A novel and efficient process for lignin fractionation in biomass-derived glycerol-ethanol solvent system

Glycerol, as a nontoxic, biodegradable and sustainable solvent, together with biomass-derived ethanol was employed in this work to fractionate and purify enzymatic hydrolysis lignin (EHL) from enzymatic hydrolysis residue (EHR). Detailed characterizations, including GPC, FTIR, 13C and 2D-HSQC NMR and Py-GC/MS analysis were conducted to provide a broad evaluation of the obtained lignin fractions. The GPC analysis revealed that the proposed method could realize the efficient fractionation of EHL with relatively narrow polydispersity, and three lignin fractions (F1, F2 and F3) with gradually increased molecular weight (M¯n from 8690 to 20110 g mol−1) and reduced polydispersity were obtained. The structural characterization (FTIR, NMR and Py-GC/MS) indicated that with the increase of molecular weight, G unit content in lignin fractions increased due to the more stable CC bond formed by C5 in G unit. Besides, F1 contained highest phenolic acids, which indicated the potential application of F1 in bioactive products. The study of glycerol recycle showed that glycerol could be recycled with high recovery rate and re-employed to fractionate lignin efficiently. Consequently, the study indicated that compared with traditional dissolving by multiple organic solvents, the sequential dissolution in glycerol-ethanol solvent was a novel, green, and efficient way to fractionate EHL and reduce its molecular weight and structural heterogeneity.