Exfoliated titanate, niobate and titanoniobate nanosheets as solid acid catalysts for the liquid-phase dehydration of d-xylose into furfural

HTiNbO5, HTi2NbO7, HNb3O8, H4Nb6O17, and H2Ti3O7 nanosheets obtained by exfoliation of protonated cation-exchangeable layered metal oxides were examined as solid acids for the cyclodehydration of xylose into furfural. The exfoliated-aggregated composites were characterized by powder X-ray diffraction, N2 gas adsorption, and scanning electron microscopy, and the acid properties were measured by IR spectroscopy of adsorbed pyridine. In general, the materials had much higher catalytic activity than the nonexfoliated acid-exchanged layered materials. The initial catalytic activities of the exfoliated materials tended to increase with the total amount of acid sites (Brønsted + Lewis, B + L). Furfural yields of up to 55% were achieved after 4 h of reaction in a water–toluene solvent mixture under batch operation at 160 °C. Most of the materials could be recycled several times without loss of performance.