Deactivation kinetics of para-selective toluene disproportionation over modified ZSM-5

Abstract

The deactivation kinetics of toluene disproportionation to yield a mixture of benzene and xylenes with a high para-selectivity has been studied using a Si-Mg modified ZSM-5 catalyst. The kinetic model developed includes the deactivation of the main and the secondary reactions and takes into account the influence of the intracrystalline diffusional steps. The best fit of the experimental data was obtained when toluene is assumed to be the coke precursor. The order of deactivation of the main reaction is close to 3, which agrees with the control of the overall reaction rate by the intracrystalline diffusion. The secondary reaction of p-xylene dealkylation is deactivated faster than the main reaction. However, the activity of the secondary reaction of p-xylene isomerization, which takes place only over the external zeolite surface, remains constant and close to unity. These differences among the deactivation rates have been related to the mechanism of deactivation and to the strength of the acid sites involved in each reaction.