Various products can be obtained after lignin depolymerization. Table 1 shows the effect of different metal chloride on bio-oil, aromatic monomer, and residue yields. The specific aromatic monomer distribution and yield are shown in Supplementary Table S1. Generally, the distribution of depolymerized lignin products was complex. According to the specific functional groups, products were classified into four categories: 2,3-dihydrobenzofuran (DHBF), phenol-type compounds (H), guaiacol-type compounds (G), and syringol-type compounds (S). It should be noted that no chloride-contained compounds were detected in products, indicating that dissolved chloride did not react with the lignin.

As shown in Table 1, the bio-oil yield increased slightly after adding Ru/C catalyst, but the promotion effect on the aromatic monomer yield was not significant. The yield of bio-oil increased or decreased when metal chloride was added. It is generally known that Cl is a high electronegativity element, and Cl⁻¹ was widely used in biomass dissolution and conversion as an excellent nucleophilic regent (Long et al., 2011; Shu et al., 2015). C-O bond in cellulose was easier to be broken in most cellulose conversion reactions using chloride ionic liquids as solvent (Swatloski et al., 2002). Lignin contained a large number of ether bonds, especially the β-O-4′ bonds. Therefore, it is considered that Cl⁻¹ in metal chloride can work in lignin depolymerization. Data in Table 1 showed that ZnCl₂, CrCl₃, and FeCl₃ significantly promoted lignin depolymerization. The common feature was the higher valence of metal cation. The result showed that metal cations also affected lignin depolymerization, which may be related to the Lewis acid strength of metal cation. It has been reported that metal cations can produce more acid centers (Miessler et al., 2013), promoting lignin depolymerization. One more interesting phenomenon, metal cations of AlCl₃ and MgCl₂ also had high valence, but the bio-oil yield was lower than that of other high-valent metal cations. This might be caused by hydrated metal chloride used in the experiment, such as AlCl₃•6H₂O and MgCl₂•6H₂O. Water present in the reagents might affect lignin depolymerization, which was also confirmed by the high residue yield.

Yields of bio-oil and aromatic monomers were the highest in the presence of ZnCl₂. The result differed from Shu et al. (2018)’s research, who believed that CrCl₃ provided a better catalytic effect. The higher valence of Cr³⁺ in CrCl₃ could produce more acidic centers, which promoted lignin depolymerization. Cr³⁺ was first conjunct with O and benzene ring of lignin molecule to form a stable complex. Synchronously, Cl⁻ with high electronegativity was connected to lignin molecule, resulting in the breakage of C-O bonds (Tang et al., 2021). Furthermore, hydrogenolysis was promoted by capturing hydrogen under the polarization effect of Cl⁻ and the synergic effect of Pd/C (Parsell et al., 2013). The product distribution was significantly different by comparing the aromatic monomers generated from ZnCl₂ and CrCl₃ synergistic Ru/C catalytic depolymerization of lignin. For example, when ZnCl₂ was used, the yield of 4-ethylphenol (H2, 0.89 wt%) was the highest among the H-type compounds, without the generation of p-coumaric acid (H4). When CrCl₃ was used, it showed the opposite result that the yield of H4 (1.55 wt%) was the highest among the H-type compounds without H2. The result indicated that the catalytic mechanism of ZnCl₂ and CrCl₃ was different. Compared with other metal chloride, using ZnCl₂ could facilitate the formation of H4 and increase the yield of G-type compounds. Besides, the highest yield (5.58 wt%) of 2,3-dihydrobenzofuran (DHBF) was obtained. Significant decreases of bio-oil and total aromatic monomers were exhibited when ZnCl₂ was replaced by Zn(OAc)₂. Meanwhile, a dramatic change of product distribution occurred, especially the S-type compounds. It indicated that the synergic effect between Zn²⁺ and Cl⁻ was contributed to improve the yields of bio-oil and aromatic monomer. Data showed that lignin could be efficiently depolymerized under the catalyst of ZnCl₂ cooperated with Ru/C.

The effect of ZnCl₂ dosage on lignin depolymerization was investigated. As shown in Table 2, the category and yield of S-type compounds decreased with the increase of ZnCl₂ dosage. Only S3 was produced when the dosage of ZnCl₂ was 1 and 2 mmol. As the dosage of ZnCl₂ was further increased to 3 mmol, no S-type compounds were produced. In view of the lower yield of S-type compounds in depolymerized products, yields of bio-oil, total monomers, DHBF, H-type, and G-type compounds were mainly discussed as follows.

As shown in Figure 1, the yield of bio-oil increased with increasing ZnCl₂ dosage. And the highest bio-oil yield of 93.4 wt% was obtained when 3 mmol ZnCl₂ was used. The yield of total monomers rose from 5.38 wt% to 13.4 wt% with increasing ZnCl₂ dosage from 0 to 2 mmol. However, as ZnCl₂ dosage was further increased to 3 mmol, yield of total monomers decreased to 11.6 wt%. This phenomenon suggested that lignin depolymerization was favored at high ZnCl₂ dosage; meanwhile, repolymerization of monomers to form oligomers was also promoted. Among the monomers, DHBF was abundantly produced; the yield first increased to 5.6 wt% and then decreased to 2.03 wt% as a function of ZnCl₂ dosage. The yield of H-type compounds increased as the amount of ZnCl₂ increased, and the highest yield (5.63 wt%) was obtained when 3 mmol ZnCl₂ was used. It should be noted that H2 was produced only in the presence of ZnCl₂, and phenol was produced only when the ZnCl₂ dosage was more than 2 mmol. The yields of H2 and phenol were proportional to ZnCl₂ dosage. This phenomenon indicated that increasing the dosage of ZnCl₂ was conductive to the formation of H1 and H2. The highest yield (1.91 wt%) of p-coumaric acid appeared when the ZnCl₂ dosage was 0.5 mmol. Further increasing ZnCl₂ dosage, no p-coumaric acid was detected in products. This indicated that high ZnCl₂ dosage inhibited the production of p-coumaric acid. For G-type compounds, the yield first increased to 6.4 wt% and then decreased to 3.92 wt% as a function of ZnCl₂ dosage. The decrease in the yield of G-type compounds was mainly caused by a decline of G3, G4, G5, and G6. The yields of G1 and G2 showed similar variation tendency with H1 and H2, respectively, when increasing ZnCl₂ dosage. The result indicated great significance for controlling the selectivity of depolymerized products by varying ZnCl₂ dosage.

