Kinetics of toluene alkylation with methanol over magnesium-modified ZSM-5

Abstract

The kinetics of toluene alkylation with methanol has been studied over both unmodified and Mg-modified ZSM-5 catalysts. On the basis of the product distribution obtained over the unmodified zeolite, the following secondary reactions have been considered: methanol dehydration, toluene disproportionation and xylene dealkylation. Power law and Langmuir-Hinshelwood-Hougen-Watson (LHHW) models have been tested for the main toluene alkylation reaction, the best fits to the experimental data being achieved with two models: a second-order power law model and a model based on the Rideal-like mechanism. The first one has been applied to the results obtained with the Mg-modified catalyst, including the effect of the diffusion rate, due to its simpler expression. Over this catalyst p-xylene is the only isomer present in the primary product, hence m- and o-xylene formation takes place by p-xylene isomerization on the external surface catalytic sites. Therefore, the kinetic model developed for the Mg-modified zeolite takes into account, as well as diffusional effects, the influence of p-xylene isomerization over the external zeolite surface, reproducing the experimental product distribution with an average relative error of 6.8%