Jointly published by React.Kinet.Catal.Lett.Akadémiai Kiadó, Budapest Vol. 73, No. 1, 99-107and Kluwer Academic Publishers, Dordrecht (2001)

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RKCL3799

KINETICS AND MECHANISM OF STYRENE OXIDATION USINGTRANSITION METAL SUBSTITUTED

DODECATUNGSTOPHOSPHATE

V. Indiraa, Shivappa B. Halligudi*, S. Gopinathan and C. Gopinathan Inorganic Chemistry and Catalysis Division,

National Chemical Laboratory, Pune 411008, India1 Chemistry Department, Sree Keralavarma College, Trichur 680011, India

*email:halligudi@cata.ncl.res.in

Received October 18, 2001Accepted May 3?, 2001-05-02

Abstract

Tetraalkylammonium (TAA) salts of transition metal substituteddodecatungstophosphate, (TAA)5PW11O39.H2O (where TAA = (C2H5}4N(TEA), (C3H7}4N (TPA), and (C4H9}4N (TBA), and M = Mn2+, Fe2+ andCo2+) catalyzed the oxidation of styrene with H2O2 at 353 K. Except forthe Mn2+ salt, other TAA salts were found to be active and gave mainlybenzaldehyde and styrene oxide products. Among the catalyst tested, TBA(tetrabutylammonium) salt of Co2+ was more active in styrene oxidation.Kinetics of TBA-PW11CoO39 catalyzed oxidation of styrene have beeninvestigated and mehanism for oxidation of styrene has been proposed.

Keywords: Oxidation, tetraalkylammonium (TAA) salts, kinetics, styrene

INTRODUCTION

A major goal in the oxidation of organic compounds is the replacement ofstoichiometric oxidation processes by catalytic oxygen transfer reactions incombinations with oxygen donors like hydrogen peroxide, t-butyl-hydroperoxide (TBHB) and molecular oxygen. Inorganic and transition metalsubstituted complexes are mainly used as catalysts in oxidation of organiccompounds. The strong acidity (oxidizing property) of transition metalsubstituted derivatives of polyoxometallates (TMSP complexes), in particularthose with Keggin structure make them good candidates for catalysis [1,2] andmany of the characteristic reactions of TMSP complexes are similar to those ofmetalloporphyrin systems [3]. The additional attractive and technologicallysignificant aspect of TMSP complexes in catalysis is their resistance towardsoxidative degradation [4,5].

100 INDIRA et al.: OXIDATION

Oxidation of alkenes is important from the synthetic point of view [6].Oxidative cleavage of alkenes to carbonyl compounds are commonly carriedout by ozonolysis or stoichiometric oxidation [7-9]. Though, ozonolysis is veryselective and high yielding, it is not acceptable for reasons of economy andsafety [10]. Therefore, the development of ozone free processes for oxidativeC=C cleavage is important and hence, much research has been focused on thedevelopment of catalyst systems for this purpose. In this communication, wereport the kinetics of transition metal substituted dodecatungstophosphatecatalyzed oxidation of styrene to benzaldehyde and styrene oxide withhydrogen peroxide as an oxidant.

EXPERIMENTAL

Tetraalkylammonium (TAA) salts of transition metal substituteddodecatungstophosphate, (TAA)5PW11O39.H2O, where TAA = (C2H5}4N (TEA),(C3H7}4N (TPA), and (C4H9}4N (TBA), and M = Mn2+, Fe2+ and Co2+ wereprepared by the general method reported for [Bu4N]5PW11CoO39·H2O [11].These compounds were characterized by TG, DTA, IR and UV spectroscopyand elemental analysis [12,13]. Electronic and ESR spectroscopic studiesindicated that the transition metal ions are present in the divalent state. Styrene oxidation was carried out using the above complexes as catalysts inpresence of hydrogen peroxide as an oxidant. In a typical reaction, the catalyst(0.025 mM) was added to a mixture of styrene (0.01 M) containing 30%aqueous hydrogenperoxide (0.01 M) and 5 g of acetonitrile. The reactionmixture was refluxed at 353 K in a temperature controlled oil bath. The productwas analyzed on a Hewlett Packard gas chromatograph model No. 5890 series-11 using HP-5 fused column with 30 m x 0.53 m x 1.0 m film thickness andFID detector. From the moles of styrene reacted, turnover numbers werecalculated. The samples were also analyzed at fixed time intervals by UV-Visible spectroscopy to understand a reaction mechanism.

