dihexylamine increased from 4 to 7%. At 300

°C, the hexylamine conversion decreased slightly when the H2S pressure was increased from

10 to 50 and 100 kPa (Fig. 3.8). At 320 °C and 5 MPa, a stronger decrease was obtained. With

increasing H2S partial pressure, the selectivity of the hexenes decreased, the hexane

selectivity was about constant, the hexanethiol selectivity increased strongly, and the

dihexylamine selectivity decreased strongly (Fig. 3.9). The weight time dependencies of the

products show that dihexylamine and hexanethiol are primary products, that 1-hexene

behaves like a secondary product, and that hexane might be formed as a primary as well as a

secondary product.

To obtain a greater amount of product, and thus increase the accuracy of the selectivity

measurements at short weight time, the reaction temperature was increased to 340 °C. Figure

10 shows the conversion of hexylamine at 340 °C and 3 MPa in the presence of 10 or 50 kPa

H2S and 5 kPa pentanethiol. Because of the fast HDS reaction, this means that the actual H2S


pressure amounted to 15 or 55 kPa. The hexylamine conversion decreased strongly when its

partial pressure was increased from 5 to 20 kPa and the dihexylamine selectivity increased

(Fig. 3.10). Their non-zero initial selectivities show that dihexylamine and hexanethiol are

primary products, while hexane and hexene behave like primary products. With increasing

H2S pressure, the dihexylamine and hexene selectivities decreased, while the hexanethiol

selectivity increased sharply. The selectivity of hexane did not change much with H2S

pressure.

0 2 4 6 8 100

20

40

60

80

100




10 kPa HA, 10 kPa H2S5 kPa HA, 50 kPa H2S

HA

con

vers

ion,

%


0 2 4 6 8 100

20

40

60

80

100

Hex

ane,

%


0 2 4 6 8 100

10

20

30

40

50

Hex

enes

, %


0 2 4 6 8 100

5

10

15

Hex

anet

hiol

, %


0 2 4 6 8 100

5

10

15

20

Dih

exyl

amin

e, %


Fig. 3.10. Conversion and product selectivities in the HDN of 5, 10, and 20 kPa hexylamine

(HA) at 340 0C and 3 MPa, in the presence of 5 kPa pentanethiol, and 10 and 50

kPa H2S.

5 kPa HA and 10 kPa H2S 20 kPa HA and 10 kPa H2S

20 kPa HA and 50 kPa H2S

The alkenes/alkane ratio in the HDN of hexylamine and HDS of pentanethiol increased

when increasing the temperature from 300 to 340 °C (cf. Figs. 3.4 and 3.11). The


hexenes/hexane ratio was equal to the pentenes/pentane ratio resulting from the HDS of

pentanethiol at two different H2S and hexylamine partial pressures (Fig. 3.11). Both ratios

decreased with increasing weight time, due to the hydrogenation of the alkene to the alkane.

The decrease was less steep at higher hexylamine as well as H2S partial pressure because of

the slower hydrogenation of the alkene under those conditions.

0 2 4 6 8 100.0

0.5

1.0

1.5

2.0

2.5


Alk

enes

/alk

ane


0.0

0.5

1.0

1.5

2.0

2.5

10 kPa HA, 10 kPa H2SAlk

enes

/alk

ane


0 2 4 6 8 100.0

0.5

1.0

1.5

2.0

2.5


Alk

enes

/alk

ane


0 2 4 6 8 100.0

0.5

1.0

1.5

2.0

2.5


Alk

enes

/alk

ane


Fig. 3.11. Pentenes/pentane ( ) and hexenes/hexane ( ) ratios in the simultaneous HDS of 5

kPa pentanethiol and HDN of 10 and 20 kPa hexylamine (HA) at 340 0C and 3

MPa, and 10 and 50 kPa H2S.

3.3.3. HDN of Dihexylamine

The conversion of dihexylamine (Fig. 3.12) was much faster than that of hexylamine

(Fig. 3.6). At 320 °C, almost complete conversion was reached already at short contact time.

The conversion of dihexylamine hardly changed when the total pressure was increased from 3

to 5 MPa (not shown). Figure 3.13 shows the product selectivities in the HDN of 5 kPa

dihexylamine at 300 °C, 3 MPa, and 50 kPa H2S. It is apparent from the selectivities at low


weight time that hexylamine, hexanethiol, and trihexylamine behave like primary products

and that hexene probably behaves like a secondary product. The hexane selectivity decreased

with decreasing weight time, but it is not clear whether it extrapolates to zero. Therefore we

cannot say whether hexane is a secondary or a primary as well as secondary product. The

selectivities did not change significantly when the total pressure was increased from 3 to 5

MPa. The main difference was observed at low weight time, where the hexanethiol selectivity

was lower and the hexane selectivity higher at 5 MPa. The selectivity patterns at short weight

time were the same at both pressures, as were the conclusions about primary and secondary

products. When the dihexylamine partial pressure was increased from 5 to 20 kPa, while the

other reaction conditions remained constant at 300 °C, 3 MPa, and 50 kPa H2S, the

conversion decreased substantially (Fig. 3.12). At the same time, the THA selectivity

increased strongly, the thiol selectivity remained the same, but the hexylamine selectivity

decreased at shorter weight time (cf. Figs. 3.13 and 3.14A). Trihexylamine, hexanethiol, and

hexylamine are primary products.

0 2 4 6 80

20

40

60

80

100

20 kPa DHA, 300 °C, 3 MPa

300 °C, 3 MPa320 °C, 5 MPa

Con

vers

ion,

%


Fig. 3.12. Conversion of 5 or 20 kPa dihexylamine (DHA) at 300 or 320 °C, 3 or 5 MPa, and

50 kPa H2S.

5 kPa DHA at 320 °C and 5 MPa 5 kPa DHA at 300 °C and 3 MPa

20 kPa DHA at 300 °C and 3 MPa

While the conversion of dihexylamine decreased only slightly when increasing the H2S

pressure from 10 to 50 kPa (not shown), it influenced the product distribution substantially.

With increasing partial pressure of H2S, the hexanethiol selectivity increased strongly, the

trihexylamine selectivity decreased sharply, the hexane and hexenes selectivities decreased,

and the hexylamine selectivity remained the same (Fig. 3.13). The much higher rate of


trihexylamine formation shows that disproportionation is favored by a low partial pressure of

H2S. At the high pressure of 20 kPa dihexylamine and low pressure of 10 kPa H2S, the

trihexylamine selectivity even reached 61% at short contact time, while the hexylamine

selectivity was 26% and the sum of the thiol, hexene, and hexane selectivities was about 15%

(Fig. 3.14B). Under these conditions, dihexylamine reacts predominantly by

disproportionation. Extrapolation to zero weight time gave a hexylamine selectivity of 25%;

thus, hexylamine is a primary product.

0 1 2 3 4 50

6

12

18

24

Hex

enes

, %


0 1 2 3 4 50

3

6

9

12

1-H

exen

e, %

W e ight tim e , g .m in /m o l

0 1 2 3 4 50

5

10

15

20

25

Hex

ane,

%

W eight tim e, g .m in /m ol0 1 2 3 4 5

0

15

30

45

60

Hex

ylam

ine,

%

W eight tim e, g.m in/m ol

0 1 2 3 4 50

4

8

12

16

Trih

exyl

amin

e, %

W e igh t tim e , g .m in /m o l0 1 2 3 4 5

0

10

20

30

40

Hex

anet

hiol

, %

W eight tim e, g .m in/m ol

Fig. 3.13. Product selectivities in the HDN of 5 kPa dihexylamine at 300 °C, 3 MPa,

and 10 kPa H2S ( ), 50 kPa H2S ( ), and 100 kPa H2S ( ).


0 1 2 3 4 50

20

40

60

80 B

C6-SH1-C6

=C6

=

C6

C6-NH2

(C6)3N

Sele

ctiv

ity, %

Weight time, g.min/mol0 2 4 6

0

15

30

45

60 A

(C6)3N

C6=

1-C6=

C6-SH

C6

C6-NH2

Sele

ctiv

ity, %


Fig. 3.14. Product selectivities in the HDN of 20 kPa dihexylamine at 300 °C, 3 MPa, and (A)

50 and (B) 10 kPa H2S.

hexylamine, hexane, hexenes, hexanethiol, 1-hexene, trihexylamine.

3.3.4 HDN of Trihexylamine

The HDN conversion of trihexylamine at 300 °C and 5 MPa was very high; already at

short weight time (0.9 g.min/mol) it reached 82% (not shown). The main products at τ = 0.9

g.min/mol were dihexylamine (43%), hexane (26%), and hexylamine (16%), while the 1-

hexanethiol selectivity was 7% (Fig. 3.15). At 320 °C, the conversion of trihexylamine was

even 92% at τ = 0.9 g.min/mol, and at this temperature dihexylamine (28%), hexane (35%),

and hexylamine (23%) were the main products. The time dependency of the products show

that dihexylamine and 1-hexanethiol are primary products, and that hexane, hexylamine, 2-

hexene, and 3-hexene are secondary products, while 1-hexene behaves like a primary product

(not shown).

0 2 4 6 8 100

15

30

45

60

C6-SH 1-C6=

C6=

C6-NH2

(C6)2NH

C6

Secl

ectiv

ity, %


Fig. 3.15. Product selectivities in the HDN of 5 kPa trihexylamine at 300 °C, 5 MPa, and 50

kPa H2S.

dihexylamine, hexane, hexylamine, hexene, 1-hexene, hexanethiol.


The conversion of 5 kPa trihexylamine at τ = 0.9 g.min/mol and in the presence of 50

kPa H2S decreased to 70% when the total pressure was decreased from 5 to 3 MPa (Fig. 3.16).

Increasing the partial pressure of trihexylamine from 5 to 10 kPa at 3 MPa decreased the

conversion even further to 50% (Fig. 3.16). Lowering the pressure from 5 to 3 MPa increased

the selectivities of the primary products hexanethiol and dihexylamine and decreased those of

the secondary products hexylamine and hexane (cf. Figs. 3.15 and 3.17). Figure 3.17 shows

the product distributions in the HDN of 5 kPa trihexylamine at 300 °C, 3 MPa, and 50 kPa

H2S. At short contact time, the main products were dihexylamine (63%) and 1-hexanethiol

(20%), both being primary products. The initial selectivity of 1-hexene was less than 3%.

Under these conditions, hexane and 1-hexene are clearly secondary products. At high weight

time, the dihexylamine selectivity decreased substantially to less than 5%. The hexanethiol

selectivity also decreased; only the hexylamine and hexane selectivities increased to 32 and

44% respectively. The product selectivities at 5 and 10 kPa trihexylamine were the same at

short contact time but changed less fast with weight time at 10 kPa than at 5 kPa

trihexylamine (Fig. 3.17).

0 1 2 3 4 50

20

40

60

80

100

10 kPa THA, 50 kPa H2S



Con

vers

ion,

%


Fig. 3.16. Conversion in the HDN of 5 or 10 kPa trihexylamine (THA) at 300 °C and 3 MPa,

in the presence of 10 or 50 kPa H2S.

5 kPa THA, 10 kPa H2S 5 kPa THA, 50 kPa H2S


Whereas the conversion of hexylamine decreased slightly and that of dihexylamine

decreased even less when the H2S pressure was increased from 10 to 50 kPa, the conversion

of trihexylamine increased (Fig. 3.16). At short contact time, the selectivities of dihexylamine


and hexylamine were the same at both H2S pressures, but the hexanethiol selectivity was

much higher and the selectivities of hexane and 1-hexene lower at 50 kPa H2S pressure (Fig.

3.17).

0 1 2 3 4 5 60

5

1 0

1 5

2 0

2 5

Hex

enes

, %

W e ig h t tim e , g .m in /m o l 0 1 2 3 4 5 60

3

6

9

12

1-H

exen

e, %


0 1 2 3 4 5 60

10

20

30

40

Hex

ylam

ine,

%

W eight tim e, g.m in/mol0 1 2 3 4 5 6

0

10

20

30

40

50

Hex

ane,

%

W e igh t tim e , g .m in /m o l

0 1 2 3 4 5 60

5

10

15

20

25

Hex

anet

hiol

, %

Weight time, g.min/mol0 1 2 3 4 5 6

0

20

40

60

80

Dih

exyl

amin

e, %

W e igh t tim e , g .m in /m o l

Fig. 3.17. Product selectivities in the HDN of 5 or 10 kPa trihexylamine (THA) at 300 °C, 3

MPa, and 10 and 50 kPa H2S.

5 kPa THA, 10 kPa H2S 5 kPa THA, 50 kPa H2S



3.4 Discussion

To determine which mechanism is responsible for the HDN reaction of an n-alkylamine

one can measure the product selectivities as a function of weight time (τ) and determine

whether these extrapolate to a non-zero or zero value at τ = 0. If a product such as 1-hexene

has a zero selectivity at zero weight time, then it cannot be a primary product and elimination

cannot play a role. If its selectivity is non-zero at τ = 0, then 1-hexene might be a primary

product, and elimination might be important. A zero selectivity at zero weight time does not

automatically mean that a product is a secondary product, however, nor does a non-zero initial

selectivity automatically mean that a product is primary. For instance, hexylamine may react

slowly by substitution to hexanethiol, which then reacts fast to hexane and 1-hexene. The

selectivities of hexane and 1-hexene may then extrapolate to a non-zero initial selectivity and

these molecules may then appear to be primary products, although they are secondary. At the

same time, the fast consecutive reaction of the real primary product hexanethiol will decrease

its initial selectivity and the contribution of the substitution mechanism will be

underestimated. With such potential pitfalls in mind, we will analyze the initial selectivities

observed in the HDN of the hexylamines and try to determine the responsible mechanism(s).

Since elimination is generally considered to be the main HDN mechanism [1,6,13,18], we

will pay particular attention to the initial selectivity of hexene. Because isomerization of 1-

hexene to 2- and 3-hexene is fast, we will use the initial selectivity of the sum of all hexenes,

rather than that of 1-hexene, as a measure of the contribution of elimination.

3.4.1. Hexylamine

Three reactions are possible in the HDN of n-hexylamine: elimination to 1-hexene,

substitution to hexanethiol, and disproportionation to dihexylamine:

6 13 2 6 12 3C H NH C H NH→ + (1)

6 13 2 2 6 13 3C H NH H S C H SH NH+ → + (2)

6 13 2 6 13 2 32 ( )C H NH C H NH NH→ + (3)


At 300 °C, the hexanethiol selectivity increased and the disproportionation selectivity to

dihexylamine decreased with increasing H2S pressure (Fig. 3.9), because the higher H2S

pressure favors the substitution of the NH2 group of hexylamine by H2S over the substitution

by another hexylamine molecule. The hexene selectivity was lower at high H2S pressure and

decreased strongly with decreasing weight time for all H2S pressures. Unfortunately,

measurements below τ = 0.8 g.min/mol were not possible because the gas flow rate could not

be increased further and using less than 50 mg catalyst led to channelling and to a decreased

conversion and thus lower accuracy of the measurement. Therefore, the values to which the

hexene selectivities extrapolate at τ = 0 could not be determined with high precision.

Nevertheless, the steep decrease of the hexene selectivity towards τ = 0 indicates that the

hexene selectivity is in any case smaller than 5% at 50 and 100 kPa H2S and smaller than 20%

at 10 kPa H2S (Fig. 3.9).

6 13 6 12 2C H SH C H H S→ + (4)

Under our conditions, the equilibrium of the decomposition of hexanethiol (reaction 4)

lies to the right [25]. This means that the hexene formed in the HDN of hexylamine will

hardly react with H2S. Also a disappearance of hexene by hydrogenation plays a minor role,

as the hydrogenation of 1-pentene showed. It was strongly inhibited by hexylamine and the

yield of pentane was only 5% at τ = 0.8 g.min/mol. This means that the observed hexene

selectivities in the HDN of hexylamine are truly representative for the discussed HDN

mechanisms.

The values of 5% for the hexene selectivity at 50 and 100 kPa H2S and of 20% at 10 kPa

H2S at short weight time demonstrate that elimination is not the major mechanism in the HDN

of hexylamine. These values are even upper limits to the contribution of elimination to the

HDN of hexylamine (Eq. 1). The reason is that 1-hexene can not only be formed by direct

elimination of hexylamine (Eq. 1), but also by substitution of hexylamine followed by

elimination of the resulting thiol to 1-hexene (Eqs. 2 and 4). The pentenes/pentane ratio from

the HDS of pentanethiol and the hexenes/hexane ratio from the HDN of hexylamine,

measured simultaneously in the same reaction mixture, are about the same (Fig. 3.4). This

similarity of the alkene/alkane branching ratio shows that substitution is the predominant

route for nitrogen removal from hexylamine and that the contribution of elimination is even


less than 5% at 50-100 kPa H2S and 20% at 10 kPa H2S. This also explains why, in the HDN

of hexylamine, the selectivity of hexene is higher at low H2S pressure. Namely, the HDS of

pentanethiol demonstrated that the pentanethiol conversion increased strongly with decreasing

H2S pressure (Fig. 2.2).

At 340 °C, hexene and hexane behaved as primary products (Fig. 2.10), but the

hexenes/hexane ratio was very similar to the pentenes/pentane ratio measured in the

simultaneous HDS of pentanethiol (Fig. 2.11). Both ratios were not only very similar, but also

reacted in the same way on changes in the H2S and hexylamine partial pressures. This shows

that also at 340 °C the main reaction of hexylamine is substitution by H2S to form

hexanethiol. At the higher temperature of 340 °C, the subsequent decomposition of

hexanethiol becomes so fast, that hexene and hexane appear to be primary products.

Hexane is a product in the HDN of all three hexylamines. It is supposed to be formed

from hexanethiol

(5) 6 13 2 6 14 2C H SH H C H H S+ → +

The mechanism of this reaction is unclear. It might be a real hydrogenolysis reaction as

on a metal surface or as reported in the homogeneous reaction of aliphatic and aromatic thiols

with the Cp’2Mo2Co2S3(CO)4 cluster (Cp’ stands for pentamethylcyclopentadienyl) [26].

Another possibility would be that the alkene formed by elimination from the alkanethiol (Eq.

4) is hydrogenated before desorbing from the catalyst surface. Both mechanisms explain the

observed lower hexene/hexane ratio at higher H2 pressure (total pressure), lower temperature,

and higher H2S and hexylamine pressure. The increase of the hexane selectivity with

increasing τ (Figs. 3.9 and 3.10) is due to increased hexanethiol decomposition (Eq. 5) as well

as hydrogenation of hexene. These two factors oppose each other in the production of hexene,

and explain the maximum in the hexenes/hexane ratio at 300 °C (Fig.3.4).

3.4.2 Dihexylamine

Like hexylamine, dihexylamine can react in three ways: by elimination to hexylamine

and 1-hexene, by substitution to hexylamine and 1-hexanethiol, and by disproportionation to

hexylamine and trihexylamine:


6 13 2 6 12 6 13 2( )C H NH C H C H NH→ + (6)

6 13 2 2 6 13 6 13 2( )C H NH H S C H SH C H NH+ → + (7)

6 13 2 6 13 3 6 13 22( ) ( )C H NH C H N C H NH→ + (8)

Furthermore, because of the much higher reactivity of trihexylamine than of hexylamine

and dihexylamine, trihexylamine can react back to dihexylamine and 1-hexene or 1-

hexanethiol (Eqs. 9 and 10).

6 13 3 6 12 6 13 2( ) ( )C H N C H C H NH→ + (9)

6 13 3 2 6 13 6 13 2( ) ( )C H N H S C H SH C H NH+ → + (10)

At 320 °C and 5 MPa we observed only a trace amount of trihexylamine, which means

that, under these conditions, either disproportionation hardly took place or that trihexylamine

reacted away faster than it was formed. The product selectivity suggests that 1-hexene is a

primary product, which would mean that 1-hexene is formed by elimination of dihexylamine.

At 300 °C and 5 MPa (not shown) and 3 MPa, however, 1-hexene behaves more like a

secondary product. The maximum hexene selectivities at τ = 0 are 3% at 50 and 100 kPa H2S

and 8% at 10 kPa H2S (Fig. 3.13). Because of the stoichiometry of Eq. 6 this means that the

maximum relative contribution of elimination to the HDN of dihexylamine are 6 and 16%

respectively. Like in the case of hexylamine, these percentages overestimate the contribution

of elimination substantially, because the hexene/hexane ratio in the HDN of dihexylamine

was not much different from that of the HDS of hexanethiol. This means that reaction 7

followed by reaction 4 is the main pathway for the formation of hexene.

Trihexylamine behaved as a primary product, meaning that dihexylamine quickly forms

trihexylamine by disproportionation. At 320 °C less trihexylamine was observed as at 300 °C,

probably because trihexylamine quickly reacts to hexanethiol and dihexylamine by

substitution. This would also explain why the conversion of dihexylamine and trihexylamine

never reached 100%, neither at 320 °C nor at high weight time. This can be explained by re-

formation of these molecules by a disproportionation reaction of two molecules of


hexylamine to dihexylamine, or two molecules of dihexylamine to trihexylamine, or of one

molecule of dihexylamine and one molecule of hexylamine to trihexylamine.

The trihexylamine selectivity in the HDN of dihexylamine increased with increasing

partial pressure of dihexylamine, while the hexanethiol selectivity stayed almost the same,

and the hexylamine selectivity became much lower (cf. Figs. 3.13 and 3.14). At first glance

this seems strange; if the thiol selectivity stays the same, one would expect that the

hexylamine selectivity is also the same (Eq. 7). We can only explain the increased

trihexylamine selectivity and decreased hexylamine selectivity by assuming that the

hexylamine obtained from the decomposition of dihexylamine reacts with dihexylamine to

form trihexylamine. Although the trihexylamine selectivity increased and trihexylamine has a

much higher reactivity, fewer active centers are available for trihexylamine to decompose at

high partial pressure of dihexylamine. This also explains why the conversion of dihexylamine

decreased at higher partial pressure of dihexylamine. We therefore suggest that at 300 °C, 3

MPa, and 50 kPa H2S dihexylamine reacts by substitution by H2S to hexylamine and 1-

hexanethiol and by disproportionation to trihexylamine and hexylamine (Scheme 3.1). 1-

Hexene and hexane are subsequently formed from 1-hexanethiol. An analysis of the mass

balance supports the conclusion that mainly substitution and disproportionation and hardly

any elimination occurred in the HDN of dihexylamine.

C6 N

C6

C6

C6HN C6 C6 NH2 C6 SH+

+ C6

C6

C6=

NH2

Scheme 3.1. HDN network of dihexylamine.

Cattenot et al. studied the HDN of n-pentylamine over unsupported MoS2 at 275 °C and

atmospheric pressure [15]. They observed dipentylamine and pentanethiol as primary


products and pentenes as secondary products. From the initially larger amount of

dipentylamine, they concluded that the majority of the pentenes formed from dipentylamine

and only a fraction from pentanethiol. Under our conditions (300-320 °C, 3 MPa,

NiMo/Al2O3), however, dihexylamine mainly reacted by substitution with another amine

molecule and with H2S, and hexanethiol was the main source of hexene.

