A Mechanism Insight into the Effect of Water on Preferential Dehydration Rather Than Retro-Aldol Condensation of Fructose over a Nb2O5 Catalyst

Nb2O5 exhibits good catalytic performance for fructose-to-5-hydroxymethylfurfural (HMF) dehydration in water and Methyl isobutyl ketone (MIBK) biphasic solutions. The reaction mechanisms for the fructose-to-HMF dehydration have been theoretically studied over the Nb2O5 catalyst in both MIBK and aqueous solutions at the GGA-PBE/DNP level, including for the retro-aldol condensation of fructose to lactic acid (LaA). Here, the Nb2O5(010) surface is preferred as the Nb2O5 catalyst model. In MIBK solution, there is only one type of active site (−Nb–O–Nb−), denoted as [Nb]. In an aqueous solution, the dissociation of H2O on [Nb] produces (−Nb(OH)–O(H)–Nb−) active site, named as [NbOH]. In either MIBK or aqueous solution, for the dehydration of fructose to HMF, the rate-determining steps are associated with the simultaneous cleavage of both the C–OH and C–H bonds. For the retro-aldol condensation of fructose to LaA, the rate-determining steps are concerned with the addition of the −CO double bond. In an aqueous solution, [NbOH] selectively promotes the dehydration of fructose to HMF, which mainly comes from the Brønsted acidity of the (−Nb–O(H)–Nb−) site, accompanied by the contribution of Brønsted basicity of the (−Nb–OH) site. In MIBK solution, [Nb] selectively facilitates the retro-aldol condensation of fructose to LaA, which stems from the Lewis acidity displayed by the exposed Nb site.