Study On Depolymerization Of Lignins Into Monomeric Phenolic Compounds

Lignin is the only renewable feedstock to replace fossil resources for the production of high-value aromatic bulk,possessing great economic value.However,due to the robust and complex structure,efficient conversion of lignin still represents a great challenge compared to that of cellulose and hemicellulose.Currently,lignin can be depolymerized into fine monomeric phenolic compounds,but there are still many issues existing,such as the advanced correlation between lignin's structure and its depolymerization monomers yield,insufficient depolymerization,and poor product selectivity,etc.Solving the above issues is significantly important for the full component utilization of biomass.In this paper,from the point of "the structure and the depolymerization effect",we firstly carried out comprehensive structural analysis of eight lignins,and then focused on the cleavage of C-O bonds in lignin dimeric model compounds of ?-O-4 linkage(most prevalent in real lignin)and the depolymerization of real lignins,through catalyzed in-situ hydrogenation and acid catalysis respectively.Finally,the specific relationship between the structure of lignin and the yield of depolymerized monomeric phenolic compounds was proposed.Firstly,the structure of eight lignins were characterized by a series of means to provide valuable information for the follow-up study,and preliminary depolymerization of the black liquor lignin was explored in subcritical water without catalyst.Results showed that the C content in hardwood lignin was slightly lower than that in softwood lignin and grass lignin(58.63-61.07%).Softwood lignin consisted of mainly guaiacyl units,and hardwood lignin had both guaiacyl and syringyl units,while grass lignin had all the three units.The linkage contents in hardwood lignin was generally higher than that in softwood lignin and grass lignin.About 34-36 ?-O-4 linkages were identified per 100 C9 units for the two birch lignins.Mild lignin preparation process helped to preserve more linkages in lignin.Preliminary depolymerization of black liquor lignin in subcritical water indicates that the highly condensed lignin is not suitable for depolymerization,and suppression of the repolymerization should also be valued while promoting the depolymerization.According to the the structural analysis,cleavage of C-O bonds in ?-O-4 dimeric model compounds was studied by catalytic in situ hydrogenation and acid catalysis respectively.Phenethyl phenyl ether(?-O-4)was sufficiently converted into 90.34%monomers with He at 160? in isopropanol/water.Adding Re on Pt/TiO2 allowed higher conversion and higher yields of monomers at long reaction time.With the existence of benzyl hydroxyl group,C-O bonds could be effectively cleaved catalyzed by formic acid,and dioxane helped to achieve the cleavage under lower temperature,and effectively inhibited the condensation reaction and esterification of formic acid with the reactant.Depolymerization of real lignin in catalytic hydrogenation system showed that the presence of H2 performed worse than He,and lignin depolymerization deepened at higher temperature with longer reaction time.Catalyzed over Pt-Re/TiO2,depolymerization of acid extracted birch lignin with He at 240? in isopropanol/water after 12 h produced 18.71%monomeric phenolic compounds,with alkyl phenolics as the main products.Yield of 4-propylsyringol reached 7.48%.The HSQC of the depolymerization product revealed that ?-O-4 linkages in real lignin have been fully cleaved.Pt-Re/TiO2 catalyst performed good hydrothermal stability,and its activity decreased slightly after three cycles.Correlation of structural information with the depolymerization results shows that a positive correlation existed between the amount of ?-O-4 bonds in lignin and the total yield of monomeric phenolics.The monomers yields of hardwood lignins were generally higher than that of softwood lignins and grass lignins,possessing higher potential for production of monomeric phenolics.Hardwood lignin prepared with mild condition should lead to further higher monomer yields.Depolymerization of real lignin in acid-catalyzed system showed that the lignin depolymerization reaction deepened at higher temperature with longer reaction time,but the distribution of product became complex at the same time.Depolymerization of organic acid-extracted birch lignin over formic acid at 180? in formic acid/dioxane after 12 h produced 8.04%monomeric phenolic compounds,with Hibbert ketones as the main products.Yield of 1-(4-hydroxy-3,5-dimethoxyphenyl)-2-propanone reached 3.10%.The present work used formic acid as the catalyst and the solvent,and formic acid can be directly prepared from biomass,which will bring better economic benefits and is more environmental-friendly.The results provide valuable information for choosing lignin with high potential to be depolymerized into monomeric phenolic compounds,and determing the appropriate depolymerization method according to the target products.