3.4.3 Trihexylamine

Trihexylamine can only react by elimination to dihexylamine and 1-hexene and by

nucleophilic substitution by H2S to dihexylamine and 1-hexanethiol (Eqs. 9 and 10). As

shown in Section 3.3.3, dihexylamine will react further to hexylamine, hexanethiol, hexane,

and 1-hexene. At higher temperature, the scission of the C-N bond of dihexylamine becomes

faster, and thus the selectivity of dihexylamine in the HDN of the trihexylamine is much

lower at 320 than at 300°C and substantially more hexylamine and hexane are formed at 320

°C. At lower total pressure, the C-N cleavage rate of trihexylamine is lower and the selectivity

of dihexylamine increases. High selectivities to dihexylamine and hexanethiol and a low

selectivity to hexenes were obtained at short weight time (Fig. 3.17). The dihexylamine

selectivity extrapolates to 67% at τ = 0 (Fig. 3.17), as is expected for both elimination and

nucleophilic substitution (Eqs. 9 and 10). Note that the selectivity is defined so as to preserve

the carbon mass balance, which is the hexyl balance in this case. The hexene selectivity

extrapolates to 2% at τ = 0 at 50 kPa H2S and to 8% at 10 kPa H2S (Fig. 3.17). With a

maximum selectivity of 33.3% for hexene (Eq. 9), this means that the relative contribution of

elimination to the HDN of trihexylamine is 6 and 24% respectively. Like in the case of n-

hexylamine and dihexylamine, these percentages overestimate the contribution of elimination

substantially, because the hexene/hexane ratio in the HDN of trihexylamine was not much

different from that of the HDS of hexanethiol. This means that reaction 10 followed by

reaction 4 is the main pathway for the formation of hexene. The reason that the hexene

selectivity was higher at the lower H2S pressure of 10 kPa, is due to the higher reactivity of

alkanethiol to alkene under these conditions. Assuming that the reactivity of hexanethiol is

similar to that of pentanethiol, we expect less hexanethiol and more 1-hexene in the HDN of

trihexylamine at lower H2S pressure. This means that elimination of 1-hexene from

trihexylamine to form dihexylamine plays a minor role at all H2S pressures. The qualitative


conclusion about the dominant role of nucleophilic substitution was substantiated by the mass

balance of the amounts of the products from the HDN of trihexylamine (Scheme 3.2).

C6 N

C6

C6

C6 NH

C6

+

+

C6=

C6C6

C6

C6

NH2 SH

SH

Scheme 3.2. HDN network of trihexylamine.

Substitution explains the higher conversion of trihexylamine at higher H2S pressure and

the higher selectivity to hexanethiol and lower selectivity to hexane and hexene. At a higher

H2S pressure there will be more SH groups at the surface and trihexylamine will react faster

with SH to dihexylamine and hexanethiol. At the same time, the hexanethiol decomposition

will be more inhibited because there are fewer vacancies on which hexanethiol can adsorb;

this causes an increase in the hexanethiol selectivity and a decrease in the hexane plus hexene

selectivity. The increased number of SH groups at the catalyst surface not only increases the

formation of dihexylamine from trihexylamine, but also the further (substitution) reaction of

dihexylamine to hexylamine and hexanethiol. As a consequence, the H2S pressure does not

have a significant influence on the dihexylamine selectivity at short weight time.

3.4.4. General Discussion

The conversion of the hexylamines increased with increasing basicity of the amine in

the gas phase in the order hexylamine < dihexylamine < trihexylamine. The conversion of all

three hexylamines increased with increasing total pressure (higher H2 pressure). Since the

primary reactions of the hexylamines (be it elimination, nucleophilic substitution, or

disproportionation) are chemically independent of hydrogen, the positive influence of

hydrogen must be due to a secondary effect. Most probably it is caused by the number of


vacancies on the catalyst surface, which is generated by the H2S/H2 ratio and, at constant H2S

pressure, increases with the H2 pressure. On the other hand, when increasing the H2S pressure

at constant total pressure, the conversion of the hexylamines did not show a strong decrease.

While it decreased slightly for hexylamine and dihexylamine, it even increased with the H2S

pressure for trihexylamine. This may be due to two competing factors. On the one hand, H2S

decreases the number of vacancies on the catalyst surface and, thus, the reaction rate. On the

other hand, more sulfur on the catalyst surface increases the nucleophilic substitution by H2S,

which was demonstrated to be the main HDN reaction. These two factors are almost equal for

hexylamine and dihexylamine, but the second factor is more important in the case of

trihexylamine. This might be due to a strong adsorption of trihexylamine and even the

replacement of H2S from the catalyst surface.

An additional explanation for the increased activity of trihexylamine and slightly

decreased activities of hexylamine and dihexylamine at increasing H2S pressure could be that

only the latter two molecules can undergo disproportionation. At low H2S pressure,

disproportionation determines the conversion of hexylamine and dihexylamine but not that of

trihexylamine. At increasing H2S pressure, nucleophilic substitution increases in importance

for all three molecules, while the contribution of disproportionation to the conversion

decreases. This would explain why the conversion of trihexylamine continuously increased

with increasing H2S pressure, while the contributions of disproportionation and nucleophilic

substitution are influenced in opposite ways by H2S.

Our results clearly demonstrate that nucleophilic substitution is the dominant HDN

mechanism for unbranched alkylamines, dialkylamines, and trialkylamines. For the first two

molecules the substitution can occur by an amine as well as by H2S, while for the

trialkylamine only substitution by H2S is possible. Portefaix et al. showed that more β-H

atoms led to a higher HDN conversion of piperidines [6,13]. We showed, however, that the

higher conversion of 2-methylpiperidine was not due to HDN but to dehydrogenation to 2-

methylpyridine [14]. In fact, 2-methylpiperidine preferentially underwent ring opening to 2-

aminohexane by C-N bond cleavage of the α carbon atom that did not carry the methyl group.

Thus, the extra three β-H atoms on this methyl group did not increase the HDN conversion.

To prove that this has nothing to do with the type of H atoms (on primary or secondary carbon

atoms), we performed an HDN experiment with 2-ethylpiperidine. Also for this molecule,

with two extra β H atoms on a secondary carbon atom, ring opening occurred mainly on the


less sterically hindered side of the piperidine ring. We take this as evidence that, also for

piperidine-like molecules, HDN occurs by nucleophilic substitution rather than by

elimination, as is also the case for dihexylamine, which is a secondary amine like piperidine.

A remaining question is how the nucleophilic substitution of alkylamines takes place at

the catalyst surface. Hydroxyl and amine groups are very poor leaving groups in nucleophilic

substitution [27,28]. Protonation of the amine group or complexing with a Lewis acid group

gives a better leaving group and this might be the main role of the catalyst surface. The

nucleophile that attacks the α-carbon atom can either be an alkylamine, or an SH- or S2- group

at the catalyst surface. Laine suggested that a metal-assisted nucleophilic substitution might

occur via a metal alkyl or alkylidene intermediate [10]. Another mechanism could be a

sequence of dehydrogenation, addition, elimination, and hydrogenation reactions, which is an

established method to replace a hydroxyl or amine group. For instance, the conversion of an

amine into an alcohol takes place via

32 2 2-NH-H H O H

2 2 2 2R-CH -NH R-CH=NH R-CHOH-NH R-CH=O R-CH -OH⎯⎯→ ⎯⎯⎯→ ⎯⎯⎯→ ⎯⎯→

Similarly, an alkylamine can be transformed into an alkanethiol. In that case, one

obtains a reaction sequence that has the same overall stoichiometry as the nucleophilic

substitution reaction of an alkylamine and H2S, giving an alkanethiol and NH3. All four

reactions in the sequence are known to take place on metals. For instance, copper is the metal

of choice in the reaction sequence from alcohol to amine [29]. If such reactions take place on

copper, they may also take place on metal sulfides. The dehydrogenation of amines to nitriles

has been reported by Portefaix et al. in the HDN of pentylamine at low pressure (0.1 MPa)

[15]. In the present work we never observed nitriles, but did observe imines in low

concentration. This may be due to the higher H2 pressure used in our work than in that of

Portefaix et al. Further work has to clarify which mechanism is responsible for the HDN of

alkylamines. For the moment, we can only conclude that, in the HDN of linear alkylamines,

elimination plays a minor role and that a reaction with the stoichiometry of nucleophilic

substitution can explain all observations.


3.5 Conclusions

Our results show that the removal of the nitrogen atom from alkylamines occurs mainly

by a nucleophilic substitution of the alkylamine to an alkanethiol, which subsequently reacts

to an alkene or alkane and H2S. This makes sense from the point of view of organic

chemistry. The aliphatic C-N bond is strong and the amine group is thus a bad leaving group.

Also, an SH- group is too weak a base to remove the hydrogen atom from the β−carbon atom.

As a consequence, Hofmann elimination of an alkylamine to an alkene and ammonia is an

unlikely reaction. Nucleophilic substitution, on the other hand, may very well occur with an

SH- group, because it is a strong nucleophile [15,27]. Even the NHR group can act as a

nucleophile. At the same time, the α-carbon atom in a n-alkylamine is easily accessible for the

nucleophile. Thus, nucleophilic substitution of the NH2 group of an amine by an SH group,

leading to an alkanethiol, as well as by an NHR group (leading to disproportionation)

occurred readily. The much higher reaction rates of dihexylamine and trihexylamine than the

hexylamine might, on the one hand, be caused by their higher number of carbon atoms and,

thus, higher adsorption constants and, on the other hand, by their higher basicities. The

nucleophilic substitution is aided by protonation of the nitrogen atom by a Brønsted acid or by

interaction of the nitrogen atom with a Lewis acid in order to create a better leaving group

[27]. The Ni atom at the catalyst surface may act as the Lewis acid site.

The reaction sequence dehydrogenation-H2S addition-NH3 elimination-hydrogenation

may explain the HDN of hexylamines equally well. This reaction scheme has the same overall

stoichiometry as nucleophilic substitution and therefore reacts in the same way on

temperature, pressure, and other parameters. Also the metal-assisted nucleophilic substitution

proposed by Laine [10] explains our results.

The reason that elimination has long been held to be the dominant nitrogen removal

reaction is due to the fact that relatively large amounts of alkenes are observed in HDN at

longer weight time. Our results show that, at least for n-alkylamines, these large amounts of

alkenes are caused by the elimination reaction of alkanethiols and not by the elimination of

alkylamines. It is the fast reaction of alkanethiols that obscures the true origin of the alkenes.

Only when measuring at short reaction time can one identify the true origin of the alkene.


3.6 References


[2] C.N. Satterfield, M. Modell, J.A. Wilkens, Ind. Eng. Chem. Proc. Des. Dev. 19 (1980)

154.

[3] R. Ramachandran, F.E. Massoth, Chem. Eng. Commun. 18 (1982) 239.

[4] R.T. Hanlon, Energy & Fuels 1 (1987) 424.

[5] M.J. Girgis, B.C. Gates, Ind. Eng. Chem. Res. 30 (1991) 2021.


10 (1991) 473.


[8] U.S. Ozkan, S. Ni, L. Zhang, E. Moctezuma, Energy & Fuels 8 (1994) 249.

[9] R. Prins, Adv. Catal. 46 (2001) 399.

[10] R.M. Laine, Catal. Rev.-Sci. Eng. 25 (1983) 459.

[11] M. Zdrazil, J. Catal. 141 (1993) 316.

[12] S. Rajagopal, R. Miranda, J. Catal. 141 (1993) 318.

[13] J.L. Portefaix, M. Cattenot, M. Guerriche, M. Breysse, Catal. Lett. 9 (1991) 127.

[14] M. Egorova, Y. Zhao, P. Kulula, R. Prins, J. Catal. 206 (2002) 263; and chapter 2.


(1998) 366.

[16] M. Breysse, J. Afonso, M. Lacroix, J.L. Portefaix, M. Vrinat, Bull. Soc. Chim. Belg.

100 (1991) 923.

[17] J.H. Lee, C.E. Hamrin, B.H. Davis, Appl. Catal. A 111 (1994) 11.

[18] P. Clark, X. Wang, P. Deck, S. T. Oyama, J. Catal. 210 (2002) 116.

[19] F. Rota, R. Prins, J. Catal. 202 (2001) 195.

[20] M. Fikry Ebeid, J. Pasek, Coll. Czech. Chem. Commun. 35 (1970) 2166.

[21] P. Hogen, J. Pasek, Coll. Czech. Chem. Commun. 39 (1974) 3696.

[22] R. Pille, G. Froment, Stud. Surf. Sci. Catal. 106 (1997) 403.

[23] S. Eijsbouts, C. Sudhakar, V.H.J. de Beer, R. Prins, J. Catal. 127 (1991) 605.


[25] J.G. Speight, in “The Desulfurization of Heavy Oils and Residua”. Marcel Dekker

Journals, 2000.


[26] M.D. Curtis, S.H. Druker, J. Am. Chem. Soc. 119 (1997) 1027.

[27] R. Bruckner, “Advanced Organic Chemistry”. Academic Press, 2002.

[28] J. Clayden, N. Greeves, S. Warren, P. Wothers, “Organic Chemistry”. Oxford Uni.

Press, 2001.

[29] T. Mallat, A. Baiker, in “ Handbook of Heterogeneous Catalysis” (G. Ertl, H.

Knözinger, J. Weitkamp, Eds.). Vol 5, P. 2334, Wiley-VCH.

HDN of Alkylamines Chapter 4 81

4. Mechanisms of the hydrodenitrogenation of alkylamines with secondary

and tertiary α-carbon atoms on sulfided NiMo/Al2O3

4.1 Abstract

The hydrodenitrogenation (HDN) of alkylamines with secondary and tertiary α-carbon

atoms (2-pentylamine, 3-methyl-2-butylamine, 3,3-dimethyl-2-butylamine, 2-

methylcyclohexylamine, 2-methyl-2-butylamine) and benzylamine as well as the

hydrodesulfurization (HDS) of corresponding alkanethiols were studied over sulfided

NiMo/Al2O3. Alkanethiols and dialkylamines were primary products in the HDN of the

amines with secondary α-carbon atoms, formed by substitution of the amine group by H2S or

an alkylamine. Alkanes and alkenes were secondary products, formed from elimination and

hydrogenolysis of the alkanethiols, as confirmed by the similar alkenes/alkane ratios in the

HDN of the alkylamines and HDS of the corresponding alkanethiols. 2-Methyl-2-butylamine

and benzylamine reacted much faster than the amines with secondary α-carbon atoms.

Methylbutenes and methylbutane were the primary products of 2-methyl-2-butylamine, and

toluene was the primary product of benzylamine. This and the different

methylbutenes/methylbutane ratios in the HDN of 2-methyl-2-butylamine and HDS of 2-

methyl-2-butanethiol indicate that 2-methyl-2-butylamine, with a tertiary α-carbon atom, and

the activated benzylamine react by means of an E1 mechanism.

4.2 Introduction

Nitrogen atoms are removed from hetero-aromatic compounds by hydrogenation of the

aromatic ring, which contains the nitrogen atom, and breaking of the resulting aliphatic C-N

bonds to form a hydrocarbon molecule and ammonia [1-5]. Hydrogenation is not required for

the removal of a sulfur atom from an aromatic ring that contains a sulfur atom, as in

(di)benzothiophene, because the relatively weak C-S bond can be broken by hydrogenolysis


[6,7]. The presence of a large amount of alkenes and a minor amount of alkanes in the

reaction product of HDN suggests that aliphatic C-N bond breaking occurs mainly by

elimination of NH3 [1,3]. Nucleophilic substitution of the alkylamine by H2S, followed by C-

S bond hydrogenolysis, explains the presence of alkanes [1,4]. Cattenot et al. showed,

however, that in the HDN of the linear n-pentylamine over unsupported MoS2 at 275 °C and

atmospheric pressure the formation of pentenes was negligible at short weight time and that

the major product was dipentylamine [8]. At higher weight times pentenes and pentanethiol

were observed and the production of the pentenes was ascribed mainly to the elimination of

pentylamine from the dipentylamine and, in part, to the elimination of H2S from pentanethiol.

We showed that, over a sulfided NiMo/Al2O3 catalyst at 300 to 340 °C and elevated pressure

(3 MPa), n-hexylamine, di-n-hexylamine, and tri-n-hexylamine react predominantly by

nucleophilic substitution of the amines by H2S and not by elimination of NH3 [9]. The

resulting hexanethiol reacts very fast to hexenes as well as to hexane. The very low selectivity

of the hexenes at low weight time demonstrates that the hexenes are secondary not primary

products in the HDN of n-hexylamines. As a consequence, the hexene/hexane ratio in the

HDN product mixture is determined by the HDS reaction, as demonstrated by the similar

alkenes/alkane ratios in the simultaneous HDN of hexylamine and HDS of pentanethiol [9].

The nucleophilic substitution by a good nucleophile such as SH- is aided by the

accessibility of the α-carbon atom in linear alkylamines. The accessibility of the α-carbon

atom decreases with substitution and, at the same time, the number of β-hydrogen atoms

increases. Thus, amines with secondary and tertiary α-carbon atoms may react differently as

linear alkylamines [8]. Therefore, we report the results of the organic part of our mechanistic

investigation of the removal of ammonia from such alkylamines in this work. In future work

we will report on the inorganic aspects, the catalytic sites and the mode(s) of adsorption.

4.3 Results

4.3.1. 2-Pentylamine and 2-pentanethiol

The conversion of 2-pentylamine at 300 °C in the presence of 10 kPa H2S was 12% at τ =

0.9 g.min/mol and 53% at 8.9 g.min/mol (Fig. 4.1). The conversion increased slightly when


the H2S pressure was increased from 10 to 100 kPa but increased considerably with an

increase in temperature from 300 to 340 °C. The products were 2-pentanethiol, di-(2-

pentyl)amine, 1-pentene, 2-pentene, and pentane (Fig. 4.2). The selectivity of 2-pentanethiol

increased with decreasing weight time, showing that 2-pentanethiol is a primary product (of

the nucleophilic substitution of 2-pentylamine with H2S). Di-(2-pentyl)amine, the

disproportionation product of two molecules of 2-pentylamine, behaved as a primary product

as well. The selectivity of 2-pentanethiol increased and that of di-(2-pentyl)amine decreased

with increasing H2S pressure.

0 2 4 6 8 100

20

40

60

80

100

3M2B

A C

over

sion

, %


0

20

40

60

80

100

2-PA

Con

vers

ion,

%


Fig. 4.1 Conversions of 2-pentylamine (2-PA) and 3-methyl-2-butylamine (3M2BA) at 300

°C ( and ) and 340 °C ( and ), and 10 kPa (drawn line) and 100 kPa H2S

(dashed line).

The selectivity of the pentenes (i.e. the sum of the two pentene isomers) as a function of

weight time was different at 300 than at 340 °C. At 300 °C the selectivity extrapolates to zero

with decreasing weight time, which indicates that pentenes are a secondary product. On the

other hand, at 340 °C the pentenes behaved like a primary product because their selectivity

extrapolates to a non-zero value at τ = 0. At both temperatures, the selectivity of pentane

decreased with decreasing weight time to a value close to zero.

The conversion of 2-pentanethiol in the presence of 10 kPa H2S and 5 kPa hexylamine

was much larger than that of the corresponding 2-pentylamine. Thus, at τ = 0.9 g.min/mol the

conversion of the thiol was 83% at 300 °C and 95% at 340 °C (Fig. 4.3), while that of the

amine was 12% at 300 °C and 42% at 340 °C (Fig. 4.1). The conversion of 2-pentanethiol

decreased strongly when the H2S pressure was increased from 10 to 100 kPa. The main

products at 300 °C and 10 kPa H2S were 1-pentene (21%), 2-pentene (52%), and pentane


(27%) at τ = 0.9 g.min/mol (not shown). All three molecules were primary products. The

selectivity of 1-pentene decreased and that of pentane increased with weight time, while the

selectivities of cis- and trans-2-pentene were constant. This difference between the pentenes

must be due to the easier hydrogenation of the terminal 1-pentene. The pentenes/pentane ratio,

obtained from the HDS of 2-pentanethiol, was similar to that obtained in the HDN of 2-

pentylamine (Fig. 4.4).

0 2 4 6 8 100

20

40

60

80

100

Pent

ane,

%


0 2 4 6 8 100

20

40

60

80

Pent

enes

, %


0 2 4 6 8 100

5

10

15

20

25

Pent

anet

hiol

, %


10

0 2 4 6 8 100

20

40

60

80

0

Di-2

-PA

, %


Fig. 4.2 Product selectivities in the HDN of 2-pentylamine (2-PA) at 300 °C ( and ) and

340 °C ( and ), and 10 kPa (drawn line) and 100 kPa H2S (dashed line).

0 2 4 6 8 100

20

40

60

80

100

2-PT

Con

vers

ion,

%


0 2 4 6 8 100

20

40

60

80

100

3M2B

T co

nver

sion

, %


Fig. 4.3 Conversions in the HDS of 2-pentanethiol (2-PT) and 3-methyl-2-butanethiol

(3M2BT) in the presence of hexylamine at 300 °C ( and ) and 340 °C ( and ),

and 10 kPa (drawn line) and 100 kPa H2S (dashed line).


0 2 4 6 8 100

1

2

3

42-PA

Pent

enes

/pen

tane


0 2 4 6 8 100

1

2

3

42-PT

Pent

enes

/pen

tane


Fig. 4.4 Pentenes/pentane ratio in the HDN of 2-pentylamine (2-PA) and HDS of 2-

pentanethiol (2-PT) in the presence of hexylamine at 300 °C ( and ) and 340

°C ( ), and 10 kPa (drawn line) and 100 kPa H2S (dashed line).

4.3.2. 3-Methyl-2-butylamine and 3-methyl-2-butanethiol

The conversion of 3-methyl-2-butylamine was slightly lower than that of 2-pentylamine

(Fig. 4.1). It increased weakly with increasing H2S pressure and increased substantially with

increasing temperature. The main products were 3-methyl-2-butanethiol, di-(3-methyl-2-

butyl)amine, 2-methyl-2-butene, 2-methyl-1-butene, 3-methyl-1-butene and 2-methylbutane.

The selectivities of 3-methyl-2-butanethiol and di-(3-methyl-2-butyl)amine increased with

decreasing weight time (Fig. 4.5). This shows that they are primary products of the

nucleophilic substitution of 3-methyl-2-butylamine with H2S and with another molecule of 3-

methyl-2-butylamine, respectively. As expected for a molecule with two chiral atoms (3-

methyl-2-butylamine has one chiral atom), the gas chromatogram of di-(3-methyl-2-

butyl)amine showed two peaks of equal intensity and only a small difference in retention

time; the corresponding products had equal mass spectra. One GC peak is due to the (R,R)

and (S,S) isomers, the other to the meso (R,S) isomer. The selectivity of di-(3-methyl-2-

butyl)amine decreased with increasing H2S pressure, while the reverse was true for the

selectivity of 3-methyl-2-butanethiol.


0 2 4 6 8 100

20

40

60

80

100

2-M

ethy

lbut

ane,

%


0

10

20

30

40

502-

Met

hyl-2

-but

ene,

%


0 2 4 6 8 100

2

4

6

8

10

3-M

ethy

l-1-b

uten

e, %


0 2 4 6 8 100

5

10

15

20

2-M

ethy

l-1-b

uten

e, %


0 2 4 6 8 100

10

20

30

40

3-M

ethy

l-2-b

utan

ethi

ol, %


0

10

20

30

40

50

Di-a

min

e, %


Fig. 4.5 Product selectivities in the HDN of 3-methyl-2-butylamine at 300 °C ( and ) and

340 °C ( and ), and 10 kPa (drawn line) and 100 kPa H2S (dashed line).

As in the HDN of its isomer 2-pentylamine (Section 4.3.1), the selectivities of the alkene

products in the HDN of 3-methyl-2-butylamine differed at 300 and 340 °C. At 300 °C the

methylbutene selectivities were low at τ = 0 and increased with weight time, while at 340 °C

and τ = 0 they were higher and decreased with weight time. At both temperatures, all three

methylbutenes behaved as primary products (Fig. 4.5) as the selectivities extrapolated to non-

zero with decreasing weight time to zero.


The conversion of 3-methyl-2-butanethiol in the presence of 10 kPa H2S and 5 kPa

hexylamine was much larger than that of the corresponding 3-methyl-2-butylamine: The

conversion of the thiol was 74% at τ = 0.9 g.min/mol at 300 °C and 100% at 340 °C (Fig.

4.3), while that of the corresponding amine was 7% at 300 °C and 29% at 340 °C (Fig. 4.1).

The main products at 300 °C and 10 kPa H2S were 3-methyl-1-butene (17%), 2-methyl-2-

butene (49%), and 2-methylbutane (29%) at τ = 0.9 g.min/mol, while the selectivity of 2-

methyl-1-butene was 5% (not shown). The methylbutenes/methylbutane ratio obtained from

the HDS of 3-methyl-2-butanethiol in the presence of hexylamine was slightly lower than that

obtained from the HDN of 3-methyl-2-butylamine at low weight time but was similar at high

weight time (Fig. 4.6).

