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

The activity and activation energy for cracking of n-hexane were measured o ver three zeolites: HZSM-5, H-MOR, H-USY (ultrastable Y zeolite), and CDHY (AHFS dealuminated Y zeolite), under conditions where the product selectivi ties were nearly identical and there was no deactivation of the catalysts. The identical selectivities implied that the same reaction mechanism for cr acking was occurring over all three catalysts. Within experimental error, t he differences in apparent activation energies could be entirely attributed to differences in heats of n-hexane adsorption, such that the intrinsic ac tivation energies were identical. These results suggest that the kinetics o f cracking is insensitive to any possible differences in acid strength amon g these catalysts, and the large enhancement in activity at lower temperatu res observed upon steam dealumination of Y zeolite is due to effects other than changes in acid strength. (C) 1999 Elsevier Science B.V. All rights re served.