Pretreatment of enzymatic hydrolysis lignin based on deep eutectic solvent containing a reversibly-soluble base

The pretreatment with green deep eutectic solvents (DESs) is conducive to realizing the high-efficiency utilization of lignin at a low cost. In this study, an innovative choline chloride/urea/calcium hydroxide (ChCl/UR/Ca(OH)₂) DES containing a reversibly-soluble base Ca(OH)₂ was developed for the pretreatment of enzymatic hydrolysis lignin (EHL). The lignin pretreatment effects of the proposed ChCl/UR/Ca(OH)₂ DES were compare with a series of DESs. The results indicated that ChCl/UR/20 % Ca(OH)₂ with a reversibly-soluble base addition of 20 % was permitted to achieve the optimal recovery yield of 81.067 % under mild conditions (80 °C, 7 h). The physical and chemical structure, molecular weight, thermal stability, content of phenolic hydroxyl and carboxyl groups, and recycling performance of DES were compared between ChCl/UR-L and ChCl/UR/20 % Ca(OH)₂-L. The results showed that ChCl/UR/20 % Ca(OH)₂-L retained the lignin skeleton structure and possessed preferable thermal stability and antioxidant activity, which was mainly attributed to the homogeneous molecular structure and abundant phenolic hydroxyl group contents. ChCl/UR/20 % Ca(OH)₂ was more advantageous for recycling, maintaining a lignin recovery yield of 57.400 % after 5 cycles, which was higher than that of most acidic and neutral DESs for regenerating lignin. This work provides a valuable reference on the valorization of lignin by using renewable DES.