0 2 4 6 8 100

1

2

3

4 3M2BA

C5= /C

5


0 2 4 6 8 100

1

2

3

4

C5= /C

5

3M2BT


Fig. 4.6 Methylbutenes/methylbutane ratio in the HDN of 3-methyl-2-butylamine (3M2BA)

and HDS of 3-methyl-2-butanethiol (3M2BT) in the presence of hexylamine at 300

°C ( and ) and 340 °C ( and ), and 10 kPa (drawn line) and 100 kPa H2S

(dashed line).

To determine whether a methyl-shift rearrangement occurred during the HDN of 3-

methyl-2-butylamine, the HDN of 2-methyl-3-pentylamine was measured. The selectivity of

the rearrangement products 3-methyl-2-pentene and 3-methylpentane was below 1%. The

main products (di-(2-methyl-3-pentyl)amine, 2-methyl-1-pentene, 2-methyl-2-pentene, 2-

methyl-3-pentene, 2-methylpentane, and 2-methyl-3-pentanethiol) corresponded to those in

the HDN of 3-methyl-2-butylamine and were present in similar amounts (Fig. 4.5). In

addition, a constant selectivity of 2% was observed for 2-methyl-4-pentene.


4.3.3. 3,3-Dimethyl-2-butylamine

The conversion of 3,3-dimethyl-2-butylamine was even lower than that of 3-methyl-2-

butylamine: only 2% at 300 °C and 10 kPa H2S at τ = 0.9 g.min/mol but 24% at τ = 8.9

g.min/mol. The conversion increased strongly with increasing temperature and increased

slightly with increasing H2S pressure (Fig. 4.7). 3,3-Dimethyl-2-butanethiol formed and

behaved as a primary product, because its selectivity increased with decreasing weight time

(Fig. 4.8). We did not observe a disproportionation product, probably because of the

combined steric hindrance of the tertiary butyl and methyl groups attached to the same carbon

atom as the NH2 group.

In addition to the normal products (3,3-dimethyl-1-butene and 2,2-dimethylbutane),

rearranged products (2,3-dimethylbutane, 2,3-dimethyl-2-butene, and 2,3-dimethyl-1-butene)

also formed. The sum of the selectivities of these rearranged products was 43% at 300 °C and

63% at 340 °C and 10 kPa H2S at τ = 0.9 g.min/mol. As in the case of 2-pentylamine and 3-

methyl-2-butylamine, the selectivity of 3,3-dimethyl-1-butene in the HDN of 3,3-dimethyl-2-

butylamine as a function of weight time was different at 300 °C than at 340 °C. At 300 °C, the

3,3-dimethyl-1-butene selectivity went through a maximum, while at 340 °C it decreased

continuously with increasing weight time.

0 2 4 6 8 100

20

40

60

80

100

3,3D

M2B

A C

onve

rsio

n, %


0 2 4 6 8 100

20

40

60

80

100

2-M

CH

A C

onve

rsio

n, %


Fig. 4.7 Conversions of 3,3-dimethyl-2-butylamine (3,3DMBA) and 2-

methylcyclohexylamine (2-MCHA) at 300 °C ( and ) and 340 °C

( and ), and 10 kPa (drawn line) and 100 kPa H2S (dashed line).


Fig. 4.8 Product selectivities in the HDN of 3,3-dimethyl-2-butylamine at 300 °C ( and )

and 340 °C ( and ), and 10 kPa (drawn line) and 100 kPa H2S (dashed line).

0 2 4 6 8 100

10

20

30

40

50

2,2-

Dim

ethy

lbut

ane,

%


0

10

20

30

402,

3-D

imet

ht

e,%

enyl

-2-b

u


0 2 4 6 8 100

10

20

30

2,3-

Dim

ethy

lbut

ane,

%

Weight time, g.min/mol100 2 4 6 8

0

5

10

15

20

3,3-

dim

ethy

l--b

uten

e,%


1

0 2 4 6 8 100

10

20

30

2,3-

Dim

ethy

l-1-lb

uten

e,%


0 2 4 6 8 100

10

20

30

40

SH

3,3-

Dim

ethy

l-2-b

utan

ethi

ol, %



4.3.4. 2-Methylcyclohexylamine

2-Methylcyclohexylamine has the same molecular structure as 3-methyl-2-butylamine,

with the amine group attached to a secondary α-carbon atom and a methyl group on the

neighboring β-carbon atom. The conversion of 2-methylcyclohexylamine was lower than that

of 3-methyl-2-butylamine, both at 300 °C and 340 °C, and H2S had a positive influence on the

conversion of both amines (cf. Figs. 4.1 and 4.7). The selectivities of the methylcyclohexenes,

methylcyclohexane, 2-methylcyclohexanethiol, and di-(2-methylcyclohexyl)amine products

(Fig. 4.9) were similar to those of the respective alkenes, alkane, alkanethiol, and

dialkylamine products in the HDN of 3-methyl-2-butylamine respectively (Fig. 4.5).

0 2 4 6 8 100

20

40

60

80

MC

H, %


0

20

40

60

80

MC

HE,

%


0 2 4 6 8 100

10

20

30

40

50

MC

HTH

IOL,

%


0 2 4 6 8 100

10

20

30

40

50

Di-2

-am

ine,

%


Fig. 4.9 Product selectivities of methylcyclohexenes (MCHE), methylcyclohexane (MCH) 2-

methylcyclohexanethiol (MCHTHIOL) and di(2-methylcyclohexyl)amine (Di-2-

amine) in the HDN of 2-methylcyclohexylamine at 300 °C ( and ) and 340 °C

( and ), and 10 kPa (drawn line) and 100 kPa H2S (dashed line).



The HDN of 2-methyl-2-butylamine occurred fast; the conversion was already 7% at 270

°C and 10 kPa H2S at low weight time (0.9 g.min/mol) and reached 90% at τ = 8.9 g.min/mol

(Fig. 4.10). The main products were 2-methyl-2-butene, 2-methyl-1-butene, and 2-

methylbutane (Fig. 4.11). The non-zero selectivities of these products at τ = 0 showed that

they behaved as primary products. The selectivity of 2-methyl-1-butene decreased and that of

2-methylbutane increased with increasing weight time due to isomerization and

hydrogenation. A separate experiment of 2-methyl-1-butene in the presence of hexylamine at

270 °C, 3 MPa and 10 kPa H2S showed 30% conversion to 2-methyl-2-butene and 10%

conversion to 2-methylbutane at τ = 10 g.min/mol. The selectivity of 3-methyl-1-butene, the

isomerization product of 2-methyl-2-butene, was less than 1% over the whole range of weight

times. A small amount of 2-methyl-2-butanethiol was observed that increased with increasing

H2S pressure. Its selectivity was 1.5% at τ = 0.9 g.min/mol and 100 kPa H2S. Increasing the

H2S pressure from 10 to 100 kPa hardly influenced the conversion and product selectivities of

2-methyl-2-butylamine.

0 2 4 6 8 100

20

40

60

80

100

BA C

onve

rsio

n, %


0

20

40

60

80

100

2M2B

A C

onve

rsio

n, %


Fig. 4.10 Conversions in the HDN of 2-methyl-2-butylamine (2M2BA) and benzylamine

(BA) at 270 °C ( and ), and 10 kPa (drawn line) and 100 kPa H2S (dashed line).


0 2 4 6 8 100

20

40

60

80

2-M

ethy

l-2-b

uten

e, %


0 2 4 6 8 100

10

20

30

40

50

2-M

ethy

l-1-b

uten

e, %


0 2 4 6 8 100

10

20

30

2-M

ethy

lbut

ane,

%


0 2 4 6 8 100

1

2

3

4

5

SH

2M2B

T, %


Fig. 4.11 Product selectivities in the HDN of 2-methyl-2-butylamine at 270 °C ( and ), and

10 kPa (drawn line) and 100 kPa H2S (dashed line).

The conversion of 2-methyl-2-butanethiol at 270 °C in the presence of 10 kPa H2S and 5

kPa hexylamine was much larger than that of the equivalent amine: It was already 66% at τ =

0.9 g.min/mol (Fig. 4.12), while the conversion of the corresponding amine was only 20%

(Fig. 4.10). The conversion of 2-methyl-2-butanethiol decreased with increasing H2S

pressure. The main products at τ = 0.9 g.min/mol were 2-methyl-1-butene (35%), 2-methyl-2-

butene (39%), and methylbutane (25%) (not shown). The methylbutenes/methylbutane ratio,

obtained in the HDS of 2-methyl-2-butanethiol, was 3 at 270 °C and 10 kPa at τ = 0.9

g.min/mol, which is very different from the value of 16.5 obtained in the HDN of 2-methyl-2-

butylamine under the same conditions (Fig. 4.13).


0 2 4 6 8 100

20

40

60

80

100

2M2B

T C

onve

rsio

n, %


Fig. 4.12 Conversion in the HDS of 2-methyl-2-butanenethiol (2M2BT) in the presence of

hexylamine at 270 °C ( and ), and 10 kPa (drawn line) and 100 kPa H2S

(dashed line).

0 2 4 6 8 100

6

12

18

2M2BT

2M2BA

C5= /C

5


Fig. 4.13 Methylbutenes/methylbutane ratio in the HDN of 2-methyl-2-butylamine (2M2BA)

and HDS of 2-methyl-2-butanethiol (2M2BT) in the presence of hexylamine at 270

°C ( and ), and 10 kPa (drawn line) and 100 kPa H2S (dashed line).

The conversion of benzylamine was lower than that of 2-methyl-2-butylamine at 270 °C

(32% for benzylamine and 52% for 2-methyl-2-butylamine at τ = 3.4 g.min/mol) and was not

influenced by the H2S pressure (Fig. 4.10). The main product at 10 kPa H2S was toluene (>


99%); only traces of α-toluenethiol (benzyl mercaptan), methylcyclohexene, and

methylcyclohexane formed. At 100 kPa H2S, the α-toluenethiol selectivity was 3% at low

weight time and even smaller at high weight time. The conversion of α-toluenethiol was

100% under all conditions at 270 °C, even at the lowest weight time, and toluene was the only

product.

4.4 Discussion

4.4.1 HDN and HDS mechanisms

As shown above, the products of the HDN of alkylamines are dialkylamines, alkanethiols,

alkenes, and alkanes, while alkenes and alkanes are the products of the HDS of alkanethiols.

Dialkylamines and alkanethiols are formed by substitution of the NH2 group in alkylamines

by an alkylamine or H2S respectively. The alkenes can be formed by elimination of NH3 from

the alkylamines or of H2S from the alkanethiols. The alkanes are formed by hydrogenolysis,

which is the rupture of the C-N or C-S bond and simultaneous hydrogenation. Dialkylamines,

alkanethiols, and alkenes can, in principle, form by acid-base catalysis as well as metal-like

catalysis, while alkanes can only form by metal-catalyzed hydrogenolysis.

4.4.1.1 Acid-base mechanisms

In acid-base catalyzed elimination [1,3] and nucleophilic substitution [1,4] reactions of

alkylamines, the amine group reacts first with a proton or a Lewis acid in order to create a

better leaving group (In the subsequent schemes, only the reaction with a proton is indicated).

Then a concerted bimolecular reaction takes place in the E2 elimination as well as SN2

nucleophilic substitution (Scheme 4.1), in which a base or nucleophile reacts with the

protonated amine and ammonia is split off. In the E2 mechanism, the base subtracts a

hydrogen atom from the β-carbon atom of the alkylamine. That is why this elimination can


only occur when β-H atoms are present and why it was proposed that the HDN of alkylamines

occurs faster when a greater number of β-H atoms is present [3]. In the SN2 reaction, the base

attacks the α-carbon atom of the alkylamine. In classic nucleophilic substitution this attack

takes place from the backside of the molecule, at the side opposite to the leaving amine group,

with inversion of the configuration at the α-carbon atom.

B + R

H2C

CH2

NH3+ E2

BH+ + R

HC

CH2+ NH3

HS- + R1 C NH3+

R2

R3

SN2SH C R1

R2

R3

+ NH3

Scheme 4.1 E2 and SN2 reactions of an alkylamine to an alkene and alkanethiol.

In the E1 elimination and SN1 nucleophilic substitution mechanisms, ammonia splits off

from the protonated amine in the rate-limiting step and forms a carbenium ion (Scheme 4.2).

The carbenium ion quickly undergoes proton abstraction to form an alkene (E1), or it reacts

with a nucleophile, such as H2S or another alkylamine, to form an alkanethiol or

dialkylamine, respectively (SN1). E1 and SN1 mechanisms are likely to occur only if relatively

stable carbenium ions (such as a benzyl, allyl, or tertiary trialkyl carbenium ion) can be

formed. Linear alkylamines will not react by E1 and SN1 mechanisms, because they would

lead to an unstable primary carbenium ion. Secondary alkyl carbenium ions are more stable

than primary ions and may form from alkylamines with a secondary α-carbon atom. They will

only form, however, if sufficiently strong acid sites are available and this is not the case on

the surface of metal sulfides. On the other hand, HDN reactions are carried out at least 300

°C. Such temperatures, which are much higher than normally used for organic reactions,

might increase the participation of carbenium ions [11].

The E1 and E2 reactions of an alkylamine lead to an alkene and the SN1 and SN2 reactions

to an alkanethiol or dialkylamine. The dialkylamine can undergo further nucleophilic

substitution to give an alkanethiol. As this and other studies [9,12-15] have shown,

alkanethiols react fast to alkenes and alkanes. Acid-base catalyzed elimination explains the


formation of an alkene from a thiol by an E1 or E2 reaction. In both cases, the reaction rate

and, thus, the conversion might be positively influenced by the H2S pressure. In the E1

reaction protonation of the SH group takes place before the rate-determining breaking of the

C-S bond. As a consequence, the E1 reaction may be aided by H2S. In the E2 reaction, a

higher H2S pressure increases the concentration of the S2- or SH- base at the catalyst surface.

However, in the HDS reactions that we studied, H2S had a strong negative influence on the

conversion of all n-alkanethiols, be it primary, secondary, or tertiary alkanethiols (Table 4.1).

This must be due to the fact that H2S adsorbs rather strongly on the catalyst surface and

inhibits the adsorption of the alkanethiol.

H2C C NH3+

R2

R3

H2C C+

R2

R3

+ NH3R1 R1

H

R2R1

R3

H2C C SH

R2

R3

R1

H2SSN1E1

+H++ H+

Scheme 4.2 E1 and SN1 reactions of an alkylamine to an alkene and alkanethiol.

4.4.1.2 Metal-like mechanisms

Substitution and elimination of alkylamines can be catalyzed not only by acids and bases,

but also by metals. Nucleophilic substitution can take place by a series of metal-catalyzed

reactions: dehydrogenation of an amine to an imine, addition of H2S, elimination of NH3, and

hydrogenation of the resulting thioaldehyde to a thiol (Eq. 1) [16]. Analogous reaction

schemes for homogeneous catalysts have been proposed by Laine [17]:

R1R2C=NHR1R2CH-NH2 R1R2C(SH)-NH2

R1R2C(SH)-NH2 R1R2C=S R1R2CH-SH (1)


Furthermore, the elimination of NH3 from an amine to form an alkene can be metal-

catalyzed [2,5]. In a first step, C-N bond hydrogenolysis would take place. This is an easy

reaction on metal catalysts [18]; on supported platinum, for example, it is already fast at

around 150 °C [19]. The resulting alkyl fragment can lose a β−hydrogen atom and form an

alkene, or it can add a hydrogen atom to form an alkane. Both reactions are well-known in

Fischer-Tropsch catalysis by metals. The fast and reversible addition of the H atom to the

alkene and rupture from the alkyl fragment explains the double bond shift in the alkene. C-S

bond breaking (hydrogenolysis) has also been demonstrated for the homogeneous

Cp*2Mo2Co2S3(CO)4 cluster catalyst (Cp* stands for pentamethylcyclopentadienyl) [20].

2-Pentanethiol and di-(2-pentyl)amine were primary products and pentane and pentenes

were secondary products in the HDN of 2-pentylamine at 300 °C (cf. Fig. 4.2). The alkenes

and alkane are formed from the corresponding alkanethiol, that is formed from the alkylamine

2 2RNH + H S RSH + NH→ 3

2

(2)

and dialkylamine by substitution of the amine group by H2S

2RNHR + H S RSH + RNH→ (3)

The pentenes/pentane ratio in the HDN of 2-pentylamine was very similar to that obtained in

the HDS of 2-pentanethiol in the presence of hexylamine (Fig. 4.4). This demonstrates that

the branching ratio between alkenes and alkane is determined by the thiol and that the thiol is

an intermediate between alkylamine and hydrocarbons.

At 340 °C, the pentenes behaved like a primary product (Fig. 4.2) due to the fast

formation of pentenes from 2-pentanethiol (Fig. 4.3). The decrease in the selectivity of the

alkenes and increase in the selectivity of the alkane with weight time at 340 °C (Fig. 4.2) is

due to hydrogenation of the alkenes. At this elevated temperature the hydrogenation of the

alkenes is relatively fast because of the high conversion (and thus decreased inhibition) of the

alkylamine. The HDN of 2-pentylamine is, thus, similar to that of n-hexylamine [9]. This

shows that amines with primary and secondary α-carbon atoms do not undergo elimination to

alkenes or hydrogenolysis to an alkane.


Table 4.1 Conversions and effect of H2S on conversion at time τ, and product selectivities at τ = 1 g.min/mol. Reactant T, τ, conv. select. °C g.min/ % effect H2S Cn

=/Cn RNHR, RSH, mol % %

2M2BA 270 3 30 (+) 16.5 0 0

300 1 46 (+) 16 0 0

2M2BT 270 1 65 -- 3.0

BA 270 3 25 0 0 0 0

TT 270 1 100 0

2PA 300 3 27 (+) 2.9 80 2

340 3 75 + 35 1

2PT 300 1 83 -- 2.9

340 1 95

3M2BA 300 3 18 + 3.5 45 9

340 3 70 + 3.5 10 3

3M2BT 300 1 75 -- 2.5

340 1 100 -- 3.2

2MCHA 300 3 10 (+) 1.0 38 4

340 1 55 (+) 3.5 10 1

3,3DM2BA 300 3 5 + 1.5 0 10

340 3 35 + 4.1 0 2

HA [9] 300 3 8 - 0.9 22 10

320 3 24 (-) 1.3 20 7

HT 300 1 90 -- 2.0

320 1 100 2.0

DHA [9] 300 1 55 0.8 14* 7

THA [9] 300 1 55 + 0.8 58 7


4.4.2 HDN of amines with secondary α-carbon atoms

The reactions of 3-methyl-2-butylamine, 2-methylcyclohexylamine, and 3,3-dimethyl-2-

butylamine showed many similarities (cf. Figs. 4.5, 4.8, 4.9). The conversions vary within a

factor of three in the order 3-methyl-2-butylamine > 2-methylcyclohexylamine > 3,3-

dimethyl-2-butylamine (Table 4.1). Alkanethiol and dialkylamine are primary products (non-

zero selectivities at τ = 0) and alkenes and alkane are primary as well as secondary products

(increasing selectivity with τ). The initial alkene selectivities at 300 °C and 100 kPa H2S were

about 22% for 3-methyl-2-butylamine, 8% for 2-methylcyclohexylamine, and 34% for 3,3-

dimethyl-2-butylamine. The non-zero selectivities of the alkenes at τ = 0 are due either to

direct elimination of NH3 from the respective alkylamine or to a relatively slow reaction of

the alkylamine to the corresponding alkanethiol followed by a fast reaction of this thiol to the

alkenes. The latter seems more feasible, because it is hardly likely that 2-pentylamine does

not undergo elimination, but that the introduction of a methyl group on the neighboring β-

carbon atom induces such a change in the mechanism. The higher initial selectivities at 340

than at 300 °C are, thus, due to an even faster formation of alkenes from the alkanethiol and

the decrease with weight time at 340 °C to a relatively fast hydrogenation of the alkenes. This

agrees with the fact that the HDS rates of the alkanethiols decrease less than the conversions

of the corresponding alkylamines as a result of methyl substitution (Table 4.1). Thus, the HDS

of the intermediate is relatively faster for the methyl-substituted thiol than for the

unsubstituted thiol and the final secondary products tend to behave like primary products.

Furthermore, the alkenes/alkane ratio, obtained in the HDN of 3-methyl-2-butylamine, is

about the same as that obtained in the HDS of the corresponding 3-methyl-2-butanethiol (Fig.

4.6) and similar to the alkenes/alkane ratio obtained in the HDN of 2-pentylamine (Fig. 4.4).

These ratios of the alkylamines and alkanethiols with a secondary α-C atom are much lower

than the ratio obtained for 2-methyl-2-butylamine, an alkylamine with a tertiary α-C atom (cf.

Section 4.4.3). In Section 4.4.3 we will show that the latter alkylamine reacts by direct

elimination of ammonia. The alkenes/alkane ratios thus confirm that the alkylamines with a

secondary α-C atom do not react by elimination of ammonia.

The amines with secondary α-carbon atoms reacted faster than the primary n-hexylamine

[9] but much slower than 2-methyl-2-butylamine with a tertiary α-carbon atom and the

activated benzylamine (section 4.3.5). 2-Pentylamine and 3,3-dimethyl-2-butylamine, both


with secondary α-carbon atoms, reacted four times faster at 300 °C than n-hexylamine and

neopentylamine [21], both with primary α-carbon atoms, respectively (Table 4.1). This

indicates that a pure SN2 mechanism cannot be responsible for the reaction of 2-pentylamine

and 3,3-dimethyl-2-butylamine. In a pure SN2 mechanism the extra methyl group on the α-

carbon atom would hinder the approach of a nucleophile, and the rate of reaction of the

alkylamine with a secondary α-carbon atom would be lower than that of the corresponding

alkylamine with a primary α-carbon atom [22]. The higher rates of the amines with secondary

α-carbon atoms are probably due to a weakening of the C-N bond because of the higher ionic

character of the C-N bond for a secondary carbon atom. The limit would be the dissociation of

the amine group with the formation of a secondary carbenium ion. This extreme situation did

not occur, however, for the amines in this study. In that case, an SN1 or E1 reaction would

have taken place and H2S would not have influenced the reaction rate.

The Wagner-Meerwein-type rearrangement of the carbon skeleton in the reaction of 3,3-

dimethyl-2-butylamine is ascribed to a nucleophilic substitution to 3,3-dimethyl-2-

butanethiol, followed (partly) by γ elimination aided by the neighboring group effect of the

methyl groups on the β-carbon atom (Scheme 4.3), analogous to reactions of branched

alcohols [23]. As to be expected, the rearrangement in the reaction of 2-methyl-3-pentylamine

was much less pronounced (< 1%).

NH2

SH

SH

+H2S

-NH3

-H2S

-H2S

H

Scheme 4.3 Reaction network of 3,3-dimethyl-2-butylamine.


The HDN of cyclohexylamine and 2-methylcyclohexylamine and the HDS of the

corresponding thiols was described recently [15,24]. These compounds also have secondary

α-carbon atoms. The removal of ammonia from cyclohexylamine and 2-

methylcyclohexylamine was mainly (60-70%) ascribed to an E2 elimination reaction to

cyclohexene and methylcyclohexenes respectively, because cis-2-methylcyclohexylamine (c-

MCHA) reacted much faster to 1-methylcyclohexene than trans-2-methylcyclohexylamine (t-

MCHA) [24]. Elimination is assumed to occur with the β-H atom in an anti-periplanar

configuration relative to the axial NH2 group. As the conformations in Scheme 4.4 show, the

elimination of c-MCHA should then be faster than that of t-MCHA because of the presence of

the hydrogen atom on the tertiary carbon atom in the anti position to the amine group [24]. In

t-MCHA the methyl group occupies this anti position and cannot be eliminated. It was

overlooked, however, that nucleophilic substitution of the amine group by an SH group may

also explain the faster reaction of c-MCHA. As in the anti-periplanar E2 elimination, SN2

substitution can only occur when the leaving group is in the axial position (Scheme 4.4). In

that case, the methyl group in the anti position in t-MCHA hinders the approach of an SH

nucleophile from the backside of the carbon atom bearing the amine group and, thus, the SN2

reaction should also be faster for c-MCHA than for t-MCHA. Thus, our conclusion, that 2-

methylcyclohexylamine reacts by substitution and not by elimination, does not contradict the

experimental results described in refs. 15 and 24.

