Physicochemical characterization of extracted lignin from sweet sorghum stem

Seven lignin fractions were isolated from dewaxed sweet sorghum stem and their physicochemical features were investigated. The dewaxed sweet sorghum stem was extracted sequentially with hot water, 50% dioxane and dimethyl sulfoxide (DMSO), and various concentrations of NaOH. The treatments resulted in dissolution of 0.5, 0.7, 0.2, 1.3, 1.6, 0.8, and 0.5% lignin, corresponding to release of 8.9, 12.5, 3.6, 23.2, 28.6, 14.3, and 8.9% of the total solubilized lignin, respectively. The seven lignin fractions were comparatively characterized by chemical composition, structural features, and molecular weights using alkaline nitrobenzene oxidation, acid hydrolysis, UV, FT-IR, 1H and 13C NMR spectroscopy, and GPC. The purity of the lignin fractions isolated by 50% dioxane and alkali was higher than that of the lignin preparations obtained by hot water and DMSO, in which the content of associated polysaccharides in the former lignin fractions (0.16–2.12%) was much lower than those of the latter lignin preparations (10.39 and 37.03%). It was found that the seven lignin fractions contained substantial amounts of non-condensed guaiacyl and syringyl units with fewer p-hydroxyphenyl units, and had weight-average molecular weights between 1270 and 3590 g mol−1. No substantial differences in the main structural features between the lignin preparations isolated with 50% dioxane and alkali were found and these treatments did not result in significant changes in lignin composition and its structure.