Theoretical Insight into the Mediation Mechanism of Water on Favorable Isomerization over Epimerization of Glucose on the Nb₂O₅ Catalyst

Abstract

Over the Nb2O5 catalyst, the reaction mechanisms of both glucose-to-fructose isomerization and glucose-to-mannose epimerization are theoretically studied in both tetrahydrofuran (THF) and aqueous solutions at the GGA-PBE/DNP level. Here, Nb2O5 catalyst is modeled as a Nb2O5(010) surface. On the Nb2O5 (010) surface, there is only one kind of catalytically active site in THF solution, i.e., (−Nb–O−), whereas in aqueous solution, due to the dissociation of H2O on (−Nb–O−), a (−Nb(OH)–O(H)−) active site is produced. For the glucose-to-fructose isomerization in either THF or aqueous solution, the rate-determining step is associated with the intramolecular [2,1]-H shift for the aldose–ketose tautomerization. Alternatively, for the glucose-to-mannose epimerization, the rate-determining step is concerned with the intramolecular [2,1]-C shift of the C3 center for the aldose–aldose epimerization. In an aprotic solvent, such as THF, the exposed Lewis acid-site (−Nb–O−) can directly interact with the aldehydic carbonyl of glucose, which promotes the aldose–aldose epimerization. Alternatively, in a protic solvent, such as H2O, the Brønsted acidic H atom of the resultantly closed Lewis acid-site (−Nb(OH)–O(H)−) reacts with aldehydic carbonyl of glucose, thus facilitating the aldose–ketose tautomerization. Over the Lewis acid-site, an aprotic solvent can selectively promote the aldose–aldose epimerization with the intramolecular [2,1]-C shift, depending on the exposed Lewis acid-site. On the other hand, a protic solvent can selectively facilitate aldose–ketose tautomerization with the intramolecular [2,1]-H shift, which depends on the Brønsted acidic H atom for the dissociation of protic solvent on Lewis acid-site. The current research results should provide some theoretical clues for designing highly selective catalyst systems toward both aldose–ketose tautomerization and aldose–aldose epimerization of carbohydrates.

Origin of Catalytic Selectivity from Sn(OTf)2 in Methanol Solution for the Conversion of Glucose to α-Hydroxyesters

“…For glucose molecules, as reported in the literature, − there are four main tautomers, involving β- d -glucopyranose, α- d -glucopyranose, chain-cis- d -glucose, and chain-trans- d -glucose. Among them, β- d -glucopyranose is energetically the most stable in methanol solution, as demonstrated in Figure S1d from Supporting Information (SI).…”

“…Based on our previous reports, − , in the ring-opening of β- d -glucopyranose to chain-glucose ( S-1-ab ), the Bro̷nsted acid ([CH 3 OH 2 ] + ) plays a major catalytic role. For concise, the geometric structures and the schematic energy diagrams for S-1-ab catalyzed by [CH 3 OH 2 ] + are depicted in Figure S2 from SI.…”