Tin modified Nb2O5 as an efficient solid acid catalyst for the catalytic conversion of triose sugars to lactic acid

Abstract

Lactic acid (LA) is a versatile platform chemical for the production of biodegradable plastics and starting materials for the chemical and pharmaceutical industries. In this study, bimetallic oxide catalysts based on niobia were prepared by a facile sol–gel method and used as heterogeneous solid acid catalysts for the conversion of triose sugars to LA under aqueous conditions. The coprecipitation of KNbO₃ and metal salts ensured the uniform dispersion of all the metal atoms and prevented the agglomeration of individual metal oxides. A phase transfer of the niobium species from corner-sharing to edge-sharing octahedra was observed after the incorporation of tetragonal tin species, endowing niobium oxides with promising catalytic activity. Pyridine Fourier transform infrared spectroscopy analysis demonstrated the presence of both Lewis and Brønsted acid sites, which played essential roles in the conversion of biomass sugars. The Brønsted-to-Lewis site ratio could be tuned by varying the amounts of the metal oxides. SnO₂-Incorporated niobia outperformed all the catalysts investigated, catalyzing the complete conversion of dihydroxyacetone at 160 °C to give a promising optimal LA yield of 98%. The product distribution depended closely on the reaction temperature, catalyst loading and substrate concentration. The metal–metal interactions between Sn and Nb could be observed by X-ray photoelectron spectroscopy, Raman spectroscopy, and UV-vis spectroscopy. In addition, a possible reaction mechanism was proposed; the anchoring of water molecules at the oxygen vacancies created by incorporating tin oxide into niobia greatly facilitated proton diffusion during the acid-catalyzed conversion of pyruvaldehyde to LA, which was found to be the rate-determining step. This method enables facile catalyst separation and recycling and provides an efficient strategy for the development of novel solid acid catalysts for converting carbohydrates to platform chemicals.