Improving the homogeneity of sugarcane bagasse kraft lignin through sequential solvents

The heterogeneous features of lignin, especially the wide distribution of its molecular weight, limit its high value-added application. To improve the homogeneity of lignin, sugarcane bagasse kraft lignin (KL) dissolved in methanol/acetone (7:3, v/v) was successively fractionated into four fractions (i.e.., F1, F2, F3, and F4) with various organic solvents of decreasing dissolving capacity (i.e.., ethyl acetate, ethyl acetate/petroleum ether 1:1 v/v, and petroleum ether). The yields of the four fractions (F1, F2, F3, and F4) were 59.6, 28.4, 10.8, and 1.2% that of KL, respectively. Gel permeation chromatography (GPC) analysis indicated the molecular weight of each fraction decreased from F1 to F4. All fractions had a lower polydispersity than KL. KL and the fractions were comprehensively characterized by chemical composition analysis, elemental composition analysis (EA), methoxyl group analysis, Fourier transform infrared spectroscopy (FT-IR), nitrobenzene oxidation analysis (NBO), and nuclear magnetic resonance (NMR) including P-31 and H-1-C-13 HSQC NMR. The results showed that the methoxyl group, hydroxyl group, interunit linkages, and thermal properties of the fractions varied with the molecular weight. The homogeneity of lignin was improved through solvent fractionation.