Crystalline niobium phosphates with water-tolerant and adjustable Lewis acid sites for the production of lactic acid from triose sugars

Abstract

Lactic acid (LA) is a versatile platform chemical for the production of biodegradable polymers as well as starting materials for the pharmaceutical industry. In this study, crystalline niobium phosphates directed by various surfactants were prepared by a facile sol–gel method and studied as heterogeneous Lewis acid catalysts for the direct conversion of triose sugars to LA under aqueous conditions. Metal oxides were used to alter the surface acidity, and pyridine FTIR analyses demonstrated the presence of both Lewis and Brønsted acid sites, which played essential roles in the conversion of biomass sugars. The incorporation of SnO₂ and the use of surfactants significantly lowered the Brønsted-to-Lewis site ratio. P–OH groups were determined to be the origin of the Brønsted acid sites, while the partially hydrolyzed framework of tin and niobium species led to the formation of Lewis acid sites. The cationic surfactant-modified samples with highly crystalline structures outperformed the amphiphilic surfactant-modified samples. The optimum cetyltrimethylammonium bromide-modified catalyst afforded an LA yield of ca. 92% with the complete dihydroxyacetone (DHA) conversion at 160 °C. Furthermore, the isomerization of pyruvaldehyde (PA) was found to be the rate-determining step, while the rehydration of PA to DHA could occur in the current reaction system. A possible reaction mechanism involving the evolution of two key catalytic intermediates was proposed. In addition, crystalline niobium phosphates were also effective in the aqueous dehydration of sugars to furan derivatives with a 44% 5-hydroxymethylfurfural yield from glucose after 1 h. The current study can hopefully serve as a model for the development of novel solid acid catalysts for the conversion of sugars to platform chemicals.