Studies On Lignin Depolymerization And Its Structure

Due to sharp decreases of the three non-renewable fossil fuel energy sources,as well as environmental issues caused by their used processes,the development of renewable energy is urgent.Biomass is one of renewable energies,and it is an abundant and widely distributed carbon-neutral resource,which has aroused a widespread concern in the society.The three most abundant components in biomass are cellulose,hemicellulose and lignin,of which cellulose is a linear molecule made of glucose connected by glycosidic bonds,hemicellulose is a macromolecule composed of five-cabon and six-carbon sugars,and their structures have been clarified.The two carbonhydrates has been widly used.Lignin is a three-dimensional reticulated macromolecular,and the majority of lignin obtained in pupl and cellulosic industry has been used as fuel for energy supply because of its complex structure.As the only aromatic structure in biomass and the most abundant aromatic compound in nature,direction combustion of lignin results in a great waste of resoureces.Effective utilization of lignin can greatly alleviate the consumption of petroleum.However,the current application of lignin is far from its industrial application.The main reasons are its complex structure,difficulty in depolymerization,poor product selectivity and low yield.Up to now.the proposed lignin structures are composed of three oxygenated phenyl propane structure units,but satisfying monomer yields cannot be obtained in all lignin conversion processes.As we know,the structure of a substance determines its properties,and it is also the basis for its effective use.In response to the current problems,this work depolymerized lignin by a series of means to obtain its products,and then inverted the lignin structure.Through the exploration of the lignin structure,we constructed a new lignin structure model,by which can we provide a theoretical guidance for the application of lignin and improve the utilization efficiency of lignin.It is usually believed that coal is converted from biomass,and coal is also rich in aromatic structure.Considering that lignin is the only aromatic substance in biomass,we compared its structure with that of lignite to find clues for the eveolution of lignin to coal.The main research contents and conclusions are as follows.（1）The structure of lignin is rich in oxygen atoms,which makes the fuel prepared from the lignin low in calorific value and unstabe,and these are the main disadvanages of lignin pyrolysis to oil and hydrogenation to fuel.However,for the oxidation of lignin to prepare carboxy-containing chemicals,the above characteristics of lignin can potentially be converted into advantages for its utilization.A series of carboxylic acid products,including formic acid,acetic acid,oxalic acid,succinic acid,and benzene carboxylic acids（BCAs）,were obtained by alkali-oxygen oxidation of lignin.The effects of alkali ratio,initial pressure of oxygen,oxidation time,and oxidation temperature on the yield of carboxylic acids were investigated,and the total yield was up to67.1wt%;on the one hand,it is due to the transformation of the phenolic aromatic structure of lignin into quinone and then the aromatic ring rupture during oxidation process,and on the other hand,the small aliphatic acids formation may be due to the the oxidation of the aliphatic structure in lignin.The total yield of benzene carboxylic acids was 8.14%,and the absence of oxygen-containing groups（phenolic groups,methoxy,et al.）on the aromatic ring is necessary for the formation of BCAs.However,none of the currently proposed lignin structure models can be oxidized to BCAs.In view of the previous study of the parent structures of the generated benzene carboxylic acids,we think that the existing lignin structure is not completely accurate.（2）For the new discovery of lignin structure and the disadvantage that the oxidation reaction is too violent to obtain the macromolecular structure,we adopted supercritical ethanol to depolymerize lignin,which could break the weak bonds such as C-O and C=O in lignin,and macromolecular structures in lignin can be extracted.Meanwhile,due to hydrogen supply by alcohols and low reaction temperature,the repolymerization of secondary products is avoided,so that the obtained extracted products can be regarded from the the native structure of lignin.The effects of alcohol type,ethanolysis time,and ethanolysis temperature on the carbon yield of lignin depolymerization were investigated,and a 54.7%carbon yield of ethanolysis liquid products was obtained under the conditions of ethanol as solvent,a time of 2 h and a temperature of 290 ^oC.The depolymerized products were detected by GC/MS and ESI-MS,we found that polycyclic aromatic clusters,including double-ring and triple-ring aromatic clusters,did exist in lignin.The presence of polycyclic aromatic clusters was also confirmed by combining with ¹³C NMR analysis.