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Chapter IVChapter IVChapter IVChapter IV

Benzene hydroxylation over Cu-containing LDHs:

Influence of co-bivalent ions and solvent

Appl. Clay. Sci., doi:10.1016/j.clay.2011.01.024 (2011)

� ~10 atom% Cu containing ZnAl-LDH showed higher activity

compared to MgAl, NiAl or CoAl containing LDHs

� XPS analysis confirmed the formation of active Cu+ for CuZnAl-

LDH

� STEM analysis confirms the presence of highly dispersed Cu2+ in

the CuZnAl-LDH

� Addition of sulfolane as co-solvent increased the phenol

selectivity

Benzene hydroxylation over Cu-containing

LDHs: Influence of co-bivalent ions and

solvent

4.1 Introduction

4.1.1 Sulfonation process

4.1.2 Chlorination process

4.1.3 Toluene-benzoic acid process

4.1.4 Cumene process

4.1.5 Direct hydroxylation of benzene

4.1.5.1 Molecular oxygen

4.1.5.2 Nitrous oxide

4.1.5.3 Hydrogen peroxide

4.2 Experimental

4.2.1 Sample preparation

4.2.2 Benzene oxidation

4.3 Results and discussion

4.4 Conclusions

4.5 References

Benzene hydroxylation Chapter-IV

71

4.1 Introduction

Phenol is common name of hydroxy benzene (C6H5OH) and belongs to the

class of compounds, containing one or more hydroxyl groups attached to an aromatic

ring. Phenol has also been called carbolic acid, phenolic acid, phenylic acid or

oxybenzene. The history of phenol goes back to 1834, when it was first isolated from

coal tar and named carbolic acid. Until the advent of synthetic phenol production, just

before World War-1, coal tar remained as the only source of phenol. In the early years

of the 20th

century, Belgian chemist entrepreneur Leo Baekeland created the world's

first completely synthetic plastic. Baekeland mixed phenol and formaldehyde,

subjected them to heat, pressure and produced the sticky, amber-colored resin named

as Bakelite [1]. Due to this finding requirement of phenol increased and forced the

synthetic production of phenol from other petroleum fraction. Several industrial

applications have been developed during this century based on phenol and their

derivatives. Demand for phenol and their derivatives has been the driving force to

discover new technologies. Worldwide production of phenol was more than 5.5

megatons in 2000 and 7.2 megatons in 2006 and it’s continuous to grow.

4.1.1 Sulfonation process

The first synthetic phenol was produced by sulfonation of benzene and

hydrolysis of sulpfonate (Scheme 4.1, BASF- 1899). More than 99% of phenol was

produced worldwide in the 1890s from synthetic processes [2].

Scheme 4.1 Sulfonation process

In spite of aggressive reagents and large amounts of waste sodium sulphite, this

process was used for about 80 years.


72

4.1.2 Chlorination process

In 1924, a new chlorination process was put in operation by Dow Chemicals in the

USA:

Scheme 4.2 Chlorination process

Independently, I. G. Farben industrie Co., developed a similar technology in Germany

(Scheme 4.2).

4.1.3 Toluene-benzoic acid process

Dow-Canada Ltd., first introduced the toluene benzoic acid process in 1961.

It accounts for 5% of the total synthetic phenol capacity in the world today (Scheme

4.3). The three chemical reactions in the toluene- benzoic acid process are oxidation

of toluene to form benzoic acid, oxidation of benzoic acid to form phenyl benzoate

and hydrolysis of phenyl benzoate to form phenol. A typical process consists of two

steps, in the first step, the oxidation of toluene to benzoic acid is achieved with air and

cobalt salt catalyst at a temperature 121-177 oC, the reactor operated at 206 kPa and

the catalyst concentration is between 0.1 and 0.3%, the reactor effluent is distilled and

the purified benzoic acid is collected. The overall yield of this process is believed to

be about 68 mol% of toluene.

Scheme 4.3 Toluene- benzoic acid process

The second processing step, in which benzoic acid is oxidized and hydrolyzed

to phenol, is carried out in two reactors in series. In the first reactor, the benzoic acid

is oxidized to phenyl benzoate in the presence of air and a catalyst mixture of copper


73

and magnesium salts. The reactor is operated at 240 oC and 147 kPa. The phenyl

benzoate is then hydrolyzed with steam in the second reactor to yield phenol and CO2.

The overall yield of phenol from benzoic acid is around 88 mol%.

4.1.4 Cumene process

The cumene process was developed and first realized in the former Soviet

Union (Scheme 4.4), 95% of phenol is produced now using this method. The route

was discovered in 1942 by a group of excellent chemists P. G. Sergeev, R. Yu. Udris

and B. D. Kruzhalov. In 1949, the first industrial plant was put in operation in

Dzerzhinsk city. There are several licensed processes to produce phenol, which are

based on cumene. Benzene on reaction with propene to give cumene, which is

oxidized with air to form cumene hydroperoxide and cumene hydroperoxide is

cleaved to yield phenol and acetone. In this process, approximately 0.46 kg of acetone

and 0.75 kg of phenol are produced per kg of cumene feedstock.

Scheme 4.4 Cumene process

Oxidation of cumene to cumene hydroperoxide is usually achieved in three to

four oxidizers in series, where the fractional conversion is about the same for each

reactor. Fresh cumene and recycled cumene are fed to the first reactor. Air is bubbled

in at the bottom of the reactor and leaves at the top of each reactor. The oxidizers are

operated at low to moderate pressure. Due to the exothermic nature of the oxidation

reaction, heat is generated and must be removed by external cooling. A portion of


74

cumene reacts to form dimethyl benzyl alcohol and acetophenone. Methanol is

formed in the acetophenone reaction and is further oxidized to formaldehyde and

formic acid. A small amount of water is also formed by various reactions. The

selectivity of the oxidation reaction is a function of oxidation conditions, temperature,

conversion level, residence time and oxygen partial pressure. Typical commercial

yield of cumene hydroperoxide is about 95 mole percent in the oxidizers. The reaction

effluent is striped off un-reacted cumene, which is then recycled as feedstock. Spent

air from the oxidizers is treated to recover 99.99% of the cumene and other volatile

organic compounds. Due to environmental consideration, many phenol plants are

equipped with a special water treatment facility where acetone and phenol are

recovered from the wastewater stream.

4.1.5 Direct hydroxylation of benzene

The market growth is quite different for phenol and acetone produced from

cumene process, so much effort is currently being devoted to decouple their

productions and the day-to-day increasing demand of phenol lead researches to find

new economical and environmental friendly processes. The practical development of

the one-step production of phenol will have advantages in cost reduction and energy

saving. To achieve direct hydroxylation of aromatic compounds, a neutral oxygen

species or radical oxygen species, such as HO. or HOO

., might work as the active

species. The efficient production of active oxygen species on a catalyst remains

difficult which can be obtained through three different methods

Molecular oxygen (O2) as oxidizing agent in presence of different reducing

agents like H2, NH3, CO, dithioalcohols and ascorbic acid etc.

Nitrous oxide (N2O) as oxidizing agent.

Hydrogen peroxide (H2O2) as oxidizing agent.

4.1.5.1 Molecular oxygen

Benzene hydroxylation using oxygen requires the activation of C–H bonds in

the aromatic ring and the subsequent insertion of oxygen. Most heterogeneous

catalysts that contain transition metals, however, supply an oxygen species that has a

negative charge, such as O-, O2

- and O

2-, all of which cause oxidation of benzene via

the electrophilic reaction mechanism [3]. More than 100 years ago, Friedel and Crafts


75

examined the conversion of benzene to phenol using oxygen, these experiments were

conducted in the gas phase in the presence of aluminium chloride [4]. Later studies

include molybdenum, tungsten, copper, uranium, and vanadium oxides as catalysts [5-

7]. The selectivity of phenol obtained was very low and hence they were of no

importance with regard to practical applications. Investigations on liquid phase

oxidation in the presence of CuCl [8], iron or palladium in heteropoly acids [9, 10] or

palladium acetate [11] also gave only low selectivities. Table 4.1 shows literature

results of benzene hydroxylation using O2 as oxidant. Tsuruya et al. worked diversely

on this reaction using O2 as oxidizing agent and ascorbic acid as reducing agent, at

>0.5 MPa and different temperatures. They used both vanadium and copper supported

materials like V/SiO2, CuO–Al2O3, CuHZAM-5, V/Al2O3, V-HPAs and Cu/Ti/H-

ZSM-5 as catalysts among them V containing materials showed higher phenol yield

as compared to Cu containing materials [12]. Both V and Cu substituted materials

showed the leaching of active materials in different quantities. Several other materials

like titanium silicate, microporous SBA-15 containing Mo or V heteropolyacid and

organo-transition metal incorporated on solid matrix also found to be active for

phenol production [13-15].

