Kinetics of the liquid-phase oxidation of toluene by air

The liquid-phase oxidation of toluene by air is an environmentally benign process to produce benzoic acid and benzaldehyde. In a bubble column (diameter of 48 mm) reactor, the kinetics of the oxidation reaction was investigated under conditions similar to those of commercial operations. Based on the compositional analysis results of the oxidation products and the experimental observations, a plausible mechanism and reaction network were proposed. A kinetic model was derived from the proposed mechanism, and it describes the kinetics data very well. The reaction is observed to be first order, with regard to both toluene and oxygen. By fitting with the experimental data under conditions that simulate the commercial process, a macrokinetics equation for a temperature range of 145-175 degrees C was obtained. The apparent activation energy was estimated to be 40.957 +/- 1.89 kJ/mol. Deducting the influence of mass transfer from the microkinetics model, an intrinsic reaction rate equation based on the liquid concentrations of toluene and dissolved oxygen was derived. The intrinsic activation energy was estimated to be 57.35 kJ/mol and the pre-exponential factor was estimated to be 53.34 m(3) mol(-1) s(-1). Calculations determined under the reaction conditions showed that the Hatta number was given as Ha < 0.3 and the gas-liquid reaction effectiveness factor was given as eta = 0.61-0.73. The reaction is suggested to be slow, in comparison to the mass transfer of oxygen.