Acid-Base Property of Tetragonal YNbO4 with Phosphate Groups and Its Catalysis for the Dehydration of Glucose to 5-Hydroxymethylfurfural

Acid-base properties of amphoteric crystalline YNbO4, which is synthesized by co-precipitation method using a water-soluble Nb peroxo complex, are adjusted by the impregnation of phosphate groups. The resulting phosphate-YNbO4 materials are tested for glucose dehydration to 5-hydroxymethylfurfural (HMF). Both activity and selectivity increase by the introduction of phosphate groups, giving the optimal catalyst with five phosphate groups per nm(2) of surface (5P-YNbO4). 5P-YNbO4 shows approximate to 50% selectivity at 75% conversion. Pyridine adsorption experiments show that 5P-YNbO4 exclusively possesses Lewis acidity. NH3- and CO2-temperature programmed desorption measurements confirm that phosphoric acid treatment increases acid strength and acid density with concurrent loss of basicity. Experiments using isotope-labeled glucose clarify that HMF is formed on 5P-YNbO4 through the stepwise dehydration reaction mechanism. Recyclability experiments show an accumulation of carbon deposits at the end of each reaction causing deactivation, but the activity can be completely recovered by successive washing with water and subsequent calcination at 400 degrees C in air.