Process And Mechanism Research On Lignin Depolymerization

With the depletion of fossil fuels as a source of high-value fine chemicals and fuels, thefraction of chemicals and fuels supplied by renewable resources such as biomass can beexpected to increase in the foreseeable future. However, most researches have focused onconversion of cellulose and hemicellulose, the relatively easy conversion components oflignocellulosic biomass, remaining lignin under-utilization. With its unique structure andchemical properties, lignin supplies an alternative raw material for the production ofhigh-value phenolics. Challenges are to develop a methodology for continuous feeding of thelignin and the maximum yield and selectivity of a limited number of target mono-phenolicsubstances at mild reaction conditions. In this work, extraction of high-purity lignin wasexpolred to prepare lignin solution; then the obtained lignin solution was directly used asfeedstock for production of phenolics by mild hydrothermal depolymerization; afterinvestigating the mechanism of lignin depolymerizaiton by model compounds, selectiveproduction of high-value4-ethylphenolics from lignin hydrogenolysis at mild conditions wasfirstly reported; finally, the mechanism of lignin hydrogenolysis was studied. The results areas follows:Lignin solution was prepared by ethanol organosolv extraction of residues ofsteam-exploded–enzymatic-hydrolyzed corn stalks without catalysts. The results show thatthe optimum extraction conditions were as follows: ethanol concentration of65%(volumepercent), extraction temperature318K, extraction time60min, and solid-liquid ratio of1:8.The yield of lignin was35%(based on Klason lignin of raw material), with Klason lignincontent (purity) high up to76%. The purity of lignin was increased significantly from55.0%to76%, while the ash content in lignin sample decreased notably from8.03%to0.1%. Theproperties of the lignins obtained from different conditions were similar. Corn stalk lignin is aclassic G/S/H-type lignin, the highest content of H-type structure unit indicated that it can beused for the preparation of phenolics.The hydrothermal depolymerization experiments were conducted in a100mL SS316Lbatch reactor. The study found that the highest yield of liquid products up to65.5%wasrecovered from lignin depolymerization at optimal conditions of523-548K,90min,65%vol.ethanol and3-7.5%lignin; meanwhile, only17%of solid residue was obtained. Up to35-40%of identified compounds such as4-ethylphenol,4-vinylphenol, guaiacol,4-ethylguaiacol and4-vinylguaiacol can be obtained. During lignin depolymerization, the cleavage of chemicalbonds in lignin coupled with the formation of new functional group. Decarbonylation and cleavage of β-O-4linkages were the major reactions during lignin depolymerization. Phenolichydroxyl and C2-alkyl were the major new functional groups which were formed duringlignin depolymerization.Some monomers, p-hydroxycinnamic acid (PCA), ferulic acid (FA), ferulic acid ethyl ester(FAEE),3,4-dimethoxycinnamic acid (DMCA), and some polymers, the H-type (HP) andG-type (GP) β-O-4polymer as well as guaiacyl phenolic glycerol-β-guaiacyl ether (GG) wereselected to be lignin model compounds. The lignin model compounds were depolymerizedunder hydrothermal conditions to simulate the real lignin depolymerization process.Experimental results showed that p-hydroxycinnamic acid, ferulic acid and β-O-4modelcompounds could be completely degraded. Decarboxylation reaction and cleavage of β-O-4bonds was the major pathway that lignin model compounds degraded. The cleavage of thesebonds and function was really the key steps that lignin depolymerized. The formationcompounds were mainly4-C2-C6phenolics and their derivatives. The formation of4-vinylphenolics prone to repolymerization to form a dimer, which might explain why the solidresidue even coke formation during lignin depolymerization. Ethanol reacted with4-vinylphenolics which on the one hand inhibited the condensation of4-vinylphenolics andon the other hand improved the selectivity of the target compounds. The alkylation ofphenolic hydroxyl and/or carboxyl group led to the high stability of the model compounds.This may explain why the lignin can not be completely degraded to monomeric phenolics.Based on the lignin depolymerizaiton process and lignin depolymeirzation mechanism,selective production of4-ethylphenolics (4-ethylphenol and4-ethylguaiacol) from lignin viamild hydrogenolysis was first reported in this work. The results showed that2.3%of4-ethylphenol and1.1%of4-ethylguaiacol were obtained simultaneously using5%Ru/C ashydrogenation catalyst under the same reaction conditions. Catalysts can be reused with highselectivity and stability. When reaction temperature is548K, other reaction conditions remainunchanged, the yields of4-ethylphenolics from lignin (3.1%of4-ethylphenol and1.3%of4-ethylguaiacol) were approximate to the yields of4-ethylphenol produced by petroleumbased routes (2.8-4.7%). The production of4-ethylphenolics from lignin via mildhydrogenolysis would be a promising method for the substitution of petrochemical route.Lignin model compounds were good tools for better understanding the pathway of lignincatalytic hydrogenolysis. The experimental results showed that the degradation pathway weredecarboxylation reaction and double bond saturation reaction by hydrogenation. Ethanolreacted with hydrogenated products containing carboxyl groups to frorm ester, whichinhibited the further degradation to form simple phenolics. The4-vinylphenolics obtained from lignin model compounds was hydrogenated to form4-ethylphenolics. Dimers producedfrom β-O-4polymer without double bonds in molecules indicated that β-O-4polymer firstcleaved at β-O-4bonds to form4-vinylphenolics. Then4-vinylphenolics on one handhydrogenated to form4-ethylphenols, on the other hand repolymerized to form dimers withdouble bonds. The formed dimers with double bonds were further hydrogenated to formstable dimers. The results suggested that the pathway of lignin hydrogenolysis could beconcluded as follow: lignin first depolymerized to monomeric4-vinylphenolics via thermallyinduced free radical depolymerization reaction, as happened in lignin hydrothermaldepolymerization; then4-vinylphenolics were hydrogenated to form stable4-ethylphenols.Hydroxycinnamic acid type structural units were the primary source of4-C2-C6phenolics.