Journal of Molecular Catalysis, 53 (1989) 209 - 217 209

KINETICS AND MECHANISM OF COBALT BROMIDE CATALYZED OXIDATION OF p-XYLENE IN THE PRESENCE OF PHASE TRANSFER CATALYSTS

M. HARUfiTIAK, M. HRONEC and J. ILAVSKP

Department of Organic Technology, Faculty of Chemistry, Slovak Technical University, 812 37 Bratislava (Czechoslovakia)

(Received July 15, 1988; accepted November 11,1988)

Summary

The kinetics and mechanism of high-temperature (120 - 140 “C) liquid- phase oxidation of p-xylene in the presence of water, catalyzed by phase transfer agent and CoBrz have been studied. Lipophilic quaternary ammo- nium cations act as phase transfer agents for Br- and Co-Br anionic com- plexes. Maximum absorption rate of oxygen can be expressed as follows:

-dOz/dt = k[PX]2[Co”]o~2[Br-]0~2[Q]o~3[H20]-0~3

with k = 2.2 X 10m4 dm3 mol-’ s-l at 120 “C. A proposed free-radical mechanism implies bromine radicals as initiat-

ing species.

Introduction

The oxidation of methyl-substituted benzenes by molecular oxygen to the corresponding carboxylic acids is carried out in commercial processes preferably using acetic acid as a solvent. This solvent acts positively in heat transfer and manipulation with solid products. Cobalt salts or their combi- nations with activators or cocatalysts are commonly used as catalysts [ 11.

As we have recently shown [2], when water is used as a solvent instead of acetic acid, a two-phase (aqueous and organic) reaction system is formed. In such a system, cobalt bromide catalysts are inactive in p-xylene oxidation even at high temperatures and oxygen pressures. A catalytic system com- posed of cobalt bromide and quatemary ammonium or phosphonium salts is very active in the above-mentioned reaction. We have proposed that lipo- philic quatemary onium cations act as phase transfer agents for catalysts species or precursors [2]. This is the main difference from obviously studied phase transfer catalyzed reactions [3] where some inorganic anions are trans- ferred into the organic phase as stoichiometric oxidants. The studied oxida- tion reaction is an example of a simultaneous phase-transfer and metal-

0304-5102/89/$3.50 0 Elsevier Sequoia/Printed in The Netherlands

210

catalyzed reaction. Such types of ‘multiple catalysis’ [4] have been exten- sively studied in oxidation, reduction and c~bonylation reactions [5,6].

As an extension of our studies on p-xylene oxidation in two-phase systems (aqueous-organic), we have investigated the function of quaternary ammonium cations, cobalt bromide catalysts, water and some radicals on the kinetics of oxidation.

Experimental

Ma teriuls Hydrocarbons were purified by distillation. Bu4NfHS0, (TBAHS)

was prepared from Bu,N+Br- (TBAB) and dimethyl sulphate by the pub- lished method [ 31. Bu,N+NO, was prepared by treating aqueous solutions of TBAB with AgNOJ. AgBr was filtered out and after evaporation of water Bu4N+NOs- was crystallized. Quaternary ammonium salts of general for- mula RN+(Et),Br-, where R is n-alkyl C2 - Cl*, were prepared by quatemi- zation of triethylamine with RBr in an acetone-ethanol solution under reflux for 20 - 40 h. After evaporation of solvents and unreacted compounds, the crude products were twice crystallized from the ethanol-benzene system.

Kinetic measurements The rate of oxidation was measured by following the oxygen consump-

tion under constant pressure and temperature in a stainless steel reactor, described previously [ 71. Reaction rates were measured over the range of speed of agitation where transport phenomena did not limit the reaction rate. Initial and maximum reaction rates (in moles of oxygen consumed per liter of reaction mixture per second), were calculated from the plots of absorbed oxygen versus time. The apparent reaction orders with respect to reagents and catalysts were estimated in the region of initial and maximum rates at 120 “C. In the measurements to evaluate reaction orders, the initial concentrations of all components of the reaction mixture, except the reagent measured, were kept constant. Approximated values of the apparent reaction orders were optimized. The minimum of the sum of the squares of differ- ences between calculated and experimental rates was determined using Rosenbrock’s rn~tip~~e~c optimization procedure [ 81,

Phase transfer equilibria measurements Phase transfer efficiency of quaternary ammonium salts and ‘salting in’

phenomena were estimated by measuring concentrations of Br- and Co(I1) transferred into the organic phase and by measuringp-xylene concentration in the aqueous phase, respectively. All components in the same concentra- tions as in kinetic measurements were put into a stainless steel autoclave and heated in nitrogen atmosphere. The mixture was stirred (20 min) and after resting (30 min) at constant temperature without mixing, samples from both phases were withdrawn. Samples from the organic phase were twice

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extracted with excess water, and concentrations of Bf and Co(I1) were determined in the aqueous extract. The concentration of Co(I1) was esti- mated by polarography and that of Br- by argentometric titration with potentiometric indication. The concentration of p-xylene in the aqueous phase (for examining ‘salting in’ phenomena) was estimated by UV spectros- copy. A sample of the aqueous phase from the autoclave was extracted with n-hexane, and the UV spectrum of the extract recorded. The concentration of p-xylene was calculated at a wavelength of 216 nm.

