Chapter-6

Heterogeneous Hydroaminomethylation over

Rhodium Exchanged Na-ETS-4 and Na-ETS-10

Chapter 6

PhD Thesis 174

6.1. Introduction

Hydroaminomethylation of alkenes was originally discovered by Reppe and

Vetter in late 1940’s [1]. It is the single pot synthesis of amines via hydroformylation,

enamine/imine formation and consecutive hydrogenation of enamine [2–3]. The

starting materials for hydroaminomethylation are olefin and primary or secondary

amine in presence of CO and H2 with a suitable catalyst [4]. The classical

hydroaminomethylations are generally carried out in homogeneous catalysis system

[5–9], where the major difficulties of the catalyst recovery and catalyst separation

from the product constitute major disadvantages. Possible solutions to these

difficulties include heterogenizing a homogeneous catalyst, either by anchoring the

catalyst on a support, or by using a liquid–liquid two-phase system. The systems

associated with biphasic hydroaminomethylation are available [10–12], but

heterogeneous systems for hydroaminomethylation are scanty and less known. The

homogeneous hydroaminomethylation are catalysed by using cationic metal

precursors. It is found that the cationic precursors gave higher activity towards this

domino reaction.

From the performed investigations mentioned in the chapter 5 of this thesis, it

is observed that the rate of formations of enamine from the aldehyde and substrate

amine is slow and hence long reaction time is needed for hydroaminomethylation. As

a result, large amounts of intermediate aldehyde are quantified. The main problem

associated with this may be the lack of an acidic site for enamine formation and

hydrogenation. The enamine formations are often acid catalysed [13] reaction. In the

absence of an acidic species hydroaminomethylation runs slowly and may take long

time about 24-72 h [14–16]. This problem may be overcome to an extent by the usage

of mixed solvent systems consisting of a protic and aprotic solvent [17]. The

enamine/imine formation and subsequent hydrogenation are enhanced in presence of a

protic solvent like methanol. The addition of acid in hydroaminomethylation reaction

was found to enhance the activity of the catalyst. Beller et al. reported that the

addition of catalytic amount of tetrafluoroboric acid significantly enhanced the yield

of amines during hydroaminomethylation of arylethylenes [18]. The exact role of acid

was not explained but the enhancement of the yield may be attributed to the formation

of imine/enamine ions, which are smoothly reduced to the corresponding amines.

Chapter 6

PhD Thesis 175

Another study of Behr et al. showed that the addition of aminium ions formed by the

reaction of amine with an acid lead to the highly selective enamine hydrogenation

resulting in to an efficient hydroaminomethylation [19]. It has been also found that the

presence of acid makes the cationic Rh-complex, which is active for hydrogenation.

These two studies showed the possible acid catalysed formation of enamine or imine

and subsequent formation of enamine/imine ions which can be easily hydrogenated to

corresponding amine. Therefore the presence of an acid is very useful for the

hydroaminomethylation [20]. Various reports are available in which the use of acetic

acid or other organic and/or mineral acids have shown significant effects on

hydroaminomethylation [21, 22].

The use of acetic acid has improved the hydrogenation of enamine to

subsequent amine. The presence of acid helps the formation of cationic rhodium

species which is active for hydrogenation [19]. The formations of such cationic

rhodium–phosphine and rhodium–phosphite complexes were investigated by

Bitterwolf by NMR-studies [23]. But from some studies on vinyl acetate

hydroformylation it was found that small amounts of acetic acid formed during

hydroformylation tends to decrease the hydroformylation activity. The acetic acid in

hydroformylation conditions forms rhodium carboxylate complexes which are

inactive for hydroformylation [24–26]. These are more prominent when the recycling

of the catalyst is performed. Therefore, the use of acetic acid or similar acids in

hydroaminomethylation may pose problems in the recovery and recycle of the

catalysts.

Hence there lies a potential in the development of heterogeneous support with

acidic properties to address the issues concerned with these drawbacks. The supports

with active acidic sites for enamine formation and subsequent hydrogenation will be a

better choice for hydroaminomethylation catalysts. Titanosilicates, mainly ETS-4 and

ETS-10 (Engelhard titanosilicate) [27–31] are the class of materials possessing such

acidic properties and are less studied for hydroformylation and/or

hydroaminomethylation in heterogeneous conditions. The framework of ETS-4 is

made up of chains of corner-sharing TiO6 octahedra along the b axis, with

neighbouring octahedra in a chain which are laterally linked by SiO4 tetrahedra. Pairs

of SiO4 tetrahedra are connected to the adjacent TiO6 chains which further joined

through the structural unit containing one TiO6 surrounded by four SiO4 tetrahedra

[32]. This arrangement produces two orthogonal sets of channels made by 12-

Chapter 6

PhD Thesis 176

membered rings perpendicular to the c axis and eight-membered rings perpendicular

to b. The effective pore size of ETS-4 is comparable to the pore size for a small-pore

zeolite. ETS-10 framework is made up of chains of TiO6 octahedral units that are

linked to two-folded chains of SiO4 tetrahedra, forming TiSi4O13 columns [33]. These

columns are packed in such a manner that the layers are parallel to (001) with the

columns in adjacent layers which are perpendicular to each other. The layers are

further connected by SiO4 tetrahedra forming a three-dimensional framework. ETS-10

framework contains three orthogonal sets of channels, defined by 12-membered rings,

along a, b and c axis. Absorption measurements showed that its effective pore size is

comparable to a large-pore zeolite [34]. The single unit structure of ETS-4 and ETS-

10 are shown in Fig. 6.1.

