Acid catalysis by dealuminated zeolite-Y: I. Methanol dehydration and cumene dealkylation

Abstract

A series of dealuminated Y-type zeolites was prepared by reaction of HY with SiCl 4 at elevated temperatures. Methanol dehydration and cumene dealkylation were used as test reactions to probe acid sites of weak and moderate strength, respectively. Zeolites dealuminated at temperatures up to 550 °C exhibited catalytic activity for both reactions, but samples dealuminated at 750 °C were relatively inactive. Both for methanol dehydration and for cumene dealkylation the activity was linearly proportional to the number of framework aluminum ions over the range from 0 to 14 aluminum ions per unit cell. Thus, assuming one acid site per framework aluminum ion, the turn-over frequency (TOF) was essentially constant in this range. The activity of catalysts containing ca. 30 Al/u.c. was less than expected for both reactions, probably because of diffusional limitations. Moreover, a normal HY zeolite exhibited much less activity than anticipated on the basis of its framework aluminum content. The difference in TOF between the dealuminated and the HY zeolite suggests that the latter material has a large number of weak acid sites. The difference was accentuated by the more demanding cumene dealkylation reaction.