Study On Lignin Depolymeriation To Prepare Aromatic Compounds And Its Mechanism By CoMoS-based Catalysts

At present,under the requirement of carbon neutrality and carbon peak,many emission reduction production methods,including the utilization of clean coal,solar energy,wind energy and so on,have become the research hotspots.Lignin is a renewable resource which has not been taken full advantage of.However,it has great potential to produce energy and chemicals and therefore it has attracted extensive attention from researchers all over the world.In this study,bifunctional Co-Mo-S catalysts were used to depolymerize lignin to obtain high value-added aromatic products.The alkaline composite medium system was employed to reduce the operating temperature and pressure of the lignin depolymerization reaction.In addition,the mechanism of the rupture between the main linking bonds（β-O-4）in lignin was investigated by using model compounds.Finally,a 2D nanolayered Co Mo S catalyst with better activity was prepared for the lignin depolymerization.Normally,in order to stabilize the depolymerized lignin free radical fragments and prevent the re-aggregation of these fragments to form carbon deposition,an initial hydrogen pressure or a hydrogen-donating solvent is required in the lignin depolymerization.The bifunctional Co Mo S/δcatalysts were investigated in this study.To depolymerize lignin and stabilize the lignin free radical fragments,the synergic effect of acid-base sites on supports and Co-Mo-S phase were utilized.On the one hand,the lignin was depolymerized effectively and the monocyclic products were fully alkylated.On the other hand,the re-aggregation of lignin fragments was inhibited,hence the process was simplified and the cost was reduced.In this study,the bifunctional Co Mo S/δcatalysts were characterized by various methodologies,such as ICP,BET,Raman,XRD,XPS,CO₂-TPD,NH₃-TPD and Py-IR,while the liquid products were tested by GC-MS.The results showed that the optimal catalyst for lignin depolymerization was Co Mo S/Zr O₂,which achieved a liquefaction rate of 95.76%and a char rate of 3.91%at the conditions of 340℃and 2.5 h.The liquid products were composed mainly of C₄-C₈ alcohols,C₄-C₁₀ esters and C₇-C₁₀ aromatic compounds.In addition,the results indicated that the Co Mo S/Zr O₂ catalyst has a relatively high stability.The liquefaction rate was only reduced by 8%after reuse for 5 times,and the accumulation of carbon deposition was slow as well.It can be seen that the purpose of replacing initial hydrogen pressure and reducing carbon deposition was implemented preliminarilyvia bifunctional catalysts.However,in order to further promote the practical application of depolymerized lignin,the method of lignin depolymerization under mild conditions was needed.In this study,the alkaline composite supercritical mediums were used to depolymerize lignin.The experimental results showed that the lignin liquefaction rate was 86.39%,and the char yield was 0.63%in the alkaline composite supercritical medium of ethanol/water/Na OH.The depolymerization conditions in the alkaline composite supercritical medium were milder,with a lower char yield and a higher concentration of monocyclic aromatic compounds in the liquid product.It was attributed to the formation of cationic compounds which was through the nucleophilic reaction between Na⁺and the phenolic hydroxyl groups in lignin,which polarized the ether bond and reduced the activation energy of its chemical bond.To determine the mechanism of lignin depolymerization,2-phenoxy-1-phenylethanol and phenoxyethylbenzene were employed as the model compounds to study the bond breakage mechanism ofβ–O–4.The experimental results indicated that the Mo S₂ component was conducived to the breakage betweenβ–O–4 bonds,while the introduction of Co could further increase the rupture rate ofβ–O–4 bonds.The unsulfided Co Mo O/Zr O₂ played a certain role in the breakage ofβ–O–4 bonds due to the Co-Zr-Mo phase.In addition,Co Mo S/Zr O₂ bifunctional catalysts showed the best effect on the bond breakage rates ofβ–O–4 bonds in 2-phenoxy-1-phenylethanol and phenoxy-ethylbenzene,and the rates were 82.01%and 10.77%,respectively.While the formation of bicyclic aromatic compounds was inhibited as well.After the addition of TEMPO（free radical inhibitor）,the breakage probability ofβ–O–4 bonds was decreased,indicating that free radicals were involved in the breakage reaction ofβ–O–4 bonds.Several possible pathways were listed when phenol was selected as the reference.Combined with the dissociation energy（BDE）value of C–O bonds inβ–O–4,the optimal path was determined.Firstly,2-Phenoxy-1-phenethylbenzene and phenoxyethylbenzene were transformed into an intermediate state Ph CH·CH₂OPh,which had a low BDE value of 66.9 k J·mol^-1 for C_β-O.Then theβ–O–4 bonds in the intermediate was broken.This provides insights for obtaining monocyclic aromatic products via lignin depolymerization.Finally,to improve the activity of Co Mo S catalysts,a series of nanolayered Co-Mo-S catalysts were prepared for the lignin depolymerization.The effects of Co content and preparation temperature of catalyst were investigated,and it was found that Co Mo S-0.37-180 was the optimal catalyst.The reaction time and catalyst dosage were both reduced compared to previous experiments.The liquefaction rate was 96.15%,and the char yield was 0.06%,which were the best in this study.In addition,the monocyclic compounds in products went through a more thorough alkylation reaction,which was beneficial to the subsequent reforming processing and utilization.When the Co Mo S-0.37-180 catalyst was reused for 5 times,the liquefaction rate was 80.24%,which showed that its catalytic performance maintained well.