Fig. 6.1. The single unit structure of ETS-4 and ETS-10

The main characteristics of ETS material lies in its high cation exchange

capacity (applicable for adsorption and waste removal), facile metal loading and

presence of mild acidic sites which finds application in catalysis, and possible photo

catalytic applications. Na-ETS materials possess basic sites along with acidic sites

that are formed due to strong electric field of cationic sites on Na-ETS-10, which

could act as a Lewis acid [35]. Na-ETS-4 [36] and Na-ETS-10 [37, 38] are used

materials for gas sorption [39–43] and other catalytic applications [44, 45]. But both

ETS-4 and ETS-10 are not well explored for its catalytic activity. Here in this chapter

the aim is focussed to exchange Rh with Na-ETS-4 and Na-ETS-10 to obtain a robust

Chapter 6

PhD Thesis 177

solid catalyst and to investigate hydroaminomethylation reaction of olefin with

amines.

6.2. Experimental

6.2.1. Materials

Sodium silicate solution, titanium butoxide, titanium dioxide, rhodium

chloride, all the olefins and amines were procured from Sigma-Aldrich, USA. Sodium

chloride, potassium chloride and sodium hydroxide were purchased from Sd- Fine

Chemicals, Mumbai. H2 and CO were procured from Ami Traders, Bhavnagar,

Gujarat, India.

6.2.2. Synthesis of Na-ETS-10 and Na-ETS-4

The synthesis of Na-ETS-10 was done by the procedure reported elsewhere

[46]. In a typical synthesis procedure, sodium silicate solution (40 g) was treated with

deionized water (70 mL) followed by the addition of NaCl (13.8 g) and KCl (2.6 g).

The gel formed was vigorously stirred and titanium dioxide (2.6 g) was added. The

pH was maintained at 10.5. The stirring was continued for about 40 min. at room

temperature and then transferred to Teflon lined autoclave, which is then heated at

200 ºC for 42 h. The crystalline product obtained was washed with deionized water

and dried at 60 ºC.

The synthesis of Na-ETS-4 was done according to the procedure reported

elsewhere [47]. Titanium (IV) butoxide was added to the aqueous NaOH solution

under stirring. The white precipitate was then dissolved by adding an aqueous

solution of H2O2, resulting in a bright yellow solution. Sodium silicate was then

dissolved in this solution under stirring. The initial composition of the solution used

for preparing the ETS-4 powder was Ti:Si:Na:H2O2:H2O = 0.9:10:14:8:675 on a

molar basis. The hydrothermal synthesis was performed at 180 ºC for 72 h in an

autoclave. The powder was filtered, washed with deionized water, and dried at 40 ºC.

6.2.3. Synthesis of rhodium exchanged ETS-4 and ETS-10

The rhodium was exchanged with Na-ETS-4 and Na-ETS-10 by the

conventional cation exchange method [48]. In a typical rhodium exchange to Na-ETS-

10, the ETS-10 was treated with 0.05 M aqueous solution of rhodium chloride with

solid to liquid ratio of 1:80 at 80 ºC for 4 h. The solid was then filtered and washed

Chapter 6

PhD Thesis 178

with hot distilled water, until the filtrate is free of chloride ions and then dried at room

temperature. Similar procedure was followed for the rhodium exchange in ETS-4.

6.2.4. Characterization techniques

Powder X-ray diffraction (P-XRD) of the samples were recorded with Phillips

X’Pert MPD system equipped with XRK 900 reaction chamber, using Ni-filtered Cu

Kα radiation (λ = 1.54050 Å) over a 2θ range of 1–10º at a step time of 0.05º s−1. The

FT-IR spectra of the samples were recorded from 400 to 4000 cm−1 with a

PerkinElmer Spectrum GX FT-IR system using KBr pellets. Inductively coupled

plasma atomic emission spectroscopy (ICP-AES) analysis (Optima 2000DV,

PerkinElmer instruments) was used to determine the rhodium content in the catalyst.

The surface area analysis and pore size distribution of the samples were measured by

nitrogen adsorption at 77.4 K using a Sorptometer (ASAP-2010, Micromeritics). All

the samples were degassed at 80 ºC for 4 h prior to the measurements.

6.2.5. Hydroaminomethylation and product analysis

Hydroaminomethylation reactions were carried out in a 100 mL stainless steel

autoclave reactor (Autoclave Engineers, USA). The experimental setup, safety

precautions and procedures were similar to that described in previous chapters. The

weighed amount of alkene and amine with catalyst was taken in the autoclave with a

mixture of 30 mL of toluene and 20 mL of methanol as solvent. The autoclave was

raised to reaction temperature and then CO and H2 were pressurized in the required

ratio. Aliquots of product mixture were withdrawn for analysis of conversion and

selectivity at regular time intervals. The product was analyzed by GC (Schimadzu

GC-17A, Japan) and GC-MS (Schimadzu GCMS-QP2010, Japan). To ensure

reproducibility the experiments were repeated under identical reaction conditions.

6.3. Results and Discussions

6.3.1. Catalyst characterization

6.3.1.1. Powder X-ray diffraction

The powder XRD patterns of the Na-ETS-4, Na-ETS-10, Rh-ETS-4 and Rh-

ETS-10 are shown in Fig. 6.2. The X-ray diffraction patterns of the as synthesized

Na-ETS-4 [49] and Na-ETS-10 [50, 51] were in good agreement with that reported in

Chapter 6

PhD Thesis 179

literatures. The major diffraction patterns of Na-ETS-10 are corresponding to the

planes 105, 202, 116 and 204 at 2θ values near 20.2, 24.7, 25.7 and 26.6º respectively

[52]. The formation of some quartz was observed and corresponding plane is

observed at 20.8º and 27.4º of 2θ [53]. But no anatase phase of TiO2 was observed.

The diffraction pattern of Na-ETS-4 showed the major planes of ETS-4 at 2θ values

of 7.6, 12.7 and 24.7º corresponding to 200, 001 and 020 planes respectively [29]. No

formation of anatase or quartz phase is observed. It is due to the usage of titanium

(IV) butoxide as the precursor in case of ETS-4 whereas the TiO2 was used in ETS-10

synthesis. The diffraction patterns of both Na-ETS-10 and Na-ETS-4 confirm the

formation of ETS framework.

