On the role of Bronsted acid sites in the reactions of ethylene over ZSM-5 and HZSM-5 zeolites

The ZSM-5 and HZSM-5 zeolite samples dehydoxylated in situ to different extent were evaluated for their activity for the reactions of ethylene in a fixed bed now reactor, operating at 1 atm pressure and at temperatures in the range 100-300 degrees C. On dehydroxylation at temperatures in the range 300-700 degrees C, the catalytic activity of ZSM-5 zeolites remained almost unchanged while that of the HZSM-5 showed a marginal increase, even though the hydroxyl group concentration in both reduced considerably. These zeolites were active for ethylene oligomerization, homologation and cyclization reactions, though at a reduced level, even when the zeolites were almost completely devoid of hydroxyl groups after a heat treatment at 900 degrees C. These studies thus reveal that the initiation of the above mentioned reactions may follow an alternate route in the absence of the Bronsted acid sites.