Lignin separation modification and high-value utilization based on low eutectic solvents: a review

Abstract Agricultural and forestry waste is rich in renewable lignin, which holds natural potential for the production of fine chemicals and functional materials. However, lignin is tightly cross-linked with cellulose and hemicellulose. Traditional extraction processes using strong acids and bases can easily cause lignin molecular condensation and damage to the molecular framework. At the same time, there are problems such as high energy consumption and heavy environmental burden, which seriously restrict its resource utilization. Due to their advantages of simple synthesis, high tunability, and environmental friendliness, deep eutectic solvents (DES) have become a research hotspot for the green and efficient extraction of lignin. This paper systematically reviews the latest research findings on DES-based extraction of lignin from agricultural and forestry waste, summarizes the characteristics of different types of DES components, investigates the effects of process parameters on lignin extraction efficiency and structural retention, and provides an in-depth analysis of the lignin's characteristic bonds, functional group composition, and other microstructural features. The study confirms that DES can achieve selective dissolution of lignin and effectively cleave ether bonds such as β-O-4; the resulting lignin has a low molecular weight, a narrow distribution, and a high content of phenolic hydroxyl groups, laying an excellent structural foundation for subsequent chemical modification and deep processing. The paper also further summarizes the high-value application potential of this type of lignin in fields such as the preparation of phenolic compounds and the synthesis of functional materials. At the same time, it objectively analyzes the practical challenges currently faced by DES technology in process optimization, solvent recycling, and industrial-scale upscaling, and makes targeted predictions regarding future key development directions. The aim is to provide a theoretical basis and practical reference for the iteration of green lignin separation technologies and the construction of high-value conversion systems for waste biomass.