RESULTS AND DISCUSSION

Five tetraalkylammmonium salts of transition metal substituted metalcomplexes were screened for styrene oxidation and the results are presented inTable 1. It was observed that cobalt substituted complexes were highly active instyrene oxidation. Oxidation of styrene under the reaction conditions gavebenzaldehyde as the major product and styreneoxide as the minor product. Thecatalytic activities for styrene oxidation were found to be in the order: Co > Fe

INDIRA et al.: OXIDATION 101

> Mn. Thus, the Mn2+ containing salt was the least active for this reaction anda similar observation has been reported in cyclohexene oxidation using p-cyano-N,N-dimethylaniline-N-oxide as oxidant [14]. However, TAA saltcontaining Mn2+ worked well for alkene oxidations in the presence ofiodosylbenzene [5]. Among the three-alkylammonium cations studied, it wasobserved that the tetrabutylammonium (TBA) salt is more active catalyticallythan others. The higher catalytic activity observed for TBA salt was attributedto the increased length of the alkyl group present in it, which in turn favors fastphase transfer in the biphasic reaction medium.

Table 1

Catalyst activity data: aConditions: Styrene = 0.01 M, H2O2 = 0.01 M, Catalyst = 0.1 g,Acetonitrile = 5 g, Temperature = 353 K, Reaction time = 30 min

Product selectivity (wt.%)Catalyst Conversion(wt.%) Benzaldehyde Styrene oxide

TONb

TBA-PW11MnO39

No significant conversion

TBA-PW11FeO39 7 76 9 86TBA-PW11CoO39 22 71 29 1467TPA-PW11CoO39 19 79 21 146TEA-PW11CoO39 18 91 9 128

b Turnover number = moles of styrene reacted per mole of catalyst

Since the TBA salt of the cobalt substituted compound was found morecatalytically active, styrene oxidation with this catalyst was studied in moredetail in order to elucidate the reaction mechanism and to evaluate theactivation energy and thermodynamic functions. The UV-visible and IR spectra of (TBA)5PCoW11O39.H2O after catalyticreaction showed only the original absorption bands with almost the sameintensities. This indicated that the compound was stable under the reactionconditions studied. Hill et al. [15] has reported that this compound self-assembles under turnover conditions for the epoxidation of olefins usingiodosylbenzene as oxidant.

Electronic spectroscopic studies

Electronic spectra of fresh Co2+ TBA catalyst and that of the reaction

mixture during the course of the reaction (after 5 and 30 min) are shown inFig. 1. Electronic spectrum (in the visible region of 400-900 nm) of the catalyst

102 INDIRA et al.: OXIDATION

showed the features expected for Co2+ ion in octahedral or near octahedralsymmetry. Three major peaks were observed at 483 and 560 nm and a weak

Fig. 1. UV-Vis spectra of the catalyst

shoulder around 600 nm. The electronic spectra of the reaction mixture weresimilar to that of the fresh catalyst, which indicated that there was no change inthe oxidation state of cobalt. This observation ruled out a redox mechanism forthe reaction involving the formation of oxospecies of Co4+. From the datapresented in Table 1, it is clear that the major oxidation product benzaldehydeis formed due to oxidative cleavage. In the above catalytic oxidation of styrenean induction period was observed for the formation of products. The high ratioof oxidation product to epoxide and an induction period for the reactiondirectly suggest a free radical mechanism for the above reaction. Besides, incase of styrene, which is electrophilic in nature, oxidative cleavage of the C=Cbond occurs rather than epoxidation [16]. Any changes in the oxidation state of

INDIRA et al.: OXIDATION 103

cobalt during the free radical initiated mechanism will not be detected in theelectronic spectrum. But, in the case of cyclohexanol oxidation using thiscatalyst and H2O2, a redox mechanism was observed, which was monitored byelectronic spectra [11]. This contrast suggests that the oxygenation mechanismis different for different substrates with the same oxidant. It is also reported thatthe oxygenation mechanism for a given catalyst and substrate is different withdifferent oxidants [17]. For the oxidation of cyclohexene using this catalyst andiodosylbenzene as oxidant, Hill et al. proposed a mechanism, which is neither aredox nor a free radical one [5]. The pink color of the catalyst was retained throughout the catalytic reaction.This is due to the presence of cobalt in the 2+ oxidation state, which alsosupports the proposed free radical mechanism. In the case of cyclohexanoloxidation using this catalyst, which involves a redox mechanism, the pinkcolored catalyst became colorless and the pink color was retained aftermaximum conversion. From the above study, the possible mechanism forstyrene oxidation is shown in Scheme 1, which is similar to the one proposedelsewhere in the literature [18].

KINETICS

A detailed kinetic study was made with the TBA salt of Co2+ in theoxidation of styrene by H2O2 to establish the dependence of rate with respect toall the reactant parameters involved in the above oxidation reaction. In all thekinetic experiments, reaction mixtures were analyzed at fixed intervals andfrom gas chromatographic analysis, the moles of styrene converted werecalculated. From the moles of styrene reacted, rates of oxidation were evaluatedfrom the graphs of moles vs. time graphs. It was found that the rate of oxidationhad a first order dependence with respect to catalyst, substrate, and half orderdependence with respect to H2O2 concentrations respectively. Most of thereactions are sensitive towards temperature. Hence, the effect of temperature onTBA salt of Co2+ catalyzed oxidation of styrene was studied. Temperature wasvaried between 323 - 353 K, keeping the concentrations of styrene, 0.01 M,catalyst, 0.025 mM, H2O2, 0.01 M and acetonitrile, 15 g. The effect oftemperature is shown in Fig. 2 as a graph of ln k vs. 1/T. From this graph, theactivation energy evaluated was 3.74 kcal mol-1.

