Toward an Understanding of Inhomogeneities in Structure of Lignin in Green Solvents Biorefinery. Part 1: Fractionation and Characterization of Lignin

To investigate the inhomogeneity of the lignin, milled wood lignin (MWL), organosolv lignin (OL), and alkaline lignin (AL) were sequentially fractionated from Tamarix spp. with dioxane, alkaline organosolv, and alkaline solutions, respectively. The carbohydrates associated with three lignins were comparatively analyzed, and the molecular weight and polydispersity of lignins were determined by gel permeation chromatography (GPC). Nuclear magnetic resonance (NMR) spectroscopy was used to measure the contents of the substructures of lignins. The results indicated that the alkaline organosolv extraction released a higher yield of lignin (17.7%, based on lignin in the raw material) than dioxane and alkaline solution extractions. Small amounts of carbohydrates (0.79%, w/w) were detected in the OL fraction, suggesting a significant cleavage of alpha-ether bonds between lignin and carbohydrates in the alkaline organosolv fractionation process. GPC results revealed that the molecular weights of lignin decreased in the order M-w.OL > M-w.MWL > M-w.AL. All lignins were characterized by predominance of beta-O-4' ether linkages (68.4-79.0%). A relatively low content of beta-beta' (17.3%) resinol substructure was detected in MWL compared with that in OL (22.3%) and AL (21.0%) fractions. In addition, the component units of lignin were also investigated. HSQC spectra indicated a lower S/G (2.52) ratio of AL compared with MWL (S/G, 2.80) and OL (S/G, 3.85).