Table 4.1 Catalytic hydroxylation of benzene using O2

S.

No

Catalyst Phenol

yield %

(Convers

ion %)a

Reducing

agent

Phenol

selectivi

ty %

Conditions Ref.

1 V (2wt%) / SiO2 3.8 Ascorbic acid - 333 K / 24 h [16]

2 CuO–Al2O3 0.9 Ascorbic acid - 303 K / 5 h [17]

3 Pt / Pd on Amberlyst

or Nafion / silica membrane

7.6 H2 56 303 K / 0.7–

1.4 MPa

[18]

4 V / Al2O3 8.2 Ascorbic acidd - 303 K / 0.4

MPa

[19]

5 CuH-ZSM-5 1.2b - - 673 K [20]

6 Pd / porous alumina 13.3 H2 85.3 523 K [21]

7 VCl3 0.3 Ascorbic acid 65 323 K / 17 h [22]

8 CuH-ZSM-5 1.3c - - 673 K [23]

9 4 µm-thick palladium film on porous

alumina tube

19-23 H2 77 473 K [24]

10 Pulse

DC corona discharge

2.2 - 28 kV / 100

Hz

[25]

11 V / 5% Pd / Al2O3 0.02 H2, Acetic acid 99 338 K / 2 h /

0.69 MPa

[26]

12 Cu–Ca phosphates 1.3 NH3 96 - [27]

13 Cu/Al2O3 2.1 Acetic acid - 303 K / 24 h / [28]


76

aBenzene conversion%, b 3% CO2 and 1.8% CO , c 2.1% CO2, 1.2% CO, d 37.6% V leaching observed.

Kubacka et al. achieved phenol yield of 10% using Cu-ZSM-5 catalyst and

both O2 and O2/H2 mixtures as oxidant. Followed by them, Gu et al. showed highest

phenol yield of 27% using VOx/CuSBA-15 as catalyst. However to overcome the

0.1 MPa

14 [ReO4] / H-ZSM-5 - NH3 40 0.1 MPa [29]

15 Copper-hydroxyapatite

3 97 723 K / 5 h [30]

16 Cu-ZSM-5 1.7 Air 26 673 K [31]

17 1% Pt–20% PMo12 / SiO2

-(0.3 ) H2 98 473 K [32]

18 [(C4H9)4N]5[PW11Cu

O39(H2O)]

-(9.2) Ascorbic acid 91.8 323 K / 12 h /

1 MPa

[33]

19 Pt / V / ZrO2 -(59.2) Acetic acid 13.9 423 K / 3.5 MPa

[34]

20 Pd membrane reactor -(15) H2 95 423 K [35]

21 Cu / H-ZSM-5 3.3 H2O 30.1 673 K [36]

22 V-HPAs 6 Ascorbic acid - 303 K / 24 h / 0.4 MPa

[37]

23 V / Al2O3 -(4.7) Ascorbic acid 423 K / 1 MPa [38]

24 Pd(Oac)2 /

Heteropolyacid (HPA)

-(12) LiOAc 74.8 393 K [39]

25 V-Substituted

Heteropoly Acid

0.9 Acetic acid 353 K / 24 h /

0.4 MPa

[40]

26 Re10 cluster on H-ZSM-5

- NH3 90.6 553 K [41]

27 Cu-ZSM-5 10 Without and

with H2

673 K [42]

28 VOx / CuSBA-15 27 Ascorbic acid 100 353 K / 5 h / 0.7 MPa

[43]

29 MEMS-based Pd

membrane

20 (56) H2 Low

level

473 K [44]

30 Pd–silicalite-1 composite membrane

2.9 (4.8) H2 100 473 K [45]

31 Py3PMo10V2O40 11.5 Ascorbic acid 100 373 K / 10 h /

2.0 MPa

[46]

32 VCl3 ( reaction-extraction-

regeneration system)

- - - - [47]

33 Schiff base manganese complex -

molybdovanadophos

phoricheteropolyacid

12.2 Ascorbic acid - 373 K / 10 h / 2 MPa

[48]

34 Cu / Ti / H-ZSM-5 4 - 673 K [49]

35 Laurylamine

modified

H5PMo10V2O40

11.6 Ascorbic acid 373 K / 10 h /

2 MPa

[50]

36 Vanadium oxide nano plate

3.7 Acetic acid 100 423 K / 10 h / 1 MPa

[51]


77

leaching of active metal ion from the material [52], which was commonly observed in

benzene hydroxylation, Laufer et al. used the novel Pt/Pd supported on acid resins

and showed best phenol yield of 7.6% in a semi continuous flow reactor. Rhenium

supported metal oxide was also reported for this reaction using ammonia as reducing

agent [53] and vanadium complex inclusion zeolite as catalyst and saccharides as

reducing agents was also used for the synthesis of phenol in acetic acid water solution

as solvent [54].

Pioneering work on hydroxylation using palladium membrane was done in

2002, Mizukami et al. [21] from Japan used Pd/porous alumina membranes for the

continuous production of phenol from benzene using H2/O2 mixture. Since H2 and O2

were not mixed simultaneously it has the advantage of low probability of causing

detonating gas reaction and phenol yield of 1.5 Kg per Kg of catalyst per hour at

150 oC was achieved. Followed by them Itoh et al. [24] used 4 µm thick palladium

film formed on a porous alumina tube as membrane and showed that appropriate

H2/O2 ratio giving a larger benzene yield (Fig 4.1).

Fig. 4.1 Palladium membrane reactor (reproduced from ref. 24).

This novel idea improved the phenol yield up to 20% at 473 K. Several other

works are also reported on palladium membranes for benzene hydroxylation; however

the stability of the Pd membrane is still a challenge for longer operations [35]. Guo et

al. prepared Pd membranes of excellent stability and flux with multi-legged anchors

by electroless plating of Pd on a porous support modified by a zeolite barrier layer

containing Pd seeds [45].

Ye et al. improved the stability of the Pd membrane by using a Pd membrane

made by chemical vapour decomposition (CVD) or a Pd-Ag membrane instead of the

Pd membrane made by sputtering in the Pd membrane micro reactor [44]. Recently


78

Shu et al. observed that, the surface area of the dense palladium membrane is low, it

cannot be sufficiently utilized because of the hydrogen combustion and concluded

that the membrane concept is not effective for the one step hydroxylation of benzene

to phenol with hydrogen and oxygen [55].

Bortolotto et al. used novel microstructured membrane reactor with distributed

dosing of hydrogen and oxygen with palladium-coated Pd/Cu foil membrane. The

highest phenol selectivity obtained at 423 K was 9.6% with carbon dioxide being the

dominant product [56].

Advantages

Low cost

Environmentally friendly

Disadvantages

Usage of reducing agents like CO, NH3 and ascorbic acid are industrially not

attractive

Oxygen-hydrogen mixture in a large-scale synthesis is not preferred

Low selectivity - over oxidation to maleic acid and CO2

4.1.5.2 Nitrous oxide

N2O is one of the best mono oxygen donor used in the literature, the O- ion

can be formed by surface decomposition of N2O in the presence of available electrons

[57, 58], so that catalytic oxidation reactions of hydrocarbons using N2O as an oxidant

are of interest in connection with usual oxidations by O2. Pioneering work on benzene

hydroxylation using N2O was done by Iwamoto et al. in 1983 [59], where they

showed O2 oxidation leads to the cleavage of C-C bond to maleic acid or maleic

anhydride where as N2O oxidation selectively forms the phenol (Scheme 4.5). They

used V2O5/SiO2 as catalyst at 823 K and showed 11% conversion of benzene with

45% phenol selectivity.

