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

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.