DEALUMINATION OF ZEOLITES .3. EFFECT OF EXTRA-FRAMEWORK ALUMINUM SPECIES ON THE ACTIVITY, SELECTIVITY, AND STABILITY OF Y-ZEOLITES IN NORMAL-HEPTANE CRACKING

A series of dealuminated Y samples with between 12 and 48 framework aluminum species per unit cell (NAl) were prepared by steaming of a NH4NaY zeolite. A treatment with dilute HCl solution was applied to the samples in order to eliminate, without modifying the framework composition, the extra-framework aluminum species (EFAL) created by steaming. The activity, stability, and the selectivity in n-heptane cracking at 350°C of the two series of samples were compared. For high values of NAl, EFAL species have a promoting effect on the rates of isomerization, cracking, hydrogen transfer, and coking but increase the deactivating effect of coke. An inductive influence of their Lewis acid sites on the protonic sites of the zeolite (superacid sites") would be responsible for these effects. For NAl < 15 the reaction rates do not depend on the EFAL species probably because they are too far from the protonic sites. With all the reactions the activities of the dealuminated samples pass through a maximum for NAl between 35 and 40. The cracking activity of the highly dealuminated samples of the series containing few EFAL species is proportional to the number of the framework acid sites whereas for hydrogen transfer and for coking the activity depends also on the acid site density. This is attributed to the fact that these two reactions require many bimolecular steps and therefore demand the participation of several acid sites in their catalysts. © 1991."