E2

SN2

c-MCHA t-MCHA

NH2

CH3

H

NH2

CH3

NH2

CH3

NH2

H

CH3

B

SHSH

Scheme 4.4 E2 and SN2 reactions of cis- and trans 2-methylcyclohexylamine.


4.4.3 HDN of 2-methyl-2-butylamine and benzylamine

2-Methyl-2-butylamine and benzylamine reacted much faster than the other amines

studied. Whereas 2-pentylamine, 3-methyl-2-butylamine, 3,3-dimethyl-2-butylamine, and 2-

methylcyclohexylamine (all with secondary α-carbon atoms) did not show appreciable

conversion below 300 °C, 2-methyl-2-butylamine, with a tertiary α-carbon atom, already

reached a conversion of 30% at τ = 3 g.min/mol at 270 °C. The product distribution was also

different. For the most part, alkenes (2-methyl-1-butene and 2-methyl-2-butene) and an alkane

(methylbutane), but no di-(2-methyl-2-butyl)amine, and only a trace of 2-methyl-2-

butanethiol were observed. This behavior indicates that 2-methyl-2-butylamine, with a tertiary

α-carbon atom, reacts by a different mechanism than the amines with secondary α-carbon

atoms. Furthermore, because H2S does not influence the reaction rate, the most likely

mechanisms are E1 elimination and SN1 nucleophilic substitution.

If 2-methyl-2-butylamine were to react by a classic organic E1 or SN1 mechanism, then it

would be protonated and would react by ammonia removal to the tertiary isopentyl carbenium

ion (Scheme 4.2). In the E1 mechanism, this ion would then react further to 2-methyl-1-

butene and 2-methyl-2-butene by proton removal and the formation of methylbutane would be

unaccounted for. However, on the metal sulfide surface, the 2-methyl-2-butylamine adsorbs

with the nitrogen lone pair on an Mo or Ni atom. After C-N bond breaking, the isopentyl

carbenium ion will either move to a neighboring Mo or Ni atom or to a sulfur atom. If the

carbenium ion binds to a metal atom, an electron transfer reaction may take place with the

formation of the isopentyl radical. As in Fischer-Tropsch chemistry on a metal surface [2],

this alkyl radical may react to an alkene by removal of a hydrogen atom, or it may add a

hydrogen atom and become an alkane. If the carbenium ion binds to a sulfur atom, then

adsorbed 2-methyl-2-butanethiol forms and the mechanism changes to the SN1 type. 2-

Methyl-2-butanethiol can react to 2-methylbutenes as well as to methylbutane.

The methylbutenes/methylbutane ratio of the products of the HDN of 2-methyl-2-

butylamine was about five times larger than that obtained in the HDS of the corresponding 2-

methyl-2-butanethiol (Fig. 4.13). This demonstrates that the methylbutenes/methylbutane

ratio in the HDN is not determined by the thiol and that 2-methyl-2-butylamine reacts by an

E1 rather than an SN1 mechanism. In agreement with this conclusion, only 0.3% thiol was

observed in the HDN of 2-methyl-2-butylamine in the presence of 10 kPa H2S at τ = 1


g.min/mol; even in the presence of 100 kPa H2S, the initial selectivity of the thiol was only

1.5% (Fig. 4.11). Figure 4.12 demonstrates that 2-methyl-2-butanethiol reacts rather slowly in

the presence of 100 kPa H2S. Thus, if this thiol had been an intermediate in the HDN of 2-

methyl-2-butylamine, then a quantity larger than 1.5% would have been observed.

Benzylamine cannot react by elimination, because it has no β-hydrogen atoms. The high

reaction rate and lack of an effect of the H2S pressure suggest that benzylamine does not react

by an SN2 but by an SN1 reaction. Protonation of the amine group and the removal of

ammonia would lead to the relatively stable benzyl carbenium ion. As indicated above for the

isopentyl carbenium ion, the benzyl carbenium ion will move to a neighboring Mo or Ni atom

or to a sulfur atom. If it binds to a metal atom, an electron transfer reaction may take place

with the formation of the benzyl radical, which can be hydrogenated to toluene. If the

carbenium ion binds to a sulfur atom, adsorbed α-toluenethiol forms and may react to toluene.

4.5 Conclusions

Our former [9] and present results show that alkylamines with the NH2 group attached to a

primary or secondary carbon atom react by substitution of the NH2 group by an SH or amine

group to form an alkanethiol or a dialkylamine. After subsequent substitution by H2S the

dialkylamine also reacts to an alkanethiol. The alkanethiol finally reacts to an alkene or alkane

and H2S. Only an alkylamine with the NH2 group attached to a tertiary or activated carbon

atom reacts directly to an alkene or alkane. The C-N bonds of alkylamines with primary and

secondary α-carbon atoms are too strong to be easily broken. For such alkylamines

elimination is, therefore, too difficult and they react by other mechanisms. The stabilization of

the tertiary or benzyl carbenium cation is necessary to weaken the C-N enough for elimination

to take place.

The proposal by Portefaix et al. [3], that alkylamines react by elimination and that the

number of β-H atoms determines their HDN rate, is thus incorrect. The fact that 2-methyl-2-

butylamine reacts much faster than n-pentylamine has nothing to do with the four times larger

number of β-H atoms but has everything to do with the fact that the NH2 group of the former

amine is attached to a tertiary α-C atom and the NH2 group of the latter amine to a primary α-


C atom. Even when elimination occurs, as for 2-methyl-2-butylamine, the selectivity for 2-

methyl-2-butene is higher than that for 2-methyl-1-butene, although there are three times

more β-H atoms on the terminal methyl groups than on the internal methylene group. We

checked that this is not due to a fast isomerization of 2-methyl-1-butene to 2-methyl-2-butene.

The higher selectivity for 2-methyl-2-butene is due to the fact that in an E1 mechanism the

leaving group is gone before the proton. The product is thus determined by thermodynamic

factors and Zaitsev’s rule applies: the double bond goes preferentially to the most highly

substituted carbon atom. We conclude therefore that the number of β-H atoms, as proposed by

Portefaix et al., does not determine the HDN rate of the alkylamines. In alkylamines with

primary and secondary α-C atoms the number of β-H atoms is of no importance, because

substitution rather than E2 elimination takes place. In alkylamines with a tertiary α−C atom

the reverse occurs: the hydrogen atom is preferentially removed from the β-C atom with the

lowest number of β-H atoms!

Even though elimination of the NH2 group from an alkylamine does not take place when it

is attached to a primary or secondary carbon atom, removal of nitrogen does take place for

such alkylamines. The substitution of the NH2 group by H2S leads to an alkanethiol and

ammonia and, thus, to total denitrogenation. Substitution by an amine leads to a dialkylamine

and ammonia and to 50% nitrogen removal. The rest of the nitrogen is removed in the

subsequent substitution of the dialkylamine by H2S to an alkanethiol and the original

alkylamine. High partial pressures of H2S and alkylamine increase the rate of transformation

of alkylamine to alkanethiol and, thus, of denitrogenation. At the same time, however, the rate

of sulfur removal from the alkanethiol decreases.

While we have pinpointed the types of reactions that alkylamines undergo, we have not

answered the question as to how these reactions are catalyzed by the supported metal sulfide.

Our future work will address the question of how the substitution of the NH2 group by H2S

takes place on the surface of nickel- and cobalt-promoted and unpromoted MoS2.

Acknowledgement.

The authors thank T. Schmid for synthesizing 2-methyl-3-pentylamine and Dr. P. Kukula for

discussions.


4.6 References




10 (1991) 473.


[5] R. Prins, Adv. Catal. 46 (2001) 399.

[6] M. Houalla, N.K. Nag, A.V. Sapre, D.H. Broderick, B.C. Gates, AIChE J. 24 (1978)

1015.

[7] J. Mijoin, G. Pérot, F. Bataille, J.L. Lemberton, M. Breysse, S. Kasztelan, Catal. Lett.

71 (2001) 139.


(1998) 366.

[9] Y. Zhao, P. Kukula, R. Prins, J. Catal. 221 (2004) 441; and chapter 3.

[10] M. Egorova, Y. Zhao, P. Kulula, R. Prins, J. Catal. 206 (2002) 263.

[11] H. Knözinger, A. Scheglila, J. Catal. 17 (1970) 252.

[12] P. Desikan, C.H. Amberg, Can. J. Chem. 42 (1964) 843.

[13] D.L. Sullivan, J.G. Ekerdt, J. Catal. 178 (1998) 226.


[15] F. Rota, R. Prins, J. Catal. 202 (2001) 195.

[16] R. Prins, in “ Handbook of Heterogeneous Catalysis” (G. Ertl, H. Knözinger, J.

Weitkamp, Eds.). Vol 4, p. 1916, Wiley-VCH.

[17] R.M. Laine, Catal. Rev.-Sci. Eng. 25 (1983) 459.

[18] G. Meitzner, W.J. Mykytka and J.H. Sinfelt, J. Catal. 98 (1986) 513.

[19] Triyono, R. Kramer, Appl. Catal. A 100 (1993) 145.


[21] Y. Zhao, J. Czyzniewska, R. Prins, Catal. Lett. 88 (2003) 155.

[22] J. Clayden, N. Greeves, S. Warren, P. Wothers, ‘’Organic Chemistry’’. Oxford Univ.

Press, 2001.

[23] H. Pines, J. Manassen, Adv. Catal. 16 (1966) 49.

[24] F. Rota, V.S. Ranade, R. Prins, J. Catal. 200 (2001) 389.

HDN over Hydrotreating Catalysts Chapter 5 107

5. Mechanisms of HDN of Alkylamines and HDS of alkanethiol on NiMo/Al2O3, CoMo/Al2O3, and Mo/Al2O3

5.1 Abstract

The simultaneous hydrodenitrogenation (HDN) of alkylamines and hydrodesulfurization

(HDS) of alkanethiols, with the NH2 and SH groups attached to primary, secondary, and

tertiary carbon atoms were studied at 270 and 270-320 °C and 3 MPa over sulfided

NiMo/Al2O3, CoMo/Al2O3, and Mo/Al2O3 catalysts. Pentylamine and 2-hexylamine reacted

by substitution with H2S to alkanethiols and with another amine molecule to dialkylamines.

Alkenes and alkanes were not formed directly from pentylamine and 2-hexylamine, but

indirectly by elimination and hydrogenolysis of the alkanethiol intermediates, as confirmed by

their secondary behaviour and the similar alkenes/alkane ratios in the simultaneous reactions

of amines and thiols. Only 2-methyl-2-butylamine, with the NH2 group attached to a tertiary

carbon atom, produced alkenes as primary products by E1 elimination. NiMo/Al2O3 and

CoMo/Al2O3 have a higher activity for the HDS of alkanethiols than Mo/Al2O3 and H2S has a

negative influence. This shows that the thiols react on vacancies on the catalyst surface

(Lewis acid sites). Mo/Al2O3 is the best HDN catalyst and H2S has a positive influence for the

HDN of amines with the NH2 group attached to a secondary and tertiary carbon atom. This

indicates that the HDN of alkylamines occurs on Brønsted acid sites.

5.2 Introduction

Environmental legislation in the coming years requires a further reduction of the sulfur

content of gasoline and diesel fuel to 10 ppm. Deep hydrodesulfurization (HDS) technology

has to be implemented to reach this low level of sulfur. Nitrogen-containing compounds are

harmful in deep HDS, as they inhibit the HDS of sulfur-containing compounds through


competitive adsorption [1-3]. Therefore, it is important to know how nitrogen-containing

molecules are removed by hydrotreating catalysts. Nelson and Levy were the first to suggest

mechanisms for the hydrodenitrogenation (HDN) of alkylamines and they suggested that

nucleophilic substitution and Hofmann β-H elimination are responsible for HDN [4]. For each

of these mechanisms evidence has been published [5-8] and it has been concluded that the

mechanism depends on the alkylamine as well as on the catalyst [7]. We found that over

sulfided NiMo/Al2O3 the substitution mechanism is predominant in the HDN of alkylamines

with the NH2 group attached to a primary or secondary carbon atom [9,10]. Hofmann β-H

elimination hardly takes place in these alkylamines. Nevertheless, a large amount of alkenes is

formed in the HDN of an alkylamine by fast decomposition of the corresponding alkanethiol,

which is formed by the substitution of the alkylamine with H2S. On the other hand, the HDN

of tertiary amines does occur via an E1 mechanism [10].

Cattenot et al. concluded that the HDN mechanism not only depends on the alkylamine

but also on the catalyst [7]. Different metal sulfides may have different acidities and therefore

different catalytic properties. According to their results of unsupported metal sulfides, the

acidity decreases in the order NbS3 > MoS2 > RuS2 > Rh2S3. The HDN reaction of

alkylamines might help to discriminate between the acid properties of NiMo/Al2O3,

CoMo/Al2O3, and Mo/Al2O3. In the HDN of an alkylamine, the products are a dialkylamine,

an alkanethiol, alkenes, and an alkane. They can, in principle, be formed by acid-base

catalysis (Scheme 5.1). The amine group reacts with a proton or a Lewis acid and, at the same

time, a nucleophile attacks the α-carbon atom. The ammonia is split off, while a substitution

product is formed. That acid-base chemistry can take place at metal sulfide surfaces was

confirmed by FTIR spectroscopy of pyridine adsorbed at high temperature [11]. Maugé and

coworkers found evidence that H2S adsorption leads to a substantial increase in the Brønsted

acidity of the sulfide phase [12-14].

Substitution may not only be catalyzed by acidic sites, but also by metallic sites [15].

Several groups have discussed the electronic properties of the ideal reactive MoS2 surface and

pointed out that this surface has electron acceptor property and that it is metallic [16-22]. As

shown in Scheme 5.2, a sequence of dehydrogenation of amine to imine, H2S addition,

ammonia elimination, and hydrogenation of the thioaldehyde transforms an alkylamine into

an alkanethiol.


For these reasons, we compared the HDN mechanisms over the three catalysts in the

simultaneous reactions of pentylamine and hexanethiol, the simultaneous reactions of 2-

hexylamine and 2-pentanethiol, the HDN of 2-methyl-2-butylamine, and the HDS of 2-

methyl-2-butanethiol in the presence of hexylamine over the three metal sulfide catalysts.

H2S + R1 C NH2

R2

H

SN2C SH

R2

H

+ NH3R1

Scheme 5.1 Direct substitution of an alkylamine to an alkanethiol by acid-base catalysis.

C N

H

H

H

C N

SH

H

HN

H-H2 H2S

S + NH3

S C SH

H

C H

H

+ H2SH2 H2

Scheme 5.2 Indirect substitution of an alkylamine to an alkanethiol by metal catalysis.

5.3 Results

As we showed for sulfided NiMo/Al2O3 [9,10] and as we will show here for sulfided

Mo/Al2O3 and CoMo/Al2O3, the HDN of alkylamines is not a simple reaction but is the sum

of two parallel reactions, each of which consists of several consecutive reactions. To

disentangle this complex network of reactions, we will use two methods. One method is to

concentrate on selectivities instead of yields. The advantage of selectivities over yield is that

they allow, in principle, to more easily distinguish between primary and secondary products.

For instance, in a series of consecutive reactions from reactant A to product D

1 2 3k k kA B C⎯⎯→ ⎯⎯→ ⎯⎯→D

the yield of the primary product B as well as of the secondary product C initially increase with

reaction time (Fig. 5.1 A), but this behaviour would also be observed for parallel reactions of

A to B and A to C. Only if one can measure at very short time, will one be able to distinguish


between consecutive and parallel reactions and prove the secondary character of C. By

plotting the same experimental results as selectivities, however, the selectivity of B decreases

and that of C increases with time initially for a consecutive reaction (Fig. 5.1B), while both

selectivities increase for parallel reactions of A to B and C. Even though the product yields

are initially low, and thus the uncertainties in the selectivities high, it is still easy to

distinguish between an increase or a decrease of the selectivity with time. Therefore, we will

use selectivities to unravel the reaction mechanisms.

0 1 2 3 4 50

20

40

60

80

100B

0 1 2 3 4 50

50

100A

D

B

A

Yiel

d, %

k*t

D

C

B

Sele

ctiv

ity, %

k*t

C

Figure 5.1 Product selectivities and yields in a consecutive reaction from reactant A to

product D (with equal rate constants for each reaction step).

The second method that we use is the comparison of the alkene/alkane ratio in the HDN of

an alkylamine with the ratio in the HDS of a similar alkanethiol. The idea is that if an

alkylamine would react to alkenes and an alkane without going through an intermediate

alkanethiol, the alkene/alkane ratio should not be the same as that in the HDS of the

alkanethiol. If, on the other hand, the alkylamine first reacts to an alkanethiol, which then

reacts to a mixture of alkenes and alkane, the alkenes/alkane branching ratio will be

determined by the alkanethiol. Thus the branching ratio will be the same in HDN and in HDS.

The ratio of elimination to hydrogenolysis of an alkanethiol, and thus the resulting

alkenes/alkane ratio, is sensitive to the coverage on the catalyst surface and thus to the

presence and partial pressures of alkylamine and alkanethiol. Therefore, the branching ratio in

HDS was always determined in the presence of an amount of alkylamine equivalent to that

used in HDN.


5.3.1. Simultaneous reaction of pentylamine and hexanethiol

The conversion of 5 kPa pentylamine in the simultaneous HDN and HDS at 320 °C in the

presence of 5 kPa hexanethiol and 10 kPa H2S was 12% at τ = 1.6 g.min/mol and 60% at 8.9

g.min/mol over NiMo/Al2O3 (Fig. 5.2); the conversion over CoMo/Al2O3 was similar. The

conversion over both catalysts decreased slightly when the H2S pressure was increased from

10 to 100 kPa. Over Mo/Al2O3, the conversion of pentylamine was much higher than over

NiMo/Al2O3 and CoMo/Al2O3 and it decreased substantially with increasing H2S pressure

from 10 to 100 kPa H2S.

Figure 5.2 Conversion of pentylamine in the presence of hexanethiol at 10 ( ) and

100 ( ) kPa H2S, and 320 °C over sulfided NiMo/Al2O3, CoMo/Al2O3

and Mo/Al2O3 catalysts.

0 2 4 6 8 100

20

40

60

80

100Mo/Al2O3

C5-N

H2 C

onve

rsio

n, %


40

60

0 2 4 6 8 100

20

NiMo/Al2O3

C5-N

H2 C

onve

rsio

n, %

Weight time, g.min/mol 0 2 4 6 8 100

20

40

60CoMo/Al2O3

C5-N

H2 C

onve

rsio

n, %



The products over the three catalysts were pentanethiol, dipentylamine, pentenes (i.e. the

sum of the two pentene isomers), and pentane (Figs. 5.3-5.5). The selectivities to

dipentylamine and pentanethiol increased in the order NiMo<CoMo<Mo, while the reverse

order was observed for pentane and the pentenes. Dipentylamine behaved as a primary

product over all three catalysts. It was present in much larger quantity over Mo/Al2O3 than

over NiMo/Al2O3 and CoMo/Al2O3. The selectivity of pentanethiol over the NiMo/Al2O3 and

CoMo/Al2O3 catalysts increased with decreasing weight time, suggesting that pentanethiol is

a primary product as well. The Mo/Al2O3 catalyst behaved differently at 100 kPa H2S (Fig.

5.5).

0 2 4 6 8 100

20

40

60

Pent

ane,

%


0

10

20

30

40

50Pe

nten

es, %


0 2 4 6 8 10

0

5

10

15

20

25

Pent

anet

hiol

, %


0

5

10

15

20

25

DPA

, %


Figure 5.3 Product selectivities in the HDN of pentylamine in the presence of

hexanethiol at 10 ( ) and 100 ( ) kPa H2S, and 320 °C over sulfided

NiMo/Al2O3 (DPA = dipentylamine).


50


hexanethiol at 10 ( ) and 100 ( ) kPa H2S, and 320 °C over sulfided

CoMo/Al2O3 (DPA = dipentylamine).

When decreasing the weight time, the selectivity of pentanethiol first increased, then reached

a maximum, and decreased at shorter weight time. This result was checked several times, by

repeating the experiments. The selectivity of pentanethiol strongly increased and that of

dipentylamine strongly decreased with increasing H2S pressure over all three catalysts.

The selectivities of the pentenes and pentane as a function of weight time were different

over the three catalysts (Figs. 5.3-5.5). The pentenes and pentane behaved as primary

products over NiMo/Al2O3 and CoMo/Al2O3, as the selectivities extrapolate to a non-zero

value with decreasing weight time. Since the selectivities initially increase with weight time,

the pentenes and pentane are secondary or even tertiary products as well. Over Mo/Al2O3,

0 2 4 6 8 100

10

0 2 4 6 8 100

20

40

60

Pent

enes

, %


40

30

20

Pent

ane,

%


40

0 2 4 6 8 100

10

0 2 4 6 8 100

10

20

30

40

DPA

, %


30

20

Pent

anet

hiol

, %



however, the selectivities extrapolate to zero with decreasing weight time, which indicates

that over this catalyst the hexenes and hexane are secondary or tertiary products only.

0 2 4 6 8 100

20

40

60

80

Pent

ane,

%


0

10

20

30

Pent

enes

, %Weight time, g.min/mol

0 2 4 6 8 100

20

40

60

Pen

tane

thio

l, %


0

20

40

60

80

100

DPA

, %



hexanethiol at 10 ( ) and 100 ( ) kPa H2S, and 320 °C over Mo/Al2O3

(DPA = dipentylamine).

The conversion of 5 kPa hexanethiol in the presence of 10 kPa H2S and 5 kPa pentylamine

(simultaneous HDS and HDN) was much higher than that of pentylamine. At τ = 1.6

g.min/mol it was already 100% at 320 °C (Table 5.1), while that of pentylamine was 12% at

320 °C over NiMo/Al2O3 and CoMo/Al2O3 (Fig. 5.2). The conversion of hexanethiol

decreased when the H2S pressure was increased from 10 to 100 kPa, most strongly over

Mo/Al2O3 (Table 5.1). Nevertheless, for all three catalysts the conversion of the thiol was

complete at 320 °C at short τ. This means that the presence of H2S during the HDN was

actually 5 kPa higher than the indicated 10 or 100 kPa due to H2S in the feed. The


hexenes/hexane ratio, obtained in the HDS of hexanethiol, was similar to the

pentanes/pentane ratio obtained in the HDN of pentylamine over all three catalysts (Fig. 5.6).

2.0

Figure 5.6 Hexenes/hexane ratio in the HDS of hexanethiol (HT) and pentenes/pentane

ratio in the HDN of pentylamine (PA) at 10 ( ) and 100 ( ) kPa H2S,

and 320 °C over sulfided NiMo/Al2O3, CoMo/Al2O3, and Mo/Al2O3

(DPA = dipentylamine).

0 2 4 6 8 100.0

0.5

1.0

1.5

HT, NiMo/Al2O3

C6= /C

6

0 2 4 6 8 100.0

0.5

1.0

1.5

2.0


PA, NiMo/Al2O3

C5= /C

5

0 2 4 6 8


10

2.0

1.5

1.0

0.5

0.0

HT, CoMo/Al2O3

C6= /C

6


0.0

0.5

1.0

1.5

2.0PA, CoMo/Al2O3

C5= /C

5


2.0

0 2 4 6 8 100.0

0.5

1.0

1.5

HT, Mo/Al2O3

C6

6


0.0

0.5

1.0

1.5

2.0PA, Mo/Al2O3

C5= /C

5


= /C


TABLE 5.1 Conversions (%) of hexanethiol, 2-pentanethiol, and 2-methyl-2-butanethiol in the presence

of pentylamine, 2-hexylamine, and hexylamine respectively at 270 and 320 °C, 10 and 100

kPa H2S at τ = 1.6 g.min/mol over NiMo/Al2O3, CoMo/Al2O3, and Mo/Al2O3

Hexanethiol 2-pentanethiol 2-methyl-2-butanethiol

320 °C 320 °C 270 °C

H2S (kPa) 10 100 10 100 10 100

NiMo 100 90 100 89 76 32

CoMo 100 79 99 83 77 26

Mo 88 24 98 59 65 22

The conversion of pentylamine over sulfided Zn/Al2O3 and Cd/Al2O3 was lower than 4%

at 320 °C, 10 kPa H2S and τ = 8.9 g.min/mol.

