Study On Heteropoly Acid Catalyzed Lignin Depolymerization And Application Of Its Depolymerization Derivatives

Recently,with the consumption of non-renewable resources,products with fossil resources as basic raw materials are in an increasingly serious shortage.Therefore,it has gradually been considered as a promising energy strategy to replace some fossil resources with biomass resource for producing refined chemicals,liquid fuel and biomass-based materials.Currently,studies about biomass mostly focus on cellulose and hemicellulose,which are relatively easy to convert among three major components of plants.In contrast,studies about lignin conversion have not been widely concerned by researchers.Meanwhile,lignin,as a by-product,is characterized by low cost and high yield for papermaking.Hence,the high-value utilization of black liquor lignin cost can somewhat be reduced and fossil fuel can be replaced by efficiently utilizing black liquor lignin in papermaking so that papermaking is less dependent on non-renewable resources.However,due to the complex structure of lignin and the strong chemical bonding within the molecule,there is still no effective means to convert it into high-value chemicals,renewable fuels and biomass-based materials.In this study,the five black liquor lignin pulping with ethanol is firstly converted into high-value phenolic compounds and lignin derived bio-oil with low molecular weight and high yield by phosphotungstic acid（PTA）catalytic depolymerization in this study,and then the lignin bio-oil is further used for developing lignin-based materials with excellent performance.The poplar,pine,straw,bulrush and corn stalk enzymolysis residue are used as raw materials,and organic solvent pulping with ethanol as main solvent is then applied for extracting highly pure lignin from these five raw materials,respectively.Subsequently,HPLC,FT-IR,2D-HSQC NMR,GPC,EA,TGA and DSC are used for investigating the purity,structure,molecular weight,elements distribution and thermal stability of five original lignin extracted from raw materials.The results indicate that five original lignin prepared by ethanol pulping are pure with fewer carbohydrate impurities,but there are different contents of structural bonding units such asβ-O-4,β-βandβ-5 in five lignin structures.Among them,broadleaf lignin is dominated by S-type and G-type structural bonding units,coniferous lignin is dominated by G-type structural bonding unit,and grass lignin contains H-type,G-type and S-type structural bonding units.Meanwhile,the difference in the contents of structural bonding unit between the five lignin samples further results in their difference in molecular weight,elements distribution and thermal stability.Generally speaking,for lignin sample,the moreβ-O-4 aryl ether bonds,the larger molecular weight,the more oxygen content,and the worse the thermal stability.After five kinds of black liquor lignin are extracted by ethanol pulping,lignin is further depolymerized under the catalytic effect of heteropoly acid（phosphotungstic acid,PTA）in the reaction system of ethanol/water solvent by taking high-temperature and high-pressure reaction still for magnetic stirring as reactor and five kinds of lignin as raw materials to prepare high-value phenolic monomer and lignin-derived bio-oil with low molecular weight and high yield.It is found that there are moreβ-O-4 aryl ether bonds in the lignin of poplar than another four kinds of lignin if the yields of phenolic compounds and lignin-derived bio-oil with low molecular weight are taken as standard for measurement.Therefore,the lignin of poplar produces phenolic compounds and lignin-derived bio-oil with low molecular weight with a yield of 7.76%and 85.41%,respectively.Meanwhile,the difference in the contents of structural bonding unit between the five lignin samples further results in the different proportions of H-type,G-type and S-type structural bonding units contained by phenolic compounds after catalytic depolymerization.After five kinds of lignin are depolymerized under the catalytic effect of PTA,this paper further investigates the optimal reaction conditions by taking grass raw materials（bulrush lignin）as substrate,PTA as catalyst and the yields of phenolic compounds and lignin-derived bio-oil with low molecular weight as standard for measurement and changing reaction temperature and reaction duration.The results indicate that when reaction temperature is250°C and reaction duration is 6 h,the maximum yields of phenolic compounds and lignin-derived bio-oil with low molecular weight are 5.11%and 85.84%,respectively.After lignin is depolymerized under the catalytic effect of PTA,the electrostatic spinning properties of lignin-derived bio-oil with low molecular weight（including ethanol soluble lignin-derived bio-oils（EL）and tetrahydrofuran soluble lignin-derived bio-oils（TL））are comprehensively studied by N,N-dimethylformamide（DMF）as solvent and polyacrylonitrile（PAN）as spinning aid.The experimental results indicate that the spinning solution of lignin-derived bio-oil with low molecular weight basically does not exhibit spinning property,but its electrostatic spinning property is significantly improved after spinning aid PAN is added into the solution.In addition,as the lignin-derived bio-oil with low molecular weight increases,the viscosity,surface tension and conductivity of the blended solution gradually decrease.Subsequently,electrostatic spinning method is used to prepare electrospun nanofilaments（lignin-based carbon nanofiber precursor）,which is further pre-oxidized and carbonized to eventually prepare high-performance lignin-based carbon nanofibers with high substitution rate by lignin and excellent morphology（ligni-deploymerized derivative carbon nanofibers with low molecular weight）.Based on an application of lignin carbon nanofibers,this study adds nickel nitrate during the preparation of blended spinning solution to form Ni-lignin carbon nanofibers（Ni-LCNF）composite catalyst.The experimental results indicate that the addition of nickel nitrate can contribute to the formation of fine fiber morphology.After electrospun nanofilaments prepared by electrostatic spinning is pre-oxidized and carbonized,Ni²⁺can still be successfully loaded on the surface of lignin-based carbon nanofibers.The Ni-LCNF composite catalyst that has been prepared is used for lignin catalytic degradation.Based on the measurement results of phenolic compounds,it is found that Ni-LCNF composite catalyst can significantly facilitate the catalytic polymerization of lignin.