Deep eutectic-like solvent-assisted isolation of lignin from Pinus densiflora and its characteristics

Lignin, the most abundant aromatic biopolymer on Earth, has great potential to replace petrochemical-based polymers in the production of value-added products. However, lignin is difficult to extract from lignocellulose because of the recalcitrance of the latter. Herein, the extraction of lignin from lignocellulose using deep eutectic-like solvents (DESs) as green solvents was investigated. Three types of DESs were used, and the effects of treatment temperature (100, 110, and 130 °C) and time (6, 12, and 24 h) on lignin yield and its characteristics were studied. For each DES, the yield of DES-lignin increased with reaction temperature and time. At the same time, the lignin yield obtained using different DESs decreased in the order of choline chloride/lactic acid > betaine/lactic acid > K2CO3/glycerol. At higher temperatures and longer reaction times, lignin with a high molecular weight was produced, which was attributed to the recondensation of lignin fragments. Furthermore, the methoxyl and hydroxyl group contents decreased with increasing reaction temperature and time. Thermal stability studies revealed that the increased molecular weight of DES-lignin enhanced its thermal resistance.