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 at 673 K have been examined. Remarkably similar behavior was observed in terms of both product selectivities and catalytic activity at alkane partial pressures below 6 kPa, low conversions and before deactivation. This supports the earlier conclusion, based on a higher temperature reaction study, that the Bronsted acid sites of both Y zeolites are chemically similar. The contribution of bimolecular and oligomeric cracking to the overall reaction rate became increasingly dominant with increasing alkene partial pressure, whether it resulted from a higher conversion of alkane or a higher alkane pressure 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 observed between chemically dealuminated and steam dealuminated Y zeolites is that the former deactivated more quickly and to a greater extent. This was explained 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 after deactivation has occurred, the steamed zeolite is much more active. (C) 2000 Elsevier Science B.V. All rights reserved.