Catalytic Dehydration of Fructose to 5-Hydroxymethylfurfural in Aqueous Medium over Nb₂O₅-Based Catalysts

The catalytic dehydration of fructose to 5-hydroxymethylfurfural (HMF) in water was performed in the presence of pristine Nb₂O₅ and composites containing Nb and Ti, Ce or Zr oxides. In all experiments, fructose was converted to HMF using water as the solvent. The catalysts were characterized by powder X-ray diffraction, scanning electron microscopy, N₂ physical adsorption, infrared and Raman spectroscopy and temperature-programmed desorption of NH₃. Experimental parameters such as fructose initial concentration, volume of the reacting suspension, operation temperature, reaction time and amount of catalyst were tuned in order to optimize the catalytic reaction process. The highest selectivity to HMF was ca. 80% in the presence of 0.5 g·L^-1 of bare Nb₂O₅, Nb₂O₅-TiO₂ or Nb₂O₅-CeO₂ with a maximum fructose conversion of ca. 70%. However, the best compromise between high conversion and high selectivity was reached by using 1 g·L^-1 of pristine Nb₂O₅. Indeed, the best result was obtained in the presence of Nb₂O₅, with a fructose conversion of 76% and a selectivity to HMF of 75%, corresponding to the highest HMF yield (57%). This result was obtained at a temperature of 165° in an autoclave after three hours of reaction by using 6 mL of 1 M fructose suspension with a catalyst amount equal to 1 g·L^-1.