Scheme 4.5 Hydroxylation of benzene using N2O


79

In 1988, three groups of researchers: Suzuki et al. [60] from Japan (Tokyo

Institute of Technology), Gubelmann et al. [61, 62] from France (Rhone Poulenc

Co.), and G. I. Panov et al. from Boreskov institute of catalysis (BIC) [63-66]

independently discovered that ZSM-5 zeolites are the best catalysts for this reaction.

In ZSM-5 zeolites, reaction proceeded at much lower temperature and more

importantly, with selectivity approaching 100% [67]. Fe-ZSM-5 with 0.055 wt% of

Fe showed 27% benzene conversion with 98% phenol selectivity at 623 K [68]. In

subsequent years several patents published in nitrous oxide hydroxylation of benzene

[69-75] using several catalysts.

In Fe-ZSM-5, ‘α-sites’ found to be the active sites for this reaction and

minimum concentration of Fe atom were needed for the generation of ‘α-sites’. The

mechanism of formation of active oxygen species, their interaction with the catalyst

and mechanism of phenol hydroxylation were well reviewed recently by G. I. Panov

[67]. Solutia Inc., is a USA chemical company was one of the major world producers

of adipic acid, an intermediate for Nylon 6,6, and BIC jointly made a process called

AlphOx (after the name of α-oxygen). They used waste nitrous oxide from adipic acid

plant for the oxidation of benzene to phenol, incorporating this reaction as a key stage

in a new modified adipic acid production (scheme 4.6).

Scheme 4.6 AlphOx process

The reaction was performed in a simple adiabatic reactor, an essential feature

of this process economy. The process has also very good environmental features.

Instead of spending efforts and money on N2O neutralization, the new phenol process

used N2O as a valuable raw material, thus mimicking the waste less strategy of nature.

Followed by these findings several other materials like Fe/Mo/DBH (partially

deboronated borosilicate) synthesized through chemical vapor deposition method

[76], unsupported and supported FePO4 [77], Fe β-zeolites (where coke formation was

higher), iron-exchanged ferrierite [78], copper–calcium–phosphates [79], tetrahedrally


80

coordinated iron in framework substituted microporous AlPO-5 (gave less selectivity)

[80] and iron-functionalized Al-SBA-15 [81] were studied however they failed to give

either good conversion or selectivity compared to Fe-ZSM-5.

To further improve this process, ZSM-5 coated on stainless steel grids [82],

Fe-ZSM-5 coated stainless-steel supports in micro reaction engineering studies [83],

(NH4)2SiF6-modified iron exchanged ZSM-5 [84], Fe/ZSM-5 prepared by chemical

vapor deposition [85], Fe-ZSM-5 sintered metal fibers composites as catalytic beds

[86], hierarchical Fe/ZSM-5 with a strongly improved lifetime [87], wet milling of H-

ZSM-5 zeolite [88] and microwave radiation during the reaction [89] were used and

obtained improved conversions and selectivity. Meloni et al. showed that the addition

of steam to the feed increased to the benzene conversion over Fe-ZSM-5 catalyst with

improved selectivity and durability [90].

Several works were done to find out mechanism and nature of active

sites for benzene hydroxylation using N2O. Nature of the active sites in H-

ZSM-5 with Fe concentration was studied [91] and showed that there was no

direct correlation between the concentrations of Bronsted and Lewis acid sites. The Fe

ions with complex oxo structure were found to be the active sites. Hensen et al.

showed extraframework Fe–Al–O species occluded in MFI zeolite as the active

species for benzene hydroxylation [92], they observed only negligible conversion in

the absence of aluminium. Yuranov et al. observed three types of Fe(II) active sites in

the zeolites upon activation and which could be assigned to Fe(II) sites in

mononuclear species, oligonuclear species with at least two oxygen-bridged Fe(II)

sites, and Fe(II) sites within Fe2O3 nanoparticles and amount of different sites were

measured by the transient response of CO2 during CO oxidation [93]. Only

mononuclear and oligonuclear Fe(II) sites were active and Fe2O3 nanoparticles were

inactive. Fe(II) ions present in Fe2O3 nanoparticles (inactive in hydroxylation) were

probably irreversibly reoxidized by N2O to Fe(III), which were known to be

responsible for the total oxidation of benzene. Sun et al. studied the mechanism of this

reaction using Fe-ZSM-5 catalyst obtained through solid state ion exchange process

and proved by spectroscopic method that oligonuclear and binuclear, iron sites

appeared most favorable for nitrous oxide decomposition, whereas the mononuclear

iron sites were active for benzene hydroxylation to phenol [94]. Recently, Pirutko et


81

al. studied the kinetics and mechanism of nitrous oxide decomposition over Fe-ZSM-

5 and showed that both N2O decomposition and benzene hydroxylation were shown to

have kindred mechanisms, which can be described by two main steps [95]. The first

step was a common one and includes oxidation of the α-site by the deposition of

oxygen from N2O:

The α-oxygen can be safely quantified. The second step was the reduction of the site.

It proceeded due to the removal of α-oxygen by the interaction of the later with either

N2O decomposition or benzene hydroxylation.

Fellah et al. have done density functional theory calculations for the oxidation

of benzene to phenol by N2O on a model (FeO)1+

-ZSM-5 cluster and

[(SiH3)4AlO4(FeO)] cluster. They suggested that the experimentally observed

preference of Fe2+

sites over (FeO)1+

on ZSM-5 for benzene oxidation to phenol by

N2O was due to the reduced formation of adsorbed phenolate, which was possibly an

intermediate for deactivation [96]. Rana et al. synthesized hierarchically mesoporous

Fe-ZSM-5 zeolite with catalytically active surface Fe species by introduction of Fe3+

in the synthesis gel of mesoporous ZSM-5 and the material synthesized with the

optimum Fe content and porosity. Samples with very high surface area, strong acidity,

low zeolitic wall crystallinity and 0.93 wt% Fe content showed 1.3 times higher

phenol formation rate than a steamed H-ZSM-5 sample [97].

Advantages

Low capital expenses

No by-product

No highly reactive intermediates

Disadvantages

Applicable only where N2O is available cheaply such as adipic acid plants


82

4.1.5.3 Hydrogen peroxide

Next to air, H2O2 is the most environmentally friendly oxidizing agent and

water alone formed as by product. Phenol is more reactive towards oxidation than

benzene, and hence substantial formation of over-oxygenated by-products (catechol,

hydroquinone, benzoquinones, and tars) usually occurs [98, 99] and hydrogen

peroxide selectivity is always low for these reactions. Since catalytic decomposition

of H2O2 is also a competitive reaction during hydroxylation and hydrogen peroxide is

more expensive, this method will be economically attractive only when H2O2

selectivity is high.

Hydroxylation of benzene to phenol was studied initially in the literature in the

view of understanding the biological processes which showed that formation of

biphenyl through cyclohexadienyl radical and in the understanding of Fenton’s

mechanism [100, 101]. It was also reported that presence of copper in the Fenton’s

system reduces the formation of biphenyl [102, 103]. After the synthesis of titanium

silicates [104] Perego et al. first reported the synthesis of phenol using titanium

silicate [105] and followed by them Thangaraj et al. reported the titanium silicate

catalyzed phenol synthesis from benzene using H2O2 as oxidant [106,107]. Balducci

et al. patented the synthesis of phenol using spherical silica/zeolite composite

materials with submicronic dispersion of titanium silicate and other titanium silicates

[108, 109]. Amorphous microporous mixed oxides containing transition metals and

copper containing molecular sieves were also found to be active for this reaction with

15 to 35% yield of phenol with 100% selectivity [110, 111]. Table 4.2 shows

literature results of benzene hydroxylation using H2O2 as oxidant. H2O2 selectivity

was calculated from the reported conversion values wherever it is needed, however

reported conversion and phenol selectivity critically depends on the experience of the

analyst and analytical instrument sensitivity.

