Direct fractionation of low molecular weight lignin from Eucalyptus kraft black liquor by green solvents

Although hardwood (Eucalyptus) kraft lignin is rich in low molecular weight (low-Mw) lignin derivates, finding green solvents or processes for fractionating low-Mw constituents with high selectivity from kraft black liquor remains challenging. This study employed ethanol/water (EW) solutions of varied proportions in combination with lignin autohydrolysis to fractionate lignin from Eucalyptus kraft liquor into EW-soluble and EW-insoluble lignin fractions of low (496 ∼ 644 g·mol−1) and high (1563 ∼ 1862 g·mol−1) molecular weight, respectively. EW-soluble and EW-insoluble fractions were comprehensively characterized by GC-MS and 2D HSQC NMR. The fractionation yield of low-Mw lignin is 41.1 % (based on total recovered lignin content) with hydrolysis temperature of 120 °C, 3 h reaction time, and a 1:1 ethanol-to-water volume ratio. S-type lignin-derived monomeric derivates are predominant in the EW-soluble fraction. In particular, 2,6-dimethoxyphenol, syringaldehyde, and acetosyringone are relatively high in the EW-solution fraction, being 30.04, 11.74, and 38.75 mg/g, respectively. In contrast, the EW-insoluble lignin fraction is dominated by stilbene structures and β−β linkages, exhibiting a more uniform dispersity than unfractionated lignin. This study has developed a direct fractionation method with high selectivity of the low-Mw components, which has great application potential for producing platform chemical compounds, fuels, and pharmaceutical precursors.