5.3.2. Simultaneous reaction of 2-hexylamine and 2-pentanethiol

The conversion of 2-hexylamine was higher than that of pentylamine over NiMo and

CoMo and increased weakly with increasing H2S pressure (cf. Figs. 5.2 and 5.7). The main

products were 2-hexanethiol, di-(2-hexyl)amine, hexenes (i.e. the sum of the three hexene

isomers), and hexane. Like for pentylamine, the selectivities for disproportionation and

substitution increased in the order NiMo<CoMo<Mo, while those for hexane and hexenes

decreased in the same order (Figs. 5.8-5.10). The selectivities of 2-hexanethiol and di-(2-

hexyl)amine increased with decreasing weight time (Figs. 5.8-5.10), which shows that they

are primary products. The selectivity of di-(2-hexyl)amine decreased with increasing H2S

pressure, while the reverse was true for the selectivity of 2-hexanethiol.

The selectivities of the hexenes and hexane as a function of weight time were different

over the three catalysts (Figs. 5.8-5.10). Hexenes and hexane behave as primary products

over NiMo/Al2O3 and CoMo/Al2O3, as the selectivities extrapolate to non-zero values with

decreasing weight time, as well as secondary products because the selectivities initially

increase with increasing weight time. Over Mo/Al2O3, however, the selectivities extrapolate


to zero with decreasing weight time, which indicates that hexenes and hexane are secondary

or tertiary products only.

0 2 4 6 8 100

20

40

60

80

100NiMo/Al2O3

2-H

exyl

amin

e C

onve

rsio

n, %


0 2 4 6 8 100

20

40

60

80

100CoMo/Al2O3

2-H

exyl

amin

e C

onve

rsio

n, %


0 2 4 6 8 10

0

20

40

60

80

100Mo/Al2O3

2-C

6-NH

2 Con

vers

ion,

%


Figure 5.7 Conversion of 2-hexylamine in the presence of 2-pentanethiol at 10 ( )

and 100 ( ) kPa H2S, and 320 °C over sulfided NiMo/Al2O3,

CoMo/Al2O3 and Mo/Al2O3


0 2 4 6 8 100

20

40

60

80

0 2 4 6 8 100

20

40

60

Hex

enes

, %


ne, %

Hex

a


0 2 4 6 80

10

20

30

10

Figure 5.8 Product selectivities in the HDN of 2-hexylamine in the presence of

2-pentanethiol at 10 ( ) and 100 ( ) kPa H2S, and 320 °C over sulfided

NiMo/Al2O3

2-H

exa

io


0

10

20

30

40

Di-2

-hex

ylam

ine,

%


l, %

neth


0 2 4 6 8 10

0

20

40

60

80

Hex

ane,

%


0

20

40

60

Hex

enes

, %Weight time, g.min/mol

0 2 4 6 8 100

10

20

30

2-H

exan

ethi

ol, %


0

10

20

30

40

50

Di-2

-hex

ylam

ine,

%




CoMo/Al2O3


0 2 4 6 8 100

20

40

60

80

Hex

ane,

%


0 2 4 6 8 100

10

20

30

40

Hex

enes

, %


0 2 4 6 8 10

0

10

20

30

40

2-H

exan

ethi

ol, %


0

20

40

60

80

100D

i-2-h

exyl

amin

e, %




Mo/Al2O3

The conversion of 2-pentanethiol in the simultaneous reaction with 2-hexylamine in the

presence of 10 kPa H2S was much larger than that of the corresponding amine. It was almost

100% at τ = 1.6 g.min/mol at 320 °C (Table 5.1), while that of the corresponding amine was

30% (Fig. 5.7). The conversion of hexanethiol decreased when the H2S pressure was

increased from 10 to 100 kPa, most strongly over Mo/Al2O3. The pentenes/pentane ratio

obtained from the HDS of 2-pentanethiol was the same as the hexenes/hexane ratio obtained

from the HDN of 2-hexylamine over NiMo/Al2O3 and CoMo/Al2O3 (Fig. 5.11). The ratio

originating from 2-pentanethiol was slightly lower than the one from 2-hexylamine over

Mo/Al2O3.


0 2 4 6 8 100.0

0.5

1.0

1.5

2.02-PT, NiMo/Al2O3

C5= /C

5


0.0

0.5

1.0

1.5

2.02-HA, NiMo/Al2O3

C6= /C

6


0 2 4 6 8 10

0.0

0.5

1.0

1.5

2.02-PT, CoMo/Al2O3

C5= /C

5


0 2 4 6 8 100.0

0.5

1.0

1.5

2.02-HA, CoMo/Al2O3

C6= /C

6


0 2 4 6 8 10

0.0

0.5

1.0

1.5

2.02-PT, Mo/Al2O3

C5= /C

5


0 2 4 6 8 100.0

0.5

1.0

1.5

2.02-HA, Mo/Al2O3

C6= /C

6


Figure 5.11 Pentenes/pentane ratio in the HDS of 2-pentanethiol (2-PT) and

hexenes/hexane ratio in the HDN 2-hexylamine (2-HA) at 10 ( ) and 100

( ) kPa H2S, and 320 °C over sulfided NiMo/Al2O3, CoMo/Al2O3, and

Mo/Al2O3



The HDN of 2-methyl-2-butylamine occurred fast; the conversion was already 17% at 270

°C and 10 kPa H2S at low weight time (1.6 g.min/mol) and reached 75% at τ = 8.9 g.min/mol

over NiMo/Al2O3 (Fig. 5.12). The conversions over CoMo/Al2O3 and Mo/Al2O3 were even

higher than over NiMo/Al2O3. In all cases, the main products were 2-methyl-2-butene, 2-

methyl-1-butene, and 2-methylbutane (not shown), and the methylbutenes were primary

products and methylbutane a secondary product.

0 2 4 6 8 100

20

40

60

80

100CoMo/Al2O3

2M2B

A C

onve

rsio

n, %


0

20

40

60

80

100NiMo/Al2O3

Figure 5.12 Conversion of 2-methyl-2-butylamine (2M2BA) at 270 °C, 10 ( ) and

100 ( ) kPa H2S over sulfided NiMo/Al2O3, CoMo/Al2O3, and Mo/Al2O3

2M2B

A C

onve

rsio

n,


%

0 2 4 6 8 100

20

40

60

80

100Mo/Al2O3

2M2B

Aon

ve


rsio

n, %

C


The conversion of 2-methyl-2-butanethiol at 270 °C in the presence of 10 kPa H2S and 5

kPa hexylamine was much larger than that of the equivalent amine. It was already 76% at τ =

1.6 g.min/mol over NiMo/Al2O3 (Table 5.1), while the conversion of the corresponding amine

was only 17% (Fig. 5.12). The conversion of 2-methyl-2-butanethiol strongly decreased with

increasing H2S pressure. The methylbutenes/methylbutane ratio, obtained in the HDS of 2-

methyl-2-butanethiol in the presence of hexylamine, was very different from the one obtained

in the HDN of 2-methyl-2-butylamine under the same conditions over all three catalysts (Fig.

5.13).

The conversion of 2-methyl-2-butylamine over sulfided Cd/Al2O3 at 10 kPa H2S and 270

°C was only 6% at τ = 8.9 g.min/mol. It increased to 25 % at 300 °C and 100 % at 320 °C.

20

Figure 5.13 Methylbutenes/methylbutane ratio in the HDN of 2-methyl-2-

butylamine (2M2BA) and HDS of 2-methyl-2-butanethiol (2M2BT) in

the presence of hexylamine at 270 °C, 10 ( ) and 100 ( ) kPa H2S over

sulfided NiMo/Al2O3, CoMo/Al2O3 and Mo/Al2O3

0 2 4 6 8 100

5

15

10

NiMo/Al2O3

2M2BT

2M2BA

C5= /C

5


40

30

20

10

0 2 4 6 8 100

Mo/Al2O3

2M2BT

2M2BA

C5= /C

5


0 2 4 6 8 100

5

10

15

20

2M2BT

2M2BA

CoMo/Al2O3

C5= /C

5



5.4. Discussion

5.4.1. Pentylamine

The conversion of pentylamine at 320 °C was similar over NiMo/Al2O3 and CoMo/Al2O3

(Fig. 5.2), but lower than over Mo/Al2O3. It decreased more stongly over Mo/Al2O3 with

increasing H2S pressure from 10 to 100 kPa H2S. At first glance, these results seem to be in

contradiction with literature, which reports that NiMo/Al2O3 is a better HDN catalyst than

CoMo/Al2O3 and much better than Mo/Al2O3 [23-25]. However, these publications are

mainly concerned with the HDN of aromatic N-containing molecules for which

hydrogenation and not the final N-removal step is rate determining. Indeed, the position of the

maximum in the selectivity of the pentenes at lower τ for NiMo (Fig. 5.3) than for CoMo (Fig.

5.4) than for Mo (Fig. 5.5) indicates that hydrogenation is fastest for the NiMo catalyst and

slowest for the Mo catalyst. Also, the product selectivities in the HDN of pentylamine should

be considered in detail. The selectivity of dipentylamine, formed by the disproportionation

reaction of pentylamine, was 20% over NiMo/Al2O3 (Fig. 5.3), 40% over CoMo/Al2O3 (Fig.

5.4), and 80% over Mo/Al2O3 (Fig. 5.5) at τ = 1.6 g.min/mol, 320 °C, and 10 kPa H2S. Thus,

the higher conversion over Mo/Al2O3 is mainly due to the higher disproportionation of

pentylamine to dipentylamine. In section 5.4.3, we will suggest an explanation for the higher

activity of Mo/Al2O3.

Hexanethiol converted very fast at 320 °C (Table 5.1). The conversion was already 100%

at τ = 1.6 g.min/mol over both NiMo/Al2O3 and CoMo/Al2O3 at 10 kPa H2S. At 100 kPa H2S,

NiMo/Al2O3 performs slightly better than CoMo/Al2O3 in the HDS of hexanethiol, but it is

fair to say that both catalysts are about equally good in the HDN of alkylamines as well as in

the HDS of alkanethiols. The HDS of hexanethiol is definitely slower over Mo/Al2O3,

however, and decreased strongly with increasing H2S pressure from 10 to 100 kPa. We

ascribe the lower H2S conversion over Mo/Al2O3 to the much lower number of sulfur

vacancies on MoS2 than on Co or Ni-promoted MoS2 [19,20] and the decreased conversion

with increasing H2S pressure to the filling of these vacancies by H2S.

Pentanethiol was a primary product in the HDN of pentylamine over all three catalysts, as

the selectivity extrapolated to a non-zero value at time zero (Figs. 5.3-5.5). At 100 kPa H2S,

the selectivity of pentanethiol increased with decreasing weight time over NiMo/Al2O3,


seemed to reach a maximum at short weight time over CoMo/Al2O3 (Fig. 5.4), and clearly

showed a maximum with decreasing weight time over Mo/Al2O3 (Fig. 5.5). This was checked

by repeating the experiments several times. Pentanethiol can be formed in two ways. One is

substitution of pentylamine with H2S

C5H11NH2 + H2S C5H11SH + NH3 (1)

and the other is substitution of dipentylamine with H2S

C5H11NH2 + C5H11NH2 C5H11NHC5H11 + NH3 (2)

C5H11NHC5H11 + H2S C5H11SH + C5H11NH2 (3)

Pentanethiol is a primary product in Eq. 1 and a secondary product in Eqs. 2 and 3. This

explains that the selectivity of pentanethiol is non-zero at τ = 0 (Eq. 1) and then increases with

increasing weight time at 100 kPa H2S (Eqs. 2 and 3). At higher weight time, the pentanethiol

selectivity decreases because of the reaction of the thiol to an alkene by elimination and to an

alkane by hydrogenolysis. The initial increase of the pentanethiol selectivity with increasing

weight time at 100 kPa H2S proves that dipentylamine, formed by disproportionation of

pentylamine, reacts fast with H2S to form pentanethiol and pentylamine over Mo/Al2O3. Over

NiMo/Al2O3, dipentylamine reacts indeed faster with H2S to form pentylamine and

pentanethiol than pentylamine reacts with H2S to pentanethiol [9]. For NiMo/Al2O3, the

decomposition of the formed pentanethiol is very fast, while for Mo/Al2O3 it is slower (Table

5.1). This explains that for NiMo/Al2O3 no maximum in the thiol selectivity is observed

(above τ = 1.6 g.min/mol, the lowest weight time that could be obtained), while for Mo/Al2O3

a clear maximum is observed. For CoMo/Al2O3, with an intermediate rate of thiol

decomposition (Table 5.1), there is an indication of a maximum (Fig. 5.4).

Pentene and pentane behaved as primary as well as secondary products over NiMo/Al2O3

and CoMo/Al2O3 (Figs. 5.3,5.4), while they behaved only as secondary products over

Mo/Al2O3. However, the pentenes/pentane ratio in the simultaneous reaction of pentylamine

and hexanethiol was very similar to the hexenes/hexane ratio originating from hexanethiol

over all three catalysts (Fig. 5.6). This demonstrates that the alkenes and alkane are

determined by the thiol, which is an intermediate between alkylamine and hydrocarbons.


Thus, substitution is the predominant reaction in the HDN of pentylamine over all three

catalysts. That the pentenes and pentane behave as primary products is due to the very fast

decomposition of the intermediate pentanethiol (Table 5.1). Only when this decomposition is

slowed down, as over Mo/Al2O3 (Table 5.1), the formation of the pentenes and pentane is

clearly not primary. The fact that their selectivities seem to extrapolate to zero for τ > 0 (Fig.

5.5), suggests that the pentenes and pentane might even mainly be tertiary products (cf. curve

D in Fig. 5.1B with the pentenes and pentane selectivities in Fig. 5.5, both at low τ). They

would then be formed from dipentylamine (Scheme 5.3).

RNH2

H 2S

-NH 3

RSH

R= + H2S

RH + H2SH2

RNH2-NH

3 RNRH

H2S -RNH2

Scheme 5.3 Reaction network for the removal of nitrogen from alkylamines with the amine

group attached to a primary or secondary carbon atom.

5.4.2. 2-Hexylamine

The reaction rate of 2-hexylamine was faster than that of pentylamine over NiMo/Al2O3

and CoMo/Al2O3, because of the weaker C-N bond of the alkylamine with the NH2 group

attached to a secondary carbon atom [10]. The conversion of 2-hexylamine increases slightly

with increasing H2S pressure, while that of pentylamine decreases with increasing H2S

pressure. The influence of H2S is at least twofold. On the one hand, it has a positive influence

as a reaction partner in the nucleophilic substitution of the alkylamine to an alkanethiol. On

the other hand, it adsorbs on the catalyst surface and hinders reaction, as shown by the

negative influence on the disproportionation of the alkylamines to dialkylamines and on the

reaction of alkanethiols (Table 5.1). The much stronger effect of H2S on the Mo/Al2O3


catalyst (Fig. 5.2), which shows the highest selectivity for dipentylamine (Fig. 5.5), suggests

that the inhibition of the disproportionation explains the negative influence of H2S on the

HDN of pentylamine. For 2-hexylamine also the increased acidity of the catalyst surface by

H2S adsorption and dissociation [12-14,26] may play a role. The ionic character of the C-N

bond for secondary carbon atoms may be increased by an increased proton concentration at

the catalyst surface.

Like for pentylamine, the selectivity to the dialkylamine (di-(2-hexyl)amine in this case) is

much higher over Mo/Al2O3 than over NiMo/Al2O3 and CoMo/Al2O3 (Figs. 5.8-5.10). With

increasing weight time, the selectivity of di-(2-hexyl)amine decreased much stronger over the

Ni(Co) promoted catalysts than over Mo/Al2O3. It shows that disproportionation easily takes

place over Mo/Al2O3, but that the dialkylamine disproportionation product reacts slower over

Mo/Al2O3 than over NiMo/Al2O3 and CoMo/Al2O3. The selectivity to hexane, the final

product in the HDN of 2-hexylamine, always increased with increasing weight time over all

three catalysts. On the other hand, the selectivity of the hexenes first increased with increasing

weight time and later decreased over NiMo/Al2O3 and CoMo/Al2O3. This must be due to the

high HDN conversion and thus decreased inhibition of the hydrogenation of the hexenes by

the amines at higher weight time. The hexenes/hexane ratio in the simultaneous reaction of 2-

hexylamine and 2-pentanethiol was very similar to the pentenes/pentane ratio originating from

2-pentanethiol over all three catalysts (Fig. 5.11). This demonstrates that the hexenes are

mainly produced from 2-hexanethiol, formed by substitution of H2S in the HDN of 2-

hexylamine over all three catalysts. The substitution of 2-hexylamine with H2S can be a direct

substitution (Eq. 1) or an indirect substitution after the disproportionation reaction of 2-

hexylamine (Eqs. 2 and 3). The indication of a maximum in the 2-hexanethiol selectivity at

short τ for Mo/Al2O3 (Fig. 5.10) suggests that at least for this catalyst both routes play a role.

Because of the secondary carbon atom in 2-hexylamine and 2-hexanethiol, these molecules

react faster than pentylamine and pentanethiol respectively. As a consequence, maxima due to

formation of intermediates shift to shorter τ for reactants with the secondary carbon atoms.

This explains why only an indication of a maximum is observed for 2-hexanethiol (Fig. 5.10)

and why the maxima in the alkenes are at shorter τ in the HDN of 2-hexylamine than in the

HDN of pentylamine (cf. Figs. 5.8-5.10 with Figs. 5.3-5.5).


5.4.3 2-Methyl-2-butylamine

Pentylamine and 2-hexylamine did not show an appreciable conversion below 300 °C, but

the conversion of 2-methyl-2-butylamine was very high already at 270 °C. Hydrocarbons and

only a trace of thiol (not shown) were formed in the HDN of 2-methyl-2-butylamine, in

accordance with our former results for NiMo/Al2O3 [10]. Methylbutenes and methylbutane

were primary products and the methylbutenes/methylbutane branching ratio in the HDN of 2-

methyl-2-butylamine was about five times higher than that obtained in the HDS of 2-methyl-

2-butanethiol in the presence of hexylamine over NiMo/Al2O3, twenty times larger over

CoMo/Al2O3, and thirty times larger over Mo/Al2O3 at τ = 1.6 g.min/mol at 270 °C and 10

kPa H2S (Fig. 5.13). The branching ratio in the HDS of the thiol with the SH group attached

to a tertiary carbon atom (Fig. 5.13) is similar to that of thiols with the SH group attached to a

primary (Fig. 5.6) or secondary carbon atom (Fig. 5.11). The branching ratio in the HDN of

the amine group attached to a tertiary carbon atom is totally different, however. Therefore, 2-

methyl-2-butanethiol cannot be the intermediate in the HDN of 2-methyl-2-butylamine.

Also the very small amount of thiol formed shows that the amine with the NH2 group

attached to a tertiary carbon atom does not, or hardly, react by substitution with H2S to the

corresponding thiol. In that case a much higher concentration of 2-methyl-2-butanethiol

should be observed. In fact, the conversion of 2-methyl-2-butanethiol in the presence of an

amine is lower at 270 °C than that of hexanethiol and 2-pentanethiol at 320 °C (Table 5.1).

Nevertheless, the thiol selectivities in the HDN of pentylamine (Figs. 5.3-5.5) and 2-

hexylamine (Figs. 5.8-5.10) were high and that of 2-methyl-2-butylamine was very low. 2-

Methyl-2-butylamine thus neither reacts by substitution with H2S to a thiol, nor by

substitution with another amine molecule to a dialkylamine, since this disproportionation

product was not observed over any of the catalysts. This may be due to steric hindrance at the

α-carbon atom. Because the main primary products in the HDN of 2-methyl-2-butylamine are

alkenes, we conclude that this HDN mechanism takes place by elimination. The weak

influence of H2S and the tertiary carbon atom suggest that the elimination is of the E1 type

over all three catalysts, as discussed in detail previously for NiMo/Al2O3 [10].

2-Methyl-2-butylamine reacted fastest over Mo/Al2O3 and slowest over NiMo/Al2O3 (Fig.

5.12). This HDN reaction is most probably catalysed by acid sites. These may be Lewis acid

sites, consisting of a sulfur vacancy on a molybdenum, cobalt and nickel atom, or Brønsted


acid sites constituted of H atoms on the sulfur atoms (protons of SH- groups). The activity

order would mean that the acidity of sulfided Mo/Al2O3 is higher than that of CoMo/Al2O3

than of NiMo/Al2O3. In an ionic model, this might be ascribed to the higher charge of Mo4+

than on Co2+ or Ni2+ (Lewis sites) or to the weaker bonding of H+ to S2- which is bonded to

Mo4+ than to Co2+ or Ni2+ (Brønsted site).

The conversion of 2-methyl-2-butylamine increased slightly with increasing H2S pressure

from 10 to 100 kPa. This cannot be explained by competition of H2S with the amine for the

adsorption sites on the catalyst surface. Since nucleophilic substitution hardly occurred for 2-

methyl-2-butylamine, this cannot explain the positive influence of H2S either. On the other

hand, H2S introduces protons at the catalyst surface and thus increases the acidity of the

catalyst. While it is impossible to turn a primary carbon atom into a carbenium ion, the

tertiary carbon atom of 2-methyl-2-butylamine can easily form a carbenium ion after

protonation by H2S and splitting off of NH3. With increasing H2S pressure, the conversion of

2-methyl-2-butylamine thus increases over all three catalysts. This explanation would mean

that Brønsted and not Lewis acid sites are responsible for the elimination of NH3 from the

amine with an NH2 group attached to a tertiary carbon atom. It explains why Mo/Al2O3, the

catalyst with the lowest number of sulfur vacancies, has the highest activity.

Mo/Al2O3 not only has the highest activity in the HDN of 2-methyl-2-butylamine, but also

in that of pentylamine (Fig. 2). The latter reactivity is based on two reactions, however, the

disproportionation to dipentylamine and the substitution to pentanethiol. Since the majority

product is dipentylamine, this suggests that also the disproportionation is acid catalysed and

that therefore the order of the three catalysts is the same for the HDN of 2-methyl-2-

butylamine and of pentylamine.

The conclusion that MoS2 has a stronger acidity than Co-MoS2 and Ni-MoS2 catalysts is

in accordance with DFT calculations of the heat of adsorption of NH3 on Lewis and Brønsted

acid sites on the surface of promoted and non-promoted M-MoS2 systems [20]. Travert et al.

found that the heat of adsorption on metal sites that were not fully coordinated by sulfur

atoms (CUS, Lewis sites) was stronger for Mo than for Co and Ni and decreased with

increasing sulfur coordination. The heat of adsorption on Brønsted SH groups was stronger

for SH groups attached to Mo atoms, as is to be expected since a stronger metal-sulfur bond

will induce a higher protonic character of the SH group. Investigation of Petit et al. showed


that H2S adsorption leads to an increase in the number of Brønsted acid sites and a decrease in

the number of Lewis acid sites on sulfided Mo/Al2O3 and CoMo/Al2O3 [12].

5.5. Conclusion

The results in the HDN of alkylamines over CoMo/Al2O3 and Mo/Al2O3 are in agreement

with our former work over NiMo/Al2O3 [9,10]. The alkylamine with the amine group attached

to a primary or secondary carbon atoms reacts by substitution of the NH2 group by SH or an

amine to form an alkanethiol or a dialkylamine respectively over all three catalysts. The

alkanethiol forms an alkene and an alkane. Only tertiary alkylamines react directly to

hydrocarbons by an E1 mechanism over all three catalysts. An E2 mechanism hardly takes

place in the HDN of alkylamines over our catalysts. The fact that Mo/Al2O3, the catalyst with

the lowest number of sulfur vacancies, has the highest HDN activity and that H2S has a

positive influence on the HDN of the alkylamines with NH2 group attached to a tertiary

carbon atom suggests that alkylamines react on Brønsted acid sites. The HDS of alkanethiols

on the other hand need vacancies because Mo/Al2O3 has the lowest HDS activity and H2S has

a strong negative influence on all these alkanethiols, with the SH group attached to a tertiary

carbon atom.