Electron spectroscopy The formation of Co-Br complexes in both phases - aqueous and

organic - was studied by electron spectroscopy in the visible region (13 X lo3 - 23 X lo3 cm-‘). A thermostatted stainless steel mixing autoclave (20 cm3) with glass windows (1 cm) was used for these measurements.

Analysis of reaction products Reaction products from p-xylene oxidation were analyzed with a

Hewlett-Packard 5830 gas chromatograph. Samples were doped with dime- thy1 terephthalate as an internal standard. The stationary phase was 15% neopentyl glycol succinate and 1% HsPO, on Chromaton NAWDMCS. The concentration of terephthalic acid was determined by gravimetry after extraction of the sample with acetone.

Results

In our previous paper [2] we proposed a phase transfer activity of lipophilic quatemary onium cations in the oxidation of p-xylene in the presence of water catalyzed by CoBr,. However, quaternary ammonium cations may also act as ‘salting in’ agents, surfactants, complexing agents or as activators of molecular oxygen. In order to elucidate the extent of these effects, we performed a series of experiments for quantitative determination of these side-effects of quatemary salts.

Quatemary ammonium salts increase the solubility of organic com- pounds in water [9, lo]. The increase of p-xylene solubility in the aqueous phase was studied by equilibria measurements at 125 “C with TBAHS in autoclave (Table 1). These data show that a large increase in quatemary salt concentration causes only a moderate increase in p-xylene solubility in water, but within the same concentration conditions the enhancement of oxidation rates is significant. This suggests that the catalytic activity of quatemary ammonium salt is not a result of ‘salting in’, and that oxidation proceeds in the organic phase.

Ohkubo et al. [ll, 121 found that quatemary onium compounds (QX) alone act as catalysts in the oxidation of some hydrocarbons by molecular oxygen at temperatures of 40 - ‘70 “C. The authors proposed a direct activa- tion of molecular oxygen by the quaternary onium cation (eqn. 1):

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TABLE 1

The influence of TBAHS concentration in reaction system (CQ) on p-xylene solubility in water (S) and on reaction rates ria

CQ x lo3 s x 103 (mol dmW3) (mol dmp3)

Pi x 105 (mol dm-3 s-l)

0 2.0 0 8.1 2.0 0.5

14.7 2.1 0.9 29.5 2.1 1.9 52.6 2.2 3.3 73.7 2.4 4.7

147.5 2.4 9.3 199.8 2.6 12.8 399.6 3.3 24.8

“p-Xylene 0.72 mol, water 1.33 mol, CoBrz 2.4 mmol.

Q++02 + Q+6...02-S~R’+HO;+Q+ (1)

The possibility of this reaction was studied in the reaction system composed of p-xylene and water. It was shown that in the presence of cobaltous sul- phate and TBAHS, or cobaltous nitrate and Bu4N+N03-, i.e. in the absence of Br-, the oxidation reaction does not proceed under the studied condi- tions. This indicates that direct activation of oxygen by quaternary ammo- nium cations does not play a significant role in the catalytic activity of Q.

Quaternary ammonium salts can thermally or chemically decompose under reaction conditions. Amines, olefins and alkylbromides are formed as byproducts [3, 91. It was found that such byproducts are catalytically inactive.

The formation of Co-Br complexes in the aqueous and organic phase was studied by spectroscopy in the visible region, under conditions similar to those of the oxidation reaction. In the aqueous phase, bromocobalt com- plexes are not formed from CoBrz and QBr, even at high temperature. Only hydrated cobaltous cations are present. In the organic phase, bromocobalt- ous complexes were detected at higher CoBrZ and QBr concentrations. Their spectra indicate that mononuclear tetrahedral CoBr,- and CoBr,-* complex anions [13] are probably formed. Concentrations of these species depend on the ratio of CoBr2 to QBr. We suggest that complex anions exist in the form of ionic pairs with quatemary ammonium cations.

