Separation of Lignin from Paulownia and Its Application in DES Gels

In this study, binary and ternary DES systems were prepared using choline chloride (ChCl) with lactic acid (LA), glycerol (GL), urea, and acrylic acid (AA) to extract lignin from Paulownia. The chemical structure of lignin was analyzed to evaluate the structural changes induced by various DES systems, and the isolated lignin was used to prepare DES gels. The results showed that lignin extracted using different DES systems shares similarities in its basic structural framework, with all samples retaining an intact benzene ring structure. However, there are certain differences in the content of the linking bonds and the S/G ratio, and the acidic DES caused the breakage of the β-O-4′ linkage in the lignin molecule, promoting its separation. The molecular weight distribution varied among the DES systems. In the ternary DES, the addition of acrylic acid disrupted lignin’s internal chemical linkages, leading to the precipitation of relatively small lignin molecules. TGA results demonstrated varying levels of thermal resistance among lignin extracted from different DES systems varied, with the best stability observed for lignin extracted from the ChCl-LA system. Lignin extracted from Paulownia using different DES systems was added to the DES gels, and the effects of lignin structure on the properties of the DES gels were investigated. The mechanical, swelling, microstructural, and thermal properties of DES gels prepared from different Paulownia lignin structures showed slight differences; however, no significant discrepancies were observed among the gels. The present work offers a novel strategy for the valorization of lignin derived from lignocellulosic biomass.