Critical role of solvent extraction sequence in the fractional separation of alkaline lignin

Abstract

Solvent-based method, especially solvent sequential extraction, is capable of tuning the molecular weight and structural characteristics of lignin for the fractional utilization of lignin. Three organic solvents, methanol, isopropanol and dichloromethane, were selected for sequential fractionation of AL following two sequences of gradually increasing and decreasing the HSP of these three solvents was reported in this work, to interrogate the role of extraction sequence in the fractional separation. GPC results revealed that the molecular weight of insoluble fractions obtained by gradually increasing the HSP value of the selected solvent was lower than that obtained from reverse extraction sequence. In addition, a decrease was found in PD of the separated fractions compared with AL no matter which extraction sequence was performed, especially the molecular weight and PD of the soluble fractions were dramatically reduced. In contrast, the PD value of the insoluble fractions was slightly smaller than that of AL, indicating that the solvent sequential extraction protocol was difficult to enrich the large molecular weight components in lignin. On the other hand, 2D HSQC NMR and thermal stability results showed that the content of β-O-4 linkages and the amount of residual carbon at 800 °C increased and decreased with the increase of molecular weight of insoluble fractions respectively, attributable to the polarity of the solvent selected and its ability to combine with lignin fragments to form hydrogen bonding. In view of these findings, the solvent extraction sequence is closely related to the molecular weight and structural properties of insoluble fractions and plays a critical role in the fractional separation.