# Deep Eutectic Solvent Pretreatment and Green Separation of Lignocellulose
## 5 Effect of DES Treatment on the Chemical Structure of Lignocellulosic Separation Products
In general, the major substructures of lignin include methoxy [120], aryl ether bonds (A, \(\beta\)-O-4), and \(\beta\)-\(\beta\) resin (B). As Figure 4 shows, during the pretreatment process, the cleavage of \(\beta\)-O-4 linkage in lignin linkage significantly increased with the increase in pretreatment temperature. It was found that the \(\beta\)-O-4 bond disappeared above 110 \({}^{\circ}\)C, indicating that the depolymerization of lignin was achieved by destroying the \(\beta\)-O-4 substructure during DES pretreatment. This is also consistent with the results of acidic DES pretreatment and the breakdown of most lignocellulosic. The lignin-carbohydrate complex (LCC) bond, as described above, shows different signals in the aromatic region corresponding to guaiaicol (G) and syringyl (S) units, which are clearly visible. This phenomenon indicates the process of lignin polycondensation during pretreatment [144]. Morphological changes of lignocellulosic biomass during DES pretreatment can be analyzed by SEM. This technique provides detailed images of the surface structure and morphology of the biomass, allowing researchers to observe alterations at the microstructural level [145]. [PERSON] et al. [146] pretreated sugarcane bagasse (SCB) with ChCl/carboxylic acid. The morphological changes of SCB surface after pretreatment were shown in Figure 5. Pictures of the natural samples showed smooth, complete, and ordered fiber surfaces (Figure 5a), while SEM analysis of the pretreated samples revealed structural differences, with rough and exposed structural morphology. Micrographs of ChCl/LA (lactic acid) showed smooth and consistent surfaces (Figure 5b), mainly indicating the presence of crystalline cellulose. The pretreated samples had higher crystallinity and more ordered cellulose structure than the natural samples [147]. In addition, images of ChCl/CA (citric acid) pretreated biomass indicated a planar porous structure and non-uniform surface formed by various fiber fragments (Figure 5c). Finally, images of the SCB pretreated with ChCl/AA (acetic acid) showed that the deformed structure had wide cracks and holes (Figure 5d). The loss of fibers and the increase in porous surfaces can be observed [146]. The effects of DES with different characteristics and the addition of auxiliary solvents on the morphology of biomass were also demonstrated by SEM analysis [148]. This technique showed that mild acidic DES pretreatment can improve cellulose reactivity by decomposing and swelling the cellulose, removing lignin and hemicellulose (mainly xylan), thereby better exposing the innermost cellulose component of the biomass and making it accessible to enzymes [149]. In contrast, alkaline DES can lead to expansion and partial dissolution of lignocellulosic materials. These DES systems typically effectively remove lignin while preserving the cellulose structure [150]. [PERSON] et al. added FeCl\({}_{3}\) to Moso bamboo pretreated with ChCl/lactic acid DES. SEM illustrated varying degree of cracks and fragments on the surface of the fibers after pretreatment. FeCl\({}_{3}\)-mediated pretreatment caused more \begin{table}
\begin{tabular}{l l l l l l l} \hline \hline
**Substrate** & **DES** & **Chemical Bonds** & **Absorption** & **Trend** & **Affected** & **Component** & **Ref.** \\ \hline Moso bamboo & ChCl: zinc acetate = 1:6 & OH/C-H & 3331/2891 & Almost disappeared & Hydrogen bonds & [136] \\ \hline  & & C=O & 1734 & Decreased & Hemicellulose & \\ \hline Rape straw & ChCl: oxalic acid = 3:1 & CO & 1625 & Significantly reduced & Lignin & [138] \\ \hline  & & CO stretching & 1050 & Reduced & \
\begin{tabular}{l} Hemicellulose- \\ cellulose- lignin \\ polymers \\ \end{tabular} & \\ \hline  & & CH or CH\({}_{2}\) bending & 896 & Reduced & \(\beta\)-glucose bond & \\ \hline Willow & ChCl: lactic acid = 1:10 &
\begin{tabular}{l} CH bending vibration \\ of aliphatic compounds \\ \end{tabular} & 1373 & Not present & Carbohydrates & [141] \\ \hline \hline \end{tabular}
\end{table}