CONICET | Buscador de Institutos y Recursos Humanos

The processes of catalytic partial-oxidation of aromatic hydrocarbons in the vapor phase, are of great importance in the chemical technology [1] and it has been subject of many research work. This is so because the oxidation catalysis is complex and involves many parallel and consecutive reactions. There have been few comprehensive studies of the kinetics of selective-oxidation reactions. In particular, for toluene oxidation, kinetic expressions are usually of the power rate law type and Mars-van Krevelen redox model type [2-6]. In many works [4-6], low hydrocarbon pressures and high oxygen pressures were used. In consequence, the rate expressions are nearly first order in hydrocarbon pressure and zero order in oxygen pressure. The data available in the literature, about vapor-phase oxidation of toluene, shows that it is common to obtain selectivity to carbon oxides of 40% and higher, even at low toluene conversion levels [7-8]. This would indicate the presence of a route for direct oxidation of toluene to carbon oxide. However, there are few works [2] considering this direct oxidation of toluene by a parallel route, in the kinetic studies. In a previous work it was found that the vanadium antimonate doped with titanium, prepared by solid state reaction and with nominal composition VSb0.8Ti0.2O4 [9], presents good catalytic performance in the toluene oxidation reaction. The products obtained with this catalytic system were benzaldehyde and carbon oxides. In the present work, a kinetic study of vapor-phase oxidation of toluene by air, using a titanium-doped vanadium antimonate as catalytic system, is reported. The aim is to propose a kinetic scheme suitable to identify the main reaction-steps, to determine a kinetic expression based on redox mechanism and to analyze the influence of the operating variables, such as, temperature, toluene/oxygen feed-ratio and residence time on the toluene conversion and benzaldehyde selectivity. The catalytic oxidation of toluene was studied in an integral fixed-bed reactor operated at atmospheric pressure and working at the appropriate operating conditions to assure the absence of internal and external diffusion limitations in the catalysts particles. From the analysis of the results obtained, a kinetic scheme that considers the partial oxidation of toluene to benzaldehyde, a consecutive oxidation of benzaldehyde to carbon oxides and a parallel oxidation of toluene to carbon oxides, was proposed. The approach is oversimplified with respect to the true reaction mechanism, but provides a better description of the experimental data than the empirical approaches (power low equations). The reaction network was modeled assuming the redox mechanism proposed by Mars and van Krevelen. A satisfactory fit of the data was found. In the range of the operating conditions, the overall rate depends on the partial pressures of the reactants and there is no rate-controlling step. Temperature was the variable with the higher effect on the kinetic behavior. Both steps of toluene oxidation have similar activation energies indicating that the same intermediate is involved. The activation energy of benzaldehyde total oxidation is lower than benzaldehyde production from toluene. This means that higher temperatures enhance the selectivity to benzaldehyde. The benzaldehyde selectivity has a maximum value depending on the rate-coefficiets ratio of total and partial oxidation of toluene. This maximum value is independent of the temperature due to the fact that the activation energies for partial and total oxidation of toluene have equal values. The toluene conversion is higher at lower values of the feed molar ratio (hydrocarbon molar fraction/oxygen molar fraction). On the other hand, the yields are independent of the feed molar ratio indicating that hydrocarbon oxidation and catalyst reoxidation take place at different active sites as it is proposed by the redox mechanism.

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