Novel ternary deep eutectic solvent for highly efficient dissolution of lignins: Dissolution behavior and mechanism study

Lignin, as an abundant natural aromatic biopolymer, has a complex structure and poor solubility in most solvents, making it difficult to achieve high-value applications and becoming the largest "natural waste". In this work, two novel ternary deep eutectic solvents (DESs) consisting of choline chloride (ChCl)/lactic acid (LA) with ethylene glycol (EG) or γ-valerolactone (GVL) were designed for separating lignin from waste wheat straw. Results showed that the delignification efficiency of wheat straw treated with two ternary DESs (ChCl-LA-EG and ChCl-LA-GVL) was twice as high as that treated with ChCl-LA under mild conditions (90 °C, 4 h). The highest delignification efficiency of 83.12% was achieved by ChCl-LA-GVL treatment at 130 °C for 4 h. Moreover, ternary DESs extracted lignin exhibited lower molecular weight, better homogeneity, and remarkably higher antioxidant activities than that extracted from ChCl-LA. Furthermore, ternary DESs extracted lignin had more S units and inhibited the destruction of the lignin substructure at high temperature. Compared with ChCl-LA treatment, cellulose fiber treated with ChCl-LA-EG and ChCl-LA-GVL had less active sites, lower total surface energy, and more alkaline (electron donor) adsorption sites on cellulose fiber surface. Theory calculations further confirm that the addition of EG and GVL changes the hydrogen bond strength and electrostatic potential of the ChCl-LA system, thus promoting the separation of lignin. These results provide new insights for understanding the mechanism of lignin dissolution in DESs and for the design of advanced DESs for lignocellulosic biomass separation and valorization.