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 Nb 2 O 5 with water-compatible Brønsted and Lewis acid sites. Nb 2 O 5 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 Nb 2 O 5 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 D 2 O and xylose suggested that furfural is formed through stepwise dehydration via a highly reactive dicarbonyl intermediate on Nb 2 O 5, whereas typical Lewis acids such as CrCl 3 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 Nb 2 O 5 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.