Lignin separation and tailoring with phenol-assisted deep eutectic solvents for enhancing alkyl phenols yield in pyrolysis

Deep eutectic solvents (DESs) offer a sustainable and effective strategy for lignin fractionation from biomass, improving the efficiency of enzymatic hydrolysis. However, the downstream utilization of the lignin extracted in high yield through DES is a difficult problem. Herein, this work employed a novel acidic DES with phenolic modifiers to investigate their dual role in lignin extraction and structural modification, aiming to optimize the production of phenolic compounds via pyrolysis. The result showed that the choline chloride/formic acid/phenol DES with a molar ratio of 1:2:0.05 exhibited an excellent lignin extraction efficiency when pretreated at 120 ^oC for 6 h, and the recovered lignin maintained a high β-O-4 content, decreased molecular weight, and low char yield. High yield and selectivity of alkyl phenols were obtained by pyrolysis of the regenerated lignin extracted under the optimal pretreatment conditions. The low-condensed lignin easily generated phenolic compounds after pyrolysis. After five cycles of reuse, the recycled DES maintained a superior delignification effect but significantly decreased pyrolysis efficiency compared to the fresh DES. This indicated that the highly condensed lignin with a large molecular weight is not favorable to producing phenolic compounds by pyrolysis. These highlight the potential of DES-based strategies for the efficient extraction and structural tailoring of lignin to maximize the production of value-added phenolic compounds.