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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 KgHMFKgCat−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 mmolH+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.