256
Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 3, 1978
The Oxidation of Substituted Toluenes by Cobalt(II1) Acetate in Acetic Acid Solution Charles F. Hendriks, Hendrik C. A. van Beek,' and Pieter M. Heertjes Laboratory of Chemical Technology, University of Technology, Delft, The Netherlands
The reaction products and kinetics of the oxidation of several substituted toluenes by Co"' acetate in acetic acid
solution were determined. Under anaerobic conditions the substituted toluenes were first converted into benzyl acetate and subsequently into benzaldehyde. Under anaerobic and aerobic conditions the same rate equation and rate constant were found for the oxidation, but the product distribution differed strongly. The main products of the aerobic oxidation are benzaldehyde and benzoic acid, derived from the substituted toluenes used. Regeneration of Co"' acetate occurs in aerobic solutions, which leads to stationary values for the concentration ratios Co"'/Co" and RCHO/RCH3. A reaction mechanism is proposed to explain the results obtained.
Introduction The oxidation of substituted toluenes in the gas and liquid phases is used in the technical production of bulk chemicals such as benzoic acid, phthalic acids, and phenol. The low reaction rates necessitate the use of a catalyst. In the liquid phase these oxidations can be catalyzed heterogeneously by metal oxides (VzOb,Co203)and homogeneously by salts of the transition metals, in particular of Co and Mn. In 1960, Brill investigated the oxidation of substituted toluenes in aerobic acetic acid solutions in the presence of Co"' acetate as catalyst at relatively low temperatures (90-110 "C) and oxygen pressures (0.2-1.0 atm). Since 1960 several investigations of the mechanism of the reaction have been published which supplied valuable but scattered additional information (Morimoto and Ogata, 1967; Kamiya and Kashima, 1972; Scott and Chester, 1972). We now present a comprehensive study of this reaction combining the results of separate studies of the autoxidation of aldehydes derived from the substituted toluenes (Hendriks et al., 1978b) and of the oxidation of these hydrocarbons by Co"' acetate under anaerobic conditions. Experimental Section Materials. The substituted toluenes were purified by distillation or crystallization. Co" acetate tetrahydrate had A.R. quality as received. Co"' acetate was prepared in acetic acid solution by oxidation of Co" acetate with a stoichiometric amount of peracetic acid. Acetic acid was purified by distillation (bp 118-120 "C at 1 atm). Water was added to the fraction used until a concentration of 2.5 M was reached in order to maintain an almost constant water concentration in the solution during the experiments. Procedures. The oxidations were carried out in a cylindrical Pyrex glass vessel (diameter 6 cm, length 15 cm), which was connected at the bottom to a Pyrex glass cell (path length 0.30 cm), placed in a Vitatron UC 200 colorimeter. The solutions, with a total volume of 100-150 mL, were stirred at a rate of 2000 rpm. Co"' acetate concentrations were measured by determining the extinction of Cornacetate at 652 nm, where absorption of Coq acetate is negligible. Hydrocarbon and aldehyde concentrations were determined gas chromatographically with a Varian Aerograph 1522-1B, using a 3-m 10% SE-30 on Chromosorb-W column. Peroxide concentrations were determined by iodometric titration. Consumption of oxygen was measured volu-
metrically. The initial hydrocarbon concentrations were varied between 0.1 and 2.0 M; the initial Co" and Co"' acetate concentrations were between and 2 X lo-' M. Reaction products were identified and determined (after conversion of carboxylic acids into the corresponding methyl esters with diazomethane) with GC/MS, using a Varianmat 111 GNOM mass spectrometer. Curve fitting of the concentration curves of reactants and products was carried out by integration of the reaction rate equation according to the procedure of Runner and the method of Runge-Kutta (Smith et al., 1970). The minimum of the sum of the squares of the differences between the experimental and calculated values was determined with the aid of the Optow procedure and the method of Powell (1964). To control the accuracy, the calculations were carried out with two time-step values with a difference of a factor of 2. Results Addition of a substituted toluene to an anaerobic acetic acid solution of Co"' acetate resulted in the initial formation of the corresponding benzyl acetate and Co" acetate as the main products and of the corresponding benzyl alcohol and 1,2-diphenylethane as by-products (1-5%). During prolonged reactions small quantities of the corresponding benzaldehyde and benzoic acid were also formed. The formation of the main products for the oxidation of toluene by Co"' acetate is given for a typical experiment in Figure 1. The average relative amounts of the products formed by the oxidation of toluene are given by eq 1for the initial stage of the reaction [CO"] = 2,05[C6HbCHzOCOCHa]
(1)
This relation changes during the later stages of the reaction, because of the increase of the concentration of benzaldehyde, into eq 2. [CO"] = 2,04[CGHbCH2OCOCH3]+ 4.02[CeHbCHO] (2) The validity of eq 1 and 2 appeared to be independent of the initial concentrations of the reactants. Analogous results were obtained for other substituted toluenes. The initial rate (95% of the hydrocarbon is converted. This is illustrated for the oxidation of toluene and p-xylene in Figures 4 and 5. The values of the concentration ratio Co"'/Co" and RCHO/RCH3 during the steady state are independent of the initial concentrations and the oxygen pressure. However, at Corn concentrations >0.05 M or oxygen pressures
