Fractionation and Characterization of Kraft Lignin by Sequential Precipitation with Various Organic Solvents

The value-added utilizations of technical lignin are restricted by its heterogeneous features, such as high polydispersity, complex functional group distribution, ununiformed reactivity, etc. Fractionation of lignin into more homogeneous parts represents a promising approach to overcome this challenge. In the present study, softwood kraft lignin was fractionated into four different portions (F1, F2, F3, and F4) by first dissolving it in a methanol–acetone mixture followed by sequential precipitation with various organic solvents (ethyl acetate, 1:1 ethyl acetate/petroleum ether, petroleum ether) of decreasing solubility parameters. The yields of various fractions F1, F2, F3, and F4 were 48%, 39%, 10%, and 3%, respectively. The results from gel permeation chromatography indicated that the molecular weights of each fraction decreased from F1 to F4. The lowest molecular weight fraction F4 contained mainly monomeric and dimeric aromatic structures such as guaiacol and vanillin formed from lignin degradation. All fractions showed lower polydispersity than the starting kraft lignin. Chemical structures of F1, F2, and F3 were elucidated by elemental composition, methoxyl-content analysis, UV spectroscopy, and various NMR techniques (31P, 13C, and 2D HSQC). The results indicated that (1) the sulfur content in each fraction was similar; (2) all residual polysaccharides only existed in fraction F1; (3) the contents of various structural linkages and thermal properties of each fraction varied as a function of its molecular weight. Overall, the solvent assisted fractionation method of kraft lignin yielded four fractions with varied molecular weights and polydispersities. As these fractions exhibited different chemical properties they can be used favorably in various applications.