Oxidation of alkyl aromatics Part II : oxidation of toluene to benzoic acid in presence of promoters

Abstract

The investigation on the catalytic vapour-phase oxidation of toluene in the presence of gaseous promoters was taken up to determine the operating conditions favourable for the maximum conversion to benzoic acid. The effects of addition of promoters were studied. Thermodynamic feasibilities of the reaction were evaluated. The vapour-phase oxidation of toluene was carried out on tin vanadate catalyst reported to be best suited for production of benzoic acid, giving a maximum conversion to benzoic acid of 15 mole per cent. Water vapour had no effect on the reaction. Introduction of bromine and nitrogen dioxide increased the conversions to about 28 mole per cent in each case. Sulphur dioxide increased the conversion to about 35 mole per cent. A 2² factorial experiment was carried out to assess the effects of temperature, time factor, air-to-toluene ratio and sulphur dioxide concentration. Effect of concentration of sulphur dioxide did not show any significance. This may be because sulphur dioxide in the concentration range studied acts as chain initiators and so does not get consumed in the reaction. This was confirmed by sending into the reactor only SO? in the concentration range used earlier, along with I? instead of air. The conversions to products were negligible. This confirms that sulphur dioxide does not get consumed in the reaction. Hence it acts as a promoter. Temperature and time factor show significant effect while air-to-toluene ratio shows very high significance. Hence the ratio must be maintained strictly at the same level. A central composite rotatable design for 4 factors was carried out to arrive at optimum combination of factor levels, with a view to maximise the conversion to benzoic acid. A second-order response surface is fitted to the data and the regression coefficients of its equation found. From the fitted equation the optimum combination of levels is determined and its nature tested by transforming the fitted equation to its canonical form and examining the coefficients. A true maximum was obtained. To develop a model and a rate expression, a 2² factorial experiment with rate constants as the observations instead of conversions themselves was carried out. The rate constants were calculated by Newton-Gauss iteration technique. The factors studied are air-to-toluene ratio, temperature and sulphur dioxide concentration. The consecutive reaction model assumed, i.e., toluene reacting with oxygen to give benzoic acid which further oxidises to carbon dioxide, was found to be adequate. None of the factors showed any significant effect on the rate constants. The negligibly small effect of temperature on the rate constants is indicative of very small apparent energies of activation of the order of 1 Kcal/mole. This, as compared to 13.5 Kcal/mole and 15.84 Kcal/mole when no promoter is used, is quite small. This confirms the promoter theory that when a substance promotes the reaction it does so by increasing the electron work function and hence decreasing the apparent energies of activation. X-ray diffraction patterns of the catalyst before and after use with promoter show that there is no change in the structure of the catalyst.