Study On Degradation Of Lignin By A New Type Of Heterogeneous Light-Fenton Catalytic System

The rapid consumption of petroleum energy has made renewable biomass resources increasingly concerned and valued,thereby promoting the development of renewable energy technologies.Lignocellulosic biomass is composed of three components:cellulose,hemicellulose,and lignin.Among them,cellulose and hemicellulose have been well industrialized-converted into fuel ethanol and sugar platform compounds.Because lignin has a complex and compact three-dimensional structure that makes it difficult to be used with high value,the depolymerization of lignin into monomeric aromatic compounds is of great significance for improving the utilization value of lignin.In this study,poplar wood was used as raw material to extract organosolv lignin using dioxane and dilute hydrochloric acid solvent system.The yield was 10±2%and the molecular weight（Mw）was 2137 g/mol.The structure of lignin was analyzed by 13C NMR and 2D HSQC NMR,and the contents of S（syringyl）,G（guaiac-based unit）and H（p-hydroxyphenyl unit）of three monomers were determined.The results show that the ratio of S to G is 1.87,the contents of S and G elements are 66.8 and 35.7/100 Ar,the amount of β-O-4 bond is 30.7-36.8/100 Ar,and the amount of β-5,β-β ’and 5-5’ of other side chain structures are 1.6,3.5 and 0.8/100 Ar,respectively.In order to overcome the disadvantages of Fe²⁺ leaching with Fenton’s reagent,which can degrade lignin under harsh acid and high energy consumption conditions,a ternary heterogeneous photo-Fenton catalyst Fe₃O₄@rGO@TiO2 was prepared in this study to degrade lignin.Characterized by Raman spectroscopy,XRD,infrared spectroscopy,etc.,showed that the ternary heterogeneous optical Fenton catalyst Fe₃O₄@rGO@TiO2 was successfully prepared,and its particle size was about 100-200nm,and the atomic percentages of Fe,C,and Ti are about 75%,15%,and 10%respectively.Using water and ethanol as solvents,under the irradiation of ultraviolet light,Fe₃O₄@rGO@TiO2 catalyst was applied to the depolymerization of lignin,the highest depolymerization yield was 34.5%,and the total monomer yield was 1.32%.As the amount of H2O2 and time increase,the degradation rate increases,but the monomer yield gradually decreases.The depolymerization product was qualitatively performed by GC-MS.The photocatalytic product has eight monomers including:p-hydroxybenzoic acid,vanillic acid,vanillin,coniferyl alcohol,conifer aldehyde,syringaldehyde,syringic acid,and eugenol.Quantitative analysis of eight monomers showed that as time and the amount of H2O2 increased,the yield of monomer aromatic alcohols and aromatic aldehydes decreased,and the yield of monomer aromatic acids increased.At the same time,the molecular weight of phenol oil is reduced to about 1000 g/mol under optimal conditions.Using Fe₃O₄@rGO@TiO2 catalyst to lignin dimer（1-（4-hydroxy-3-methoxyphenyl）-2-（2-methoxyphenoxy）-1,3-propanediol）（M1）and（1-（3,4-dimethoxyphenyl）-2-（2-methoxyphenoxy）-1,3-propanediol）（M2）depolymerization study,according to 2D HSQC NMR Based on the results of GC-MS,the possible mechanism of the catalyst TiO2@rGO@Fe₃O₄ to depolymerize the lignin model is speculated:·OH oxidizes the y position of the model,and then the Cγ carboxylic acid undergoes intramolecular esterification or Grob-Fragmentation reaction and then decarboxylates and then eliminates the enol ether intermediate.Then·OH attacks the enol ether intermediate to generate monomer compounds.Studies have proved the effectiveness of heterogeneous photo-Fenton composite catalytic degradation of poplar wood lignin,and explored its degradation mechanism,which provides a useful reference for the high-value utilization of lignin.