10 20 30 40 50

(020)

Q

Q

(204)(116)

(001)(200)

(202)

(105)

Rh-ETS-4

Na-ETS-4

Rh-ETS-10

Na-ETS-10

coun

ts/s

(a.u

)

2θθθθ

Fig. 6.2. P-XRD patterns of the synthesized support and catalyst

As the rhodium was exchanged with the Na-ETS-4 and Na-ETS-10 the

diffraction pattern shows that there were similar peaks confirming that the ETS

framework is undisturbed during the ion-exchange. All the major planes were seen

and there was some drop in the intensities of the peaks showing a possible loss in

crystallinity after Rh exchange. The decrease in crystallinity during the transition

metal ion-exchange may be due to the hydrated transition metal ions, which is

hydrolyzed within the titanosilicate channels leading to breakage of framework [48].

The decrease in diffraction intensity could also be due to presence of transition metal

cations impregnated on the surface, other than those are ion-exchanged. The

percentage of loading determined by ICP-AES technique and the % crystallinity

calculated from the PXRD are given in Table 6.1.

Chapter 6

PhD Thesis 180

Table 6.1. Crystallinity and wt% of Rh exchanged

Sample Wt % of rhodium exchanged % Crystallinity

Na-ETS-4 - 100

Rh-ETS-4 3.2 83

Na-ETS-10 - 100

Rh-ETS-10 4.8 80

6.3.1.2. FT-IR spectroscopy

The vibrational spectroscopy of ETS-4 and ETS-10 gave almost similar

transmittance, depicting similar kind of -Si-O-Ti- like bonding. The FT-IR spectrum

of Rh-ETS-4 and Rh-ETS-10 are given in Fig. 6.3. The FT-IR bands observed are in

well agreement with that reported in literature [53]. The broad band near 1020-1050

cm-1 corresponds to the Si-O stretching. The band at 660-690 cm-1 corresponds to the

Ti-O stretching. The band near 540-560 cm-1 corresponds to the Si-O rocking and O-

Ti-O bending vibrations. The broad peak near ~3400-3500 cm-1 is for the surface

hydroxyl groups and also for the stretching of adsorbed water molecule in the ETS

framework. The adsorbed water molecule gave H-O-H bending vibration near 1634

cm1.

4000 3500 3000 2500 2000 1500 1000 500

1647

1634

3442

3437

562

555

690

676

10241051

Rh-ETS-4

Rh-ETS-10

%T

, a.u

cm-1

Fig 6.3. FT-IR spectra of Rh-ETS-4 and Rh-ETS-10

Chapter 6

PhD Thesis 181

6.3.1.3. N2 adsorption analysis and surface area measurements

The surface area analysis was carried out by N2 adsorption-desorption

experiments and the isotherm obtained for Rh-ETS-10 and Rh-ETS-4 are given in

Fig. 6.4 and 6.5.

0.0 0.2 0.4 0.6 0.8 1.00

20

40

60

80

100 Rh-ETS-10

Vol

ume

adso

rbed

(cm

3 /g) a

t ST

P

Relative pressure (P/P0)

Fig 6.4. N2 adsorption isotherm of Rh-ETS-10

0.0 0.2 0.4 0.6 0.8 1.00

1

2

3

4

5

6

7

8

9

Rh-ETS-4

Vol

ume

adso

rbed

(cm

3 /g) a

t ST

P

Relative pressure (P/P0)

Fig 6.5. N2 adsorption isotherm of Rh-ETS-4

The isotherm of Rh-ETS-10 showed the characteristic adsorption isotherm of

ETS materials (Fig. 6.4). The BET surface area for Rh-ETS-10 is 248 m2/g. The BET

Chapter 6

PhD Thesis 182

surface area of Na-ETS-10 is 278 m2/g and the decrease in surface area after Rh

exchange may be due to the loss in crystallinity, and also the pore blocking by the

transition metal cations sitting closer to the channels. The isotherm of Rh-ETS-4 did

not show the typical microporous material (Fig. 6.5). Also the surface area observed is

very low (10 m2/g) showing that the N2 adsorption at 77 K was not taking the whole

internal surface related to the zeolitic cages. The narrow pore size of the ETS-4 may

be the reason for inaccessibility of N2 molecule at liquid nitrogen temperatures.

Similar observations of very low surface area and abnormal isotherm for ETS-4

samples are reported in literature [54].

6.3.1.4. Scanning electron microscopy (SEM) and EDX analysis

The SEM images of Rh-ETS-4 and Rh-ETS-10 are given in Fig. 6.6. It

showed the well-defined ETS-4 and ETS-10 crystals. These were the characteristic

crystal structure of ETS-4 and ETS-10 materials. Rh-ETS-4 showed a layered

structure stacked on one another whereas the Rh-ETS-10 showed a non-layered well

defined square structure. The size of each particle was in the range of 3 to 5 µm for

both the materials.

Rh-ETS-4

Rh-ETS-10

Fig. 6.6. SEM images of Rh-ETS-4 and Rh-ETS-10

The EDX spectrum of Rh-ETS-10 is given in Fig. 6.7. This shows the

presence of rhodium in the synthesized catalyst. This is further confirmed by the ICP-

AES analysis of the catalysts showing that the Rh-ETS-4 and Rh-ETS-10 contains 3.2

and 4.8 wt% of Rh.

Chapter 6

PhD Thesis 183

Fig. 6.7. EDX spectrum of Rh-ETS-10

6.3.2. Hydroaminomethylation

Hydroaminomethylation was investigated with various olefins and amines as

substrates using Rh-ETS-4 and Rh-ETS-10 catalysts. The possible product formation

of hydroaminomethylation of olefin with an amine using Rh-ETS-4 and/ or Rh-ETS-

10 catalyst is given in Scheme 6.1.

