Lewis Acid-Facilitated Deep Eutectic Solvent (DES) Pretreatment for Producing High-Purity and Antioxidative Lignin

Due to its low cost and green nature, pretreatment of lignocelluloses using deep eutectic solvents (DESs) is a promising approach for dissociation of lignocelluloses and extraction of lignin. However, pureChCl/glycerol DES has been found to be ineffective for this purpose. In this work, we develop a strategy of introducing Lewis acids (AlCl3, FeCl3, FeCl2, ZnCl2, and CuCl2) into ChCl/glycerol for efficient pretreatment of hybrid Pennisetum energy grass in order to produce high-purity and antioxidative lignin. Furthermore, the structural characteristics of the regenerated lignin isolated using the DES pretreatment were comprehensively investigated. The results revealed that the FeCl3-catalyzed DES pretreatment facilitated fractionation of the regenerated lignin. It was observed that the amount of ether linkages in the regenerated lignin decreased gradually with increasing pretreatment severity. Furthermore, it was found that the amount of phenolic hydroxyl groups in the regenerated lignin was sharply increased. The regenerated lignin isolated after the DES pretreatment showed excellent antioxidant activity. It is concluded that a deep understanding of the fundamental chemistry of lignin during the Lewis acid-catalyzed DES pretreatment will be beneficial for the optimization of the pretreatment process.