(324e) A Density Functional Theory Study of Hydride Transfer Over Zeolites Utilizing a Periodic Modeling Approach
Significant efforts have been directed toward the discovery of a solid acid catalyst to replace the hazardous strong acids currently used to produce alkylation gasoline. An optimal catalyst for this application must be able to readily promote hydride transfer between alkyl groups in order to facilitate sustained activity in such a process. In this study, periodic density functional theory (DFT) is used to evaluate the energetics of hydride transfer over acidic zeolites for various combinations of hydride donor and acceptor substitution. Dispersion corrections (DFT-D) are implemented to include interaction energies between the reactive species and zeolite framework. Incorporation of the donor molecule into the zeolite pore shows a monotonic trend between donor size and pore entry energy, becoming endothermic for isobutane. However, the inclusion of dispersion energy corrections shifts all of these energies to exothermic values and breaks the monotonic trend. The transition states for hydride transfer are typically found to be carbenium ions, with shared hydride carbonium ions representing metastable intermediates. Generation of a shared hydride intermediate from species already localized within the pore shows an inverse monotonic trend between formation energy and both donor and acceptor substitution; inclusion of dispersion energy corrections does not affect this trend. Activation energies for the hydride transfer reaction also showed an inverse monotonic trend with respect to donor and acceptor substitution; however, this trend diminishes as the substitution of the hydride acceptor increased, converging to effectively the same value for a tertiary acceptor cases. The effects of the zeolite framework on the hydride transfer reaction are discussed.