Facile Formation of Lactic Acid from a Triose Sugar in Water over Niobium Oxide with a Deformed Orthorhombic Phase

Hydrothermal treatment of NH4[NbO(C2O4)(2)(H2O)(2)]nH(2)O in water at 448 K for 3 days produced crystalline Nb2O5 with a deformed orthorhombic structure and a high surface area (208 m(2) g(-1)). Fourier-transform infrared spectroscopy measurements of pyridine adsorption revealed that the Nb2O5 catalyst has both high densities of Bronsted and Lewis acid sites that can work in the presence of water. One feature of the Nb2O5 catalyst is its high density of water-compatible Lewis acid sites (0.21 mmol g(-1)), which is much larger than that of Nb(2)O(5)nH(2)O (0.03 mmol g(-1)). The Nb2O5 catalyst was studied as a solid acid catalyst for the formation of lactic acid from 1,3-dihydroxyacetone and pyruvaldehyde in water at 373 K, and was determined to be a highly active and selective catalyst, compared with typical acid catalysts (H2SO4, Sc(OTf)(3), and Nb(2)O(5)nH2O). A high Lewis acid density with moderate acid strength is a crucial factor for the high catalytic performance exhibited for the former reaction. High densities of both Bronsted and Lewis acid sites in the catalyst promote the fast and selective production of lactic acid in the latter reaction. In addition, Such Lewis acidity of the Nb2O5 is also effective over conventional acid catalysts in xylose dehydration to furfural in water, with respect to reaction rate and furfural selectivity. The combination of aqueous-phase dehydration of xylose over the Nb2O5 and the continuous extraction of furfural with an immiscible organic solvent resulted in a high selectivity toward furfural of similar to 78.5% with high xylose conversion (97%).