The effect of Ru/C dosage had also been carefully examined. Table 3 shows the distribution of aromatic monomers. When the amount of ZnCl₂ was 2 mmol, there were basically no S-type phenolic compounds in the products regardless of the amount of Ru/C. As can be seen from Figure 2, the yield of bio-oil increased after adding Ru/C catalyst. The highest bio-oil yield (91.1 wt%) and total aromatic monomer yield (13.4 wt%) were obtained when 0.2 g Ru/C was used. Further adding Ru/C dosage would decrease the yields of bio-oil and total aromatic monomer. According to Figure 2, the amount of Ru/C had little effect on the yield of DHBF, but mainly affected the yield of H-type and G-type compounds. The yield of H-type compound was proportional to the amount of Ru/C, and the main products were H1 and H2. For G-type compounds, the highest yield (6.4 wt%) was obtained when 0.2 g Ru/C was used. The yield of G1 increased after the addition of Ru/C, but the amount of Ru/C had little effect on the yield, basically keeping at 0.9 wt%. The increase of Ru/C dosage was beneficial to the production of G2, but decreased the yield of G4. According to Table 4, when the amount of Ru/C increased, the yield of the products containing carbonyl group or double bonds in the side chain showed a decreasing trend. In contrast, the yield of products without unsaturated bonds in the side chain tended to increase. The result demonstrated that Ru/C facilitated hydrodeoxygenation in the side chain of benzene ring (Chen et al., 2015). The reason was that formic acid, as an in situ hydrogen source, could generate active hydrogen species using Ru/C catalyst under solvothermal conditions (Kristianto et al., 2017), which could facilitate hydrodeoxygenation. The role of formic acid was different from our previous study (Shao et al., 2018), in which formic acid acted through acid-catalyzed cleavage of linkages in lignin. In the presence of Ru/C, formic acid played two roles: (1) acid-catalyzed cleavage of linkages; (2) acted as an in situ hydrogen donor.

The effect of reaction temperature on lignin depolymerization was investigated (Figure 3). It could be found that lignin depolymerization was highly dependent on temperature. Yields of bio-oil and total aromatic monomers were sharply increased with the elevation of reaction temperature. Remarkably, a noticeable rise was exhibited in the temperature range of 140–160°C. Wherein, yield of bio-oil increased from 77.6 wt% to 91.1 wt%, and yield of total aromatic monomers increased from 8.22 wt% to 13.4 wt%. This indicated that a higher reaction temperature seemed to have a positive influence on lignin depolymerization. However, further increasing the temperature to 180°C, yields of bio-oil and total aromatic monomers decreased to 83.9 wt% and 8.69 wt%, where repolymerization of monomers and oligomers was considered to be responsible for this (Long et al., 2014). As can be seen in Table 4, no other S-type phenolic compounds were produced except for S3 at 160°C. The yield of DHBF first increased to 2.48 wt% and then decreased to 1.45 wt% as a function of temperature. It should be noted that yield of H-type phenolic compounds increased with the elevated reaction temperature, and the highest yield (4.97 wt%) appeared at 180°C. The yield of total G-type phenolic compounds showed a similar tendency with that of bio-oil and total aromatic monomers when reaction temperature rose. One more interesting phenomenon, G4 could be detected only reaction temperature was 160°C. Product distribution showed excessive high temperature promoted the production of H-type compounds, but inhibited G-type compounds, aggravating the repolymerization (Lin et al., 2015). An appropriate temperature is adopted to optimize the yield of product.

Figure 4 shows the effect of reaction time on lignin depolymerization, and product distribution is shown in Table 5. As reaction time increased from 15 min to 30 min, bio-oil yield increased from 75.3 wt% to 91.1 wt%, and the total aromatic monomers yield reached 13.4 wt%. Further prolonging the reaction time to 45 min, significant decrease occurred in bio-oil and total aromatic monomers yield, which were 81.8 wt% and 10.6 wt% respectively. Data in Table 5 showed that there were fewer kinds of aromatic monomers when reaction time was 15 min. DHBF and H-type compounds were the major products, which yields were 2.28 wt% and 3.87 wt% respectively. These indicated that short reaction time led to insufficient bond rupture. For G-type compounds, the total yield first increased and then decreased as reaction time increased from 15 min to 45 min. It should be noted that yields of G1 and G2 had been rising with the increase of time. However, other G-type compounds appeared in the products only when the reaction time reached 30 min, which decreased in the case of prolonging the reaction time. These results illustrated that appropriate reaction time was favorable for lignin depolymerization. Short reaction time would make the reaction incomplete. Repolymerization of lignin-derived intermediates was promoted at long reaction time (Miller et al., 2002; Toledano et al., 2014; Shu et al., 2016a). Therefore, it is necessary to determine the ideal reaction time for the efficient depolymerization of lignin.