Deactivation and coking of HZSM5 catalysts during alkylation reactions

The catalytic methylation of toluene over HZSM5 post-synthesis modified by silylation was studied in order to understand the dependence of the catalysts activity, selectivity and stability upon type and concentration of sorbed surface species. Activity and selectivity are strongly influenced by the concentration of slowly diffusing products (e.g. m-xylene, trimethylbenzenes) in the zeolite pores. While small particle ZSM5 samples did not deactivate, large particle HZSM5 and surface silylated samples decreased in activity with time on stream. Deactivation was attributed to blocking of the catalytically active sites by trimethylbenzene molecules which cannot dealkylate or leave the catalyst under our reaction conditions (570K). The more severe the steric restrictions for the transport of the bulkier products in the pores are, the higher is the p-selectivity of the sample and the stronger is the effect of deactivation due to site blocking.