Decisive role of transport rate of products for zeolite para-selectivity: Effect of coke deposition and external surface silylation on activity and selectivity of HZSM-5 in alkylation of toluene

Toluene alkylation with methanol and ethylene over HZSM-5 zeolites has been investigated in order to understand the effects of deposited "coke" and postsynthesis surface silylation on the toluene conversion and product selectivity. Long time-on-stream kinetic runs, performed under conditions in which total toluene conversion did not decrease but concentration of coke continuously increased were used to follow the changes in the individual xylene selectivity, the activity of the external surface, and the transport rates of individual xylene isomers. A very small increase in ortho- and para-xylene selectivity, no changes in xylene diffusivities, but a substantial decrease in the activity of the external surface were observed with increasing coke deposition. Under severe reaction conditions when coking caused a substantial decrease in toluene conversion, nearly no increase in para-xylene or para-ethyltoluene selectivity was found. The silylation of ZSM-5 zeolite with tetraethylorthosilicate blocked active sites on the external zeolite surface and decreased considerably the diffusivities of xylenes and ethyltoluenes. This modification also increased substantially the para-selectivity at high toluene conversions. Comparing the effect of coke deposition and surface silylation on the para-selectivity it is concluded that a decrease in the transport rates of xylenes is decisive for an enhancement of the para-selectivity, whereas secondary isomerization of para-isomers on the external surface plays a minor role.