Oxidative dehydrogenation of 4-vinylcyclohexene to styrenecatalyzed by PV2Mo10O5ÿ

40 heteropolyacids

Ronny Neumann*, Ishai Dror

Casali Institute of Applied Chemistry, Graduate School of Applied Science, The Hebrew University of Jerusalem, Jerusalem 91904, Israel

Received 27 November 1997; received in revised form 19 February 1998; accepted 17 March 1998

Abstract

The gas-phase oxidative dehydrogenation of 4-vinylcyclohexene (VCH) to styrene in high selectivities was successfully

carried out at moderate temperatures, 200±2608C, using a vanadium substituted polyoxometalate, PV2Mo10O5ÿ40 , supported on

carbon as catalyst. The major co-product was ethylbenzene and only a small amount of over-oxidation to COx was observed.

Maximum conversions and selectivity were obtained at a O2/VCH ratio of �1.9. The identity of the counter cation also

affected the results with activity and selectivity decreasing in the following order H5�(NH4)4K>Cs3H2�(NH4)5.

Ethylbenzene and styrene are not formed by the same reaction pathway. For ethylbenzene formation, oxydehydrogenation

is preceded by isomerization of the exocylic double bond to an endocyclic position, whereas for styrene formation there is no

such isomerization. A mechanism is proposed whereby the active catalyst is a polyoxometalate ± carbon support complex,

which yields in the presence of oxygen quinone/hydroquinone or aroxy/phenol redox couples responsible for the

oxydehydrogenation. # 1998 Elsevier Science B.V. All rights reserved.

Keywords: Polyoxometalate; Heteropolyanions; Oxidative dehydrogenation; Vinylcyclohexene; Styrene

1. Introduction

Major interest in the use of heteropoly acids in

heterogeneous oxidation catalysis has been directed in

the past toward new processes for methacrylic acid.

Initially, research which afterward was industrialized

had shown that methacrylic acid can be obtained by

aerobic oxidation of methacrolein with PVxMo12ÿx

O�3�x�ÿ40 (0<x<2) as catalyst [1]. Similarly, important

research has described the oxidative dehydrogenation

of isobutyric acid to methacrylic acid. As in the

methacrolein oxidation, the most effective catalysts

appear to have one or two vanadium atoms in the

Keggin structure [2]. It was found that there is a

crucial role for both, the acidic and oxidative proper-

ties of the catalyst [3]. For this reaction, catalyst

deactivation has thwarted scale-up and much effort

has been devoted to understanding and preventing this

deactivation [4]. More recently, formation of

methacrylic acid from isobutane has also been

described in both, the patent [5] and open literature

[6], using various heteropolyanions as catalysts. The

yields and selectivities in these oxygenation±oxyde-

hydrogenation reactions are, however, still quite

low. Analogous oxidation reactions of other alkanes,

Applied Catalysis A: General 172 (1998) 67±72

*Corresponding author.

0926-860X/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved.

P I I S 0 9 2 6 - 8 6 0 X ( 9 8 ) 0 0 1 0 3 - 3

ranging from methane to pentane, have also been

reported again with only fairly low yields [7]. Other

heterogeneous oxidation reactions catalyzed by

polyoxometalates which have been studied include

methanol to formaldehyde oxidative dehydrogenation

[8], furan formation from butadiene [9], oxidation of

acetaldehyde to acetic acid [10] and acrolein oxidation

to acrylic acid [11].

Conspicuously, the oxidative dehydrogenation of

non-functionalized alkenes and dienes to polyenes

and/or aromatic compounds has received scant atten-

tion. In the past, we thoroughly investigated the liquid-

phase aerobic dehydrogenation of dienes such as 9,10-

dihydroanthracene, limonene and a-terpinene cata-

lyzed by H5PV2Mo10O40 [12]. The oxydehydrogena-

tion of less reactive substrates such as cyclohexene,

tetralin and 4-vinylcyclohexene failed or showed only

much lower conversions. The use of 4-vinylcyclo-

hexene (VCH), available from 1,3-butadiene via the

Diels-Alder reaction, as a substrate for styrene pro-

duction, Eq. (1), is an interesting alternative [13] to

the classical ethylbenzene dehydrogenation process

[14]. Therefore, we set out to investigate the possible

application of oxydehydrogenation of VCH to styrene

using PV2Mo10O5ÿ40 heteropoly acids as catalysts.