Table 4.2 Catalytic hydroxylation of benzene using H2O2

S.

No

Catalyst Benzene

conversion

%a

H2O2

selectivity,

%

Phenol

selectivity,

%

Conditions Ref.

1 [Zr]-ZSM-5 2 16 91.8 333 K / 4 h [112]

2 Ti / MCM-41 68 23.4 >98 335 K / 2 h [113]

3 TS-1 28.8

(Triphase)

90.5 95 333 K / 2 h [114]

4 V-MCM-41 18 6 50 333 K / 2 h [115]


83

5 Fe2+

and 5-Carboxy-2-

methylpyrazine-N-oxide

7.54 73.2 85 308 K / 4 h /

Acetic acid

[116]

6 1% CuAPO-5 28 16.2 100 343 K / 3 h [117]

7 25-Ti-MCM-41 75 12.4 NM 333 K / 3 h [118]

8 Ti grafted / MCM-41 92 30.7 95 338 K / 1.5 h [119]

9 FeSO4 pyrazine-3-

carboxylic acid N-oxide

8.1 81 97 310 K / 4 h /

Acetic acid

[120]

10 Ts-1B 8.6 80.5 94 373 K / 2 h [121]

11 Bi-ZSM-5 (5wt%

bismuth oxide)

14 14 NM 343 K / 1.5 h [122]

12 VO2+

(APTS)

functionalized / MCM-

41

58.6 58.6 18.5 333 K / 1 h [123]

13 1.1% V / MCM-41 10 7.5 67 343 K / 20 h [124]

14 Nb-MCM-41 19.4 6.5 84.5 343 K / 48 h [125]

15 Ferric tri

(dodecanesulfonate)

Fe(DS)3 in OMImBF4

54 54 100 H2SO4 [126]

16 Ferrocene-SBA-15 - - 84 RT / 3 h /

H2SO4

[127]

17 Fe3+

/ Al2O3 12 2.5 100 333 K / 6 h [128]

18 LaMn-MCM-41 65 24.8 NM 323 K / 2 h [129]

19 Co (V, Nb, La)-MCM-

41

60 20 NM 343 K / 24 h [130]

20 V / Activated carbon 13.2 4.4 49.4 338 K / 5 h [131]

21 Iron / Activated carbon 50 16.7 40 336 K / 5 h [132]

22 Fe (DDS)3

Microemulsion (H2O /

C6H6 / CH3COOH /

NaDDS)

21.9 93.1 92.9 323 K / 5 h [133]

23 CuM(II)Al-HT 4.8 14.4 100 338 K / 24 h [134]

24 H4PMo11VO40.3H2O 26 15.5 91 RT / 1.67 h /

Acetic acid

[135]

25 V / Clay (composed of chlorite, illite,

attapulgite, etc.,)

14 10 94 313 K / 4 h / Acetic acid

[136]

26 NaVO3 13.5 7.9 94 298 K / 10 h [137]

27 FeSO4 / hydrophobic PTFE porous support

(pore size 0.2 mm)

1.2 (Biphasic

system)

96.8 99.9 308 K / 4 h / Acetic acid

[138]

28 FeSO4 /

polydimethylsiloxane membrane

0.47

(Biphasic system)

37.99 99.9 308 K / 6 h /

Acetic acid

[139]

29 H4PMo11VO40 10.1 4.7 93 343 K / 1.67

h / Acetic acid

[140]

30 Na3VO4 12H2O 12 13.1 NM 323 K / 1.5 h

/ Acetic acid

[141]

31 Fe-TpyP–Al-MCM- 41 <1 <1 100 343 K / 20 h [142]

32 Macrocomplex KU-2-8 / Fe

3+

32 4.6 NM 323 K / 0.25 h

[143]

33 Pd / c-Al2O3–NH4VO3 0.57 26.6 100 313 K / 2 h [144]


84

(Insitu)

34 H5PMo10V2O40 34.5 11.5 100 338 K / 6 h /

Acetic acid

[145]

35 Fe / Activated carbon 19.6 4.5 89.3 303 K / 7 h [146]

36 Modified Titanium

Silicalite (TS-1B)

8.6 78.4 91 333 K / 2 h [147]

37 Ferrocene-SBA-15 15.1 15.1 88.3 293 K / 5 h / H2SO4

[148]

38 VB-ZSM-5 19.1 57.5 98 353 K / 8 h [149]

39 Fe (III) / TiO2 14.8 7.4 18 303 K / 4 h [150]

40 Cu / MCM-41 21 15.2 94 RT / 1.67 h / Acetic acid

[151]

41 [(CH3)4N]4PMo11VO40 12.4 0.6 85.7 333 K / 2 h [152]

42 Pt-Fe / TiO2 6.5 3.25 91 303 K / 4 h /

Ascorbic acid

[153]

43 Rh6(CO)16 17 17 95 343 K / 7 h [154]

44 Fe / V / Cu / TiO2 14.8 7.4 49 303 K / 4 h [155]

45 Py1-PMo10V2O40 20.5 6.8 98 353 K / 5 h Acetic acid

[156]

46 TS-PQTM 3.8 19.4 92.4 [157]

47 Co / V / MCM-41 48.2 16.1 81.2 343 K / 24 h [158]

48 Vanadyl

tetraphenoxyphthalocyanine (VOPc)

22.4 22.4 100 337 K / 8 h [159]

49 H5PV2Mo10O40 24.4 4.9 100 333 K / 2 h [160]

50 Tetrakis(2-pyridylmethyl)

ethylenediamine–

iron(II)

complex((TPEN)FeII complex)

0.4 59 NM 293 K / 2 h [161]

51 5% Cu / Al-PILCs 54.9 20.9 80.7 333 K / 3 h [162]

52 CuAPO-11 10.5 21 100 343 K / 6 h [163]

53 V2O5–Al2O3 13.8 1.8 100 333 K / 6 h [164]

54 V-HMS 11.1 6.4 NM 333 K / 3 h /

Acetic acid

[165]

55 TS-PQTM 3.9 19.4 92 343 K / 3 h [166]

56 Fe2O3 / MCM-41 11 3.26 NM 348 K / 6 h / Acetic acid

[167]

57 Fe3+

/ MgO 36 6 100 333 K / 6 h [168]

58 MWCNTs 1.6 1.3 98.5 333 K / 2.5 h [169]

59 Fe2O3 / MWCNTs 16 13 90 333 K / 3.5 h [170]

60 VOx / SBA-16 14 4.1 98 333 K / 4 h [171]

61 Ferrocene / Mesoporous

organosilica

6.8 20.5 65.2 RT / 5 h /

H2SO4

[172]

62 Nanocrystalline Fe2O3 17.4 6.7 NM 343 K / 3 h /

Acetic acid

[173]

a As per the initial mol of benzene, NM - Not Mentioned

Bhaumik et al. used triphasic catalysis using TS-1 as catalyst and showed

28.8% conversion with 95% phenol selectivity and 90% H2O2 selectivity [114].


