Continuous aqueous-phase cascade conversion of trioses to lactic acid over Nb2O5 catalysts

Different soft chemical syntheses were applied to synthesize eight Nb2O5 catalysts that were fully characterized in terms of structure, porosity, surface acidity and catalytic performance. The results showed that the catalysts indeed presented different structures and acidity. The distorted orthorhombic phase rendered solids with an improved porosity and allowed tuning higher concentrations of water-tolerant Lewis acid sites and higher Brønsted acidity when compared to a more crystalline pseudohexagonal phase. Catalysts holding the distorted orthorhombic structure were more active and selective to chemoconversion of trioses into lactic acid. It was shown though that product distribution was far more sensitive to changes in crystalline structure and Lewis acidity in the cascade reaction than the overall catalytic activity. A consistent linear correlation was found between lactic acid selectivity and the amount of water-tolerant Lewis acid sites, disclosing their dependence. It is the first time that such a clear correlation is demonstrated for triose upgrading cascade reaction.