(703a) 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.