(461d) 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 into platform chemicals such as 5-(hydroxymethyl)furan and furfural. However, aldose epimerization is also catalyzed at Sn-Beta active sites and competes with isomerization, and understanding the mechanism for both processes is important for informing further improvements. Recent experimental studies probing the mechanism and active site for glucose isomerization (to fructose) and 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). Additionally, glucose conversion, along with corresponding fructose and mannose yields, were increased in methanol compared to in water. In order to address the 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. We conclude that mannose formation via two subsequent HT steps is feasible, that Na exchange influences the mechanism by electrostatic stabilization of CS relative to HT, and that the solvent effect on conversion can be explained by the difference in solvation of the hydrophobic pores.