A First-Principles Approach for Predicting the Rate Coefficient for n-Alkane Cracking in Zeolites

            Acid zeolites are important materials for a variety of petrochemical reactions, including cracking of alkanes to olefins. Cracking of short n-alkanes has been used as a model reaction in numerous studies attempting to understand the relationship between zeolite acidity, adsorption, and apparent reaction rates. We have developed a first principles computational approach to predict the apparent first-order rate coefficient, k_app, for cracking C₃-C₆ n-alkanes in zeolites MFI, FAU, MOR, and BEA. The value of k_app is represented by the product of the intrinsic rate coefficient for alkane cracking, k_int, and the equilibrium constant for alkane adsorption, K_ads. Density functional theory and absolute rate theory are used to determine k_int, whereas statistical mechanics is used to predict K_ads. Our computational approach reproduces the observed dependence of k_app on the carbon chain length in MFI. Agreement with experimental data is good for all zeolite frameworks studied. In particular, there is good agreement between our approach and experimental data from several authors for n-hexane cracking in FAU between 673 and 823 K. Our work supports the hypothesis that k_int is independent of zeolite framework structure and differences in k_app are due primarily to differences in K_ads.