## 3 Results and Discussion
### Reaction Routes and Mechanisms  
Compared to KL depolymerization without DES treatment, KL depolymerization using a ZnCl\({}_{2}\)-based DES solvent under the same conditions improved the yields of phenolic compounds, the molecular weight of the bio-oil, and the HHV of the RL (Figure 7, Tables 1 and 2). This improvement is attributed to ZnCl\({}_{2}\), a Lewis acid catalyst, which facilitates the cleavage of lignin linkages during depolymerization. For instance, during the cleavage of \(\beta\)-O-4, Zn\({}^{2+}\) ions coordinate with the O atom in the \(\beta\)-O-4 linkage, weakening the C\({}_{\alpha}\)-C\({}_{\beta}\) bond. This bond weakening leads to its cleavage, resulting in the formation of vanillin--a process already documented in the literature [44]. FT-IR results suggest that the content of phenolic hydroxyl groups in the RL increased over time, while the content of alcoholic hydroxy groups remained almost constant (Figure 5B). The cleavage of the \(\beta\)-O-4 aryl ether bonds in lignin contributes to the formation of phenolic hydroxyl groups, while that of the C\({}_{\alpha}\)-C\({}_{\beta}\) bond leads to an increase in the content of alcohol hydroxyl groups in the RL [45]. Additionally, the molecular weight of the bio-oil and its higher HHV are enhanced, supporting the notion that ZnCl\({}_{2}\)-induced bond cleavage generates reactive intermediates that undergo further reactions, contributing to larger molecular weight products and a higher energy content. The presence of aromatic aldehydes and alcohols in the GC-MS analysis further corroborates the mechanism of lignin breakdown into valuable small molecules. In contrast, control experiments without ZnCl\({}_{2}\) show lower yields of phenolic compounds and a lower molecular weight of bio-oil, confirming the critical role of ZnCl\({}_{2}\) in promoting the depolymerization process. These experimental observations align with the reaction pathway where ZnCl\({}_{2}\) acts as a catalyst to break key lignin linkages, improving both the quantity and quality of the bio-oil.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & **Reaction** & \multirow{2}{*}{**M\({}_{\text{w}}\)**} & \multirow{2}{*}{**M\({}_{\text{n}}\)**} & \multirow{2}{*}{**PDI (M\({}_{\text{w}}\)/M\({}_{\text{n}}\))**} \\  & **Conditions** & & & \\ \hline \multirow{4}{*}{Kraft lignin} & & 2450 & 1493 & 1.64 \\ \cline{2-5}  & 110 \({}^{\circ}\)C for 8 h & 1856 & 1261 & 1.47 \\  & 150 \({}^{\circ}\)C for 8 h & 1813 & 1235 & 1.46 \\  & 170 \({}^{\circ}\)C for 8 h & 1653 & 1155 & 1.43 \\  & 190 \({}^{\circ}\)C for 8 h & 1498 & 1061 & 1.41 \\  & 210 \({}^{\circ}\)C for 8 h & 1651 & 1146 & 1.44 \\ bio-oil & 4 h at 190 \({}^{\circ}\)C & 1734 & 1142 & 1.52 \\  & 6 h at 190 \({}^{\circ}\)C & 1652 & 1112 & 1.48 \\  & 8 h at 190 \({}^{\circ}\)C & 1498 & 1061 & 1.41 \\  & 10 h at 190 \({}^{\circ}\)C & 1817 & 1254 & 1.45 \\ \hline \multicolumn{5}{c}{Without DES} & 190 \({}^{\circ}\)C for 8 h & 1872 & 1259 & 1.49 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Molecular weights of the bio-oil (M\({}_{\text{n}}\): number-averaged molecular weight; M\({}_{\text{w}}\): weight-averaged molecular weight; and PDI: polydispersity index).  
Figure 8 illustrates a possible degradation pathway for lignin in the acidic (ChCl:ZnCl\({}_{2}\)) DES based on the chemical structure analysis of the RL and the GC-MS analysis of small-molecule degradation products. Initially, under the acidic conditions created by the DES, protons attack the \(\alpha\)-position of the hydroxyl groups in the lignin alkyl side chains, leading to the formation of carbocations. These unstable carbocations are then deprotonated to form end ethers, which, in the future, can degrade into small molecules under the acidic conditions. Concurrently, the generated carbocations can react with small lignin degradation products to form new C-C bonds at the C\({}_{5}\) position of the aromatic rings ([PERSON] et al., 2013; [PERSON] et al., 2015) [46; 47].