Scheme 6.1. Schematic depiction of the reaction pathways for

hydroaminomethylation over Rh-ETS-4 or Rh-ETS-10 catalyst (R = R’ = R’’ = H,

alkyl or aryl group)

Chapter 6

PhD Thesis 184

The olefin with syngas pressure, catalyst and temperature, first gets

hydroformylated to n- and iso-aldehyde. This formed n-aldehyde then undergoes

condensation with an amine substrate to yield n-enamine (B) and iso-aldehyde gives

iso-enamine (D). The n-enamine (B) gets hydrogenated in presence of H2 with the

help of Rh-ETS-4 or Rh-ETS-10 catalyst to yield n-amine the final product (A).

Under the same reaction protocol formed iso-enamine (D) also gets hydrogenated

thereby forming iso-amine (C). The possible side product of the reaction is the

formylation of amine substrate to give N-formyl product (E). The results obtained for

hydroaminomethylation of various olefins and amines using Rh-ETS-10 is given in

Table 6.2 and hydroaminomethylation using rhodium exchanged ETS-4 is shown in

Table 6.3.

Table 6.2. Hydroaminomethylation using Rh-ETS-10

Olefin Amine %

Conv.

% Selectivity

A B C D E

1-Hexene Morpholine 100 47 - 52 - 1

1-Hexene Hexylamine 100 42 9 41 8 -

1-Hexene Cyclohexylamine 100 49 - 52 - -

1-Hexene Pyrrolidine 100 48 - 52 - -

Cyclohexene Morpholine 100 99 1 - - -

1-Heptene Morpholine 100 50 - 49 - 1

1-Pentene Morpholine 100 51 - 49 - -

1-Hexene Diethylamine 100 43 5 41 8 3

1-Hexene Piperidine 100 48 - 49 - 3

1-Pentene Pyrollidine 100 48 - 50 1 1

Reaction conditions: olefin = 11.9 mmol, amine = 11.9 mmol, catalyst = 10 mg, temp. = 100 ºC, pCO = 13.5 bar, pH2 =54 bar, methanol/toluene = 20/30 mL and time = 4 h. A= n-amine, B = n-enamine/imine, C = iso-amine, D = iso-enamine/imine, E = formylated product of amine substrate.

Chapter 6

PhD Thesis 185

Table 6.3. Hydroaminomethylation using Rh-ETS-4 Olefin Amine %

Conv.

% Selectivity

A B C D E

1-Hexene Morpholine 100 44 - 54 - 2

1-Hexene Hexylamine 100 38 12 39 10 1

1-Hexene Cyclohexylamine 100 46 - 52 1 1

1-Hexene Pyrrolidine 100 45 - 52 - 3

Cyclohexene Morpholine 100 95 4 - - 1

1-Hexene Diethylamine 100 40 3 45 9 3

1-Hexene Piperidine 100 48 1 47 2 2

Reaction conditions: olefin = 11.9 mmol, amine = 11.9 mmol, catalyst = 10 mg, temp. = 100 ºC, pCO = 13.5, pH2 = 54 bar, methanol/toluene = 20/30 mL and time = 4 h. A= n-amine, B = n-enamine/imine, C = iso-amine, D = iso-enamine/imine, E = formylated product of amine substrate.

Both the catalysts are active for the hydroaminomethylation giving 100%

conversion for the studied olefins with high selectivity to amines. The reaction was

completed in 4 h showing that the heterogeneous catalyst was highly active for

hydroaminomethylation. Rh-ETS-4 and Rh-ETS-10 were more effective with

secondary amines like morpholine and pyrrolidine for higher selectivity to amines (n-

and iso-amine). Using Rh-ETS-10, 1-hexene with morpholine gave selectivity of 47%

to n-amine and with pyrrolidine and piperidine gave 48% selectivity to n-amine. 1-

Heptene and 1-pentene with morpholine gave selectivity of 50 and 51% respectively

to n-amine. The primary amine, hexylamine gave higher amounts of n- and iso-imines

of about 17% when 1-hexene was used as the alkene substrate using Rh-ETS-10.

Diethylamine, even being secondary amine, showed lower selectivity to n- and iso-

amine of 84%. Rh-ETS-4 also gave comparable selectivities as that of Rh-ETS-10 for

all the substrates. 1-Hexene with morpholine, pyrrolidine and piperidine gave

selectivities of 44, 45 and 48% respectively towards n-amine in case of Rh-ETS-4. As

expected the hydroaminomethylation of 1-hexene with hexylamine using Rh-ETS-4

gave significant amounts of (22% selectivity) corresponding imines.

Hydroaminomethylation of 1-hexene with diethylamine gave only 95% selectivity to

amines over Rh-ETS-4 catalyst.

Chapter 6

PhD Thesis 186

As compared to the previous chapter (Chapter 5) the reaction completed in

shorter time of 4 h showing the contribution of support towards the

hydroaminomethylation. All the olefins 1-pentene, 1-hexene, 1-heptene and

cyclohexene gave 100% conversion with high selectivity to amines (n and iso) with

various amine substrates. From the initial studies using Rh-ETS-4 and Rh-ETS-10,

showed that the support is having significant effect on hydroaminomethylation. From

the reported studies it is known that the presence of acid in hydroaminomethylation

can enhance the activity [20–22]. Hence the acidic nature of the titanosilicate (ETS-4

and ETS-10) may be responsible for the higher activity of Rh-ETS-4 and Rh-ETS-10.

Also the formation of N-formylated products of the amine substrate is very low in

both catalysts. The secondary amines like morpholine, pyrrolidine and piperidine

were highly active for the hydroaminomethylation of linear alkenes. The

hydrogenation of enamine was faster in these cases. But the use of primary amine

substrate, hexylamine, gave sufficient amounts of the intermediate imine. The

hydrogenation of this imine was slow showing that the hydrogenation of enamine was

faster than that of imines. Rh-ETS-10 was subjected for the detail investigations for

parametric variations using 1-hexene and pyrrolidine as a representative alkene and

amine. The reaction pathway for the hydroaminomethylation reaction of 1-hexene

with pyrrolidine is shown in Scheme 6.2. The reaction of the hydroaminomethylation

of 1-hexene with pyrrolidine was studied as the function of the various reaction

parameters like, catalyst amount, temperature, pressure and the ratio of 1-hexene:

pyrrolidine.

