# Deep Eutectic Solvent Pretreatment and Green Separation of Lignocellulose
## 3 Definition of Deep Eutectic Solvent Effects
Hydrothermal pretreatment is a chemical-free, economical and environmentally friendly process with little effect on the structure of lignin and cellulose. In addition, hydrothermal pretreatment uses water at high temperatures and pressures to hydrolyze the lignocellulosic structure, especially hemicellulose, which is more easily hydrolyzed due to its highly amorphous structural properties [105]. Compared to biomass material, the pretreatment significantly shortened the treatment time while maintaining the same lignin removal rate [57,64]. Overall, different types of DESs have varied separation effects on biomass due to their specific compositions and ability to disrupt and catalyze the breakdown of lignocellulosic structures. By modifying the components of DESs and introducing assisted solvent, the lignin solubility of DES can be adjusted, thereby protecting the structure and function of raw materials after separation [106]. Numerous of assisted treatment methods have been applied to lignocellulosic separation process [107]. The assisted treatment can improve the separation capacity of DES or simplify the separation conditions through chemical or physical means [108]. By comparing the different kinds of auxiliary treatment on the separation effect of lignocellulosic, it can be found that microwave-assisted pretreatment and ball-milling pretreatment often have a higher lignin removal rate and cellulose retention rate. This is due to the potential of these two methods to be applied in industry. Figure 2 provides a comprehensive and intuitive overview of the steps involved in the separation of lignocellulosic biomaterials using DES. This includes the composition and type of DES, pretreatment of biomass, separation of lignocellulosic by DES, and the resulting cellulose-enriched residue and lignin-enriched filtrate obtained through filtration. Due to the high viscosity of DES, an organic solvent-assisted filtration process is necessary to effectively separate the cellulose-enriched component from the DES + lignin/hemicellulose component. Commonly used organic solvents in such studies include aqueous solutions of ethanol or acetone [49; 51], which serve to reduce the viscosity of the filtrate without causing dissolution of solid cellulose residues. The changes in biomass microstructure during DES treatment were analyzed by SEM after drying the solid residue. At present, DES is widely used as a catalyst, co-solvent, and extraction solvent for various conversion and downstream processes [109]. The recycling of DES is one of the major challenges in achieving cost-effective industrial applications [110; 111]. The relatively low solvent consumption and recyclability of DES make it an important green solvent in the field of green separation of lignocellulosic [112]. There are many methods to recover and purify DES, including the addition of anti-solvent, crystallization, membrane filtration, solid-liquid extraction, liquid-liquid extraction, short path distillation, supercritical fluid extraction, and density differences-based separation [24; 113; 114; 115; 116; 117]. Specific recovery methods can be selected based on the types and characteristics of DES, the properties of target compounds, energy consumption, and equipment costs [118; 119; 120; 121; 122]. Although DES is widely known as a green solvent, it does not remain environmentally friendly under all conditions due to decomposition and volatilization. Therefore, many strategies have been studied and applied to improve the greenness of DES as potential solutions to achieve Green Chem [123]. For example, strategies include choosing more natural ingredients [124; 121], specific functionalization [125], increasing molecular weight and surface area by polymerization for separation, sensing and catalysis [126], tuning types of intermolecular interaction and component numbers [127], constructing a switchable system for efficient separation [128], avoiding air, water, high  
Figure 2: DES separation process of lignocellulosic.