The very low conversions of pentylamine over sulfided Zn/Al2O3 and Cd/Al2O3

demonstrate that just acid-base chemistry at a metal sulfide surface is not enough for the HDN

of alkylamines.

Acknowledgment

We thank Dr. M. Breysse for stimulating discussions.


References

[1] T. Kabe, A. Ishihara, W. Qian, Hydrodesulfurization and Hydrodenitrogenation:

Chemistry and Engineering, Wiley-VCH, 1999.

[2] T.C. Ho, J. Catal. 219 (2003) 442.

[3] M. Egorova, R. Prins, J. Catal. 221 (2004) 11.



10 (1991) 473.



(1998) 366.

[8] P. Clark, X. Wang, P. Deck, S.T. Oyama, J. Catal. 210 (2002) 116.

[9] Y. Zhao, P. Kukula, R. Prins, J. Catal. 221 (2004) 441; chapter 3.

[10] Y. Zhao, R. Prins, J. Catal. 222 (2004) 532; chapter 4.

[11] N.Y. Topsøe, H. Topsøe, J. Catal. 139 (1993) 641.

[12] C. Petit, F. Maugé, J.C. Lavalley, Stud. Surf. Sci. Catal. 106 (1997) 157.

[13] G. Berhault, M. Lacroix, M. Breysse, F. Maugé, J.C. Lavalley, H. Nie, L. Qu, J. Catal.

178 (1998) 555.

[14] A. Travert, F. Maugé, Stud. Surf. Sci. Catal. 127 (1999) 269.

[15] R. Prins, in: G. Ertl, H. Knözinger, J. Weitkamp (Eds.), Handbook of Heterogeneous

Catalysis, vol. 4, Wiley-VCH, New York, 1997, p. 1916.

[16] P. Raybaud, J. Hafner, G. Kresse, H. Toulhoat, Surf. Sci. 407 (1998) 237.

[17] L.S. Byskov, J.K. Nørskov, B.S. Clausen, and H. Topsøe, J. Catal. 187 (1999) 109.

[18] P. Raybaud, J. Hafner, G. Kresse, S. Kasztelan, H. Toulhoat, J. Catal. 189 (2000) 129.

[19] S. Cristol, J.F. Paul, E. Payen, D. Bougead, S. Clémendot, F. Hutschka, J. Phys.

Chem. B 104 (2000) 11220.

[20] A. Travert, H. Nakamura, R.A. van Santen, S. Cristol, J.F. Paul, E. Payen, J. Am.

Chem. Soc. 124 (2002) 7084

[21] V. Alexiev, R. Prins, T. Weber, Phys. Chem. Chem. Phys. 2 (2002) 1815.

[22] T. Todorova, V. Alexiev, R. Prins, T. Weber, Phys. Chem. Chem. Phys. 6 (2004)

3023.


[23] M. Egorova, Y. Zhao, P. Kukula, R. Prins, J. Catal. 206 (2002) 263; chapter 2

[24] R. Prins, V.H.J. de Beer, G.A. Somorjai, Catal. Rev. Sci. Eng. 31 (1989) 1.

[25] H. Topsøe, B.S. Clausen, F.E. Massoth, Hydrotreating Catalysis in Catalysis, Science

and Technology, (J. Anderson, M. Boudart, Eds.) Springer Berlin, 11 (1996 )

[26] R.J. Girgis, B.C. Gates, Ind. Eng. Chem. Res. 30 (1991) 2021.

[27] J. Quartararo, S. Mignard, S. Kasztelan, J. Catal. 192 (2000) 307.

[28] J. Clayden, N. Greeves, S. Warren, P. Wothers, Organic Chemistry, Oxford Unv.

Press 2001 p. 483.

[29] M. Breysse, E. Furimsky, S. Kasztelan, M. Lacroix, G. Perot, Catal. Rev. 44 (2002)

651.

HDN of Adamantylamine Chapter 6 133

6 Mechanism of the hydrodenitrogenation of adamantylamine and

neopentylamine on sulfided NiMo/Al2O3

6.1 Abstract

The hydrodenitrogenation of 1-adamantylamine, 2-adamantylamine and neopentylamine

and the hydrodesulfurization of 1-adamantanethiol were studied over sulfided NiMo/Al2O3.

None of these amines can react by ammonia elimination and a classic SN2 substitution is not

possible for the adamantylamines either. The adamantanethiols and neopentanethiol were

the primary products of the adamantylamines and neopentylamine by substitution of the

NH2 group with H2S. These alkanethiols reacted by C-S hydrogenolysis to adamantane and

neopentane. It is proposed that the NH2-SH substitution in adamantylamine takes place by

adsorption of the amine group at the metal sulfide surface and migration of the adamantyl

group to a neighbouring sulfur atom. The fact that neopentane and adamantane were

secondary products demonstrates that hydrogenolysis of the aliphatic C-N bond of these

amines does not take place over sulfided NiMo/Al2O3 at 3 MPa and 270-340 °C.

6.2 Introduction

The cleavage of a carbon-sulfur (C-S) or a carbon-nitrogen (C-N) bond is the final step in

hydrodesulfurization (HDS) or hydrodenitrogenation (HDN) reactions. C-S bond breaking

may occur through elimination and homolytic C-S bond scission (hydrogenolysis), as

demonstrated in the HDS of alkanethiols [1-3] and aromatic thiols like (di)benzothiophene

[4,5]. Hydrogenolysis has also been observed in the homogeneous reaction of aliphatic and

aromatic thiols to hydrocarbons with the Cp’2Mo2Co2S3(CO)4 cluster [6].


The C-N bond in aromatic molecules like pyridine can only be broken after hydrogenation

of the aromatic heterocycle [7-9]. Even the removal of ammonia from aniline and alkylaniline

takes mainly place after hydrogenation of the (alkyl)aniline to (alkyl)cyclohexylamine and

less than 10% (alkyl)benzene was formed [8-10]. This hydrogenolysis of the aryl C-N bond is

only apparent, however [11]. In reality the breaking of the aryl C-N bond takes place by

partial hydrogenation of the arylamine. In the case of aniline it occurs by hydrogenation to

1,2-dihydroaniline and elimination of the NH2 group to form benzene. This NH2 elimination

is of the E1 type, because of the conjugation between the empty 2pz orbital on the carbon

atom and the butadiene structure in the resulting cyclohexadienyl C6H7+ carbocation [11].

Aliphatic C-N bond breaking has been claimed to occur exclusively by elimination [7,12]

and nucleophilic substitution by H2S followed by C-S bond hydrogenolysis [7,13]. Although

the C-N bond in aliphatic amines is weaker than in arylamines, alkanes were not observed as

primary products in the reaction of n-alkylamines [3]. The absence of hydrogenolysis might

be due to the fact that n-alkylamines can react faster by other reactions. To test that

possibility, we studied the removal of ammonia from 1-aminoadamantane, 2-

aminoadamantane and neopentylamine, which hardly have other possibilities to react than by

hydrogenolysis of the aliphatic C-N bond. 1-Adamantylamine has the amino group at a

carbon bridgehead and cannot, therefore, react via elimination or via SN2 substitution.

Furthermore, an SN1 reaction does not seem logical [14]; thus, 1-adamantylamine might react

via hydrogenolysis. 2-Adamantylamine cannot react by elimination either, and because SN1

and SN2 substitution do not seem likely, hydrogenolysis would be a good possibility to react.

The same holds for neopentylamine: The lack of a β-hydrogen atom makes elimination

impossible, the heavy substitution at the β-carbon atom makes SN2 substitution very difficult,

and SN1 substitution is unlikely at a primary carbon atom [14,15].

6.3 Results

6.3.1. Neopentylamine

The conversion of neopentylamine was very low at 300 °C and 3 MPa, about 4% at τ = 9

g.min/mol. It increased strongly when increasing the temperature to 340 °C and decreased


slightly when increasing the H2S pressure from 10 to 100 kPa (Fig. 6.1). Neopentylamine

reacted to 2,2-dimethylpropanethiol (neopentanethiol), di-neopentylamine and di-

neopentylimine, which behaved as primary products, and 2,2-dimethylpropane (neopentane),

which behaved as a secondary product (Fig. 6.2).

0 2 4 6 8 100

10

20

30

40

300 oC

340 oC, 100 kPa H2S

340 oCC

onve

rsio

n, %

Weight time, g.min/ mol

Fig. 6.1 Conversion of neopentylamine at 3 MPa and 300 or 340 °C, and 10 or 100 kPa H2S.

0 2 4 6 8 100

20

40

60

80

100

2,2-

dim

ethy

lpro

pane

, %

Weight time, g.min/ mol0 2 4 6 8 10

0

10

20

30

40

50

2,2-

dim

ethy

lpro

pane

thio

l, %


0 2 4 6 8 100

10

20

30

40

50

di-im

ine,

%

Weight time, g.min/ mol0 2 4 6 8 10

0

10

20

30

40

di-a

min

e, %


Fig. 6.2 Product selectivities in the HDN of neopentylamine at 3 MPa and 300 ( ) or 340 °C

( and ), and 10 or 100 kPa H2S (open symbols).


6.3.2. Adamantylamines and adamantanethiol

The conversion of 1-adamantylamine (1-AdNH2) at 300 °C and 3 MPa in the presence of

10 kPa H2S was lower than that of 2-adamantylamine (2-AdNH2). Increasing the H2S pressure

from 10 to 100 kPa had a positive effect on the conversion of 1-AdNH2 and a very strong

positive effect on that of 2-AdNH2 (Fig. 6.3). Only two products were observed in the HDN

of 1-AdNH2, adamantane and 1-adamantanethiol (1-AdSH) (Fig. 6.4); no products of

isomerization, like 2-AdNH2 and 2-AdSH, were observed. The sum of the adamantane and 1-

adamantanethiol selectivities thus was always 100%. The same holds true for adamantane and

2-AdSH in the HDN of 2-AdNH2 at 100 kPa H2S. Therefore only the adamantanethiol

selectivities of 1-AdSH at 10 and 100 kPa H2S and of 2-AdSH at 100 kPa H2S are presented

in Figure 6.4. They show that the adamantanethiols are the primary products of the HDN of

the adamantylamines, which reacted further to adamantane. At 10 kPa H2S, 2-AdNH2 not only

reacted to 2-AdSH and adamantane, but also to di(2-adamantylamine) and di(2-

adamantylimine) (Fig. 6.5).

0 2 4 6 8 10 120

20

40

60

80

100

2-AdNH2, 10 kPa H2S

1-AdNH2, 10 kPa H2S

1-AdNH2, 100 kPa H2S

2-AdNH2, 100 kPa H2S

Con

vers

ion,

%


Fig. 6.3 Conversion of 1-aminoadamantane (1-AdNH2) and 2-aminoadamantane (2-AdNH2)

at 300 °C, 3 MPa and 10 and 100 kPa H2S.


1-AdSH reacted much faster than 1-AdNH2 and 2-AdNH2 in the presence of 10 kPa H2S,

but in the presence of 100 kPa H2S it reacted just a bit faster than 1-AdNH2 (Fig. 6.6). In both

cases, adamantane was the only product. In both cases, hexylamine was added to the reactants

to simulate the presence of an alkylamine during the HDS of the alkanethiol.

0 2 4 6 8 100

20

40

60

80

100

1-AdSH, 10 kPa H2S

2-AdSH, 100 kPa H2S

1-AdSH, 100 kPa H2S

Sel

ectiv

ity, %


Fig. 6.4 Selectivity of 1-adamantanethiol (1-AdSH) in the HDN of 1-aminoadamantane at 300

°C, 3 MPa and 10 and 100 kPa H2S and selectivity of 2-adamantanethiol (2-AdSH)

in the HDN of 2-aminoadamantane at 300 °C and 100 kPa H2S.

0 2 4 6 8 100

20

40

60

80

di-aminedi-imine

2-AdSH

Ad

Sele

ctiv

ity, %


Fig. 6.5 Product selectivities in the HDN of 2-adamantylamine at 300 °C, 3 MPa and 10 kPa

H2S.


0 2 4 6 8 100

20

40

60

80

100

100 kPa H2S

10 kPa H2S

AdSH

con

vers

ion

%


Fig. 6.6 Conversion of 1 kPa 1-adamantanethiol in the presence of 5 kPa hexylamine at 300

°C, 3 MPa and 10 and 100 kPa H2S.

6.4 Discussion

6.4.1 HDN of neopentylamine

Neopentylamine has no β-H atoms and Hofmann elimination can therefore not take place

[12]. In an SN1 reaction a primary carbenium ion has to be formed. This is highly unlikely, as

demonstrated by the fact that no rearrangement products such as methylbutane or

methylbutene were observed in the HDN of neopentylamine. Thus, neopentylamine can only

react by SN2 substitution or hydrogenolysis.

Portefaix et al. already reported that neopentylamine hardly reacted at 270 °C [12]. Our

results show that even at 300 °C neopentylamine reacted very slowly and an appreciable

conversion (16% at τ = 3 g.min/mol) was only obtained at 340 °C (Fig. 6.1). The main

products were neopentanethiol (2,2-dimethylpropanethiol), dineopentylamine and

dineopentylimine. The selectivity of neopentane decreased strongly with decreasing weight


time and was zero at τ = 0 at 300 °C. At 340 °C the selectivity extrapolated to zero at τ = 0 as

well (Fig. 6.2). This demonstrates that the direct hydrogenolysis reaction of neopentylamine

to neopentane does not occur at 300 and 340 °C.

The behaviour of the neopentanethiol selectivity as a function of weight time at 300 °C

suggests that this molecule is formed by a primary as well as by a secondary reaction, because

the selectivity is non zero at τ = 0 and increases with τ. The primary reaction may be

2 2RNH H S RSH NH+ → + 3 (1)

and the secondary reaction

2 2RNHR H S RSH RNH+ → + (2)

The secondary reaction would be in agreement with the high initial selectivity of

dineopentylamine and dineopentylimine at 300 °C and with the about 20 times higher

reactivity of dihexylamine than hexylamine [3]. At 340 °C the rate of reaction (1) increases

and this would explain why at 340 °C and 100 kPa H2S neopentanethiol seems to be formed

only as a primary product (cf. the continuously decreasing selectivity with τ in Fig. 6.2).

The low reactivity of neopentylamine is easy to understand. Hofmann elimination is

impossible, SN1 substitution is unlikely and hydrogenolysis does not take place. The

occurrence of neopentanethiol, dineopentylamine and dineopentylimine as primary products

indicates that nucleophilic substitution of neopentylamine by H2S and by neopentylamine are

responsible for the formation of neopentanethiol and dineopentylamine respectively. The

secondary nature of neopentane suggests that the majority of neopentane is formed from

neopentanethiol by C-S hydrogenolysis. SN2 substitution of neopentylamine is hindered by

the tertiary butyl group at the α-carbon atom [14] and neopentylamine therefore reacts much

slower than the non-branched n-hexylamine [3]. The formation of dineopentylimine may be

explained by dehydrogenation of neopentylamine to neopentylimine, followed by addition of

neopentylamine and elimination of NH3 (Scheme 6.1). These reactions are well known in the

metal-catalyzed hydrogenation of nitriles [16] and amination of alcohols [17].


R CH2 NH2

H

N

HR R CH2 NH2

R CH2 NH C NH2

H

R

N C

RH2CR

H

+ NH3

Scheme 6.1 Formation of dialkylimine from an alkylamine.

The fact that C-N bond hydrogenolysis did not occur demonstrates how difficult

hydrogenolysis of an aliphatic C-N bond is over a sulfided NiMo/Al2O3 catalyst. C-N bond

hydrogenolysis on metal catalysts is an easy reaction [18], for instance on supported platinum

it is already fast around 150 °C [19]. Primary amines react much faster on Pt than secondary

amines, who react again about an order of magnitude faster than tertiary amines.

Hydrogenolysis of the C-N bond takes place after adsorption of both neighbouring N and C

atoms on the catalyst surface. Substitution on the α-C atom makes adsorption of this α-C

atom difficult, thus decreasing the rate of hydrogenolysis. This reactivity pattern on Pt is

completely opposite to that observed for the alkylamines on sulfided NiMo/Al2O3,

demonstrating that HDN on metal sulfides occurs by a different mechanism than on metals.

6.4.2 HDN of AdNH2 and HDS of AdSH

The sulfided NiMo/Al2O3 catalyst catalyzes the removal of the NH2 group from 1- and 2-

adamantylamine and of the SH group from 1-adamantanethiol. A Hofmann elimination is

impossible for these molecules because a bridgehead double bond as in adamantene is highly

strained and will therefore not form easily, as expressed in Bredt's rule [14]. Adamantene has

been synthesized by irradiation of diazo compounds, but its lifetime is very short [20]. 1-

Adamantyl compounds cannot react by a standard SN2 reaction either, because the

nucleophile cannot approach the reaction center through the adamantane cage, from the


backside, as required for a classic organic SN2 reaction [14,15]. Since adamantane was a

secondary product, hydrogenolysis cannot explain the HDN of the adamantylamines either.

Also an SN1 reaction is unlikely, since the H2S pressure has a strong positive influence on the

conversion of both adamantylamines. This leaves a non-classic SN2 reaction as the only

mechanism to explain the HDN of 1-adamantylamine. One could envisage that the NH2-SH

substitution occurs by alkyl migration on the metal sulfide surface. In that case, the amine

would adsorb at the vacancy on a Mo or Ni atom and then the alkyl group would migrate to a

sulfur atom on the neighbouring metal atom (Scheme 6.2).

Mo

S

S S

S

Mo

S

S

S N

H

H HR

Mo

S

S S

S

Mo

S

S

S N

H

H HR

Mo

S

S S

S

Mo

S

S

S N

H HHR

Scheme 6.2 NH2-SH substitution by alkyl migration on the metal sulfide surface.

2-Adamantylamine reacted faster than 1-adamantylamine, leading to a high conversion

already at small weight time (Fig. 6.3). As a consequence, 2-adamantanethiol reacted faster to

adamantane (Fig. 6.4), because of less inhibition by the amine. At 340 °C 2-adamantylamine

reacted to 2-adamantanethiol and adamantane only, and 2-adamantanethiol was a primary and

adamantane a secondary product. At 300 °C the selectivity pattern of 2-adamantylamine was

more like that of neopentylamine at 340 °C. Diadamantylamine and diadamantylimine were

primary products and adamantane was a secondary product. The selectivity of 2-

adamantanethiol was unequal to zero at τ = 0 and initially increased with weight time. Thus,

2-adamantanethiol is a primary product at 300 °C, formed directly from a nucleophilic

substitution of the adamantylamine by H2S (Eq. 1), as well as a secondary product formed

from di(2-adamantylamine) and di(2-adamantylimine) (Eq. 2). Figure 6.5 shows that, at 300

°C, the latter two molecules are initially formed faster than the thiol. At 340 °C this situation

is reversed, or the di(2-adamantylamine) and di(2-adamantylimine) react very fast with H2S to

2-adamantanethiol.

While 1-adamantylamine could, in principle, react with H2S by a classic SN1 reaction but

not by an SN2 reaction [21,22], 2-adamantylamine and di(2-adamantylamine) can do neither


reaction. An SN1 reaction would involve the secondary 2-adamantyl carbenium ion. This

mechanism is ruled out, because the H2S pressure has a strong positive influence on the

conversion of 2-adamantylamine (Fig. 6.3). An SN2 reaction would mean that the SH- or H2S

nucleophile has to react from the inside of a cyclohexane ring on the adamantane surface

(Scheme 6.3) [23]. This reaction is equivalent to that of SH- or H2S with cyclohexylamine

with the amine group in equatorial position, which is known to be impossible; a substituent

can only react when in axial position [14]. We therefore conclude that also the reaction of 2-

adamantylamine to 2-adamantanethiol occurs most likely by a non-classic substitution. In

addition to the possibility suggested above for the reaction of 1-adamantylamine, alkyl

migration on the metal sulfide surface (Scheme 6.2), 2-adamantylamine can undergo

substitution of the amine group by a sequence of dehydrogenation, addition, elimination and

hydrogenation reactions (Scheme 6.4).

HH

H

H2N H

GSH

HH

H

NH2

H

GSH

Scheme 6.3 SN2 reaction of 2-adamantylamine.

C NH H

C NHS H

C SH H

C N

C S

-H2 H2S

-RNH2

H2

Scheme 6.4 NH2-SH substitution by dehydrogenation of an amine to an imine, addition of

H2S, elimination of alkylamine, and hydrogenation of the thioketone.


Unlike 1-adamantylamine, 2-adamantylamine can adsorb with its C-N bond parallel to the

metal sulfide surface and become dehydrogenated to 2-adamantylimine. Addition of H2S and

elimination of NH3 leads to 2-thioadamantanone and further hydrogenation gives 2-

adamantanethiol. Similarly, addition of 2-adamantylamine to 2-adamantylimine and

elimination of NH3 gives di(2-adamantylimine) (cf. Scheme 1). After addition of H2S and

elimination of 2-adamantylamine one obtains 2-thioadamantanone. This sequence of

dehydrogenation, addition, elimination and hydrogenation reactions is very well known in

metal catalysis and used in the synthesis of amines from alcohols [17]. The high initial

selectivities of di(2-adamantylimine) and di(2-adamantylamine) suggest that they may indeed

be intermediates in the substitution reaction of 2-adamantylamine by H2S.

The hydrogenolysis of the C-S bond may occur as observed by Curtis and Druker for the

reactions of alkyl and aryl thiols in the homogeneous reaction with a Mo2Co2 cluster [6].

After adsorption of the neopentane- or adamantanethiol with the sulfur atom on a Mo or Ni

atom at the metal sulfide surface, the neopentyl and adamantyl groups may move to a

neighbouring metal atom. The alkyl intermediate can then be hydrogenated to an alkane.

Alternatively, alkanethiol and SH groups adsorbed on neighboring metal atoms may react to

an alkane and two adsorbed sulfur atoms.

Only 2-adamantanethiol, and no 1-adamantanethiol or 1-adamantylamine, was observed in

the reaction of 2-adamantylamine. Neither were 2-adamantanethiol and 2-adamantylamine

observed in the reaction of 1-adamantylamine. That no isomerization of 1-adamantyl to 2-

adamantyl compounds, nor the reverse reaction, was observed is due to the fact that the

isomerization of the 1-adamantyl to the 2-adamantyl carbenium ion and back is extremely

difficult. The reason is that the 2pz orbitals on the C1 and C2 carbon atoms must be parallel in

the transition state. Because of the rigid cage structure of adamantane, this is impossible [24].

6.5 Conclusion

Neopentylamine reacts to dineopentylamine, dineopentylimine, neopentanethiol and

neopentane at the high temperature of 340 °C. The formation of dineopentylimine can be

explained by dehydrogenation of neopentylamine to neopentylimine. The NH2 group from


another neopentylamine molecule can add to the neopentylimine to form an intermediate.

After elimination of ammonia, dineopentylimine will be formed, which after hydrogenation

leads to dineopentylamine. Substitution of neopentylamine with H2S leads to neopentanethiol.

This thiol could also be formed by dehydrogenation of a neopentylamine to an imine, addition

of H2S, elimination of ammonia, and hydrogenation of the thioketone. Neopentane was a

secondary product in the HDN of neopentylamine. This demonstrates that neopentane was

formed by decomposition of neopentanethiol formed by substitution of neopentylamine with

H2S. The direct hydrogenolysis of neopentylamine to neopentane hardly took place.

1-Adamantylamine reacts only to 1-adamanethiol and adamantane, while 2-

adamantylamine reacts to di-2-adamantylamine, di-2-adamantylimine, 2-adamanethiol and

adamantane. No isomerization took place between 1-adamantyl and 2-adamantyl compounds.

The very low reactivity of 1-adamantylamine at 300 °C shows that a carbenium ion cannot be

formed. Therefore, SN1 or E1 reactions cannot take place in the HDN of 1-adamantylamine.

H2S cannot attack the α carbon from the back through the adamantane cage, which shows that

a SN2 reaction cannot take place either. An E2 mechanism is unlikely, as no double bond

cannot be formed on a bridge-head carbon atom. Our explanation of the formation of 1-

adamantanethiol in the HDN of 1-adamantylamine is that the alkyl group shifts to a

neighboring sulfur atom.

