(147c) A Theoretical Investigation of the Methylation of Olefins by Surface Methyl Groups On Zeolites

Emerging research in the conversion of methanol to hydrocarbons (MTH) over zeolitic acids suggests that observed catalytic rates and selectivity in the indirect “hydrocarbon pool” mechanism are primarily a result of olefin and arene methylation reaction sequences. In this research, cluster-based and periodic density functional theory studies were used to examine the kinetics and mechanism of olefin methylation by surface methyl groups on CHA, FER, BEA, MOR and MFI zeolite frameworks. The location of the Al atom was chosen such that the bridging O atom had access to the largest open volume of each framework. In each case, the transition state was found to consist of a nearly coplanar CH₃ species that was coordinated with both the original bridging O atom and the olefin suggesting that an sp² hybridized carbon species is involved in the transition state. Activation energies of 70, 86, and 98 kJ mol^-1 for ethylene methylation were found for FER, CHA, and BEA, respectively; in each case, lower than that reported for a mechanism involving the co-adsorption of methanol and ethylene (104 kJ mol^-1)(Svelle et al. JACS 2008, 131, 816-825). Observed trends in the activation energies and transition state geometries will be discussed with respect to the zeolite pore size and the local pore configurations.