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.
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