Catalytic dehydration of xylose to furfural: vanadyl pyrophosphate as source of active soluble species

Abstract

The acid-catalysed, aqueous phase dehydration of xylose (a monosaccharide obtainable from hemicelluloses, e.g., xylan) to furfural was investigated using vanadium phosphates (VPO) as catalysts: the precursors, VOPO 4·2H 2O, VOHPO 4·0.5H 2O and VO(H 2PO 4) 2, and the materials prepared by calcination of these precursors, that is, γ-VOPO 4, (VO) 2P 2O 7 and VO(PO 3) 2, respectively. The VPO precursors were completely soluble in the reaction medium. In contrast, the orthorhombic vanadyl pyrophosphate (VO) 2P 2O 7, prepared by calcination of VOHPO 4·0.5H 2O at 550 °C/2 h, could be recycled by simply separating the solid acid from the reaction mixture by centrifugation, and no drop in catalytic activity and furfural yields was observed in consecutive 4 h-batch runs (ca. 53% furfural yield, at 170 °C). However, detailed catalytic/characterisation studies revealed that the vanadyl pyrophosphate acts as a source of active water-soluble species in this reaction. For a concentration of (VO) 2P 2O 7 as low as 5 mM, the catalytic reaction of xylose (ca. 0.67 M xylose in water, and toluene as solvent for the in situ extraction of furfural) gave ca. 56% furfural yield, at 170 °C/6 h reaction.