Probing Topology and Reactant Effects on Hydride Transfer in Various Zeolites from First Principles

Theoretical Insights into the Effect of the Zeolite Environment and Reactant Identity on Alkane-Alkoxide Hydride Transfer

Thomas Chen and Matthew Neurock, Deparment of the Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN, 55455.

The thermal stability and tunability of the pore topology of zeolites make them attactive solid acid catalysts for carrying out the elementary transformations involved in hydrocarbon chain growth processes important in the production of chemicals and fuels.  Hydride transfer is ubiquitous to hydrocarbon transformation processes. However, detailed mechanistic studies of hydride transfer are limited as it is a secondary reaction that reactive intermediates can undergo, and direct experimental observations of hydride transfer are difficult to obtain. Herein, first principle density functional theory calculations are used to examine light alkane and olefin hydride transfer in TON, MFI, MOR, and BEA zeolites. We examine the effects of the alkane, the alkoxide, and the zeolite and show how transition state selectivity is determined by the zeolite topology. The transition state for hydride transfer are carbenium ions whose stability increase with reactant substitution thus increasing reactivity. However, calculations with TON show the existence of pore size limitations where hydride transfer reactivity decreases when either alkane or alkoxide exceeds a certain size threshold. Geometric and charge analyses are used to describe the competing effects of destabilizing steric interactions and stabilizing solvation effects. These interactions cause the pore size limitation where repulsive interactions outweigh zeolite solvation that delocalizes the positive charge of the carbenium ion transition state. Zeolite comparisons show the relative reactivity of alkane and alkoxide pairs for hydride transfer shifts to bulkier reactants as the pore size and dimensionality increase. Detailed comparison between TON and BEA show differences in energetics and transition state structure. Due to the larger pore size of BEA, no size threshold is observed for the molecules studied indicating a size threshold which is shifted towards bulkier species.