A Simple Two-Step Method for the Selective Conversion of Hemicellulose in Pubescens to Furfural

A two-step method was adopted to produce furfural from the selective dissolution and conversion of hemicellulose in pubescens. First, in GVL(γ-valerolactone)-H2O co-solvent at 160 °C, H2O promoted the cleavage of chemical bonds linking hemicellulose, lignin, and cellulose, and GVL further helped the co-dissolution of hemicellulose (93.6 wt %) and lignin derivatives (80.2 wt %), leaving a high purity cellulose (83.3 wt %). Heating to 200 °C, the liquid system obtained with NaCl and THF added, achieved the maximum yield of 76.9 mol % with 82.2% selectivity to furfural based on the moles of converted hemicellulose using a 5 wt % pubescens to solvent ratio. It was demonstrated that NaCl with GVL promoted the depolymerization of oligomers to small molecular products (Mw < 150 Da), while the co-contribution of NaCl and co-solvent improved the selectivity to furfural. Cl– could interact strongly with C-OH-2,3,4 of the xylose unit, and the dehydration of xylose to form furfural might first occur on C-OH-4 of xylose, then on C-OH-2,3 of xylose, which enhanced the dehydration and ring open reaction via the cleavage of C1–O6 bonds, then promoted the formation of furfural by inhibiting the retro-aldol reaction to form lactic acid. The co-contribution of NaCl and co-solvent was benefical not only for the selective conversion of the mixture containing hemicellulose-derived monomers and oligomers to furfural but also for obtaining a lower molecular weight lignin derivatives (150–500 Da) that could be further used.