Analysis of the Reaction
Mechanism and Catalytic Activity
of Metal-Substituted Beta Zeolite for the Isomerization of Glucose
to Fructose
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Abstract
Glucose–fructose isomerization
mediated by Sn-BEA is investigated
using an extended QM/MM model containing 208 tetrahedral atoms. The
isomerization mechanism consists of a sequence of ring-opening, isomerization,
and ring-closing processes, consistent with the previously reported
experimental observations. In agreement with the experimentally observed
kinetic isotope effect, the rate-determining step is found to involve
a hydride shift from the C<sub>2</sub> carbon to the C<sub>1</sub> carbon. The apparent activation energy for the rate-limiting step
is 22.3 kcal/mol at 343 K. The difference in the reaction barriers
for the partially hydrolyzed and the fully coordinated Sn sites was
investigated using energy decomposition analysis. It is found that
the higher activity of the partially hydrolyzed site comes from the
extra flexibility provided by the defect in the lattice. The effect
of substituting Sn in the active site by Ti, Zr, V, Nb, Si, and Ge
was examined, and it was found that Sn and Zr are metals that result
in the lowest reaction barrier for glucose isomerization. By using
energy decomposition analysis, two physical properties are shown to
contribute to the magnitude of the reaction barrier: the polarizability
of the metal atom in the active site and the Brønsted basicity
of the oxygen atom bound to the metal atom