# Deep Eutectic Solvent Pretreatment and Green Separation of Lignocellulose
## 3 Definition of Deep Eutectic Solvent Effects  
In the current research on lignocellulosic separation by DES, it is widely recognized that DES acts by disrupting chemical bonds, hydrogen bonds, and covalent bonds within lignin in biomass materials as well as between lignin and hemicellulose, thereby facilitating the separation of lignocellulosic components. By comparing the lignin removal efficiency of different DES, it is evident that the binary systems of ChCl (choline chloride) + Alcoholamine/benzene sulfonic acid and some quaternary ammonium salt + Bronsted acid/Lewis acid have higher lignin removal capabilities [21; 78]. The principle can be attributed to several factors: alkaline and acidic DES have excellent lignin solubility, and alkaline DES has the additional benefit of protecting polysaccharides in the material [23; 79]. Benzene sulfonic acid demonstrates a significant lignin removal rate at relatively low temperatures due to its strong proton-supplying capacity, which increases its ability to solubilize lignin [6; 80]. Furthermore, ternary DES is often used to increase lignin and hemicellulose removal rates [81], and when Bronsted and Lewis acids coexist in DESs, these two different acids can work synergistically to destroy the lignocellulosic structure and catalyze the cleavage of hemicellulose \(\beta\)-1, 4-glucoside bonds [77]. This combination of properties and synergistic effects makes these DES formulations particularly effective for lignocellulosic separations. Due to the easy degradation of hemicellulose into toxic compounds in a strong acidic environment, such as furfural or acetic acid, and the low yield of aromatic monomers caused by lignin recondensation [82], recent research has focused on reducing the intensity of component degradation and condensation while maintaining a high degree of lignocellulosic separation [83]. This approach aims to enhance lignocellulosic reuse and the downstream conversion of compounds in crop residues. One promising strategy is to introduce polyols into acidic DES to form a mixed-solvent system that preserves the chemical structure of the isolated components. For instance, [PERSON] et al. developed a polyol-based DES composed of ChCl, glycerol, and AlCl\({}_{3}\cdot\)6H\({}_{2}\)O, which successfully removed over 90% of lignin from garlic skin [48]. [PERSON] et al. utilized a DES composed of ChCl, oxalic acid, and ethylene glycol to remove lignin from corn stover, which obtained better cellulose preservation and shortened the reaction time [74]. Similarly, [PERSON] et al. used the neutral DES of ChCl/Xylitol and moso bamboo to obtain a lignin removal rate of about 85% [49]. The enhanced performance of these DES systems can be attributed to the presence of hydroxyl groups in the structure of polyols, which significantly boost the hydrogen bond supply capacity of the DES. This increased capacity for hydrogen bonding facilitates higher lignin removal rates [84]. As a binary neutral DES composed of HBD, polyols exhibit high cellulose preservation and monosaccharide conversion due to their mild chemical properties [85]. However, to overcome the limitation on lignin removal, dilute acids are usually used to pretreat raw materials [86]. On the other hand, polyols can be combined with quaternary ammonium salts and Bronsted/Lewis acids to form ternary acid DES. In these systems, polyols can be used as pH regulators to balance the lignin removal ability and component structure protection ability. Furthermore, DESs typically exhibit high viscosity, especially at close to room temperature, which can hinder mass transfer and prolong lignin removal times. The addition of polyols effectively reduce the viscosity of most DESs to improve its mass transfer efficiency and shorten the lignin removal time [87]. Many kinds of assisted treatments are used in lignocellulosic separation processes [88]. Different treatments often have varied effects on the separation process and the material [89; 90].