Comparative study of shape-selective toluene alkylations over HZSM-5

Abstract

The alkylation of toluene with methanol, ethanol, and 2-propanol was studied in a comparative manner using medium-port HZSM-5-type zeolite catalysts differing in crystal size and morphology and in the presence of an amorphous silica-boria phase. For comparison with large-port zeolites, propylation was also performed on H-mordenite (HM) and HY. A typical small-crystal HZSM-5 sample is non- para-selective in methylation and ethylation but exhibits substantial para-selectivity in propylation. When the catalyst is a large-crystal HZSM-5, para-selectivity is established in the case of methylation and ethylation and improves in the case of propylation; it is further enhanced with a large-crystal sample containing the amorphous phase. In contrast with HM and HY, which in toluene propylation produce only cymene, the HZSM-5 samples also catalyze the production of n-propyltoluene whose concentration in the product mixture can be twice as high as that of cymene. Cymene becomes the preferred product (up to ~10: 1 ratio) upon increasing space velocity or time on stream. It is proposed that in the case of C 3 and perhaps also larger ring substituents, skeletal isomerization is operative in ZSM-5 in addition to positional isomerization. Shape selectivity as revealed in the above comparison can be explained by assuming both internal and external catalytic activity in HZSM-5, as opposed to the essentially exclusively internal activity of HM and HY. It is further postulated that the proposed external effect is shape-selective, discriminating between molecules at a range in between that of the medium-port 10-ring channels as found in HZSM-5 and that of the large-port 12-ring channels characterizing HM and HY.