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