85

Several other reports on Ti or V containing MCM-41 were reported however showed

poor H2O2 selectivity [113, 115, 118, 119]. Bianchi et al. used homogeneous catalysis

using Fe2+

co-ordinated with different bidentate ligands with (N,N) (N,O) and (O,O)

co-ordination out of which (N,N) ligand was almost inactive (O,O) ligand showed

good activity but poor selectivity and (N,O) gave good activity as well as good

selectivity. 5-Carboxy-2-methylpyrazine-N-Oxide gave highest H2O2 conversion of

94%, 78% H2O2 selectivity and 85% benzene conversion whereas Fe SO4 pyrazine-3-

carboxylic acid N-oxide gave 8.1% benzene conversion with 80% H2O2 selectivity

and 95% phenol selectivity (116, 120). Balducci et al. showed for the first time

heterogeneously catalyzed reaction with 8.6% conversion with 80.5% H2O2

selectivity and 94% phenol selectivity using TS-1 as catalyst (121,174). They used

sulfolane as solvent for the first time for this reaction and the complex formation of

sulfolane with phenol increased the phenol selectivity. Several other attempts were

made to improve the benzene conversion using VO2+

functionalized (APTS)/MCM-

41, LaMn-MCM-41, Co (V, Nb, La)-MCM-41 and Fe/activated carbon as catalyst and

achieved conversion of benzene from 50 to 65% whereas H2O2 selectivity of 17 to

50% [123, 129, 130, 132]. Recent patents claims the catalytic application of different

materials like vanadyl pyrophosphate, vanadium phthalocyanine complex, vanadium

loaded SBA-16 and vanadium containing aluminium phosphate molecular sieves over

these reactions [175-178].

Peng et al. used biphasic hydroxylation using 1-n-octyl-3-methylimidazolium-

tetrafluoroborate (OMImBF4) ionic liquid as solvent and ferric tri(dodecanesulfonate)

Fe(DS)3 catalyst. They screened different ionic liquids and found that length of alkyl

chains on the 1-alkyl-3-methylimidazolium cations and the anions had some effect on

the conversion and selectivity however detailed mechanism was not mentioned clearly

[126]. After the reaction, oxidation products and un-reacted substrate were extracted

from the aqueous and ionic liquid phase with ether at the end of each reaction and

aqueous–ionic liquid catalyst system was reused, however this method has the

disadvantage of using 50 mM H2SO4.

Liu et al. recently reported that novel micro-emulsion system, which was

made of water, benzene, acetic acid, sodium dodecylbenzene sulfonate as a surfactant

and ferric dodecane sulfonate as a catalyst [133]. They achieved a maximum


86

conversion of 21.9% with 93.1% H2O2 selectivity, 92.9% phenol selectivity and this

can be reused for several times however it has the disadvantage of using acetic acid as

co-solvent.

Bianchi et al. used modified titanium silicate (TS-1B) obtained by the post-

synthesis treatment of TS-1 with NH4HF2 and H2O2, as catalyst and obtained 8.6%

benzene conversion with 78.4% H2O2 selectivity and 91% phenol selectivity [147].

Several other reports were also done using both vanadium and copper containing

materials however they failed to give good H2O2 selectivity or benzene conversion;

further research is needed in order to make this process commercially attractive.

Recently, Barbera et al. studied the effect of solvent and crystallite size of

TS-1 over the benzene hydroxylation reaction in both biphasic and triphasic

conditions and also in presence of co-solvent sulfolane. Small-sized TS-1 crystallites

produced the highest primary selectivity to phenol and different titanium

environments and defective hydroxyls population was evaluated by means of diffuse

reflectance UV–vis and IR spectroscopy [179]. Mesoporous aluminium oxide

containing transition metals were also found to be active for this reaction under mild

reaction conditions [180].

Advantages

Environmentally friendly oxidant

Only water formed as by-product

Disadvantages

Highly expensive with respect to oxygen

H2O2 selectivity is always low for benzene oxidation

Unproductive decomposition of H2O2

Molinari and Poerio, recently reviewed the direct production of phenol in

conventional and membrane reactors. Attention was devoted to phenol production

processes using various configurations of membrane reactors (MRs) and a

photocatalytic membrane reactor (PMR). They classified the oxidation according to

oxidant type such as N2O, O2, and H2O2 and each of them shows that direct oxidation

of benzene to phenol was a difficult task. Concluded that further efforts are needed to


87

search and replace the three step traditional process of converting benzene into phenol

with a process of direct oxidation [181].

With this knowledge we have attempted to study this challenging reaction

using H2O2 as oxidant. It was already reported that copper-containing LDHs with high

Cu content (Cu/M(II)=75/25) catalyzed the benzene hydroxylation [134] at a substrate

to oxidant mol ratio of 3:1 using pyridine as the solvent. One of the difficulties

encountered in that work was that the catalysts were difficult to reuse. The present

study discusses the utility of both Cu-containing LDHs with low copper content and

their calcined oxides as catalysts for this reaction using environmentally acceptable

water as the solvent under triphasic conditions. Since peroxide oxidation is vigorous

in water and the formation of over-oxidation products are possible; we have used ~10

atom% copper containing LDHs (Cu/M(II)=10/90) diluted with different co-bivalent

metal ions (M(II) = Co, Ni, Mg or Zn) and at low substrate to oxidant mol ratio of

10:1.

4.2 Experimental

4.2.1 Sample preparation

Samples were prepared by co-precipitation under conditions of low

supersaturation as reported earlier [182]. In brief, a solution (A) containing the desired

amount of 1M solution of metal nitrates (M(II), Cu, Al, where the (Cu+M(II)/Al)

atomic ratio was 3.0 and the Cu:M(II) ratio was 10:90; M(II) = Co, Ni, Mg or Zn)

were mixed thoroughly to make a homogeneous solution. Slowly (1ml min-1

) and

simultaneously, solution (A) was mixed with a solution (B) containing precipitating

agents (i.e., NaOH and Na2CO3; 2 M NaOH and 0.2 M Na2CO3 in 80 ml) in a Schott

electronic titrator, at a constant pH 9.5 0.1. During addition the solution was

vigorously stirred at room temperature. The addition took approximately 90 min and

the final pH was adjusted to 10 by adding few drops of precipitant solution. The

samples were aged in the mother liquor at 65 °C for 18 h in an oil bath, filtered,

washed (until nitrates and sodium were totally absent in the washing liquids), and

dried in an air oven at 100-110 °C for 12 h. These samples are referred here as

CuM(II)Al-LDH. All materials were calcined at 700 oC for 12 h in air and the

calcined samples are referred to as CuM(II)Al-CLDH-700.


88

4.2.2 Benzene oxidation

Benzene oxidation was done in a two-neck 50 ml glass reactor using 5 g (64.1

mmol) of benzene, the calculated amount of catalyst and 15 ml of solvent. The reactor

was placed in a pre-heated oil bath at 65 oC. As soon as the solution reached 65

oC,

0.7 ml (6.2 mmol) of H2O2 (30% W/V) was added through a septum at once, and the

reaction mixture was kept for 6 h with stirring at 800 rpm. Immediately after the

reaction, 5 ml of ethylacetoacetate and 5 ml of water were added, mixed well, and the

aqueous and organic layers were collected separately. The yields of phenol and other

hydroxylated aromatics (given here as total hydroxylated products) were determined

in both the organic and aqueous layers by gas chromatography (Varian 450-GC with a

Factor Four VF-1ms capillary column) by injecting 0.25 μl of sample.

Yield and selectivity were calculated as follows:

Yield of hydroxylated products = [(Moles of hydroxylated products in organic layer +

Moles of hydroxylated products in aqueous

layer) / Initial moles of benzene]*100

Selectivity of phenol = [Total moles of phenol formed (organic + aqueous) / Total

moles of hydroxylated products]*100

Our experimental error in the yield of phenol was ± 0.1 as determined from the GC

calculations. Further, each measurement was done in triplicate and the average yield

and selectivity are given.

4.3 Results and discussion

Table 4.3 summarizes the elemental compositions and the calculated formulae

of the as-synthesized LDH samples. The carbonate content was computed from the

M(II)/Al atomic ratio and the water content was calculated from the TG analysis.

Here we have assumed that carbonate was the only interlayer anion because excess

Na2CO3 was used in the synthesis. This assumption was substantiated by PXRD and

FT-IR measurements, the results of which are shown below. BET specific surface

area (SA) measurements showed the highest surface area for CuNiAl-LDH and the

lowest surface area for CuZnAl-LDH.


