Observation of a compensation relation for monomolecular alkane cracking by zeolites: The dominant role of reactant sorption

Monomolecular cracking of alkanes was investigated over zeolites of different structure types and after different treatments. Large deviations in apparent rates of reaction and apparent kinetic parameters were detected for zeolites of different structure types; smaller changes were observed within a structure type. The Si/Al ratio and different crystal sizes had no effect on turnover frequency; however, steaming of mordenite and ZSM-5 enhances the rate of reaction by a factor of 2-5 due to the formation of Lewis acid sites with enhanced heat of adsorption, although the number of Brønsted sites decreased. The apparent activation energies and preexponential factors showed a linear compensation relation in a Constable plot. Using the Langmuir-Hinshelwood kinetics formalism, the apparent kinetic parameters include enthalpy and entropy of adsorption and surface reaction terms. If the enthalpy and entropy of adsorption are linearly related, which is experimentally observed for unsteamed and steamed mordenite, then the apparent compensation effect implies that the observed differences in rate are dominated by differences in adsorption of the reactant in the pores of the zeolites, rather than by differences in acid strength. Zeolites with smaller pores show enhanced adsorption, and consequently the rate of reaction increases in the order H-Y<H-MOR<H-ZSM-5.