(1)

2. Experimental

2.1. Catalyst preparation

The H5PV2Mo10O40 heteropoly acid was prepared

according to the procedure reported in the literature

and was recrystallized in a vacuum desiccator over

P2O5 [15]. Impregnation of the catalyst onto the active

carbon (BDH cat # 33034; 0.85±1.70 mm) support

was carried out by dissolving the catalyst in excess of

methanol and then adding the active carbon. In a

typical example, 5% H5PV2Mo10O40/C was prepared

by dissolving 2.5 g H5PV2Mo10O40 in 100 ml metha-

nol, to which 47.5 g active carbon were added. After

stirring the mixture gently for 15 min at room tem-

perature the solvent was evaporated and the supported

catalyst was dried overnight in an oven at �1008C.

H5PV2Mo10O40 on other supports were prepared in an

identical manner. Cation±anion exchange was carried

out on the impregnated catalyst by the addition of

appropriate amount of an aqueous carbonate solution,

®ltration and subsequent drying overnight at 1008C.

For example, 50 g of 5% H5PV2Mo10O40/C

(�1.1 mmol H5PV2Mo10O40) was treated with

540 mg (1.65 mmol) of Cs2CO3 dissolved in 100 ml

water to form Cs3H2PV2Mo10O40/C. Thermogravi-

metric measurements (Mettler 50) of both the sup-

ported and the bulk compounds showed that the

catalysts were all completely stable (no weight loss

due to decomposition) up to 3808C. IR spectra of the

supported catalysts after various treatments were

taken using a Nicolet 510M FTIR spectrometer in

the attenuated re¯ectance mode.

2.2. Catalytic reactions

Reactions were carried out in a ®xed bed, tempera-

ture controlled, stainless-steel tubular reactor (length ±

12 cm, I.D. ± 1 cm) ®lled with 4 cm glass beads at both

ends and 4 cm supported catalyst in the middle so that

the actual reactor volume was �3.1 cm3 for 2 g of

supported catalyst. The organic substrate was deliv-

ered with a constant volume ¯ow (typically 0.15 ml

liquid/min) and preheated to 2008C and mixed with air

in a baf¯ed gas-phase mixer within an oven at 2008C.

The reactor was connected on-line to a HP 5880 gas

chromatograph equipped with a 3 m�1/8" stainless-

steel 10% SP2100 on Chromosorb WAW packed

column and a thermal conductivity detector. Under

isothermal GC column oven conditions at 1008C, the

retention times were as follows: CO/CO2 ± 0.6 min;

vinylcyclohexene ± 3.8 min; ethylbenzene ± 4.3 min;

and styrene ± 5.1 min.

3. Results and discussion

In the ®rst stage of the investigation of the oxyde-

hydrogenation of 4-vinylcyclohexene to styrene, the

effectiveness of the catalytic reaction using 5%

H5PV2Mo10O40/C as catalyst was tested at different

reaction temperatures, Fig. 1. One may observe that

68 R. Neumann, I. Dror / Applied Catalysis A: General 172 (1998) 67±72

the conversion increased steadily over the temperature

range studied. This increased conversion with tem-

perature, however, was combined with a decrease in

the styrene-to-ethylbenzene ratio (6.4 at 2008C and

2.6 at 2608C). Notable is the fact that the oxydehy-

drogenation is the dominant reaction with only limited

formation of COx and no observable formation of

other volatile compounds. Recovery of the supported

catalyst after reaction for 200 min at 2208C also

showed no increase in mass of the catalyst bed. This

indicates that polymerization to non-volatile products

is insigni®cant. In Fig. 2, a conversion vs. time plot

shows that the catalytic reactivity was more or less

constant with a sudden deactivation after a certain

time period. This time period decreased with increas-

ing temperature. Catalyst lifetimes ranged from

820 min at 2008C to 360 min at 2608C. At tempera-

tures <2008C, there was no reaction, whereas >2608Cthe catalyst lifetime was only ca. 60 min. An IR

spectrum of an inactive, spent catalyst showed the

heteropolyacid unchanged (see also below); however,

the carbon support was visibly burnt or oxidized and

the activity could not be regenerated.

In Fig. 3 one may observe that the O2/VCH ratio

had a strong effect on the conversion and the amount

of styrene obtained and only a small effect on the

formation of ethylbenzene and COx. The maximum

values were found for a O2/VCH ratio of �1.8 while

the reaction stoichiometry requires one equivalent

dioxygen per 4-vinylcyclohexene.

The effectivity of the H5PV2Mo10O40/C catalyst

was compared to other similar catalyst formulations.

The effect of changes in the counter cation is given in

Fig. 4. The (NH4)4KPV2Mo10O40/C catalyst gave

almost exactly the same results as the original

Fig. 1. Oxydehydrogenation of VCH catalyzed by 5%

H5PV2Mo10O40/C at various temperatures. Air (333 ml/min,

3.31 mmol O2/min) and VCH (0.15 ml/min, 1.66 mmol/min) were

passed over a 2 g 5% H5PV2Mo10O40/C catalyst bed at the noted

temperature. The results are given as an average of five

measurements spaced equally over the duration of the reaction.

