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

摘要

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.