Fractionation of lignin macromolecules by sequential organic solvents systems and their characterization for further valuable applications

Lignin solvent fractionation is one of the promising methods for homogenizing and utilizing lignin commercially. In this work, fractionation characteristics of two lignin fractions were compared to investigate the potential of utilization of fractionated lignin. Two lignins [milled wood lignin(MWL) and organosolv lignin(OL) from yellow poplar] were sequentially fractionated with ethyl acetate(F1), 2-butanone(F2), methanol(F3), acetone(F4), and dioxane/water(F5). Yields of five MWL fractions F1 to F5 were 11.7%, 11.7%, 15.3%, 11.8%, and 49.6%, respectively, and yields of OL fractions were 26.2%, 26.1%, 18.7%, 3.7% and 25.4%. Average molecular weight of F1 (lowest molecular weight fraction) ranged from 1000 to 2400 Da, whereas that of F5 (highest molecular weight fraction) was above 10000 Da. According to functional group analysis, contents of phenolic hydroxyl groups and methoxyl groups decreased gradually with increasing molecular weight. DFRC analysis was performed to investigate the frequency of β-O-4 linkages and it revealed that the higher molecular weight fractions yielded larger amounts of DFRC monomers, indicating that those fractions more frequently contain aryl ether linkages. TG/DTG showed that the low molecular weight fractions generally have lower initial thermal stability. Tg of the fractions ranged from 126 °C to 156 °C, increasing as the molecular weight of the lignin fraction increased.