Environmentally benign and continuous 5-hydroxymethylfurfural (HMF) production from concentrated sugars over NbOPO4 supported TiO2 catalyst in a dimethyl carbonate-water biphasic system

Abstract

NbOPO₄ supported TiO₂ materials were demonstrated to be excellent catalysts for selective conversion of C6-carbohydrates to 5-hydroxymethylfurfural in an environmentally benign dimethyl carbonate-water solvent system. The materials presented high activity and excellent stability in sub-critical water conditions (upto 180 °C and 20 bar), enabling continuous 5-hydroxymethylfurfural production from highly concentrated sugars (35 wt% fructose, glucose and glucose:fructose mixtures) in a micro fixed-bed reactor. The high catalytic activity (6–27.3 Kg_HMFKg_Cat⁻¹day⁻¹) and good-to-excellent HMF selectivity (55–91 %) and exceptional hydrolytic stability and regenerability observed under process conditions was attributed to a highly crystalline NbOPO₄ phase with Q₂ and Q₃ phosphates stabilized on a TiO₂ framework with tunable acidity (0.11–0.52 mmol_H⁺g⁻¹ and Brønsted/Lewis ratio) and acidic strength comparable to bulk-NbOPO₄. Furthermore, our experiments also revealed the importance of organic solvents in modulating catalyst acidic properties, particularly the strength of Brønsted acid sites which, in turn, influenced the HMF productivity.