6.6 References

[1] B.C. Wiegand, C.M. Friend, P. Uvdal, M.E. Napier, Surf. Sci. 355 (1996) 311.



[4] M. Houalla, N.K. Nag, A.V. Sapre, D.H. Broderick, B.C. Gates, AIChE J. 24 (1978)

1015.

[5] J. Mijoin, G. Pérot, F. Bataille, J.L. Lemberton, M. Breysse, S. Kasztelan, Catal. Lett.

71 (2001) 139.





[9] C. Moreau, C. Aubert, R. Durand, N. Zmimita, P. Geneste, Catal. Today 4 (1988) 117.

[10] M. Jian, F. Kapteijn, R. Prins, J. Catal. 168 (1997) 491.

[11] Y. Zhao, J. Czyzniewska, R. Prins, Catal. Lett. 88 (2003) 155; and chapter 7.


10 (1991) 473.


[14] M.B. Smith, J. March, Advanced Organic Chemistry (Wiley, New York, 5th Ed.,

2001).

[15] J. Clayden, N. Greeves, S. Warren, P. Wothers, Organic Chemistry. Oxford Univ.

Press, 2001.

[16] C. De Bellefon, P. Fouilloux, Catal. Rev. Sci. Eng. 36 (1994) 459.

[17] T. Mallat, A. Baiker, in “ Handbook of Heterogeneous Catalysis” (G. Ertl, H.

Knözinger, J. Weitkamp, Eds.). Vol 5, P. 2334, Wiley-VCH.

[18] G. Meitzner, W.J. Mykytka and J.H. Sinfelt, J. Catal. 98 (1986) 513.

[19] Triyono, R. Kramer, Appl. Catal. A 100 (1993) 145.

[20] E.L. Tae, Z. Zhu, M.S. Platz, J. Phys. Chem. A 105 (2001) 3803.

[21] G.A. Olah, Cage Hydrocarbons (Wiley, New York, 1990).

[22] P. von R. Schleyer, R.D. Nicholas, J.Am. Chem.Soc. 83 (1961) 2700.

[23] J.L. Fry, C.J. Lancelot, L.K.M. Lam, J.M. Harris, R.C. Bingham, D.J. Raber, R.E.

Hall, P. von R. Schleyer, J. Am. Chem. Soc. 92 (1970) 2538.

[24] D.M. Brouwer, in ‘Chemistry and Chemical Engineering of Catalytic Processes’ (R.

Prins and G.C.A. Schuit, Eds.), Sythof-Noordhof, Alphen, 1980, p. 137.

HDN of Naphthylamine Chapter 7 147

7. Mechanism of the direct hydrodenitrogenation of naphthylamine on

sulfided NiMo/Al2O3

7.1 Abstract

The hydrodenitrogenation of 1-naphthylamine was studied over a sulfided NiMo/Al2O3

catalyst between 300 and 350 ºC. 1-Naphthylamine reacted to tetralin, naphthalene, 1,2-

dihydronaphthalene and 5,6,7,8-tetrahydro-1-naphthylamine. To elucidate the reaction

mechanism, the reactions of the intermediates 1,2,3,4-tetrahydro-1-naphthylamine, 1,2-

dihydronaphthalene and 5,6,7,8-tetrahydro-1-naphthylamine were studied as well. The results

show that 1-naphthylamine reacts through hydrogenation to 1,2,3,4-tetrahydro-1-

naphthylamine, which reacts by NH3 elimination to 1,2-dihydronaphthalene. The latter

molecule subsequently reacts by hydrogenation to tetralin as well as by dehydrogenation to

naphthalene. In addition, naphthalene is formed by direct denitrogenation from 1-

naphthylamine. This direct denitrogenation may take place by hydrogenation of 1-

naphthylamine to 1,2-dihydro-1-naphthylamine, followed by NH3 elimination or followed by

a Bucherer-type NH2-SH exchange, dehydrogenation and C-S bond hydrogenolysis.

7.2 Introduction

The cleavage of a carbon-nitrogen (C-N) or a carbon-sulfur (C-S) bond is a crucial step in

hydrodenitrogenation (HDN) and hydrodesulfurization (HDS) reactions respectively.

Hydrogenation of the aromatic ring that contains the S atom does not seem to be required for

the removal of the S atom, because thiophenol reacts almost exclusively to benzene under

HDS conditions [1], while about 80% of dibenzothiophene reacts to biphenyl [2]. This


suggests that the relatively weak C-S bond can be broken by hydrogenolysis. Hydrogenolysis

is used here in the mechanistic sense, meaning a reaction on the catalyst surface in which a C-

X bond is broken and C-H and H-X bonds are formed before the product molecule leaves the

catalyst surface. Homolytic C-S bond breaking (hydrogenolysis) was demonstrated in the

homogeneous reaction of aliphatic and aryl thiols on sulfur–containing Mo-Co clusters [3].

Breaking of the C-S bond might occur by nucleophilic aromatic substitution by a hydride ion

as well [4]. It has also been suggested, however, that the hydrogenolysis of the C-S bond (also

called direct desulfurization) is only apparent and actually occurs by hydrogenation of a

neighbouring C-C bond, followed by H2S elimination [2,5].

The main reactions involved in the removal of an N atom from aromatic compounds are

hydrogenation of the aromatic ring which contains the nitrogen atom, and breaking of the

resulting aliphatic C-N bonds to a hydrocarbon molecule and ammonia [1,6-9]. Aliphatic C-N

bond breaking occurs either by elimination [6,7], or by nucleophilic substitution by H2S

followed by C-S bond hydrogenolysis [6,8]. The main HDN product of aniline is therefore

cyclohexane, which is formed via cyclohexylamine and cyclohexene [1,8]. Similarly, the

main product in the HDN of quinoline is propylcyclohexane [9].

Direct breaking of the C-N bond in aniline (also called direct denitrogenation) occurs to a

minor degree as well. For instance, in the HDN of o-propylaniline the selectivity to

propylbenzene was 7% over a NiMo/Al2O3 catalyst and 24% over a Mo/Al2O3 catalyst [10].

For fused aromatic amines like naphthylamine and anthracylamine direct C-N bond breakage

is even more important [11]. If this direct C-N bond breakage in an arylamine occurs by real

hydrogenolysis in the mechanistic sense, then one should expect that hydrogenolysis of an

alkylamine is even easier. The reason for this is that the C-N bond in alkylamines is weaker

than the C-N bond in arylamines because of the conjugation of the NH2 group with the

aromatic ring. Nevertheless, alkylamines seem to react exclusively by β hydrogen elimination

and nucleophilic substitution by H2S followed by C-S bond hydrogenolysis [6-9]. This

suggests that hydrogenolysis in arylamines may not be real but apparent, meaning that the

reaction occurs via an indirect, multi-step mechanism.

To determine whether the hydrogenolysis of an aryl C-N bond is real or apparent, we

studied the HDN of 1-naphthylamine and the reactions of the possible intermediates 1,2,3,4-

tetrahydro-1-naphthylamine, 1,2-dihydronaphthalene and 5,6,7,8-tetrahydro-1-naphthylamine.


7.3 Results

Both catalysts were active in the HDN of 1-naphthylamine in the autoclave and the

conversion after 1 h at 300 °C was 22% over NiMo/Al2O3 and 33% over CoMo/Al2O3. 1-

Naphthylamine reacted to tetrahydronaphthalene (tetralin), 1,2-dihydronaphthalene,

naphthalene and a small amount of 5,6,7,8-tetrahydro-1-naphthylamine. The selectivity to 1,2-

dihydronaphthalene decreased with reaction time and no 1,2-dihydronaphthalene was

observed after 30 min over CoMo/Al2O3, while over NiMo/Al2O3 the selectivity to 1,2-

dihydronaphthalene decreased more slowly (Fig. 7.1). At short reaction time, the selectivities

to naphthalene and 1,2-dihydronaphthalene were high and increased with decreasing reaction

time, while the reverse was true for tetralin (Fig. 7.2). This indicates that naphthalene and 1,2-

dihydronaphthalene are formed earlier and tetralin later in the reaction network. At the higher

temperature of 350 °C, the conversion was 70% after 1 h and more naphthalene and less

tetralin were formed than at 300 °C. 1,2-Dihydronaphthalene was only produced at short

reaction time. The conversions in the experiments in the microflow reactor over NiMo/Al2O3

were always much higher than in the autoclave; the main reason being the five times higher

H2 pressure. Conversions and product selectivities are presented in Figures 7.3 and 7.4

respectively. At 300 °C and 3 MPa, in the presence of 10 kPa H2S, tetralin, 5,6,7,8-tetrahydro-

1-naphthylamine, naphthalene and 1,2-dihydronaphthalene behaved like primary products

with non-zero selectivities at zero weight time. The selectivity to 1,2-dihydronaphthalene was

very sensitive to the temperature and H2 pressure. It decreased with increasing H2 pressure

and increasing temperature (Table 7.1). At low weight time the selectivity to 5,6,7,8-

tetrahydro-1-naphthylamine was 30%, but it decreased to zero at high conversion (Figure 7.4).

The naphthalene selectivity was constant at about 10%. Since the tetralin selectivity increased

with weight time, the naphthalene to tetralin ratio decreased with weight time (Figure 7.5).

This ratio was more sensitive to temperature than to H2 pressure (Table 7.1).


15 30 45 600

20

40

60

80

100

1,2-DHN

5,6,7,8-THNA

naphthalene

tetralinS

elec

tivity

, %

Time, min

Fig 7.1 Selectivities in the HDN of 1-naphthylamine over NiMo/Al2O3 (closed symbols) and

CoMo/Al2O3 (open symbols) catalysts at 300 °C and 0.6 MPa in the autoclave (1,2-

DHA = 1,2-dihydronaphthalene, 5,6,7,8-THNA = 5,6,7,8-tetrahydro-1-

naphthylamine).

3 4 5 6 7 8 90

15

30

45

60

naphthalene

tetralin

1,2-DHN

B

Sele

ctiv

ity, %

Time, min3 4 5 6 7 8 9

0

15

30

45

60

5,6,7,8-THNA

1,2-DHN

naphthalene

tetralin

A

Sele

ctiv

ity, %

Time, min

Fig. 7.2 Selectivities in the HDN of 1-naphthylamine over CoMo/Al2O3 (A) and NiMo/Al2O3

(B) catalysts at 300 °C and 0.6 MPa at short reaction times in the autoclave (1,2-

DHN = 1,2-dihydronaphthalene, 5,6,7,8-THNA = 5,6,7,8-tetrahydro-1-

naphthylamine).


0 1 2 3 4 50

20

40

60

80

100

300°C

320°C

350°C

Con

vers

ion,

%


Fig. 7.3 Conversion of 1-naphthylamine over NiMo/Al2O3 in the microflow reactor at 3 MPa,

10 kPa H2S and 300, 320 and 350 °C.

0 1 2 3 4 50

20

40

60

80

100

1,2-DHN

naphthalene

5,6,7,8-THNA

Tetralin

Sel

ectiv

ity, %


Fig. 7.4 Product selectivities in the HDN of 1-naphthylamine over NiMo/Al2O3 at 300 °C, 3

MPa and 10 kPa H2S in the microflow reactor (1,2-DHN = 1,2-dihydronaphthalene,

5,6,7,8-THNA = 5,6,7,8-tetrahydro-1-naphthylamine).


0 1 2 3 4 50.0

0.1

0.2

0.3

0.4

300°C

320°C

350°C

naph

thal

ene/

tetr

alin


Fig. 7.5 Naphthalene to tetralin ratio in the HDN of 1-naphthylamine over NiMo/Al2O3 at 3

MPa, 10 kPa H2S and 300, 320 and 350 °C.

Table 7.1 Selectivity to 1,2-dihydronaphthalene (DHN) in the reaction of 1-naphthylamine

(NA), and the naphthalene to tetralin ratio (N/T) in the reactions of NA, 1,2,3,4-tetrahydro-1-

naphthylamine (THAN) and DHN at 10 kPa H2S and τ = 1.07 g.min/mol.

Conditions % DHN N/T

T (°C) P (MPa) NA NA THAN DHN

300 1 13 0.22

300 2 6 0.20

300 3 3 0.20 0.10 0.10

320 3 0 0.26

350 3 0 0.37 0.19 0.22

To compare the reaction rates of potential intermediates in the HDN of 1-naphthylamine,

we also measured the HDN of 1,2,3,4-tetrahydro-1-naphthylamine and 5,6,7,8-tetrahydro-1-

naphthylamine and the hydrogenation of 1,2-dihydronaphthalene, the latter in the presence of

aniline to simulate the inhibiting effect of an arylamine. At 300 °C, 3 MPa and 10 kPa H2S,


the conversions of 1,2,3,4-tetrahydro-1-naphthylamine and 1,2-dihydronaphthalene were

already complete at the lowest weight time possible in our microflow reactor (τ = 1.07

g.min/mol). These conversions of 100% were much higher than that of 1-naphthylamine

(20%). The only products were tetralin and naphthalene. Figure 7.6 shows the naphthalene to

tetralin ratio as a function of time for 1,2,3,4-tetrahydro-1-naphthylamine at 300 and 350 °C

and at 10 kPa H2S; almost identical curves were obtained for 1,2-dihydronaphthalene.

Because of the complete conversion of 1,2,3,4-tetrahydro-1-naphthylamine and 1,2-

dihydronaphthalene already at short weight time, the naphthalene to tetralin ratios decreased

with time because of the hydrogenation of naphthalene to tetralin and the fact that initially a

larger amount of naphthalene was produced than corresponding with thermodynamics. The

naphthalene to tetralin ratios, extrapolated to τ = 0 g.min/mol for the reactions of 1,2,3,4-

tetrahydro-1-naphthylamine and 1,2-dihydronaphthalene, were 0.24 and 0.27 respectively at

350 °C and 3 MPa, and the ratio was 0.11 for both reactions at 300 °C and 3 MPa. These

values are much lower than the values of 0.37 (350 °C, 3 MPa) and 0.21 (300 °C, 3 MPa)

obtained in the HDN of 1-naphthylamine itself (Fig. 7.5).

0 1 2 3 4 50.00

0.05

0.10

0.15

0.20

0.25

0.30

300°C

350°C

naph

thal

ene/

tetra

lin


Fig. 7.6 Naphthalene to tetralin ratio in the HDN of 1,2,3,4-tetrahydro-1-naphthylamine over

NiMo/Al2O3 at 3 MPa, 10 kPa H2S and 300 and 350 °C.


The conversion of 5,6,7,8-tetrahydro-1-naphthylamine at 300 °C, 3 MPa and 10 kPa H2S

was only 3% at τ = 1.07 g.min/mol. This indicates that its low selectivity in the HDN of 1-

naphthylamine is not due to a fast subsequent reaction, but to a relatively slow rate of

formation.

7.4 Discussion

7.4.1 Direct denitrogenation

Our results confirm that hydrogenolysis of arylamine C-N bonds is possible, since

naphthalene behaved like a primary product on sulfided NiMo/Al2O3 and CoMo/Al2O3

catalysts. The other main products of the reaction were 1,2-dihydronaphthalene, tetralin and

5,6,7,8-tetrahydronaphthylamine; they result from hydrogenation. The ratio of hydrogenolysis

to hydrogenation strongly depended on the reaction temperature: the higher the reaction

temperature, the higher the ratio. Similar behaviour was observed in the HDN of aniline over

a sulfided NiW/Al2O3 catalyst [13].

The experiments at lower pressure in the autoclave as well as in the microflow reactor

demonstrated that both naphthalene and 1,2-dihydronaphthalene behave as primary products,

with selectivities increasing at shorter reaction time. Whereas naphthalene can be envisaged to

be formed directly from 1-naphthylamine (e.g. by hydrogenolysis), the formation of 1,2-

dihydronaphthalene from 1-naphthylamine has to occur through at least one intermediate. A

logic intermediate would be 1,2,3,4-tetrahydro-1-naphthylamine. This intermediate is very

reactive under our conditions, as shown in the separate experiment in which 1,2,3,4-

tetrahydro-1-naphthylamine already completely reacted to tetraline and naphthalene at the

lowest weight time possible in our microflow reactor. 1,2-Dihydronaphthalene, the expected

primary product obtained by NH3 elimination of 1,2,3,4-tetrahydro-1-naphthylamine, was not

observed in this experiment, as it reacts very fast as well. The very high reactivity of 1,2,3,4-

tetrahydro-1-naphthylamine also explains why it was not observed in the HDN of 1-

naphthylamine even after a short reaction time. This is in accordance with results obtained in


the HDN of o-methylaniline, for which the first hydrogenation step was also much slower

than the subsequent nitrogen-removal step [14]. In that reaction, the hydrogenated

intermediate o-methylcyclohexylamine was only observed when a large amount of

cyclohexene was added during reaction, to cause the intermediate to leave the catalyst surface.

When an arylamine is hydrogenated at the catalyst surface, the intermediate cyclohexylamine

apparently undergoes ammonia elimination faster than that it desorbs from the surface and

diffuses out of the catalyst pores. Furthermore, in the case of 1,2,3,4-tetrahydro-1-

naphthylamine, the denitrogenation might be even very fast because it can take place by an E1

elimination mechanism. The reason is that the carbocation resulting from

tetrahydronaphthalene (Scheme 7.1) is strongly stabilized by conjugation with the aromatic

ring and by electron donation from the β CH2 group.

Because 1,2-dihydronaphthalene reacts fast but not extremely fast, it could be detected

and seen to behave as a primary product. Higher temperature and H2 pressure increase the rate

of the reaction of 1,2-dihydronaphthalene. This explains why at 3 MPa and 300 or 350 °C this

intermediate was not observed. The microflow experiment showed that 1,2-

dihydronaphthalene reacts to a 9:1 mixture of tetralin and naphthalene at short weight time.

Apparently, hydrogenation as well as dehydrogenation can take place quickly.

+

Scheme 7.1 Carbocation of tetrahydronaphthalene.

The much higher naphthalene to tetralin ratios observed in the HDN of 1-naphthylamine

than in the reaction of 1,2-dihydronaphthalene (Figs. 7.5 and 7.6 and Table 7.1) indicate that

1,2-dihydronaphthalene is not the only source of naphthalene. The different behaviour of

naphthalene and tetralin as a function of reaction time confirms this (Fig. 7.2): naphthalene

behaves as a primary product and tetralin as a secondary product. This means that additional

naphthalene must be formed by a reaction that takes place earlier in the reaction network than

the formation of 1,2-dihydronaphthalene from 1,2,3,4-tetrahydro-1-naphthylamine. 1,2-


Dihydro-1-naphthylamine or 1-naphthylamine could be intermediates for the formation of this

naphthalene.

Moreau et al. proposed that the naphthalene that forms in the HDN of 1-naphthylamine

could be partially hydrogenated to tetralin [11]. To check this, we performed a hydrogenation

of 1-methylnaphthalene in the presence of 1-naphthylamine over sulfided NiMo/Al2O3 and

CoMo/Al2O3 at 300 ºC in the autoclave, but we did not observe any products of the

hydrogenation of 1-methylnaphthalene. This shows that the naphthalene-to-tetralin step does

not take place during the HDN of 1-naphthylamine as long as the 1-naphthylamine

concentration is high enough to inhibit the hydrogenation of aromatic molecules. This

corroborates the results obtained in the reaction of ethylbenzene in the presence of o-

propylaniline, in which ethylbenzene hydrogenation was only 1% over a NiMo/Al2O3 catalyst

at 350 °C and at 60% conversion of o-propylaniline [12].

On the basis of our results we propose a mechanism for the HDN of 1-naphthylamine over

sulfided NiMo/Al2O3 and CoMo/Al2O3 catalysts (Scheme 7.2) that differs in some points

from that proposed by Moreau et al. [11]. There are two pathways, the main one being a

multi-step reaction pathway. First, 1-naphthylamine is partially hydrogenated to 1,2,3,4-

tetrahydro-1-naphthylamine. This intermediate eliminates NH3 and the resulting 1,2-

dihydronaphthalene reacts to tetralin and naphthalene.

NH2 NH2 NH2 NH2

SH

4231

Scheme 7.2 Reaction mechanism for the HDN of 1-naphthylamine.


In the second pathway, 1-naphthylamine undergoes direct breaking of the C-N bond to

naphthalene. The question is, if this reaction is really taking place, for instance by homolytic

splitting of the C-N bond and fast hydrogenation of the resulting radicals (like the C-S bond

breaking in thiols [3]) or by the substitution of the amine group by a hydride ion [4]. It is also

possible that the formed naphthalene gives the impression that 1-naphthylamine reacts by

hydrogenolysis, but that this is not the case. For instance, the transformation of 1-

naphthylamine to 1-naphthylthiol by NH2-SH exchange [9] and fast reaction of 1-

naphthylthiol to naphthalene by hydrogenolysis of the C-S bond would look like C-N

hydrogenolysis if 1-naphthylthiol were not observed (route 1 in scheme 7.2). The NH2-SH

exchange (Scheme 7.3) would certainly be enhanced in arylamines with fused aromatic rings

like naphthyl- and anthracylamine, because the aromaticity of fused rings decreases with

increasing number of rings. Thus, the 1,2-C-C bond in naphthalene has more double bond

character than a C-C bond in benzene and the enamine character of 1-naphthylamine is

stronger than that of aniline. The higher enamine contribution in turn means that also the

imine character is higher because of the enamine-imine tautomeric equilibrium. This is

analogous to the greater importance of the keto form in the enol-keto tautomeric equilibrium

for naphthol than for phenol [16]. As a result of the greater imine character, the addition of

H2S to 1-naphthylamine will be easier.

NH2 NH

SH

SH NH2

H2S -NH3

S

Scheme 7.3 Reaction from 1-naphthylamine to 1-thionaphthol by enamine-imine

tautomerism, NH2-SH exchange by addition of H2S and elimination of NH3,

and thioenol-thioketo tautomerism.


Also the partial hydrogenation of 1-naphthylamine to 1,2-dihydro-1-naphthylamine

followed by the fast elimination of NH3 and the formation of naphthalene, would give the

impression that hydrogenolysis had occurred (route 2 in scheme 7.2). The formation of a

dihydro compound seems feasible and has already been proposed as an explanation for the

apparent hydrogenolysis of dibenzothiophene [5]. At first sight, this explanation seems flawed

because, due to the planar structure of the cyclohexadiene molecule, the NH2 group on the C1

atom and the H atom on the neighbouring C2 atom are in the eclipsed conformation. This

would mean that the subsequent elimination (e.g. of 1,2-dihydro-aniline to benzene and

ammonia) must occur by syn-elimination although elimination tends to occur in the anti-

periplanar rather than in the syn-antiplanar conformation [16]. A closer look at 1,2-dihydro-

aniline suggests, however, that the elimination will not occur by an E2 mechanism, but by an

E1 mechanism. The reason is that the carbon atom that bears the NH2 group is in α position to

the C3-C6 butadiene fragment. As a consequence, the cyclohexadienyl carbocation resulting

from scission of the C-N bond will be strongly stabilized by conjugation with this butadiene

fragment (Scheme 7.4). An E1 elimination mechanism means, however, that the eclipsed

conformation of the NH2 group on the C1 atom and the H atom on the C2 atom in 1,2-

dihydro-aniline is not an obstacle anymore against elimination.

++

Scheme 7.4 Cyclohexadienyl carbocation.

Another explanation for the direct denitrogenation would be to assume that aniline is

hydrogenated to tetrahydroaniline, which undergoes elimination to cyclohexadiene.

Cyclohexadiene then quickly reacts to cyclohexene or benzene. Since tetrahydroaniline is not

flat, the elimination of ammonia is possible in the anti conformation. In our case of 1-

naphthylamine, this means that the naphthalene would be formed via 1,2,3,4-tetrahydro-1-

naphthylamine (route 4, scheme 7.2). This is, however, in contradiction to the naphthalene-to-

tetralin ratio observed in the reaction of 1,2,3,4-tetrahydro-1-naphthylamine, which is two

times lower than that observed in the reaction of 1-naphthylamine. Also the ratio in the

reaction of 1,2-dihydronaphthalene, the primary product of 1,2,3,4-tetrahydro-1-


naphthylamine is lower by a factor two. We conclude that a tetrahydro intermediate cannot

explain the direct denitrogenation of 1-naphthylamine to naphthalene.