89

Table 4.3 Elemental analysis, textural and catalytic activity of the as-synthesized

samplesa

Name Molecular formula SA

(m2/g)

PV

(cm3/g)

APD

(Å)

Yield

(%)b

Phenol

Selectivity

(%)

CuCoAl-

LDH

[Cu0.07Co0.71Al0.22(OH)2]-

(CO3)0.11.0.66H2O 125 0.49 157 0.52 100

CuNiAl-

LDH

[Cu0.07Ni0.70Al0.23(OH)2]-

(CO3)0.11.0.64H2O 168 0.51 120 0.52 100

CuMgAl-

LDH

[Cu0.08Mg0.69Al0.23(OH)2]-

(CO3)0.12.0.65H2O 117 0.48 165 0.93 100

CuZnAl-

LDH

[Cu0.07Zn0.70Al0.22(OH)2]-

(CO3)0.11.0.70H2O 49 0.14 113 0.98 100

ZnAl-LDH

[Zn0.77Al0.23(OH)2]-(CO3)0.11.0.50H2O

28 0.05 71 0 0

MgAl-

LDH

[Mg0.74Al0.26(OH)2]-

(CO3)0.13.0.62H2O 100 0.45 181 0 0

aSubstrate: Benzene (5 g); Solvent: Water (15 ml); Oxidant: H2O2 (0.7 ml); Catalyst: 10 mg;

Temperature/Time: 50 oC/6 h, bTotal hydoxylated products formed

PXRD patterns of the as-synthesized materials are shown in Fig. 4.2; for

comparison, diffraction patterns of as-synthesized binary MgAl-LDH and ZnAl-LDH

compounds are also included.

10 20 30 40 50 60 70

11

3

1000

11

0

01

8

01

5

01

2

00

6

00

3

f

e

d

c

b

a

Inte

ns

ity

, C

/s

2 Theta, deg

Fig. 4.2 PXRD of as-synthesized LDHs (a) CuCoAl-LDH, (b) CuNiAl-LDH, (c)

CuMgAl-LDH, (d) CuZnAl-LDH, (e) ZnAl-LDH, (f) MgAl-LDH

Irrespective of the co-bivalent ion, all samples showed patterns similar to the

HT-like structure with no observable peaks from crystalline impurity phases. The

samples showed sharp and symmetric reflections at low diffraction angles (peaks

close to 2 = 11 and 24o; ascribed to diffraction by basal planes (003) and (006)

respectively) and broad asymmetric reflections at higher angles (peaks close to 2 =

38, 46, and 60o; ascribed to diffraction by (012), (015) and (110) planes). These

diffraction features are characteristic of hydrotalcite [183]. Among the co-bivalent


90

metal ions studied, zinc resulted in the maximum crystallinity followed by Mg, Ni,

and Co respectively. This is probably a consequence of the similarity of the octahedral

ionic radii of Zn2+

and Cu2+

(Zn2+

= 0.74 Å; Cu2+

= 0.73 Å; Mg2+

= 0.72 Å; Ni2+

=

0.69 Å; Co2+

= 0.65 Å) [184]. Further, no significant variation in the crystallinity

upon substitution of 10 atom% of Cu2+

as evidenced by PXRD profiles of binary and

ternary LDHs (see Fig. 4.2 c and f or d and e). The d-spacing values calculated for

(003) diffraction peaks for all materials were between 7.6 to 7.8 Å, which was

consistent with interlayer gallery height of 2.8 – 3.0 Å, allowing 4.8 Å for the

thickness of the brucite-like sheets. This gallery height was consistent with the

presence of carbonate ion [185]. FT-IR spectra of the as-synthesized materials, shown

in Fig. 4.3, support the presence of carbonate ions.

4000 3000 2000 1000

d

c

b

a

Tra

ns

mit

tan

ce

, %

Wavenumber, cm-1

Fig. 4.3 FT-IR spectra of (a) CuCoAl-LDH, (b) CuNiAl-LDH, (c) CuMgAl-LDH, (d)

CuZnAl-LDH

The sharp and intense band around 1350-1380 cm-1

was attributed to the ν3

asymmetric stretching vibration of carbonate, probably present in D3h symmetry.

CuZnAl-LDH alone exhibited an additional shoulder band around 1490-1510 cm-1

,

which might be due to symmetry lowering of free carbonate (to C2v or C3v symmetry)

and/or to partial co-ordination of the carbonate ion with Cu2+

in the cationic layer,

owing to Jahn-Teller distortion of the Cu2+

coordination environment. Such a

symmetry change for the carbonate anion was known for LDHs on mild heating

[186]. The TG/DTG curves of the samples recorded in a flow of nitrogen are shown in

Fig. 4.4. They showed two transitions; the first one, which was reversible,

endothermic, and occurs at low temperature, corresponds to the loss of interlayer

water.


91

100 200 300 400 500 600 700 800

d

c

b

a

dw

/dt

Temperature, oC

Fig. 4.4 DTGA traces of (a) CuCoAl-LDH, (b) CuNiAl-LDH, (c) CuMgAl-LDH, (d)

CuZnAl-LDH

The second one, which was endothermic, at higher temperature and was due to

the loss of hydroxyl groups from the brucite-like layers as well as decomposition of

the carbonate anions. The higher second transition temperatures for Mg- and Ni-

containing samples (around 350 oC) confirm the presence of strongly held hydroxyl

groups. However, for CuZnAl-LDH, high temperature transitions around 560 oC and

765 oC were also observed. These transitions were probably due to the release of

strongly bound carbonate anions, as substantiated by FT-IR spectroscopy.

Catalytic activities of the as-synthesized LDHs for benzene hydroxylation are

given in Table 4.3 (P. No. 88). Among the materials studied, CuZnAl-LDH showed

the highest activity whereas CuCoAl-LDH showed the lowest yield of phenol. Binary

LDHs under similar conditions did not show any conversion substantiating the fact

that copper was indispensible for this reaction. Because all four samples have nearly

the same concentration of copper, the variations in the activity suggested the influence

of the co-bivalent metal ion. Further, it was worth mentioning that although CuZnAl-

LDH has the lowest surface area it showed the maximum yield. This suggested that

the surface area was not the factor that determines the activity. To probe whether the

kinetics of benzene oxidation or the catalytic competitive decomposition of H2O2 was

controlling the yield, we carried out independent experiments in which the amount of

H2O2 remaining in the reaction mixture was quantified under similar conditions after

30 min. These experiments were done in the absence of benzene, because benzene

oxidation produces dark brown to brownish black colored products that complicate

the titrimetric analysis of H2O2. These experiments showed that almost all of the H2O2

remained in the solution with CuZnAl-LDH (> 85%), but no H2O2 was detected for


92

CuCoAl-LDH after 30 min. The CuNiAl-LDH and CuMgAl-LDH samples gave

intermediate values of 43 and 57%, respectively. These results suggest that

competitive decomposition of H2O2 is the predominant factor in limiting the yield of

phenol. However, one must also consider that the redox chemistry of Cu2+

, which was

influenced by the presence of the co-bivalent metal ion, was also critical. To quantify

this effect, X-ray photoelectron spectroscopy (XPS) measurements were done. Fig.

4.5 shows the Cu 2p3/2 core level XPS spectra of CuZnAl-LDH and CuCoAl-LDH.

930 935 940 945

b

a

Binding Energy, eV

Cu2+

Cu+

Inte

ns

ity

, a

.u.

Fig. 4.5 Cu 2p core level X-ray photoelectron spectra of (a) CuZnAl-LDH, (b)

CuCoAl-LDH

The main line at 933.4 eV in both CuZnAl-LDH and CuCoAl-LDH

corresponded to Cu2+

. An additional peak could be discerned at 932.1 eV for CuZnAl-

LDH (missing for CuCoAl-LDH) arising from Cu+ species [187]. Since the zinc

content of the material was very large (in comparison with copper), it masked the Cu-

LMM Auger signal. It has been reported that Cu+ was the active intermediate for the

hydroxylation of phenol using H2O2 [188], a reaction that follows Fenton’s

mechanism. From these observations, one can conclude that the formation Cu+, which

is an active intermediate in benzene hydroxylation, was regulated by the nature of the

co-bivalent metal ion. Among the metal ions studied, Zn2+

most evidently facilitated

the formation of Cu+ species. The previously reported presence of Cu

+ species in

thermally treated CuZnAl-LDHs by XPS measurements corroborates our findings

[189, 190].

