Parameters in Addition to the Unit Cell That Determine the Cracking Activity and Selectivity of Dealuminated HY Zeolites

The cracking activity and selectivity for n-heptane and gas-oil, of a series of dealuminated HY zeolites (SiCl4 or steam dealuminated) have been measured. The results have been related with the different types of aluminium and acid groups present and to the pore volume distribution. It is clearly shown that the unit cell parameter (framework Si/Al ratio) can not explain, by itself, the activity and selectivity of dealuminated zeolites. Moreover, the sharp changes in selectivity observed for samples with ao below 24.24 Å cannot be explained only on the bases of the strength and density of the acid sites. In order to explain those results, we must consider the different types of tetrahedral and octahedral extraframework aluminium as well as the texture and adsorption hydrophobicity characteristics of the zeolites. Finally, the existence of radical-type cracking, which becomes especially important on highly dealuminated HY zeolites, must be taken into account.