1,2-Dihydro-1-naphthylamine may not only react to naphthalene by elimination of NH3,

but also by a Bucherer-like NH2-SH exchange of the amino group of 1,2-dihydro-1-

naphthylamine by addition of NH3 and elimination of H2S, via enamine-imine and thioenol-

thioketo tautomeric equilibria (scheme 7.5 and route 3 in scheme 7.2). The resulting 1,2-

dihydro-1-thionaphthol may dehydrogenate to 1-thionaphthol, which quickly undergoes

hydrogenolysis to naphthalene. A dihydro intermediate could thus explain direct

hydrogenation via a Bucherer-type NH2-SH exchange reaction (Scheme 7.4). Alternatively,

NH2-SH exchange could occur directly in the arylamine via the imine form of the enamine-

imine tautomeric equilibrium (Scheme 7.3).

NH2 NH2 NH

SH SH S

HS NH2

H2H2S

-H2

-NH3

Scheme 7.5 Reaction from 1-naphthylamine to 1-thionaphthol by hydrogenation to dihydro-

1-naphthylamine, followed by a Bucherer-like NH2-SH exchange and

dehydrogenation of the resulting dihydro-1-thionaphthol.

Three mechanisms have thus been identified which can explain the apparent direct C-N

bond breaking in 1-naphthylamine to naphthalene. In the first mechanism (Scheme 7.3, route

1 in scheme 7.2), NH2-SH exchange via the imine form of 1-naphthylamine is followed by

hydrogenolysis of the C-S bond of 1-thionaphthol, while in the second mechanism

hydrogenation of 1-naphthylamine to 1,2-dihydro-1-naphthylamine is followed by NH3

elimination (route 2 in scheme 7.2). A third mechanism would be that 1-naphthylamine is

hydrogenated to a dihydro intermediate which undergoes a Bucherer-type NH2-SH exchange

reaction (Scheme 7.5, route 3 in scheme 7.2). Some experimental observations speak in


favour of the second and third alternatives. It has been observed that the ratio of direct versus

indirect C-N bond breaking depends on the catalyst. Thus, a change from Al2O3 to silica-

alumina and fluorination of these supports did not change the toluene to methylcyclohexane

product ratio in the HDN of o-toluidine, although they did improve the activity [17]. This

suggests that direct and indirect C-N bond breaking go through a common intermediate,

which could be a dihydro intermediate. That would eliminate route 1 (Scheme 7.2), but would

still leave route 2 (Scheme 7.2) (hydrogenation followed by NH3 elimination) and route 3

(hydrogenation followed by a Bucherer-type NH2-SH exchange, dehydrogenation and C-S

hydrogenolysis) as possibilities to explain the direct C-N bond breaking. The ratio of direct to

indirect C-N bond breaking would then be determined by the ratio of the two reactions that

dihydro-1-naphthylamine can undergo: further hydrogenation to 1,2,3,4-tetrahydro-1-

naphthylamine or elimination of NH3 to naphthalene (cf. scheme 7.2).

A final comment is appropriate about the small amount of 5,6,7,8-tetrahydro-1-

naphthylamine observed in the HDN product of 1-naphthylamine. This product is due to

hydrogenation of the non-substituted aromatic ring. Since the reactivity of 5,6,7,8-tetrahydro-

1-naphthylamine itself is low, its low concentration in the HDN of 1-naphthylamine points to

a slow rate of formation. This may be due to a weaker adsorption of the benzene part than of

the aniline part of the naphthylamine. The behaviour of 1,2,3,4-tetrahydro-1-naphthylamine

and 5,6,7,8-tetrahydro-1-naphthylamine in the HDN of 1-naphthylamine would then be

similar to that of 1,2,3,4-tetrahydroquinoline and 5,6,7,8-tetrahydroquinoline respectively in

the HDN of quinoline [9]. The small amount of 5,6,7,8-tetrahydro-1-naphthylamine produced

and its low reactivity show that 5,6,7,8-tetrahydro-1-naphthylamine plays only a minor role in

the HDN of 1-naphthylamine under our conditions.

7.4.2 Direct desulfurization

Aliphatic and aromatic thiols undergo HDS with high selectivity to alkanes and aromatic

hydrocarbons, respectively. This suggests that in both cases hydrogenolysis of the C-S bond

actually takes place. However, if amines do not react by real hydrogenolysis, then the

question arises as to whether or not the direct C-S bond breaking in thiols takes place by real

or apparent hydrogenolysis. An alternative explanation for the arylthiol would be partial


hydrogenation followed by H2S elimination (e.g., thiophenol reacts to 1,2-dihydrothiophenol

and then to benzene and H2S), as suggested above for the reaction of arylamines to aromatic

molecules. This is not possible, however, for alkane thiols. Alkanes are the main product of

the HDS reaction of alkane thiols at lower temperatures; the only possible explanation is

actual hydrogenolysis. Homolytic C-S bond scission (hydrogenolysis) was demonstrated by

Curtis and Drucker in the homogeneous reaction of aliphatic and aromatic thiols with the

Cp*2Mo2Co2S3(CO)4 cluster (Cp* stands for pentamethylcyclopentadienyl) [3]. By

spectroscopic and kinetic measurements they showed that the thiols react as indicated in

scheme 7.6, in which only the bare structure of the complexes is indicated. The µ3-mode of

coordination of the RS thiolate leads to the activation of the C-S bond for homolytic cleavage

by decreasing the C-S bond dissociation energy. The soft character of sulfur apparently

enables a relatively strong interaction with the soft, low-valent Mo atoms in the metal sulfide;

this promotes C-S bond homolysis, as observed in thiolate complexes [18]. The interaction of

the nitrogen atom in an amine with such Mo atoms does, on the other hand, not lead to C-N

bond scission because the µ3-bonded, harder N atom is less strongly bonded as the equivalent

S atom.

MCo

M S

SCo

S

SH

R

MCo

MS

SCo

S

S

H R

MCo

MS

S

Co

S

S-RH

RSHM

Co

M S

S

Co

S

Scheme 7.6 Reaction of alkyl- and arylthiols on homogeneous Cp*2Mo2Co2S3(CO)4 clusters

(Cp* = C5(CH3)5) [3].


7.5 Conclusions

HDN of 1-naphthylamine over sulfided NiMo/Al2O3 and CoMo/Al2O3 catalysts leads to

the formation of tetralin and naphthalene. The high selectivity to 1,2-dihydronaphthalene at

low conversion of 1-naphthylamine shows that one of the HDN pathways is partial

hydrogenation of 1-naphthylamine followed by NH3 elimination. Naphthalene forms with

high selectivity, even at low conversion. This apparent hydrogenolysis can be explained by

hydrogenation of 1-naphthylamine to 1,2-dihydro-1-naphthylamine followed by NH3

elimination. Another possible mechanism is hydrogenation of 1-naphthylamine to dihydro-1-

naphthylamine which undergoes a Bucherer-type NH2-SH exchange, followed by

dehydrogenation to 1-thionaphthol and hydrogenolysis to naphthalene.

7.6 References

[1] H. Schulz, M. Schon and N.M. Rahman, Stud. Surf. Sci. Catal. 27 (1986) 201.

[2] M. Houalla, N.K. Nag, A.V. Sapre, D.H. Broderick and B.C. Gates, AIChE J. 24

(1978) 1015.

[3] M.D. Curtis and S.H. Druker, J. Am. Chem. Soc. 119 (1997) 1027.

[4] C. Moreau, J. Joffre, C. Saenz, J.C. Afonso and J.L. Portefaix, J. Mol. Catal. 161

(2000) 141.

[5] J. Mijoin, G. Pérot, F. Bataille, J.L. Lemberton, M. Breysse and S. Kasztelan, Catal.

Lett. 71 (2001) 139.

[6] N. Nelson and R.B. Levy, J. Catal. 58 (1979) 485.

[7] J.L. Portefaix, M. Cattenot, M. Gerriche, J. Thivolle-Cazat and M. Breysse, Catal.

Today 10 (1991) 473.

[8] L. Vivier, V. Dominguez, G. Perot and S. Kasztelan, J. Mol. Catal. 67 (1991) 267.

[9] R. Prins, Adv. Catal. 46 (2001) 399.

[10] M. Jian and R. Prins, Catal. Today 30 (1996) 127.


[11] C. Moreau, L. Bekakra, J.L. Olivé and P. Geneste, in: Proc. 9th Int. Congr. on

Catalysis, Vol. 1, eds. M.J. Philips and M. Ternan (Chem. Inst. of Canada, Ottawa,

1988) p. 58.

[12] M. Egorova, Y. Zhao, P. Kukula and R. Prins, J. Catal. 206 (2002) 263; and chapter 2.

[13] P. Geneste, C. Moulinas and J.L. Olivé, J. Catal. 105 (1987) 254.

[14] F. Rota and R. Prins, Topics in Catal. 11/12 (2000) 327.

[15] F. Rota and R. Prins, J. Mol. Catal. 162 (2000) 359.

[16] M.B. Smith and J. March, Advanced Organic Chemistry (Wiley, New York, 5th Ed.,

2001).

[17] L. Qu and R. Prins, J. Catal. 217 (2003) 284.

[18] J.S. Kim, J.H. Reibenspies and M.Y. Darensbourg, J. Am. Chem. Soc. 118 (1996)

4115.

Concluding remarks Chapter 8 165

8. Concluding remarks

8.1 Conclusion

Since the publication of Nelson and Levy, it has been widely accepted that the

nucleophilic substitution and Hofmann β hydrogen elimination are the main

hydrodenitrogenation (HDN) mechanisms [1-5]. Direct hydrogenolysis was discussed as well.

Direct breaking of the C-N bond in aniline occurs to a minor degree. In the HDN of o-

propylaniline the selectivity to propylbenzene was 7% over a NiMo/Al2O3 catalyst and 24%

over a Mo/Al2O3 catalyst [6]. For naphthylamine, direct C-N bond breakage is even more

important [7]. In order to clarify whether direct C-N bond takes place, the HDN of

neopentylamine and naphthylamine was studied [8,9]. In the HDN of neopentylamine,

neopentane was formed as a secondary product, as the selectivity extrapolates to zero with

decreasing weight time. Neopentane was formed by hydrogenolysis of neopentanethiol, which

was formed by substitution of neopentylamine with H2S. Direct hydrogenolysis of

neopentylamine to neopentane can hardly take place. In the HDN of naphthylamine, the

intermediate 1,2-dihydronaphthalene was detected, which could be formed by elimination of

1,2,3,4-tetrahydronaphthylamine. The HDN studies of the intermediates 1,2,3,4-tetrahydro-1-

naphthylamine, 1,2-dihydronaphthalene and 5,6,7,8-tetrahydro-1-naphthylamine show that 1-

naphthylamine reacts through hydrogenation to 1,2,3,4-tetrahydro-1-naphthylamine, which

reacts by NH3 elimination to 1,2-dihydronaphthalene. The latter molecule subsequently reacts

to tetralin and naphthalene. Direct denitrogenation of 1-naphthylamine to naphthalene could

also occur by hydrogenation of 1-naphthylamine to 1,2-dihydro-1-naphthylamine, followed

by NH3 elimination or followed by a Bucherer-type NH2-SH exchange, dehydrogenation and

C-S bond hydrogenolysis. This suggests that hydrogenolysis in arylamines is not true but

apparent, meaning that the reaction occurs via an indirect and multi-step mechanism.

As direct hydrogenolysis of amines does not take place under normal experimental

conditions, elimination and substitution will be the only HDN mechanisms. To determine


which mechanism is responsible for the HDN reaction of an alkylamine, one can measure the

product selectivities as a function of weight time (τ) and determine whether they extrapolate

to a non-zero or zero value at τ = 0. If a product such as an alkene has a zero selectivity at

zero weight time, then it cannot be a primary product and elimination cannot play a role. If its

selectivity is non-zero at τ = 0, then the alkene might be a primary product, and elimination

might be important. The alkenes/alkane ratios could also be measured to differentiate the

mechanisms [10-12]. The similar ratios of hexenes/hexane originating from hexylamine and

pentenes/pentane originating from pentanethiol, in the simultaneous reactions of hexylamine

and pentanethiol, show that hexylamine reacts with H2S to form hexanethiol. The latter

molecule reacts to hexenes and hexane. It demonstrates that nucleophilic substitution is the

predominant mechanism. The same mechanism was proved to operate in the HDN of 2-

hexylamine [11]. However, the ratio of methylbutenes/methylbutane in the HDN of 2-methyl-

2-butylamine is totally different as that obtained in the HDS of 2-methyl-2-butanethiol in the

presence of hexylamine, which shows that an E1 mechanism operates in the HDN of an

alkylamine with the amine group attached to a tertiary carbon atom [11]. The results from the

simultaneous HDN of pentylamine and HDS of hexanethiol, and the simultaneous HDN of 2-

hexylamine and HDS of 2-pentanethiol, show that nucleophilic substitution is also the

predominant mechanism over sulfided CoMo/Al2O3 and Mo/Al2O3 catalysts. The HDN of 2-

methyl-2-butylamine, as well as the HDS of 2-methyl-2-butanethiol in the presence of

hexylamine, show that, over all three catalysts, E1 is the main HDN mechanism. Furthermore,

the formation of thiols in the HDN of amines was explained by an acid-base mechanism as

well as by a metal-like catalyzed mechanism [11,12].

In the HDN of 1-adamantylamine, 1-adamanethiol and adamantane were formed. 1-

Adamantanethiol was the primary product, which reacts by C-S hydrogenolysis to

adamantane. As 1-adamantylamine can not react by ammonia elimination, nor by a classic

SN2 substitution, it is proposed that the NH2-SH substitution in adamantylamine takes place

by adsorption of the amine group at the metal sulfide surface and migration of the adamantyl

group to a neighbouring sulfur atom.


8.2 Outlook

It is clear that substitution is the predominant mechanism in the HDN of alkylamines

with the amine group attached to a primary or secondary carbon atom [9-12]. Only an E1

mechanism operates in the HDN of an alkylamine with the amine group attached to a tertiary

carbon atom [11-12] over NiMo/Al2O3, CoMo/Al2O3 and Mo/Al2O3 catalysts.

A remaining question is how the substitution of the alkylamine with H2S or another

alkylamine takes place on the catalyst surface. The formation of a dialkylamine and

alkanethiol in the HDN of an alkylamine with the NH2 group attached to a primary or

secondary carbon atom could be explained by either an acid-catalysis mechanism or a metal-

catalysis mechanism: dehydrogenation of an amine to an imine, addition of H2S or another

alkylamine, elimination of NH3, and hydrogenation of the resulting thioaldehyde to a thiol

[13].

The disproportionation reaction of two dialkylamine molecules to a trialkylamine and an

alkylamine allows to distinguish between acid and metal catalysis. If the surface is metallic,

N-ethylbutylamine will dehydrogenate to form N-butylethylimine. The NH2 group from

another N-ethylbutylamine molecule can add to the N-butylethylimine to form a diaminal

intermediate. After elimination of butylamine, hydrogenation of the resulting molecule leads

to N,N-diethylbutylamine (Scheme 8.1). N-ethylbutylamine can also dehydrogenate to form

N-ethylbutylimine. In that case, N,N-dibutylethylamine will be formed by imine formation,

addition of an amine, elimination, and hydrogenation of the resulting elimination

intermediate. In the HDN of N-methylhexylamine, however, only dihexylmethylamine should

be formed if the surface is metallic. N,N-dimethylhexylamine cannot be generated, as the

elimination of hexylamine cannot take place because there is no β-H atom available (Scheme

8.2). Therefore, if the surface is only metallic, the molar ratio of N,N-

dihexylmethylamine/N,N-dimethylhexylamine in the HDN of N-methylhexylamine should be

much higher than 1, while the ratio of N,N-dibutylethylamine/N,N-diethylbutylamine in the

HDN of N-ethylbutylamine should be about 1. If the surface is acidic, then the

disproportionation occurs by classic substitution and both ratios should be close to 1.


C2

HN

C4

HN

C4

C2N

C4

HN

C2

C2N

C4

C2N

C4

+ C4

C2

C2N

C4

C2N

C4

C4

C2N

C4

+ C2

C2

HN

C4

NC4

NC2

H2N

H2N

Scheme 8.1 Reaction mechanism in the disproportionation of N-ethylbutylamine.

HN

C6

C1N

C6

C5HN

C1N

C6

C1

C1N

C6+ C6

C1N

C4

C4

C6

C1N

C6

+ C1

C1

HN

C6

C1

HN C6

NC6

NC5

H2N

H2N

Scheme 8.2 Reaction mechanism in the disproportionation of N-methylhexylamine.

The HDN of a chiral alkylamine can be another test to distinguish between acid-base

and metal catalysis over sulfided NiMo/Al2O3. For instance, in the HDN of R-2-butylamine,

only R,S-di(2-butylamine) can be formed by classic nucleophilic substitution, if the catalyst

surface is acidic (Scheme 8.3). If the catalyst surface is metallic, the disproportionation

product of R-2-butylamine should be a 1:1 mixture of R,R- and R,S-di(2-butylamine), which

can be explained by a sequence of reactions of amine-imine-amine addition-ammonia

elimination-dialkylimine hydrogenation (Scheme 8.4).


NH2

R

NH2

R

-NH3 NH

R

S

Scheme 8.3 Disproportionation of R-2-butylamine by acid-base catalysis.

NH

NH2

R

NH

R

NH2

R-H2

NH2

-NH3N

R

H2 NH

R

R,S

Scheme 8.4 Disproportionation of R-2-butylamine by metal-like catalysis.

Similar methods can be used to investigate the mechanism of the substitution of the

alkylamine by H2S. If this substitution would occur by classic substitution with Walden

inversion, then R-2-butylamine should form S-2-butanethiol (Scheme 8.5). If the substitution

would be indirect, via an imine, then a racemic 2-butanethiol mixture is expected (Scheme

8.6). It is unclear what the stereochemistry of the substitution proposed in Scheme 8.7 will be.

Maybe it leads to retention of the configuration.

Such and similar methods might help to elucidate the path of the HDN reactions at the

catalyst surface.

NH2H2S + NH2H2S NH3+HS

Scheme 8.5 Classic nucleophilic substitution of R-2-butylamine with H2S to S-2-butanethiol.


NH

SH

NH2

R-H2

NH2-NH3

SH2

SH

R,S

H2S

Scheme 8.6 Substitution of R-2-butylamine with H2S to R(S)-2-butanethiol by means of an

imine.

Mo

S

S S

S

Mo

S

S

S N

H

H HR

Mo

S

S S

S

Mo

S

S

S N

H

H HR

Mo

S

S S

S

Mo

S

S

S N

H HHR

Scheme 8.7 NH2-SH substitution by alkyl migration on the metal sulfide surface.

8.3 References



10 (1991) 473.


[4] F. Rota, V.S. Ranade, R. Prins, J. Catal. 200 (2001) 389.

[5] P. Clark, X. Wang, P. Deck, S. T. Oyama, J. Catal. 210 (2002) 116.

[6] M. Jian, R. Prins, Catal. Today 30 (1996) 127.

[7] C. Moreau, L. Bekakra, J.L. Olivé and P. Geneste, in: Proc. 9th Int. Congr. on

Catalysis, Vol. 1, eds. M.J. Philips and M. Ternan (Chem. Inst. of Canada, Ottawa,

1988) p. 58.

[8] Y. Zhao, J. Czyzniewska, R. Prins, Catal. Lett. 88 (2003) 155; and chapter 7.

[9] Y. Zhao, J. Czyzniewska, R. Prins, to be published (2004); and chapter 6.



[11] Y. Zhao, R. Prins, J. Catal. 222 (2004) 532; and chapter 4.

[12] Y. Zhao, R. Prins, to be published 2004; and chapter 5.

[13] R. Prins, in: G. Ertl, H. Knözinger, J. Weitkamp (Eds.), Handbook of Heterogeneous

Catalysis, vol. 4, Wiley-VCH, New York, 1997, p. 1916.

Acknowledgements

I greatly appreciate that many people contributed to my work especially:

Prof. R. Prins for offering me the opportunity to join his group so that I can fulfil my

dream studying as Ph.D. in western countries. I am very proud of his supervision.

Prof. M. Breysse of Université Pierre et Marie Curie for taking on the task of co-

examiner.

Dr. P. Kukula for many discussions and helpful suggestion and for the friendly

collaboration during my thesis. Dr. L.L. Qu for fruitful help with my hydrotreating unit,

wonderful discussion to the HDN work and teaching me how to be a standard Chinese. Dr. M.

Egorova, one of the my best colleagues in Prof. Prins groups, for kind help and endure to my

bad habits in my first year of PhD, and wonderful collaboration during my Ph.D. work. Ms.

A. Röthlisberger, my nice colleague, for the help of English (French tongue), sharing the

pressure and happiness during my Ph.D study. Samantha and Achim my excellent Swiss

friends for the help to European life and the help of skiing. I enjoyed the wonderful time

driving to ULM for Catalysis Lectures with Daniele and Lukas. Thanks Lukas to translate the

abstract of my English dissertation to German. Ms. M. Schoenberg for her kind help of

English and friendly collaboration. Dr. G. Pirngruber for the kind help and suggestions in

training my oral presentation. T. Schmid and L. Adad for the help of organic synthesis. T.

Todorova for the thoughtful discussion of DFT calculation. M. Lüchinger for the help of

AAS. S.F. Yang, B. Bin, N. Weiher, D. Sirbu, A. Leone, J. Kovacovic, M. Kuba, P. Kanti

Roy, Prof. C., Giambattista, E. Bus, J. van Bokhoven, A. Abraham, and all other members of

the group for their advises and their friendship.

I greatly thank my wife, Xueying Jin, for her love, consistent support during my PhD

work.

Publications

1. M. Egorova, Y. Zhao, P. Kukula and R. Prins

“On the role of β-hydrogen atoms in the hydrodenitrogenation of 2-methylpyridine and 2-

methylpiperidine“

J. Catal. 206 (2002) 263.

2. Y. Zhao, J. Czyzniewska, and R. Prins

“Mechanism of the direct hydrodenitrogenation of naphthylamine over sulfided NiMo/γ-

Al2O3“

Catal. Lett. 88 (2003) 155.

3. Y. Zhao, P. Kukula, and R. Prins

“Mechanisms of the hydrodenitrogenation of alkylamines with secondary and tertiary α-

carbon atoms over sulfided NiMo/γ-Al2O3“

J. Catal. 221 (2004) 441.

4. Y. Zhao and R. Prins

“Mechanisms of the hydrodenitrogenation of alkylamines with secondary and tertiary α-

carbon atoms over sulfided NiMo/γ-Al2O3 “

J. Catal. 222 (2004) 532.

5. Y. Zhao, J. Czyzniewska, and R. Prins

“Mechanism of the hydrodenitrogenation of adamantylamine and neopentylamine over

sulfided NiMo/γ-Al2O3“

To be published (2004).

6. Y. Zhao and R. Prins

“Mechanism of the hydrodenitrogenation of alkylamines over NiMo/γ-Al2O3, CoMo/γ-

Al2O3 and Mo/γ-Al2O3“

To be published (2004).

Curriculum Vitae

Name: Yonggang Zhao

Date of birth: 21th September 1974

Place of birth: Huaiyin (Jiangsu, China)

Nationality: Chinese

Education 1982-1987 Hedong Primary school, Huaiyin county, Jiangsu province

1987-1990 Wuji town Middle school, Huaiyin county, Jiangsu province

1990-1993 Huaiyin County High school, Jiangsu province

1993-1997 Fushun Petroleum Institute, Liaoning Province

Bachelor of Chemical Engineering (refinery)

1997-2000 Fushun Petroleum Institute, Liaoning province

Master of Chemical Engineering

Alkylation of long chain alkene with benzene over solid acid catalyst.

2000-2004 PhD Thesis at the ETH Zürich in the group of Prof. Dr. R. Prins:

Hydrodenitrogenation of Amines over Sulfided NiMo/Al2O3,

CoMo/Al2O3, and Mo/Al2O3

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