Chapter 6

PhD Thesis 187

CO+H

2, Rh -E TS -1 0, Te mp . C

O+ H

2 , Rh- E T S- 10, T e m

p.

Scheme 6.2. Reaction scheme for hydroaminomethylation of 1-hexene with

pyrrolidine

The hydroaminomethylation of 1-hexene with pyrrolidine proceeds through

the pathways shown in Scheme 6.2. 1-Hexene first undergoes hydroformylation to

form n- and iso-heptanal (2-methyl hexanal). Also the side reaction, isomerization of

1-hexene to 2- and 3-hexene takes place. This formed iso-hexene can again get

hydroformylated to aldehyde (n- and iso-heptanal). The formed aldehyde undergoes

condensation with pyrrolidine to form the enamine. The n-enamine formed from n-

heptanal and pyrrolidine is 1-(hept-1-en-1-yl) pyrrolidine. The iso-enamine formed

from iso-heptanal and pyrrolidine is 1-(2-methylhex-1-en-1-yl) pyrrolidine. These n-

Chapter 6

PhD Thesis 188

and iso-enamines get hydrogenated to 1-heptylpyrrolidine (n-amine) and 1-(2-

methylhexyl) pyrrolidine (iso-amine) respectively. In the further text for simplicity it

is preferred to mention, 2/3-hexene for 2- and 3-hexene, aldehyde for n- and iso-

heptanal, enamine for n- and iso-enamine and amine for n- and iso-amine. The n/iso

ratios are given with respect to the final product amine.

6.3.2.1. Effect of catalyst amount on hydroaminomethylation

The effect of catalyst amount on conversion and selectivity for

hydroaminomethylation of 1-hexene with pyrrolidine was studied over a range of 5-

100 mg at 11.9 mmol of 1-hexene and pyrrolidine, partial pressure of H2 at 60 bar and

CO at 15 bar at 100 ºC in methanol/toluene mixed solvent. The results of effect of

catalyst amount on hydroaminomethylation are given in Table 6.4 and Fig. 6.8.

Table 6.4. Effect of catalyst amount

Entry Catalyst

amount

(mg)

Time

(h)

%

Conv.

% Selectivity

2/3-

hexene

aldehyde amine enamine n/iso

1 5 2 70 31 - 69 - 1.74

4 100 7 - 93 - 1.1

2 10 2 100 19 - 81 - 1.27

4 100 5 - 95 - 1.01

3 50 2 100 9 - 91 - 0.96

4 100 3 - 97 - 0.96

4 100 2 100 1 - 99 - 0.94

Reaction conditions: 1-hexene = 11.9 mmol, pyrrolidine = 11.9 mmol, temp. = 100

ºC, pCO = 13.5 bar, pH2 = 54 bar and methanol/toluene = 20/30 mL.

Even 5 mg of the catalyst was found to be enough for active

hydroaminomethylation giving good conversion and selectivity. At 2 h of reaction

time, 5 mg of the catalyst gave 70% conversion and was increased to 100% for 10 mg

catalyst. Further increase in catalyst amount gave 100% conversion within 2 h. The

selectivity to the product amines (n and iso) is also increased on increasing the

catalyst amount. It can be seen that the selectivity of amine is increased from 69% for

5 mg catalyst to 81% for 10 mg catalyst at 2 h reaction time. This is again increased to

91 and 99% at 2 h for 50 and 100 mg of catalyst respectively.

Chapter 6

PhD Thesis 189

0 20 40 60 80 1000

20

40

60

80

100

% C

onve

rsio

n/se

lect

ivit

y

Catalyst amount, mg

% conversion % iso-hexene % amine

Fig. 6.8. Plot showing the effect of catalyst amount on conversion and selectivity at 2 h; 1-hexene = 11.9 mmol, pyrrolidine = 11.9 mmol, temp. = 100 ºC, pCO = 13.5 bar, pH2 = 54 bar and methanol/toluene = 20/30 mL

The conversion and selectivity to amine was increased further on increasing

the reaction time to 4 h for each catalyst amounts. It is interesting to note that the

selectivity is arising from only two products, iso-hexene and amine. No intermediate

aldehyde or enamines were found throughout the catalyst variation any time. It means

that the formation of enamine and subsequent hydrogenation to amines are too fast

without showing the intermediates. The n/iso ratio of product amine is found to

decrease as the amount of the catalyst was increased. The n/iso ratio for 5 mg catalyst

at 2 h was 1.74 which decreased to 0.94 for 100 mg catalyst. The n/iso ratio was also

decreased as the reaction time is increased. It may be because of the fact that the

increase in catalyst amount will increase the acidic sites of the catalysts which will

increase the formation of iso-hexene and thus the hydroformylation products will be

dominated by iso-aldehyde which in turn will produce more iso-amines. Also as the

time is increased the formed iso-hexene will turn into iso-aldehydes and then to iso-

amines which decreases the n/iso ratio.

6.3.2.2. Effect of 1-hexene: pyrrolidine on hydroaminomethylation

The effect of the substrates ratio (olefin to amine) is a parameter that must

have an effect on hydroaminomethylation. The effect of 1-hexene: pyrrolidine ratio on

hydroaminomethylation was studied for ratios 1:1 to 1:3 at conditions of 11.9 mmol

of 1-hexene, catalyst amount of 10 mg, partial pressure of H2 and CO at 60 and 15 bar

Chapter 6

PhD Thesis 190

respectively at temperature of 100 ºC. The results are given in Table 6.5 and Fig. 6.9.

It is found that as the 1-hexene: pyrrolidine ratio is increased from 1:1 to 1:3, the

conversion got decreased. The conversion of 100% for 1-hexene: pyrrolidine ratio of

1:1 got decreased to 75% for the ratio of 1:4 at 2 h reaction time. The reason may be

the increased adsorption of pyrrolidine on the ETS surface due to increase in the

concentration. This may cause the lower adsorption of 1-hexene on the surface and

thus decreases the conversion. But the selectivity to amine was increased as the 1-

hexene: pyrrolidine ratio is increased. The selectivity to the product amine got

increased from 81% (1-hexene: pyrrolidine ratio of 1:1) to 92% (1-hexene:

pyrrolidine ratio 1:4) at 2 h. The similar trend was observed when the time is

increased to 4 h.

