Catalytic Hydrogenolysis And Enzymolysis/oxidative Depolymerization Of Lignin To Produce Aromatic Monomers

With the rapid development of the global economy,the increasing population and consumption of fossil energy and the environmental pollution are caused more and more attention.Lignocellulose with the nature of renewable and being depolymerized is increasingly important.Lignin is difficult to use and thus becomes a bottleneck for biomass utilization because of its complicated structure,compared with cellulose.Therefore,it is of great significance to explore the depolymerization method of lignin to prepare the aromatic monomers for the high utilization of lignin.In this paper,V_xO_y/ZSM-5 molecular sieve catalyst was prepared.The effects of the loading of V₂O₅?1%-10%?,the ammonia?0.4-2.8 mL?,the calcination temperature?400-700??on the morphology of catalyst and the catalytic performance were investigated in the process of catalyst preparation.The composition,structure and properties of the catalysts were characterized by various characterization techniques.The morphology and the V base valence state of the catalyst are the best for the highest yields of bio-oil and aromatic monomers were 79.67% and 12.93%,respectively,and the conversion of lignin were 92.57%,when preparation catalyst with 2.8 mL of ammonia and 5% of loading of V₂O₅ at 600 ?.The results showed that the ammonia,the calcination temperature and the loading of V₂O₅ are very important for the physical and chemical properties of the catalyst.They can effectively restore the original high specific surface area and pore volume of ZSM-5,and significantly increase the average particle size of the catalyst.Ammonia can effectively promote the inner diffusion of precursor into the ZSM-5 and make the precursor distributed evenly.And ammonia partly reduced high-valent state of the loaded V₂O₅ to change the crystal structure,enhancing the catalytic activity of the catalyst.On the basis of the above,it was studied the oriented depolymerization of lignin into aromatic monomers by V₂O₅/ZSM-5 as the catalyst.The effects of the formic acid?0.5-2.5mL?,catalyst?0.01-0.08g?,reactive time?30-150 min?and reactive temperature?260-340°C?were investigated,in the process of hydrogenolysis of lignin.The hydrogenolysis of lignin is the best for the highest yields of bio-oil and aromatic monomers with 83.8% and 15.8% respectively,and the highest rates of conversion of lignin with 95.4%,when the lignin reacted with 2.5mL of formic acid and 0.06 g of catalyst at 340 ? for 90 min.The composition and structure of lignin,catalyst,depolymerized products and residues were analyzed by various characterization techniques.The results showed that the co-solvent ofmethanol/water was beneficial to the dissolution of lignin,which increased the accessibility of the substrate.On the other hand,methanol/water was as the blocking reagent against the re-polymerization of aromatic monomers,thereby increasing the depolymerization degree of lignin.The active metal oxide in the catalyst can effectively break the ?-O-4,?-?,and ?-1connected bonds in the lignin at higher temperature,and promote the conversion of the oligomers in the bio-oil into the aromatic monomers,thereby increasing the yield of the aromatic monomers.Formic acid,as a main hydrogen supply reagent,can not only effectively promote the yield of aromatic monomers with catalyst and higher temperature,but also greatly make all the intermediate products and aromatic monomers hydrodeoxygenation,decreasing the oxygen content to oriented depolymerization of lignin for obtaining higher quality of liquid oil.In the end,it was firstly studied the oxidative depolymerization of lignin by copper oxide and hydrogen peroxide followed by enzymolysis pretreatment with laccase.The effects of enzymolysis time,laccase activity,enzymolysis solvent and volume ratio of ethanol in co-solvent on the lignin depolymerization were investigated.The final depolymerization degree and the yield of aromatic monomers were 93.15% and 21.37% respectively,when the lignin was reaction with 5.24 U of laccase in 10/10 of ethanol/buffer at 50 ? for 2h.Moreover,the mechanism of oxidative depolymerization of lignin followed by enzymolysis was illuminated by analyzing the distribution of the identified aromatics in depolymerized products.It was found that as lignin has low solubility in acidic aqueous solution,ethanol was added into enzymatic system to improve the efficiency.Enzymolysis of lignin resulted in the increase in content of methoxyl,and hence promoted the efficiency of lignin depolymerization.The YAM increased to 21.37% from 14.10% and the DD increased to93.15% from 81.83% obtained conventional oxidative depolymerization,which indicated that depolymeirzation of lignin was enhanced significantly by enzymolysis.