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

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(2)O, VOHPO(4)·0.5H(2)O and VO(H(2)PO(4))(2), and the materials prepared by calcination of these precursors, that is, γ-VOPO(4), (VO)(2)P(2)O(7) and VO(PO(3))(2), respectively. The VPO precursors were completely soluble in the reaction medium. In contrast, the orthorhombic vanadyl pyrophosphate (VO)(2)P(2)O(7), prepared by calcination of VOHPO(4)·0.5H(2)O 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)(2)P(2)O(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.