Enhancing the Value Added of Lignin Extracted from Pinus massoniana Lamb. via DES Pretreatment

This study systematically investigated the pretreatment effects of diol-based DESs (deep eutectic solvents) on Pinus massoniana Lamb. (P. massoniana). A diol-based DES system (Choline chloride (ChCl): AlCl3: BDO) was developed to degrade and disassemble P. massoniana, thereby facilitating saccharification and achieving the utilization of high-value lignin. The DES-based pretreatment achieved a glucan recovery yield of 92.95% and a xylan yield of 71.73% at 130 °C. Meanwhile, the lignin removal yields reached 61.96% at 130 °C, and the lignin recovered from DES fractionation was also preserved well; moreover, the β-O-4′ linkage content was retained at approximately 51.63%. DES was also demonstrated to be promising for promoting cellulose saccharification, lignin fractionation and enzymatic hydrolysis. The preservation mechanism was speculated to involve the introduction of diol -OH groups at the Cα-position of the lignin β-O-4′ structure via etherification. In addition, FT-IR indicated that the main structure of cellulose in P. massoniana remained unchanged after pretreatment. The grafting of diol onto the Cα-position of the β-O-4′ linkages was confirmed by 2D-HSQC, which could inhibit lignin further condensation; 31P NMR revealed that the total phenolic -OH content increased significantly and was enhanced by pretreatment, which indicated that methoxy and ether bond groups were reduced.