（3）In response to the contradiction between the current existing lignin structures and above experimental results,we proposed a new method to construct a lignin structural model based on the BCAs yield distribution,and we proposed a new lignin structural model.The molecular weight of the proposed structural model was first determined by the benzene carboxylic acid yield distribution,and then the molecular formula of the model was determined based on the elemental analysis,which was combined with the results of ¹³C NMR characterization and ethanolysis products to construct a lignin structure model with the molecular formula of C₆₄₀₇H₆₇₃₈O₂₅₉₀N₁₄₇S₃ and molecular weight of 127214.The lignin structure mainly consists of sing-,double-and triple-ring aromatic clusters,whose numbers account for 52.8%,22.1%and 24.1%,respectively.Besides,the aromatic clusters were divided into two types:one could be converted to benzene carboxylic acids,and the other could not.The model not only satisfies the results of instrumental characterization,but also the results of alkali-oxygen oxidation and supercritical ethanolysis.（4）Ruthenium ion oxidation（RICO）is a highly selective oxidation method that specifically oxidizes aromatic carbons while retains aliphatic carbons during the oxidation process,and multi-ring aromatics could be oxidized into BCAs.RICO facilitates the study of the linkages between the aromatic clusters,aliphatic structure,and multi-ring aromatics in lignin.By RICO,we obtained a series of mono-,di-,and ternary carboxylic acids and BCAs.The formation of mono-carboxylic acids indicates the existence of n-alkyl with different carbon numbers substituted aromatic ring clusters in lignin;the formation of di-carboxylic acids indicates the existence of aliphatic chains with different carbon numbers between two aromatic clusters in lignin;the formation of ternary carboxylic acid indicates the existence of three aromatic clusters linked by aliphatic chains of different carbon numbers in lignin;and the formation of BCAs confirms the existence of polycyclic aromatic clusters in lignin.（5）Since lignin is intertwined with cellulose and hemicellulose in lignocellulose,different treatments are needed to separate lignin.In order to compare the structures of lignin obtained by different methods,we applied ¹³C NMR characterization,alkali-oxygen oxidation and RICO to compare the structures of enzymatic lignin,dioxane lignin,dealkaline lignin and Kraft lignin.Through the characterization of four structural parameters,namely aromatic carbon rate,aliphatic carbon rate,aromatic nucleus structure size,and aromatic ring substitution,we found that the carbon structures of these four lignins have some similarities,such as all of them contain multi-ring aromatic clusters and more than half of the aromatic carbon is substituted.All four lignins were oxidized to obtain benzene carboxylic acids with similar yield distribution,and the highest yield was obtained by enzymatic lignin oxidation.Three lignins were treated by ruthenium ion oxidation,and long-chain fatty acids and BCAs were also obtained,but with different yields.The above result indicate that the different extraction methods do not affect the structure of lignin,while they affect the yield of lignin extraction.（6）Traditionally,coal is thought to be transformed from biomass.We compared the similarities and differences between the structures of lignin and lignite.The results of elemental analysis,¹³C NMR,FTIR,alkali-oxygen oxidation,and supercritical alcoholysis revealed that lignin and lignite have some structural similarities.They are both macromolecules composed of aromatic structures,and both have parent structures that can generate BCAs and polycyclic aromatic clusters;meanwhile,there are some differences between the structure of lignin and lignite;the aromatic clusters in lignite are connected by more aliphatic carbon chains（C-C bonds）than that in lignin;besides,more aromatic clusters in lignin are linked by C-O ether bonds,and more oxygen-containing groups exist in lignin aromatic clusters.We speculate that lignite is formed from preserved lignin in biomass,but there are more oxygenated structures on the lignin aromatic rings than lignite.The reason is that a deoxygenation reaction happens during the coalification process,or a reactive nature of the oxygenated aromatic clusters,which are depolymerized during coalification.