Monomolecular cracking of n-hexane on Y, MOR, and ZSM-5 zeolites

The activity and activation energy for cracking of n-hexane were measured over three zeolites: HZSM-5, H-MOR, H-USY (ultrastable Y zeolite), and CDHY (AHFS dealuminated Y zeolite), under conditions where the product selectivities were nearly identical and there was no deactivation of the catalysts. The identical selectivities implied that the same reaction mechanism for cracking was occurring over all three catalysts. Within experimental error, the differences in apparent activation energies could be entirely attributed to differences in heats of n-hexane adsorption, such that the intrinsic activation energies were identical. These results suggest that the kinetics of cracking is insensitive to any possible differences in acid strength among these catalysts, and the large enhancement in activity at lower temperatures observed upon steam dealumination of Y zeolite is due to effects other than changes in acid strength.