The influence of the structure and lipophilicity of Q was studied with RN+(Et)sBr-, where R is n-alkyl with the number of carbon atoms (nc) 2 - 12. From Fig. 1 it is clear that the more lipophilic Q show higher cata- lytic activity in the studied oxidation, probably due to the preferential transfer of Br- and Co(U) complexes into the organic phase. This assumption was supported by equilibria measurements of Co(I1) and Br- concentrations in the organic phase (Fig. 2).

213

o 0 o 0

“.’ -9

0 l : -3

yy 0 y 10 - O-e

7 3- 4

0 l : -2 -

“4 0 0

-0

2 +e

0 -$I - a 0 E

Oh -7 ,*-

s -2

z2- 0 l

-1; &S6_ o-1

0

:’ 0

I I I gee -6

4 a 12 16 “C

2 4 6 8 10 R “C

Fig. 1. Depedence of the initial and maximum rates at 140 “C and 0.9 MPa on the number of carbon atoms (nc) in alkyl chains of Q; p-xylene 60 mmol, water 111 mmol, CoBr2 0.2 mmol, QBr 0.4 mmol.

Fig. 2. Dependence of Co(I1) and Br- concentrations in the organic phase on the number of carbon atoms in the alkyl chain of Q. Conditions identical to those in Fig. 1.

Oxidation of p-xylene in the presence of water, CoBr, and Q proceeds at temperatures above 11.0 “C and sufficient oxygen pressure. Usually only a short (5 - 10 min) induction period is observed and oxygen absorption curves exhibit autocatalytic reaction features.

The influence of reagent concentration was studied at 120 “C and 0.9 MPa oxygen pressure. The experimental results are depicted in Figs. 3 - 6. In the measurements of the influence of p-xylene concentration, the organic phase was maintained at constant volume by addition of chlorobenzene. In determining the influence of the concentration of quaternary ammonium cations, TBAHS was used as catalyst. This allowed the total initial Br- con- centration to be kept constant, since it is known [4] that the Br- anion is preferred over the HS04- anion in phase transfer into the organic phase. Kinetic data, based on the initial and maximum reaction rates are consistent with the following empirical rate equations.

-9

1 E

-a

2 3 ‘” ‘H20

Fig. 3. Effect of p-xylene concentration on the initial and maximum reaction rates; water 111 mmol, CoBr2 0.2 mmol, TBAB 0.4 mmol.

Fig. 4. Effect of water concentration on the initial and maximum reaction rates; p-xylene 60 mmol, CoBrz 0.2 mmol, TBAB 0.4 mmol.

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-11 j- I f ,

i;;/zyb;i ;y -,:::-=-: :j l -

l - -7#5

-11 - , I I I I I

-4 -5 -6 -3 -4 -5

'"cCD klc*

Fig. 5. Effect of Co(I1) concentration on the initial and maximum reaction rates; p-xylene 60 mmol, Hz0 111 mmol, TBAB 0.4 mmol.

Fig. 6. Effect of TBAHS concentration on the initial and maximum reaction rates; p- xylene 60 mmol, water 111 mmol, CoBrz 0.2 mmol.

ri = ki [PX][Co11]0~5[Br-]0~5[Q]0~8

[Hz,]‘.*

k [PX]2[Co11]0.2[Br-]o.2[Q]0~3

r max = max [H20]0.3

where the value of rate constants hi = 5.0 X lop5 dm3 mol-’ s-l and k,,, = 2.2 X 10e4 dm3 mol-’ s-l at 120 “C.

The influence of temperature on the rate of oxidation was measured with alkylaromatic hydrocarbons having different energies of the attacked C-H bond in the alkyl group. The apparent activation energies (E,) calcu- lated in the region of maximum reaction rates were found to be affected in terms of the C-H bond energy (Table 2).

The main reaction products and intermediates in the studied p-xylene oxidation are p-toluoylalcohol, p-toiualdehyde, p-toluic acid and terephthalic acid (Fig. 7). The concentration of aldehyde and alcohol intermediates

TABLE 2

Apparent activation energies and C-H bond energiesa

Hydrocarbon C-H bond energyb (kJ mol-1 )

E* (kJ mol-r)

PC

(MBa)

toluene 355 118 f9 1.0 140 - 160 p-xylene 355 120 f 9 0.9 120 - 140 ethylbenzene 343 86 + 6 0.8 100 - 122 cumene 331 67 +7 0.8 85 - 110

“Hydrocarbon 60 mmol, water 111 mmol, CoBrz 0.2 mmol, TBAB 0.4 mmol. bThe values of C-H energies are from [14]. cP = total pressure. dTI = temperature interval.

