Chemical structure change of lignin extracted from bamboo biomass by maleic acid.

Maleic acid (MA) is an effective solvent for lignin extraction from raw biomass under mild conditions through acidolysis and hydrophobic interaction. To improve the valorization of lignin, the lignin extracted with MA was subjected to successive organic solvent fractionation to make the narrow M and M distribution and the homogeneity structure of lignin fragments. After successive organic solvent fractionation, the dispersity index (Đ) of lignin reduced from 2.86 to 1.25, and the purity and homogeneity were greatly improved. The phenolic hydroxyl content of lignin extracted by ethyl acetate was the highest (2.45 mmol/g), and the aliphatic-OH concentration was the lowest (0.65 mmol/g). While the βO4 bonds were partially cleaved, more hydrophobic substrates with CC linkages were recovered by ethyl acetate. Lignin fragments with a variety of main structures and functional groups were obtained effectively with successive fractionation. This method supplied broad prospects for the preparation of lignin and its applications in epoxy resin, rubber, thermoplastic additives and polymer raw materials.