One of the limitations encountered with the as-synthesized materials was that

they could not be reused. This was mainly because of the deposition of significant

amounts of coke (the recovered catalyst was brown in color). Attempts to remove the

coke at low temperature (as one would like to retain the layered structure) by Soxhlet


93

extraction in various organic solvents were unsuccessful. The PXRD profile of the

recovered sample showed an amorphous pattern probably due to the presence of large

amount of high molecular weight organic residue (referred to here as coke) in the

interlayer galleries. However, when the recovered sample was calcined, the PXRD

showed the oxide phase (white in color) confirmed the presence of the catalyst (Fig.

4.6).

10 20 30 40 50 60 70

c

b

a

Inte

ns

ity

, C

/s

2 Theta, deg

500

Fig. 4.6 PXRD patterns of the CuZnAl-LDH (a) as-synthesized (b) after the reaction

referred here as used catalyst (c) ‘b’ calcined at 700 oC for 12 h

Likewise, when CuO was tested as an oxidation catalyst for this reaction,

although it had high activity (the copper content in CuO was five times that of

CuM(II)Al-CLDH), it could not be recovered for reuse after the reaction. In an effort

to prepare catalysts that could be reused, the as-synthesized materials were calcined at

700 oC in air for 5 h, and their PXRD profiles are given in Fig. 4.7.

10 20 30 40 50 60 70

500

++++

+

+

+

+

+

##

#

@@@

*****

** d

c

b

a

Inte

ns

ity

, C

/s

2 Theta, deg

Fig. 4.7 PXRD of (a) CuCoAl-CLDH-700, (b) CuNiAl-CLDH-700, (c) CuMgAl-

CLDH-700, (d) CuZnAl-CLDH-700, * Co3O4 (JCPDS-43-1003), @ NiO (JCPDS-22-1189),

# MgO (JCPDS-01-1235), + ZnO (JCPDS-36-1415)


94

All materials showed reflections of the oxide phase typical of the co-bivalent metal

ion namely, ZnO, MgO, NiO and Co3O4. No discrete reflections of CuO were seen for

these samples, suggested that CuO was well dispersed in the host oxide. Catalytic

activity of calcined LDHs and their surface properties are also given in Table 4.4.

Table 4.4 Benzene hydroxylation over calcined materialsa

Name SA

(m2/g)

PV

(cm3/g)

Yield

(%)b

Phenol

Selectivity (%)

CuCoAl-CLDH-700 41 0.16 0.65 89 CuNiAl-CLDH-700 106 0.41 0.76 90

CuMgAl-CLDH-700 53 0.11 0.85 100

CuZnAl-CLDH-700 38 0.17 0.88 100 ZnAl-CLDH-700 35 0.08 0.2 100

MgAl-CLDH-700 23 0.39 0 0

CuOc - - 1.42 92

Without catalyst - - Nil aSubstrate: Benzene (5 g); Solvent: Water (15 ml); Oxidant: H2O2 (0.7 ml); Catalyst: 10 mg;

Temperature/Time: 50 oC/6 h, bTotal hydoxylated products formed, c5 times excess Cu

The surface areas and pore volumes of the calcined materials followed the

same trend as those of the as-synthesized materials. Here again, CuZnAl-CLDH

showed the highest activity while CuCoAl-CLDH showed the lowest, similar to the

trend of as-synthesized LDHs.

Fig. 4.8 SEM analysis of calcined materials


95

In order to gain insight into the distribution of CuO on the oxide matrix and to

correlate the variation in the activity with sample morphology, SEM measurements

were done, and the results are summarized in Fig. 4.8. None of the samples showed

discrete CuO oxide domains, again supporting the idea that the CuO phase was well

dispersed and intermixed with the host oxide phase. Magnesium-containing CLDH

showed agglomerated platelets with a sponge-like morphology whereas zinc-

containing CLDH showed a platelet-like morphology with less agglomeration.

Nickel- and cobalt-containing CLDHs showed severe agglomeration of platelets. We

presume that the well-defined platelet morphology exhibited by CuZnAl-CLDH

offered appropriate surface for the adsorption of reactant molecules compared to the

severely agglomerated spongy structure. The influence of the calcinations temperature

on the activity of CuZnAl-CLDH was also studied.

10 20 30 40 50 60 70

2000

@

@@@

@@

##

##

#

#

*

***

*

*

*

*

A

c

ba

Inte

ns

ity

, C

/s

2 Theta, deg

400 500 600 700 800 900

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4 B

Yie

ld,

%

Calcination Temperature, oC

Fig. 4.9 (A) PXRD of (a) CuZnAl-CLDH-450, (b) CuZnAl-CLDH-700, (c) CuZnAl-

CLDH-900, * ZnO (JCPDS-36-1415), # ZnAl2O4 (JCPDS-01-1146), @ CuO (JCPDS-05-0661);

(B) Catalytic activity of CuZnAl-LDH calcined at different temperatures, (Substrate:

Benzene (5 g); Solvent: Water (15 ml); Oxidant: H2O2 (0.7 ml); Catalyst: 20 mg; Temperature/Time:

65 oC/6 h)


96

Fig. 4.9 shows the PXRD of materials calcined at different temperatures (450,

700 and 900 oC as deduced from the TG profiles; three different regimes are evident,

namely the structural decomposition of the LDH lattice below 450 oC, high

temperature transitions around 700 oC and no further decomposition above 800

oC)

along with their catalytic activity. CuZnAl-LDH calcined at 450 oC and 700

oC

showed mixed oxide phases without any discrete reflections of CuO. However, when

the temperature was increased to 900 oC, the calcined material showed the reflections

of a discrete CuO phase along with ZnO and ZnAl2O4. Activity studies revealed

nearly similar activity for the samples calcined at 700 and 900 oC. Since it was known

from our earlier experiments that the discrete CuO phase may facilitate the formation

of further oxidation products and pose difficulty for reuse, further studies were done

with the sample calcined at 700 oC (CuZnAl-CLDH-700).

In anticipation of increasing the yield of phenol, substrate:catalyst mass ratio

(Fig. 4.10) variation studies were done with CuZnAl-CLDH-700.

1000:1 500:1 250:1 125:1 62.5:1

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Yield

Yie

ld,

%

Substrate:Catalyst

0

20

40

60

80

100 Selectivity

Ph

en

ol

Se

lec

tiv

ity

, %

Fig. 4.10 Influence of substrate:catalyst mass ratio over CuZnAl-CLDH-700, Substrate:

Benzene (5 g); Solvent: Water (15 ml); Oxidant: H2O2 (0.7 ml); Catalyst: 10 mg; Temperature/Time:

50 oC/6 h)

Although a 500:1 substrate:catalyst ratio gave high selectivity of phenol, the

net yield of hydroxylated products was lower compared to 250:1 (500:1, 0.9% vs

250:1, 1.2%). Further, when the reaction was carried out at lower than 250:1

substrate:catalyst mass ratio, coke was formed. Hence the ratio of 250:1 was chosen

for further studies, which will also be reasoned later in the manuscript. Temperature

variation studies were done over CuZnAl-CLDH-700 and the results are given in Fig.

4.11. As the temperature of the reaction increased, the yield of phenol increased;

however, the formation of coke also increased at higher temperature.


