Kinetics of the hydroxylation of benzene with N 2O on modified ZSM-5 zeolites

Abstract

A detailed kinetic study of the hydroxylation of benzene to phenol using nitrous oxide (N 2O) is performed on different, well characterised modifications of ZSM-5 type zeolites. After process studies in a fixed bed reactor for the first time a recycle reactor (Berty-type) is successfully used in direct measurements of the molar rates of reactant consumption and product formation. From those data, an extended reaction network is developed which contained mechanistic steps derived from additional studies. Besides the formation of a chemisorbed, monoatomic oxygen species from N 2O and phenol formation, it can be shown that a consecutive reaction of phenol without the participation of oxygen and a non-selective oxygenation of strongly adsorbed hydrocarbons by oxygen play an important role. The former step is attributed to the accumulation of phenol inside the ZSM-5 crystal due to strong adsorption and slow diffusion of phenol. The latter step results in an accumulation of oxygen within the adsorbed hydrocarbon species until a critical oxygen content is reached and total oxidation starts. An isothermal two-site kinetic model is deduced containing the sorption of the reactants and products. The values of the model parameters allow a comparison of the efficiency of the catalysts on the rates of each step. Simulations of the selectivity–conversion behaviour impressively demonstrate the quality of the kinetic model as predictive tool for process studies.