Lewis Acid Promoted the Formation of Competitive Hydrogen Bonds in Deep Eutectic Solvent to Extract Lignin from Cotton Straw

Pretreatment of cellulosic materials with a Lewis acid is used to remove hemicelluloses and lower the reaction temperature of pretreatment systems. Adding Lewis acid in the pretreatment process of lignin extraction can form competitive hydrogen bonds to improve the pretreatment efficiency and the extraction rate of lignin. In this experiment, a Lewis acid (AlCl₃) was introduced to promote the separation of lignin by using the choline chloride/formic acid DES system. The dissolution rate of lignin was studied, as well as the effects of the molar ratio of Lewis acid/choline chloride/formic acid, reaction temperature, and reaction time on lignin structure, extraction rate, and solid residue recovery ratio. The experimental results showed that the optimal ratio of formic acid:choline chloride:AlCl₃ was 1:1:0.05. When the reaction temperature was 100 °C and the reaction time was 6 h, the solid residue recovery ratio was 33%, the extraction rate of lignin was 95.6%, and the purity was 95.0%. 2D HSQC NMR, X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared (FT-IR) spectroscopy were used to confirm the effect of Lewis acid on the extraction efficiency and structure of lignocellulose. The extracted lignin and commercial lignin showed similar structures, and the main structural units were G units and S units. In short, the addition of a Lewis acid improved the separation of lignin.