Fig. 2. Conversion vs. time plot of the oxydehydrogenation of

VCH catalyzed by 5% H5PV2Mo10O40/C at various temperatures.

Air (333 ml/min, 3.31 mmol O2/min) and VCH (0.15 ml/min,

1.66 mmol/min) were passed over a 2 g 5% H5PV2Mo10O40/C

catalyst bed at the noted temperature.

Fig. 3. Oxydehydrogenation of VCH catalyzed by 5%

H5PV2Mo10O40/C at various oxygen/ VCH ratios. Air at various

rates and VCH (0.15 ml/min, 1.66 mmol/min) were passed over a

2 g 5% H5PV2Mo10O40/C catalyst bed at 2208C. The results are

given as an average of five measurements spaced equally over the

duration of the reaction.

R. Neumann, I. Dror / Applied Catalysis A: General 172 (1998) 67±72 69

H5PV2Mo10O40/C catalyst. The Cs3H2PV2Mo10O40/C

gave a slightly higher conversion, but yielded a similar

amount of styrene, while the ethylbenzene yield rose.

The (NH4)5PV2Mo10O40/C catalyst, on the other hand,

was signi®cantly less active. Interestingly, the catalyst

support is crucially important for catalytic activity.

Other supports tested at standard conditions (333 ml/

min air, 0.15 ml/min VCH, 2 g supported catalyst with

5% loading at 2208C) such as silica, g-alumina, mor-

denite and amorphous silica±alumina all showed no

activity whatsoever. Also the use of a pure carbon

support without a polyoxometalate showed no cata-

lytic activity. It is noteworthy, however, that a 1 : 1

mechanical mixture of H5PV2Mo10O40/SiO2 and acti-

vated carbon resulted in a 7% conversion with >90%

styrene selectivity. Furthermore, the use of bulk

H5PV2Mo10O40 at up to 2808C showed no 4-vinylcy-

clohexene conversion. At temperatures �3008C, the

reactor was almost immediately plugged by formation

of a polymeric solid. Oxydehydrogenation of an

alkene using pure bulk H5PV2Mo10O40 at �3008Ccould, however, be con®rmed as was shown by using

cyclohexene as substrate in place of VCH. Over a 24-h

period, benzene was constantly formed at a 85�5%

conversion at >98% selectivity with no indication of

catalyst deactivation.

The reaction mechanism may be discussed from

several points of view. First, concerning the pathway

of 4-vinylcyclohexene dehydrogenation, one may

invoke two possible scenarios, reaction (2):

(2)

Styrene formation may be explained by (a) an iso-

merization from an exocyclic diene to an endocyclic

diene (only one possible isomer is given for simpli-

city) [12]. This isomerization is then followed by two

consecutive dehydrogenation steps to form (b) ethyl-

benzene and then (c) styrene. Alternatively, styrene

can be formed directly without isomerization from an

exocyclic to endocyclic diene, in two oxydehydro-

genation steps (d and e), wherein ethylbenzene is not

an intermediate. In a control experiment using ethyl-

benzene as substrate in place of VCH, under standard

conditions, no styrene formation was observed. This

leads to the conclusion that for the VCH oxidative

dehydrogenation, the formation of styrene is not from

or through ethylbenzene as an intermediate, i.e. step

(c) does not occur. Instead, ethylbenzene and styrene

are formed in parallel competitive reaction pathways,

whereby an isomerization to an endocyclic diene

followed by dehydrogenation to ethylbenzene is less

favored than the direct dehydrogenation of the exo-

cyclic VCH to styrene. The isomerization from an

exocyclic to an endocyclic diene is apparently irre-

versible, as when a- and g-terpinene (1,3 and 1,4

dienes, respectively) were used as substrates only p-

cymene (1-methyl-4-isopropyl benzene) was formed

as product. Interestingly, the oxydehydrogenation of

limonene (1-methyl-4-isopropenyl cyclohexene)

yields only p-cymene and no 1-methyl-4-isopropenyl

benzene. This indicates a strong positive substitution

effect for the isomerization reaction.

The second subject for discussion concerns the

mechanism of the oxydehydrogenation as concerns

the function of the catalyst. There are many reactions

for which it has been shown [12,16], especially in the

liquid phase, that H5PV2Mo10O40 catalyzes reactions

by a redox-type mechanism, Eq. (3), whereby elec-

trons and protons are transferred from the organic

substrate (SH2) to the catalyst yielding the product (S)

and reduced catalyst in the ®rst stage. This is followed

Fig. 4. Oxydehydrogenation of VCH catalyzed by 5%

PV2Mo10O5ÿ40 =C with various counter cations. Air (333 ml/min,

3.31 mmol O2/min) and VCH (0.15 ml/min, 1.66 mmol/min) were

passed over a 2 g 5% PV2Mo10O5ÿ40 =C catalyst bed at 2208C. The

results are given as an average of five measurements spaced equally

over the duration of the reaction.

