An explanation for the differences in catalytic hydrocarbon cracking activity between steam and chemically dealuminated Y zeolites

The effects of varying alkane and alkene partial pressures in the catalytic cracking of n-hexane over a chemically or a steam dealuminated Y zeolite a t 673 K have been examined. Remarkably similar behavior was observed in ter ms of both product selectivities and catalytic activity at alkane partial p ressures below 6 kPa, low conversions and before deactivation. This support s the earlier conclusion, based on a higher temperature reaction study, tha t the Bronsted acid sites of both Y zeolites are chemically similar. The co ntribution of bimolecular and oligomeric cracking to the overall reaction r ate became increasingly dominant with increasing alkene partial pressure, w hether it resulted from a higher conversion of alkane or a higher alkane pr essure at constant conversion. Even at the lowest alkane partial pressures studied, reactions between the alkene products and the reactant contributed to the apparent cracking activity of the zeolite. The main difference obse rved between chemically dealuminated and steam dealuminated Y zeolites is t hat the former deactivated more quickly and to a greater extent. This was e xplained by the different porosities of the two zeolites. With less severe treatment of the zeolite structure, the chemically dealuminated zeolite is more sensitive to deactivation, and consequently, when rates are measured a fter deactivation has occurred, the steamed zeolite is much more active. (C ) 2000 Elsevier Science B.V. All rights reserved.