97

40 50 60 70 80 90

0.8

1.0

1.2

1.4

1.6

1.8

Yie

ld,

%

Temperature, oC

Yield

0

20

40

60

80

100

120 Sel-Phenol

Sel-CAT

Sel-HQ

Se

lec

tiv

iy,

%

Fig. 4.11 Influence of reaction temperature on benzene hydroxylation over CuZnAl-

CLDH-700, (Substrate: Benzene (5 g); Solvent: Water (15 ml); Oxidant: H2O2 (0.7 ml); Catalyst: 20

mg; Time: 6 h)

In other words, reactions at high temperatures favored further oxidation products,

such as dihydroxybenzenes. This was also evidenced by the darker color of the

reaction mixture at higher temperatures, and under these conditions it was difficult to

recover the catalyst from the reaction mixture. Hence, further studies were carried out

at 65 oC. Attempts were then made to increase the yield of phenol by using mixed

solvents.

Table 4.5 Solvent variation studies over CuZnAl-CLDH-700a

Solvent (15 ml) Sulfolane % (V/V) Yield (%)b

Phenol

Selectivity (%)

Nil - 0.17 83

Acetonitrile - 0.13 100 Sulfolane - 0.14 100

Acetone - 0.04 100

Pyridine - 0.57 100 Water - 1.47 78

Water 0.6 1.22 100

Water 1.2 1.36 100 Water 1.8 1.26 100

Water 2.4 1.25 100

Water 3 1.27 100

Water 4.1 1.23 100 Water 6.7

c 0.44 100

Water 33.3d 0.41 100

aSubstrate: Benzene (5 g); Solvent: Water (15 ml); Oxidant: H2O2 (0.7 ml); Catalyst: CuZnAl-CLDH-

700 (20 mg); Temperature/Time: 65 oC/6 h, bTotal hydoxylated products formed, cSolvent: Water (14

ml), dSolvent: Water (10 ml)

Solvent variations studies were done and the results are given in Table 4.5.

Interestingly, among the solvents screened, water showed the highest yield, but with

lower selectivity for phenol. The high activity using water as a solvent under triphasic

conditions was advantageous from an environmental perspective. To improve the


98

selectivity for phenol, the addition of co-solvents was attempted and this was another

reason for using the 250:1 substrate:catalyst mass ratio (which showed lower

selectivity for phenol) in order to discern the effect of co-solvents on the selectivity of

the reaction. Sulfolane is one of the interesting solvents reported in literature for the

hydroxylation of benzene because it controls the further oxidation of easily oxidizable

(relative to benzene) phenol to dihydroxybenzenes [191, 147]. The reaction using

sulfolane alone as the solvent over CuZnAl-CLDH-700 failed to give high product

yields under our conditions. Hence, attempts were made to add sulfolane as a co-

solvent along with water, and the results are also given in Table 4.5. The addition of

larger amounts (33.3 vol.% and 6.7 vol.%) of sulfolane decreased the conversion of

benzene whereas the addition of small amounts (0.6 vol.% to 4.1 vol.%) gave nearly

equivalent conversion of benzene with 100% phenol selectivity. Under equivalent

conditions, water alone gave slightly higher conversion but with only about 80%

selectivity. To surmise, addition of suflolane enhance the selectivity for phenol as

reported in literature; however the quantity also has an influence on the activity,

probably due to change in the polarity of the medium. Due to its high dielectric

constant (43.26), sulfolane has the peculiar property of forming complexes with

phenolic compounds [191], and this sterically hindered complex does not allow

further hydroxylation of phenol, thus improving the selectivity. A time variation study

was done under optimized conditions in the hope of improving the yield of the

reaction, and the results are given in Fig. 4.12.

0 5 10 15 20 25

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Yield

Yie

ld, %

Time, h

0

20

40

60

80

100

120

Ph

en

ol S

ele

cti

vit

y, %

Selectivity

Fig. 4.12 Time variation studies under optimized conditions

A maximum yield of 1.95% of hydroxylated products was obtained with 95%

selectivity for phenol. This was the highest yield obtained in this study.


99

The reusability of the CuZnAl-CLDH-700 was assessed as follows.

Immediately after the reaction, the catalyst was separated using centrifuge, washed

with water and calcined at 700 oC to remove the adsorbed organics. Fig. 4.13 shows

the activity of the recycled catalysts (referred to here as cycle 2 and cycle 3) along

with PXRD profiles of the corresponding used catalysts. Activity studies indicated

that the catalyst could be reused for up to three cycles without significant change in

the yield of phenol.

10 20 30 40 50 60 70

A

c

b

a

Inte

ns

ity

, C

/s

2 Theta, deg

1000

1 2 3

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4 B

Yie

ld,

%

No. of cycles

Fig. 4.13 (A) Powder X-ray diffraction patterns of used catalysts (a) CuZnAl-CLDH-

700, (b) CuZnAl-CLDH-700-cycle 2, (c) CuZnAl-CLDH-700-cycle 3 ; (B) Recycle

studies for CuZnAl-CLDH-700, (Substrate: Benzene (5 g); Solvent: Water (15 ml) and Sulfolane

(180 μl); Oxidant: H2O2 (0.7 ml); Catalyst: 20 mg; Temperature/Time: 65 oC/6 h)

However, PXRD showed an interesting trend. Immediately after the first use,

specific leaching of the zinc oxide occurred. This was evident from the absence of

characteristic reflections at 2θ = 34.6o, 36.4

o, 47.7

o 56.7

o and 63.0

o for zinc oxide

(JCPDF: 36-1415). However, on further recycling, no phase change was observed,

that showed characteristic reflections of ZnAl2O4 (JCPDF: 1-1146). Similar activity


100

even after leaching of ZnO suggested that the nature of active copper species and its

dispersion were not affected.

Fig. 4.14 STEM-EDAX analysis of (A) CuZnAl-CLDH-700 (B) CuZnAl-CLDH-700-

cycle 3

Cu

Zn

Al Al

Zn

Cu


101

Further, no discrete CuO domains could be seen even after repetitive

calcinations. To confirm the absence of leaching of copper during the reaction and

also to find the morphological changes that were anticipated from leaching of ZnO,

TEM-STEM-EDAX analysis was done and the results are shown in Fig. 4.14.

Significant changes in the morphology of the catalyst were found after the reaction.

Large irregular platelets (~ 200 - 300 nm) with nano-sized punctures (due to

decarbonation and dehydroxylation of the layered lattice) for the calcined catalyst

before the reaction, whereas oblong condensed particles of smaller size (~ 20 - 50 nm)

were seen after the reaction. This contraction of the particles could be due to

intermittent calcinations of the catalyst between cycles. However, the elemental

mapping analysis revealed a uniform distribution of copper both before and after the

reaction (on both large platelets and oblong particles respectively) with no Cu-

rich/poor domains. This suggested that although the variation in the morphology

occurs after repetitive calcinations, the dispersion/distribution of copper was not

influenced significantly. This retention of dispersion could be responsible for

recyclability of the catalyst.

4.4 Conclusions

Hydroxylation of benzene using H2O2 as the oxidant was studied using ~10

atom% copper containing LDHs with different co-bivalent ions (Mg, Zn, Ni or Co)

both in their as-synthesized and calcined forms, using water as the solvent. Zinc-

containing samples are the most active in both the as-synthesized and calcined forms.

The high activity of the zinc-containing LDH was attributed to the formation of active

Cu+ species as identified by XPS measurements. Solvent variation studies reveal that

addition of small amounts of sulfolane significantly increases the selectivity for

phenol. High activity (1.36% yield of phenol) with 100% phenol selectivity was

achieved using a substrate:oxidant molar ratio of 10:1 at 65 oC for CuZnAl-CLDH-

700. This catalyst was reused for up to three cycles without any significant change in

the activity. Although significant changes in the phase and morphology exist between

the calcined catalyst and the used catalyst, STEM-EDAX measurements showed that

the dispersion of copper was not significantly affected. The retention of good copper

dispersion was the likely reason for the recyclability of the catalyst. Although the

catalysts reported in the study show lower yields of phenol, (compared to the cumene

process), it has the advantage of avoiding environmentally unfriendly catalysts and


102

dangerous intermediates. Water can be used as a benign solvent and 100% selectivity

of phenol can be obtained using an environmentally friendly LDH-derived catalyst.

Because of the low product yield and low H2O2 selectivity, further research is needed

in order to make this process commercially attractive.

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