Study On The Liquid-Phase Depolymerization Of Lignin Under Ethanol/Isopropanol For Monophenolic Compounds

Lignin,one of the three major components of biomass,is a natural aromatic supramolecular compound formed by bonding of several types of C9 phenylpropane type structural units.By breaking the linkage bonds between lignin structural units,high-value monophenolic compounds such as guaiacol,syringol and p-hydroxyphenol can be obtained.In this paper,the effects of different separation methods and raw material sources on the structure and depolymerization products of lignin were investigated,as well as the composite non-precious metal catalysts were prepared and applied to lignin liquid-phase depolymerization experiments,which laid the experimental foundation for the research of high-value chemicals from lignin liquidphase depolymerization.The elemental composition,relative molecular weight,monomer ratio and linkage bond content of lignin produced by different separation methods were characterized,and the results of liquid phase depolymerization experiments of different lignins in ethanol/isopropanol system were compared.The experiments showed that:organosoluble lignin had higher carbon element content and lower oxygen element content;the molecular weight distribution range of organosoluble lignin was more concentrated;the β-O-4,β-β and β-5 bond content in organosoluble lignin was more than that of lignin separated by inorganic reagents.The overall depolymerization efficiency of organosoluble lignin was higher than that of lignin separated by inorganic reagents,where the liquid phase depolymerization of organo-acid-soluble lignin yielded up to 5.99 wt% of monomeric compounds,and the organic acid-soluble separation method was superior to other separation methods.Different organic acid soluble lignins were prepared from different types of biomass,and the qualitative and semi-quantitative analyses of lignin composition and structure were carried out,and the results of liquid phase depolymerization experiments of different lignins were compared.The results showed that the carbon and hydrogen contents of woody lignin were higher than those of graminic lignin;the relative molecular weights of woody lignin were larger than those of graminic lignin,and their corresponding molecular weight distributions were broader.The number of linkages per 100 structural units was the highest in fir lignin,about 48%,and the number of linkages in grass-based lignins was lower than that of wood-based lignins.The higher the depolymerization efficiency of softwood lignin,the higher the yield of monophenols was 7.38⁸.06 wt%.Although the yield of mono-phenols was lower for grass-like lignin,its oil phase yield was higher.Softwood lignin is more suitable for the preparation of monophenols by depolymerization.Composite non-precious metal catalysts were prepared by co-precipitation method for the application of lignin liquid phase depolymerization.The results showed the presence of nickel atoms in NiAlOx-red in addition to Ni O and amorphous Al₂O₃.The liquid-phase depolymerization of lignin catalyzed by NiAlOx-red yielded 12.24 wt%of monophenols,and the Ni atoms in the catalyst could effectively catalyze the linkage bond breaking in lignin.The catalyst Ni5M5 Al Ox-red（M = Mg,Ca and Zn）is mainly composed of spinel phase except for Ni atoms.The liquid phase depolymerization of lignin catalyzed by Ni5Zn5 Al Ox-red yielded up to 15.49 wt% of monophenols.In NiZnAlOx-red catalysts,the Zn O and Al₂O₃ phases gradually transformed into spinel phases with the increase of nickel content;the spinel phases gradually disappeared with the further increase of nickel content to form Ni O and Al₂O₃ phases.The yield of monophenolic products from lignin liquid phase depolymerization increased firstly and then decreased with the increase of nickel content in the catalyst;under Ni₃Zn₇ AlOx-red catalysis,the lignin liquid phase depolymerization produced up to 17.18 wt% of monophenolic compounds.The bio-oil yield gradually increased with the increase of nickel content;the highest bio-oil yield of about 74.47 wt% was produced by the liquid phase depolymerization of lignin catalyzed by NiAlOx-red.Based on the existing studies and the analysis of experimental results,the possible reaction pathways for the lignin depolymerization experiments were inferred.In summary,this paper found that the efficiency of depolymerization of organicacid-soluble lignin was higher than that of lignin separated by other methods,and the depolymerization efficiency of softwood lignin was higher than that of hardwood lignin and graminic lignin.Among the composite metal catalysts,lignin depolymerization under Ni₃Zn₇ AlOx-red catalysis yielded the most monophenolic products;lignin depolymerization under NiAlOx-red catalysis produced the highest yield of bio-oil.The results of the study have implications for the high-value utilization of biomass.