Table 6.5. Effect of 1-hexene: pyrrolidine ratio

Entry 1-Hexene:

pyrrolidine

ratio

Time

(h)

%

Conv.

% Selectivity

2/3-

hexene

aldehyde amine enamine n/iso

1 1:1 2 100 19 - 81 - 1.27

4 100 5 - 95 - 1.01

2 1:2 2 91 12 - 88 - 1.13

4 100 5 - 95 - 0.92

3 1:3 2 84 10 - 90 - 1.0

4 100 4 - 96 - 0.9

4 1:4 2 75 8 - 92 - 0.92

4 100 2 - 98 - 0.85

Reaction conditions: 1-hexene = 11.9 mmol, catalyst = 10 mg, temp. = 100 ºC, pCO = 13.5 bar, pH2 = 54 bar and methanol/toluene = 20/30 mL.

Chapter 6

PhD Thesis 191

1:1 1:2 1:3 1:40

20

40

60

80

100

% C

onve

rsio

n/se

lect

ivit

y

hexene: pyrrolidine ratio

% conversion % iso-hexene % amine

Fig. 6.9. Plot showing the effect of 1-hexene: pyrrolidine ratio on conversion and selectivity at 2 h; 1-hexene = 11.9 mmol, catalyst = 10 mg, temp. = 100 ºC, pCO = 13.5 bar, pH2 = 54 bar and methanol/toluene = 20/30 mL

The intermediates aldehyde and enamines were not observed in any of the

varied ratios. The reason for decrease in the iso-hexene on increasing the pyrrolidine

may be because of the higher adsorption of pyrrolidine on the acidic ETS surfaces

which may decrease the adsorption of 1-hexene and thus the isomerization of 1-

hexene over ETS surface is hindered. This also led to the lower conversion on

increasing the ratio. The n/iso ratio of product amine was decreased as the ratio is

increased. This may be because of the higher isomerization of 1-hexene occurring

during the coordination of 1-hexene with active Rh center. Also there is possibility of

formation of Rh-pyrrolidine complexes, which may have property of isomerization of

alkene.

6.3.2.3. Effect of ratio of partial pressure of H2 and CO on hydroaminomethylation

The effects of ratio of partial pressure of H2 and CO were determined by

varying H2/CO ratio from 1:1 to 4:1 (15/15 to 60/15 bar) and the results are given in

Table 6.6 and Fig. 6.10, at 1-hexene and pyrrolidine of 11.9 mmol each, catalyst

amount of 10 mg and at 100 ºC. The conversion got increased on increasing the ratio

of the partial pressures of H2 and CO. At the H2/CO ratio of 1:1 (Table 6.6, Entry 1)

the conversion was 30% at 2 h which increased to 98% for H2/CO ratio of 3:1 (Table

6.6, Entry 3). Further increase in the H2/CO ratio increased the conversion to 100% at

Chapter 6

PhD Thesis 192

2 h. The conversion levels were also increased on increasing the time of

hydroaminomethylation.

Table 6.6. Effect of ratio of partial pressure of H2 and CO

Entry H2:CO

(bar)

Time

(h)

%

Conv.

% Selectivity

2/3-

hexene

aldehyde amine enamine n/iso

1 1:1 2 30 63 10 4 23 3.0

4 64 56 8 6 30 2.1

2 2:1 2 58 62 7 13 18 2.4

4 97 35 - 58 5 1.18

3 3:1 2 98 32 - 68 - 1.55

4 100 13 - 87 - 1.12

4 4:1 2 100 19 - 81 - 1.27

4 100 5 - 95 - 1.01

Reaction conditions: 1-hexene = 11.9 mmol, pyrrolidine = 11.9 mmol, catalyst = 10 mg, temp. = 100 ºC and methanol/toluene = 20/30 mL.

1:1 2:1 3:1 4:1 --0

20

40

60

80

100

% C

onve

rsio

n/se

lect

ivit

y

H2:CO ratio

% conversion % iso-hexene % aldehyde % amine % enamine

Fig. 6.10. Plot showing the effect of H2:CO ratio on conversion and selectivity at 2 h; 1-hexene = 11.9 mmol, pyrrolidine = 11.9 mmol, catalyst = 10 mg, temp. = 100 ºC and methanol/toluene = 20/30 mL

For the H2/CO ratio of 1:1 the conversion increased from 30% at 2 h to 64% at

4 h. The more and significant effect of H2/CO ratio is more pronounced on the

selectivity. The selectivity increased on increasing the H2/CO ratio. Higher selectivity

Chapter 6

PhD Thesis 193

is observed for iso-hexenes (2/3-hexene) towards the lower H2/CO ratio. Selectivity of

63% and 62% were observed towards iso-hexene at lower H2/CO ratios of 1:1 and 2:1.

The lower ratios of H2/CO also gave significant amounts of aldehyde and

enamine showing the effect of ratio of H2/CO on hydroaminomethylation. No

aldehyde or enamine intermediates were observed for higher H2/CO ratios of 3:1 and

above. The formation of amine was very low at H2/CO ratio of 1:1. Only selectivity of

4 and 6% was obtained at 2 and 4 h respectively. This clearly indicates the inefficient

ratio which has no potential to hydrogenate the enamine to form amines. The 23% and

30% of enamine at 2 and 4 h respectively for H2/CO ratio of 1:1 confirms this

inefficient hydrogenation of enamine. Also the smaller amounts of aldehyde found

towards lower H2/CO ratio may be due to this inefficient hydrogenation of enamine.

