Density Functional Theory Study of Alkane-Alkoxide Hydride Transfer in Zeolites

Abstract

A solid acid catalyst for the production of alkylation gasoline is sought to replace hazardous strong liquid acids currently used. An optimal catalyst for this application must be able to readily promote hydride transfer between alkyl groups to facilitate sustained activity. Herein, periodic density functional theory (DFT) is used to evaluate the energetics of hydride transfer over acidic mordenite for various combinations of hydride donor and acceptor substitution. Dispersion corrections to DFT (DFT-D) are implemented to better represent non-bonding interactions between the reactive species and the zeolite framework. DFT predicts an increasingly endothermic energy to incorporate the alkane into the zeolite pore as the hydride-donor molecule size increases. However, the inclusion of dispersion energy corrections shifts these energies to exothermic values, and a non-monotonic trend indicates a size dependent competition between attractive dispersion interactions and steric repulsion. The transition states for hydride transfer are mainly carbenium ions. Shared hydride carbonium ions represent metastable intermediates in most cases, with the notable exception of transfer between isobutane and the t-butyl alkoxide, for which the shared hydride species is more stable than its precursor and represents a global minimum along the hydride transfer reaction coordinate. Generation of a shared hydride intermediate from species localized within the pore shows an inverse monotonic trend between formation energy and both donor and acceptor substitution. Activation energies for the elementary step of shared hydride complex formation also show an inverse monotonic trend with respect to donor and acceptor substitution. Dispersion energy considerations decrease the overall barrier to hydride transfer in most cases.