Influence of the level of dealumination on the selective adsorption of olefins and paraffins and its implication on hydrogen transfer reactions during catalytic cracking on USY zeolites

The ratio of cracking to hydrogen transfer for n-heptane and gasoil increases with increasing dealumination of HYUS samples. In gasoil cracking, a sharp increase in this ratio occurs for samples with less than 10 Al per unit cell (uc). The adsorption of n-butane and 1-butene on HYUS samples shows that the amount of these gases adsorbed decreases with increasing dealumination. A decrease in the effective concentration of the product on the zeolites would favour monomolecular (cracking) over bimolecular (hydrogen transfer) reactions. The ratio of 1-butene to n-butane adsorbed decreases considerably below 10 Al/uc, indicating a less selective adsorption of olefins with respect to paraffins when the hydrophobicity (framework silicon-to-aluminium ratio) of the zeolites increases. These adsorption effects have an important influence on the cracking-to-hydrogen transfer ratio observed, as the hydrogen transfer reactions necessarily involve olefins, whereas cracking of n-heptane at low conversions and of paraffinic gasoil involves mainly paraffins.