The relationship between the acidity and the hydrocarbon cracking activity of ultrastable H-Y zeolite

Changes in the structural, acidic, and catalytic properties of H-USY (acidic ultrastable Y zeolite) that occur during steam dealumination were investigated. This study focused on three factors that previously have been suggested to cause the enhanced activity of H-USY: (1) increased Brønsted acid strength caused by nonframework Al; (2) increased Brønsted acid strength caused by decreased framework Al content; and (3) direct participation of Lewis acid sites in the cracking reaction. Acidity was characterized by microcalorimetry and FTIR of NH₃ adsorption.; The 2-methylpentane cracking activity of H-USY at 573 K was 35 times higher than that of H-Y that had not been steamed. With further steaming of H-USY, the cracking activity decreased, although the activity per strong Brønsted acid site remained essentially constant. H-USY, with both Brønsted and Lewis acid sites, had a heterogeneous acid strength and many acid sites with heat of NH₃ adsorption >130 kJ/mol.; In contrast, zeolites containing only Brønsted acid sites had a rather homogeneous acid strength. The heat of NH3 adsorption did not exceed 130 U/mol for (H,NH₄)-USY, in which the strongly acidic Lewis acid sites were covered by NH₃, but its activity was equal to that of H-USY. Thus, Lewis acid sites are inactive for hydrocarbon cracking. Dealumination by ammonium hexafluorosilicate, which produces very little nonframework Al, resulted in a zeolite with a low heat of NH₃ adsorption equal to that of H-Y, and activity only three times higher than that of H-Y.; The mechanism of coke deactivation in H-USY was studied. Coke caused a proportionally larger decrease in n-hexane cracking activity than in the number of acid sites, but did not cause pore blockage or reduced n-hexane diffusivity. The evidence is consistent with a site poisoning deactivation model for a diffusion-limited reaction.; In conclusion, the enhanced cracking activity of USY is not caused by Lewis acid sites nor by Brønsted acid sites with a very high heat of NH₃ adsorption. However, if cracking is diffusion-limited, then the mesopores and fractures formed during steam dealumination may increase its observed cracking activity by increasing mass transport rates to acid sites located near the mesopores.