Rapid microwave-assisted pretreatment using a ternary deep eutectic solvent for lignin fractionation of eucalyptus

This work developed a strategy for rapid microwave-assisted refining of eucalyptus wood in 30 min using a ternary DES (lactic acid: choline chloride: polyethylene glycol 400, 10:1 + 5%) with an excellent delignification rate of 89.73%. The test of Kamlet-Taft solvatochromic parameters of DESs shows that the acidity and polarity of the solution greatly influence the delignification rate. The polyethylene glycol 400 increased the solubility of lignin in the solvent, improved the delignification rate, and protected cellulose from degradation. The gel permeation chromatography, 2D-HSQC NMR, and 31P NMR analysis show that the lignin obtained after DES treatment had low molecular weight, small polydispersity index, and relatively rich content of beta-O-4 and phenolic hydroxyl groups, which has great application potential in high-value-added products from lignin. At the same time, the main products of the simulated reaction of the lignin model compound were monophenols and Hibbert ketone, and the possible reaction mechanism of changes in lignin during treatment was given. Our study lays a foundation for applying DES in biomass refining and the high-value utilization of lignin.