104 INDIRA et al.: OXIDATION

Fig. 2. Effect of temperature

MECHANISM

Oxidation of styrene by H2O2 is catalyzed by Co2+ in TBA-PW11CoO39

complex to give benzaldehyde as the major product. The mechanism proposedinvolving the catalytic intermediate species is as shown in Scheme.

The free radical mechanism (Scheme) involves the interaction of H2O2 withthe TBA salt of Co2+ 1 to form a hydroperoxy species 2 in a preequilibriumstep. Species 2, being unstable, undergoes one-electron reduction with a loss ofwater molecule and rearranges to give the Co3+ peroxo species 3. In anotherpreequilibrium step, species 3 interacts with styrene to form a π-bondedtransient species 4. The transfer of oxygen to olefinic bond in a rate-determining step forms a metalloepoxy intermediate species 5, whichdissociates to form products, while regenerating the active species. Theproposed mechanism for the oxidation of styrene is consistent with theobservations made in kinetic and electronic spectroscopic studies of thereaction.

INDIRA et al.: OXIDATION 105

Co2+

Co2+O

O

H

H-H2O

Co3+

CH CH2

CH CH2

Co3+ O

(rds)

CH CH2O

Co3+

products

k

K1

K2. O.

H2O21

2

4

5

3

Scheme

fast

Rate law

Based on the free radical mechanism proposed in the Scheme, and on thekinetic dependences, the rate law for TBM salt of Co2+-catalyzed oxidation ofstyrene to benzaldehyde or styrene oxide can be written as:

Rate = kK1K2[catalyst][styrene][H2O2]1/2 (1)

Where, [substrate] = concentration of styrene, [catalyst] = concentration ofTBA salt of Co2+ catalyst and [H2O2] = concentration of oxidant and k = overall rate constant and K1 and K2 are preequilibrium constants of the steps shownin the Scheme. Considering the steady state approach and if the total catalyst concentrationis expressed as [catalyst]T, which includes the concentrations of all theintermediate catalyst species, the rate can be given as:

kK1K2 [catalyst][styrene][H2O2]1/2

rate= ---------------------------------------------------- (2) 1 + K1 [H2O2]

1/2 + K1K2[styrene] [H2O2]1/2

For graphical evaluation of rate and equilibrium constants of the above

106 INDIRA et al.: OXIDATION

reaction, eq. 2 is rearranged in slope and intercept forms as:

[catalyst]T 1 1 1 1

------------- = ------------- ( ------------------- + -------) + ----- (3) rate [styrene] kK1K2 [H2O2]

1/2 kK2 k

[catalyst]T 1 1 1 1

------------- = ---------- ( ---------------------- ) + --- (1 + ----------------) (4) rate [H2O2]

1/2 kK1K2 [styrene] k K2 [styrene]

The value of rate constant k was obtained from the intercept of the graph of[catalyst]T/ rate vs. 1/[styrene] which was a straight line having slope andintercepts. Similarly, if a graph of [catalyst]T/ rate vs. 1/[H2O2]

1/2 which gave astraight line and substituting the value of k in slope and intercept, the values ofK1 and K2 were obtained. Graphically evaluated values of k, K1 and K2 for theoxidation of styrene to benzaldehyde catalyzed by TBA salt of Co2+ at 338 Kare k = 0.17 min-1, K1 = 0.27 M-1 and K2 = 47.61 M-1, respectively. From the activation energy Ea determined from the graph of ln k vs. 1/T(Fig. 2), the other activation parameters were calculated using standardequations. These activation parameters are Ea = 3.74 kcal mol-1, ∆H# = 2.96kcal mol-1 , ∆S# = + 0.009 e.u. and ∆G# = + 2.94 kcal mol-1, respectively.

From the kinetic constants evaluated, K2 = 47.61 M-1 is at least sixteen timeshigher than K1 = 0.27 M-1. This signifies that (Scheme) the formation of species2 via catalytic activation of H2O2 is less facile than the formation of species 4.∆G# was found to be positive and hence the oxidation of styrene does not occurspontaneously and needs further improvement in the catalytic activity of TBAsalts for the above reaction.

CONCLUSIONS

TBA salt of Co2+ was found to be the best catalyst for the oxidation ofstyrene to give benzaldehyde as the major product. Finding more active catalystsystems for the oxidation of styrene has been suggested for oxidative C=Ccleavage to give the corresponding aldehyde. Free radical mechanism suggestedfor the above oxidation was consistent with the spectroscopic and kineticobservations made in the above study.

INDIRA et al.: OXIDATION 107

Acknowledgements. One of the authors Ms.V. Indira thanks Dr. P.A. Joy forhelpful suggestions and discussions made in this study.

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