Because the formation of enamine from an aldehyde and amine is a reversible

reaction and hence the presence of water produced from enamine formation can make

the enamine to reverse the reaction to form aldehyde again [55]. The

hydroaminomethylation is done in a closed high pressure reactor and hence the

formed water is not getting removed from the reaction medium. This water reverses

the enamine to aldehyde because the enamine was not hydrogenated efficiently to the

product amine. The n/iso ratio of product amine was decreased as the ratio of H2/CO

was increased. It may be because of the better hydrogenation of n-enamine than iso-

enamine at lower H2/CO ratio.

6.3.2.4. Effect of total pressure on hydroaminomethylation

The effect of total pressure by keeping the H2/CO ratio at 4:1 was studied at

pressure ranging from 37.5 to 80 bar at 1-hexene and pyrrolidine of 11.9 mmol,

catalyst amount of 10 mg and temperature of 100 ºC and the results are shown in

Table 6.7 and Fig. 6.11. The increase in total pressure had significant effect towards

selectivity to amine. The total pressure had not much significant effect on conversion

when the ratio of H2/CO kept constant. A conversion of 98% was obtained for the

pressure of 37.5 bar at 2 h and all other conversions were reached 100%. The

selectivity to amine was found to be increasing as the pressure is increased.

Selectivity of 62% for pressure of 37.5 bar got increased to 81% for pressure of 67.5

bar at 2 h. The similar trend was observed at higher time of 4 h also. The n/iso ratio

was decreased as the total pressure was increased. At lower pressure the selectivity of

Chapter 6

PhD Thesis 194

iso-hexene was higher leading to formation of iso-aldehyde and subsequently iso-

amines results in lower n/iso ratio.

Table 6.7. Effect of total pressure at constant H2:CO ratio of 4:1

Entry Pressure

(bar)

Time

(h)

%

Conv.

% Selectivity

2/3-

hexene

Aldehyde Amine Enamine n/iso

1 37.5 2 98 38 - 62 - 0.96

4 100 24 - 76 - 1.05

2 55 2 100 31 - 69 - 1.12

4 100 10 - 90 - 1.0

3 67.5 2 100 19 - 81 - 1.27

4 100 5 - 95 - 1.01

4 80 2 100 5 - 95 - 1.48

Reaction conditions: 1-hexene = 11.9 mmol, pyrrolidine = 11.9 mmol, catalyst = 10 mg, temp. = 100 ºC and methanol/toluene = 20/30 mL.

40 60 800

20

40

60

80

100

% C

onve

rsio

n/se

lect

ivit

y

Total pressure (H2+CO), bar

% conversion % iso-hexene % amine

Fig. 6.11. Plot showing the effect of total pressure on conversion and selectivity at 2 h; 1-hexene = 11.9 mmol, pyrrolidine = 11.9 mmol, catalyst = 10 mg, temp. = 100 ºC and methanol/toluene = 20/30 mL

6.3.2.5. Effect of temperature on hydroaminomethylation

The effect of temperature on hydroaminomethylation was studied in the

temperature range of 80 to 140˚C at 1-hexene and pyrrolidine of 11.9 mmol each,

Chapter 6

PhD Thesis 195

catalyst amount of 10 mg, H2/CO of 60/15 bar in methanol-toluene mixture of 20/30

mL and the results are shown in Table 6.8 and Fig. 6.12.

Table 6.8. Effect of temperature

Entry Temp. Time,

h

%

Conv.

% Selectivity

2/3-

hexene

aldehyde amine enamine n/iso

1 80 2 59 40 15 41 4 1.6

4 86 25 13 60 2 1.18

2 100 2 100 19 - 81 - 1.27

4 100 5 - 95 - 1.01

3 120 2 100 16 - 84 - 1.21

4 100 3 - 97 - 1.04

4 140 2 100 15 - 85 - 1.18

4 100 2 - 98 - 0.97

Reaction conditions: 1-hexene = 11.9 mmol, pyrrolidine = 11.9 mmol, catalyst = 10 mg, pCO = 13.5 bar, pH2 = 54 bar and methanol/toluene = 20/30 mL.

80 90 100 110 120 130 1400

20

40

60

80

100

% C

onve

rsio

n/se

lect

ivit

y

Temperature, oC

% conversion % iso-hexene % aldehyde % amine % enamine

Fig. 6.12. Plot showing the effect of temperature on conversion and selectivity at 2 h; 1-hexene = 11.9 mmol, pyrrolidine = 11.9 mmol, catalyst = 10 mg, pCO = 13.5 bar, pH2 = 54 bar and methanol/toluene = 20/30 mL

The conversion got increased on increasing the temperature from 80 to 100 ºC.

From 100 ºC and above the conversion was 100%. The selectivity was influenced by

Chapter 6

PhD Thesis 196

the increase in temperature. At lower temperature of 80 ºC the selectivity to amine

was low. Significant amount of aldehyde formation and also smaller amounts of

enamine was observed. The selectivity to amine was 41% only at 80 ºC and increased

to 81% at 100 ºC at 2 h of reaction. The n/iso ratio was decreased on increasing the

temperature. The n/iso ratio of 1.18 at 80 ºC was dropped to 0.97 at 140 ºC for 4 h

reaction time. The n/iso ratio was higher at lower reaction time of 2 h. The decrease in

n/iso ratio with temperature may be because of the higher formation of iso-heptanal at

higher temperature.

6.3.2.6. Catalyst recycling

The catalyst was recycled to ensure the heterogeneous characteristic of the

catalyst. The Rh-ETS-10 catalyst was recycled at 100 ºC using 1-hexene and

pyrrolidine of 11.9 mmol each, catalyst amount of 10 mg, partial pressure of H2 and

CO of 60 and 15 bar respectively in a methanol/toluene mixture of 20/30 mL. The

catalyst was recycled by filtration with subsequent washing with methanol/toluene

mixed solvent. The results of recycling are shown in Table 6.9.

Table 6.9. Recycling with Rh-ETS-10

Entry Recycle

Time

(h)

%

Conv.

