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

Aqueous-phase dehydration of xylose into furfural was studied in the presence of amorphous Nb2O5 with water-compatible Bronsted and Lewis acid sites. Nb2O5 was determined as a more active and selective catalyst for xylose dehydration than typical homogeneous Bronsted 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 Nat-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.