Amorphous Nb₂O₅ as a Selective and Reusable Catalyst for Furfural Production from Xylose in Biphasic Water and Toluene

Abstract

Aqueous-phase dehydration of xylose into furfural was studied in the presence of amorphous Nb2O5 with water-compatible Brønsted and Lewis acid sites. Nb2O5 was determined as a more active and selective catalyst for xylose dehydration than typical homogeneous Brønsted and Lewis acid catalysts including HCl and Sc(OTf)3, and Nb2O5 converted 93% of xylose with 48% selectivity toward furfural in water at 393 K. No significant loss of the original catalytic activity was observed after Na+-exchange treatment, which indicates that the reaction proceeded only on Lewis acid sites. Isotope-labeling experiments using D2O and xylose suggested that furfural is formed through stepwise dehydration via a highly reactive dicarbonyl intermediate on Nb2O5, whereas typical Lewis acids such as CrCl3 and Sc(OTf)3 convert xylose to furfural in water through hydride transfer and subsequent dehydration via xylulose as a ketose-type intermediate. The difference in the reaction mechanism accounts for the lower activation energy (83 kJ mol–1) with Nb2O5 than those with Sc(OTf)3 and HCl (107–131 kJ mol–1). Continuous extraction of evolved furfural with toluene enabled a large increase in the selectivity toward furfural from 48% to 72% and prevented deactivation of the Lewis acid sites by covering with heavy byproducts, represented by humin.