% Selectivity

2/3-

hexene

aldehyde amine enamine n/iso

1 Fresh

catalyst

2 100 19 - 81 - 1.27

4 100 5 - 95 - 1.01

2 First

recycle

2 100 18 - 82 - 1.30

4 100 3 - 97 - 1.04

3 Second

recycle

2 99 19 - 81 - 1.30

4 100 4 - 96 - 1.00

4 Third

recycle

2 98 20 - 80 - 1.29

4 100 5 - 95 - 1.02

Reaction conditions: 1-hexene = 11.9 mmol, pyrrolidine = 11.9 mmol, catalyst = 10 mg, temp. = 100 ºC, pCO = 13.5 bar, pH2 = 54 bar and methanol/toluene = 20/30 mL.

The catalyst had shown good recyclability ensuring the stability of the catalyst.

The ion-exchange process is very stable under the employed reaction conditions. The

conversion of 100% was obtained for all the recycles at 4 h. A slight decrease in

Chapter 6

PhD Thesis 197

conversion was obtained for higher recycles which was pronounced at 2 h of reaction

time. This slight decrease in conversion may be attributed to the loss of some

adsorbed Rh that is not ion-exchanged. But no loss of Rh metal was observed by the

ICP technique which was analyzed from the reaction mixture. Therefore the amounts

of Rh that are simply adsorbed are considered to be very low and are out of the ICP

detection limits. The selectivity of the product amine was unaffected by the repeated

usage of the catalyst. The n/iso ratio was also not much affected by the catalyst

recycles ensuring the robust and stable nature of the catalyst.

6.3.2.7. Reaction kinetics

The catalyst Rh-ETS-10 was subjected to investigate the reaction kinetics of

the hydroaminomethylation reaction. The kinetics was investigated with 11.9 mmol

each of 1-hexene and pyrrolidine, catalyst amount of 10 mg, partial pressure of CO

and H2 at 13.5 and 54 bar respectively at 100 ºC. From the conversion and selectivity

the concentration (M) of 1-hexene and the products, amine and 2/3-hexene, were

determined. The corresponding kinetic profile of the consumption of 1-hexene and

formation of products are given in Fig. 6.13.

0.0 0.5 1.0 1.5 2.00

5

10

15

20

25

[Con

c], M

x102

Time, h

1-hexene 2/3-hexene amine

Fig. 6.13. Kinetic profile for the consumption of 1-hexene and formation of products; 1-hexene = 11.9 mmol, pyrrolidine = 11.9 mmol, catalyst = 10 mg, pCO = 13.5 bar, pH2 = 54 bar, temp. = 100 ºC and methanol/toluene = 20/30 mL

The 1-hexene was consumed as the reaction was progressed with time with

formation of products and attained complete conversion in 2 h. The products were

Chapter 6

PhD Thesis 198

distributed between isomerized hexene, 2- and 3-hexene (2/3-hexene), and the desired

product, amine. The catalyst was highly active and gave higher amount of amine. The

formation of 2/3-hexene got increased on increasing the time and then started

decreasing as the 1-hexene got completely consumed and also the iso-hexene started

forming aldehyde and subsequently to amine. There was no intermediate aldehyde or

enamine detected showing the faster reaction of these intermediates into the product

amine.

The rates of consumption of 1-hexene and formation of 2/3-hexene and amine

were calculated form the slope of early linear portions of the corresponding curves in

the kinetic profile from the following equations.

Rate of consumption of 1-hexene, v = -d[1-hexene]/dt (6.1)

Rate of formation of 2/3-hexene, v1 = d[2/3-hexene]/dt (6.2)

Rate of formation of amine, v2 = d[amine]/dt (6.3)

The rates of reaction were calculated using Eq. (6.1) to (6.3). The rate of

consumption of 1-hexene (v) or the overall rate of reaction was found to be 17.14 x

10-2 M/h. The rate of formation of 2/3-hexene (v1) gave 5.65 x 10-2 M/h and amine

(v2) gave 11.48 x 10-2 M/h. This clearly showed that the initial rate of formation of

amine was double than that of formation of 2/3-hexene.

6.4. Conclusions

The rhodium exchanged titanosilicates (ETS-10 and ETS-4) were synthesized

and characterized. These rhodium exchanged titanosilicates were found to be efficient

and stable catalysts for hydroaminomethylation reaction. The formation of ETS is

confirmed by XRD, FT-IR and SEM analysis. From the XRD, it has been found that

there was a slight drop in crystallinity of the support on exchange with rhodium. The

exchange of rhodium was confirmed by ICP-AES and EDX measurements. About 4.8

and 3.2 wt% of Rh was exchanged in Rh-RTS-10 and Rh-ETS-4 catalysts. Both the

catalysts were active for hydroaminomethylation and various olefin and amine

substrates are investigated. The catalyst was highly active and selective towards

product amine within lower reaction time of 4 h. 1-Hexene and pyrrolidine are taken

as model substrates for parametric variations using Rh-ETS-10 catalyst.

Chapter 6

PhD Thesis 199

The catalyst amount, 1-hexene: morpholine ratio, ratio of partial pressures of

H2 and CO, total pressure and temperature are investigated in detail. All these

parameters have significant effect on conversion and/or selectivity. The increase in

catalyst amount increased both the conversion and selectivity. The increase in

morpholine ratio gave some significant information showing a decrease in conversion

with increase in selectivity. The reason may be the higher adsorption of pyrrolidine on

the acidic ETS surfaces which may decrease the adsorption of 1-hexene giving lower

conversions. The ratios of H2 and CO have highly influenced the

hydroaminomethylation and gave that a ratio of 1:4 is the best under the studied

conditions. Also a higher total pressure gave best results than the lower pressures.

Increase in temperature gave higher conversion and selectivity. As a whole the

identification of intermediate aldehyde and amine was minimal showing the high

activity of the catalyst towards hydroaminomethylation. The kinetics of the reaction

showed that the initial rate of formation of amine was double than that of the

isomerization to 2/3-hexene. The catalyst is recycled effectively without much loss in

its activity and selectivity.

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