Computational Insight into the Effect of Sn-Beta Na Exchange and Solvent on Glucose Isomerization and Epimerization

Sn-substituted
zeolite Beta (Sn-Beta) is a promising catalyst for
efficient aldose to ketose isomerization, a key step in the conversion
of biomass to platform chemicals such as 5-(hydroxymethyl)­furan and
furfural. Recent experimental studies probing the mechanism and active
site for glucose isomerization (to fructose) and competing epimerization
(to mannose) have found that isomerization proceeds via a 1,2 intramolecular
hydride transfer (HT) and epimerization by either two subsequent HT
steps (on pure Sn-Beta; water and methanol) or one 1,2 intramolecular
carbon shift (CS) step (on Na-exchanged Sn-Beta; water and methanol).
In order to address remaining atomic-level mechanistic questions raised
by this data, we investigate the various pathways with computational
methods using several sizes of cluster models of Sn-Beta and density
functional theory. First, we find an energetically plausible pathway
for mannose formation via two subsequent HT steps that is consistent
with experimental observations. Additionally, we conclude that Na
exchange influences the mechanism by electrostatic stabilization of
CS relative to HT and that this effect is relatively independent of
geometric and flexibility constraints (even though the exact details
of the mechanism are not). Finally, we find that the experimentally
observed increase in glucose conversion when methanol is used as a
solvent instead of water can be explained by the difference in solvation
of the hydrophobic pores.