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.

Lewis Acid and Base Catalysis of YNbO4 Toward Aqueous‐Phase Conversion of Hexose and Triose Sugars to Lactic Acid in Water

“…We have previously reported that the conversion of glucose and xylose to HMF and furfural over Lewis acid sites on Nb 2 O 5 and TiO 2 surfaces is based on step‐wise dehydration via the formation of dicarbonyl compounds typically represented by 3‐deoxyglucosone . In the case of glucose conversion, the Lewis acid sites facilitated the subsequent dehydration of 3‐deoxyglucosone to yield HMF as a main product.…”

Continuous pervaporation-assisted furfural production catalyzed by CrCl3

“…Most studies in the field use a reaction temperature of approximately 130°C. [21][22][23][24][25][26][27][28][29][30][31] We used simulations to explore the performance of a membrane reactor operating at that temperature. There was no difficulty performing the batch experiments without extraction and with liquidliquid extraction (LLE) at 130°C.…”

“…These reactors are typically operated at temperatures of 130-180°C. [21][22][23][24][25][26][27][28][29][30][31] Solvents include toluene, [21][22][23][24] methyl isobutyl ketone, [23][24][25][26] cyclopentylmethyl ether, [27][28][29] among others, and are typically used in organic : aqueous volumes ratios greater than 1 in order to increase furfural extraction. The furfural yield depends on solvent, catalyst, xylose loading, etc.…”

Lewis Acid Catalysis of Nb2O5 for Reactions of Carboxylic Acid Derivatives in the Presence of Basic Inhibitors

“…5–8), (3) dehydration reactions of alcohols and polyols (No. 9–16),, (4) nucleophilic substitution reactions of carboxylic acids (and their derivatives),, and (6) selective oxidation/dehydrogenation reactions …”

“…9), have led to the new research area Nb 2 O 5 ‐catalyzed dehydration of biomass‐derived polyols (No. 10–16) . The hitherto reported Nb 2 O 5 ‐catalyzed dehydrative transformations of polyols are concerned with the dehydration of glycerol to acrolein (No.…”