70 R. Neumann, I. Dror / Applied Catalysis A: General 172 (1998) 67±72

by reoxidation of the catalyst by molecular oxygen in

the second stage.

H5PV2Mo10O40�OX� � SH2

! H5PV2Mo10O40�red� � 2H� � S

H5PV2Mo10O40�red� � 2H� � 1=2O2

! H5PV2Mo10O40�OX� � H2O (3)

At ®rst glance it could be suggested that the same

mechanistic pathway could be invoked in this reac-

tion. At >3008C, pure H5PV2Mo10O40 oxydehydro-

genates cyclic alkenes, e.g. cyclohexene, to aromatics

in high yield. In order to carry out the 4-vinylcyclo-

hexene oxidative dehydrogenation at lower tempera-

tures (200±2608C) to prevent polymerization, the

catalyst, however, must be dispersed on a support to

increase surface area and catalytic activity. Here,

somewhat unexpectedly, only activated carbon was

a useful support with others showing no catalytic

activity whatsoever. This was a strong indication that

the activated carbon may have an active role in the

catalytic cycle. This conclusion is strengthened by the

fact that a mechanical mixture of activated carbon

mixed with 5% H5PV2Mo10O40/SiO2 showed catalytic

activity although activated carbon by itself was inac-

tive. An IR spectrum of H5PV2Mo10O40/C as pre-

pared, Fig. 5 bottom, shows the clear signature of the

H5PV2Mo10O40 heteropoly acid (1051 cmÿ1 P±O;

958 cmÿ1 Mo±Oterminal; 883 cmÿ1 Mo±Ocorner±Mo;

795 cmÿ1 Mo±Oedge±Mo) [16]. After recovering an

active catalyst the IR spectrum was again measured,

Fig. 5 top. First, the heteropoly acid remains

unchanged indicating that it was stable under reaction

conditions. However, a new weak peak was observed

at 1650 cmÿ1, possibly attributable to the carbonyl

group of a quinone-type moiety. This observation lead

us to suggest that for VCH oxydehydrogenation, at

200±2608C, the polyoxometalate PV2Mo10O5ÿ40 ®rst

activates the carbon support to an active intermediate.

Such an active intermediate may be the result of

complexation of the electron-rich carbon (graphite)

with PV2Mo10O5ÿ40 followed by oxidation to yield

oxidized graphite containing quinone/hydroquinone

or alternatively an aroxy/phenol redox couples on

the catalyst surface as seems to be indicated by the

IR spectrum. Although the postulation of the forma-

tion of a quinone on the carbon surface is speculative,

these types of quinone/hydroquinone couples have

indeed already been suggested by others in the past

as effective catalysts for oxydehydrogenation reac-

tions [17,18]. A proposed generalized mechanism is

summarized as follows:

POM� C! POMÿC! POMÿC-quinone

POMÿC-quinone� VCH

! POMÿC-hydroquinone� styrene (4)

POMÿC-hydroquinone� O2 ! POMÿC-quinone

Some support for such a mechanism was also obtained

by reacting 5 mmol VCH, 0.5 mmol 2,3,5,6-tetra-

chloro-1,4-benzoquinone and 0.05 mmol

H5PV2Mo10O40 dissolved in 10 ml butanol under

1 atm O2 at 1008C for 12 h. The yield of ethylbenzene

was 88 mol% vs. 4 mol% without the quinone.

An effective catalytic system, PV2Mo10O5ÿ40 =C for

the oxidative dehydrogenation of 4-vinylcyclohexene

to styrene in relatively high selectivities has been

presented. From the literature [14,17], there is a clear

Fig. 5. IR spectra of 5% H5PV2Mo10O40/C. Bottom ± spectrum of

the catalyst before reaction; top ± spectrum of an active catalyst

after reaction for 60 min at 2208C under conditions of Fig. 1. The

transmittance from 1200±1800 cmÿ1 is enlarged 10 times com-

pared to the transmittance from 650±1200 cmÿ1.

R. Neumann, I. Dror / Applied Catalysis A: General 172 (1998) 67±72 71

indication that the use of other more oxidatively stable

carbon supports may very signi®cantly improve the

catalyst lifetime and reaction selectivity.

Acknowledgements

This work was supported by the Basic Research

Foundation administered by the Israel Academy of

Science and Humanities.

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