215

30 - 0

region of r,,,

0

mol% 0

20 - 0

0

10 - 0

g;“.: ;I:

I 1

10 20 30

Fig. 7. Yields of terephthalic acid (x), p-toluic acid (a), p-toluoyl alcohol (0) and p-tolu- aldehyde (0) during oxidation of a mixture of p-xylene (60 mmol) and water (111 mmol) at 140 “C and 0.9 MPa, catalyzed by CoBrz (0.2 mmol) and TBAB (0.4 mmol).

reached almost a steady value soon after the reaction started. At higher con- version, both p-toluic and terephthalic acid were formed.

Discussion

It is clear from experimental data that the catalytic activity of quater- nary ammonium cations during p-xylene oxidation in a two-phase system is based on the ability to transfer Br- from the aqueous into the organic phase. In the phase boundary, concentrations of Co(I1) and Br- are high. The in- crease in concentration of Br- transferred into the organic phase shifts the equilibria of formed Co-Br complexes, and results in an increase in the concentration of Co-Br complexes in the organic phase. Bromocobaltous complexes are precursors of the oxidation catalyst.

As we have previously reported [ 181, the initial rate of p-xylene oxida- tion in the presence of water catalyzed by quaternary ammonium salts and Br- only (in the absence of any transition metal) increases linearly with p-xylene concentration. In such a system, reaction (2) is the main initiation

Br- + RCHzOOH ----+ Br’ + RO’ + OH- (2)

step. Reaction orders with respect to p-xylene and Br- concentrations for initial rates in this metal-free system are the same as in the reaction with cobalt catalyst, described in this paper. This fact suggests that, during the initiation stage, the role of the cobalt catalyst is insignificant. Initiation in this stage is by bromine radicals formed from Br- anions by reaction (2). Bromide anions are transferred from the aqueous phase to the organic phase as lipophilic ionic pairs with Q.

In the later stages, Br- reacts with Co(II1) by reactions (3) - (5). The formation of Br’ radicals has been studied [ 16, 171 by reaction of Co(II1) and Bf in aqueous media, where the cobalt cation is hydrated. The oxida- tion of Br- occurs via formation of Co”‘--Br- complexes in rapid pre-

216

equilibria followed by a slow redox step, wherein the radical ion Br,‘- is an important intermediate:

rapid Co”’ + Br- - Co”’ Br-

ComBr- + Br- 5 Con + Br,‘-

Con1 + Br rapid

2 ‘- ---+ Con + 2Br’

In such a reaction system, reaction rates are proportional to both cobalt and bromide concentrations. Co(II1) ions [19] are regenerated via reaction (6).

co” RCHzOOH, RC03H

+ Co”’ + radicals (6)

Bromine radicals formed in reactions (2) and (5) abstract hydrogen from the hydrocarbon and terminate.

Br’ + RCH3 --+ RCH; + HBr (7)

Br’ + Br’ __f Br, (6)

Br’ + RCH2’ ---+ RCH2Br (9)

The last assumption can be deduced from a decrease in Br- concentration during oxidation. Thus, at higher conversion, the concentration of Br- in the reaction system is only 2 - 6% of its initial value.

The mechanism of cobalt bromide and phase transfer-catalyzed oxida- tion of p-xylene in aqueous medium in the latter stages is similar to the free- radical scheme published for cobalt-carboxylate catalysts in the absence of bromide promoters [19,21]. This supports experimental observations on the similarity of (i) the rate law for rmax, (ii) the distribution of reaction pro- ducts, and (iii) the activity of various metal catalysts [22]. In such reaction systems, the main function of the cobalt catalyst is to decompose peroxidic compounds. The electron transfer reaction of cobalt in the trivalent state with the hydrocarbon substrate plays only a minor role in the extended oxidation process [20,21].

In the oxidation of p-xylene catalyzed by amino complexes of CoBr2 [20], the nature of the solvent has significant influence on the reaction. Published results indicate that oxidation in acetic acid, benzoic acid and chlorobenzene is second order with respect to cobalt catalyst concentra- tion. In these reaction systems, reaction (10) is the main propagation step.

+ RCH200’ + Con 5 RCH200H + Co”’ (10) In contrast to these solvents, in aqueous system the reaction rate is only slightly dependent on the catalyst concentration. These facts are explained by assuming that water as a solvent suppresses the complexing of peroxy radicals with cobalt (in reaction (lo)), and propagation via reaction (11) proceeds.

217

RCH200’ + RCHs - RCH,OOH + RCH2’ (11)

The same situation also occurs in the phase transfer catalyzed system studied, where hydrated cobalt complexes and free radicals are both present.

As oxidation proceeds, p-toluic acid formed reacts with cobalt and thus improves its solubility in the organic phase. In this stage, the chain initiation by bromine species is masked by the predominant initiation with intermediately-formed peroxidic species. This effect results in the same kinetic equation in systems in both the presence and absence of phase transfer agents.

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