Study On Efficient Depolymerization Of Lignin In Hydrogen Donor Solvents For The Preparation Of Phenolic Chemicals

Lignin is the only renewable resource containing aromatic carbon in nature and is naturally suitable for the preparation of high value-added aromatic chemicals.Reductive depolymerization of lignin has been the focus of laboratory and industrial studies due to higher product selectivity and yield.At present,there are still many problems in the reductive depolymerization study of lignin:only focusing on the H₂pressure or solvent loading,but failing to recognize the effect of the hydrogen amount on the depolymerization reaction;lignin cannot be completely converted to aromatic monomer products,and solid residues are always generated,but the structure of the solid residue and the reasons for limiting its further depolymerization are not fully understand;due to the complex structure and numerous products of lignin,the reaction pathway of lignin depolymerization in hydrogen donor solvents to generate monomer products is also not yet clear;in addition,the study of lignin depolymerization mainly focuses on the cleavage of interunit ether linkages,and the monomer yield is limited by the content of ether linkages,and there is a lack of research on the cleavage of interunit carbon-carbon linkages.Based on the above analysis,this paper firstly investigated the effect of hydroliquefaction of industrial alkaline lignin（AL）in different hydrogen donor solvents（HDS）,explored the hydroliquefaction capacity of the hydrogen donor solvents,and revealed the relationship between the amount of hydrogen donation and the yields of liquid products and monophenol products,as well as the structural evolution law of the solid residue;secondly,the depolymerization of enzymatic hydrolysis lignin（EHL）in9,10-dihydroanthracene was investigated,and the effects of reaction conditions on the monomer yields were explored,and the reaction pathways for lignin depolymerization to produce monophenols as well as the kinetics of lignin depolymerization bond breaking were constructed;in addition,the depolymerization behavior of native lignin from different species of biomass（softwood,hardwood and herbaceous）in hydrogen donor solvents was investigated and compared with the method of reductive catalytic fractionation（RCF）;finally,the bond-breaking behavior of six lignin dimer model compounds in in hydrogen donor solvents was investigated,and the effect of different molecular sieve catalysts on the cleavage of carbon-carbon linkages was explored,and the following main conclusions were obtained:（1）Hydrogen donor solvents can significantly facilitate the conversion of lignin to liquid and monomer products,and their hydroliquefaction capacities depend on their theoretical hydrogen donation and C_al-H bond dissociation energy.The optimal conditions for monophenols from alkaline lignin were:350°C,8 min,9,10-dihydroanthracene（DHA）as hydrogen donor solvent,m _DHA:m _AL=2:1,and the highest monophenol yield was 10.7%.Liquid and monophenol products are formed mainly by two pathways,i.e.,initially liquid and monophenol products are formed by internal H transfer from lignin and additionally liquid and monophenol products are formed by the H donation of hydrogen donor solvent.The solid residue of lignin depolymerization underwent interunit ether linkage cleavage reaction along with the neighboring substitution reaction of substituent-containing conjugate structure on the benzene ring,which led to an increase in the content of interunit carbon-carbon linkages and an increase in the degree of condensation.The interunit carbon-carbon linkages,especially the carbon-carbon linkages forming a conjugated structure with the benzene ring,were responsible for the inability of further conversion of the solid residue.（2）The enzymatic hydrolysis lignin can be efficiently depolymerized to monomeric phenols in 9,10-dihydroanthracene（DHA）with the highest monomer yield of 20.2%,and the selectivity of monophenols and polyphenols were 78%and 21%,respectively.A similar amount of bond cleavage,regardless of reaction conditions,yields a similar number-average molecular weight of the product.About 16%of phenolic-OH in liquid product originates from the demethylation of methoxy group and 36%from the cleavage ofβ-O-4 andα-O-4 linkages.The depolymerization routes of lignin in hydrogen donor solvents were starts with the cleavage of ether linkages and decarboxylation of acrylate linkages,which follows the removal of aliphatic-OH,hydrogenation of C_al=C_al,decarbonylation of C_al=O,dealkylation of alkyl substituents and demethylation of methoxy group attached to the benzene ring.The activation energy（E_a）of bond cleavage of EHL is 36.08 k J/mol when the demethylation of methoxy is not considered.（3）Native lignin from pine（softwood）,poplar（hardwood）and corncob（herbaceous）could be efficiently depolymerized to monophenols in DHA,with the highest monomer yields of 17.6%,26.9%and 14.4%,respectively.Compared with the RCF method,the HDS method has the advantage of requiring only a small amount of solvent,no hydrogenation catalyst and high H₂pressure,and relatively high monomer yields in a short reaction time（<1 h）,with the disadvantage of requiring a high reaction temperature（350-400°C）.In addition,the HDS method is more suitable for softwood and herbal lignin,yielding the same level of monomer yield as the RCF method.（4）The essence of lignin depolymerization to generate monomers is the process of cleaving the interunit linkages and stabilizing the generated reactive radical fragments,and the key issue is the matching of the cleavage rate with the hydrogen donation rate.When the hydrogen donation rate meets the need of radicals generated by cleavage,the cleavage products can be stabilized quickly to produce monomers.When the hydrogen donation rate is not sufficient to meet the radicals generated by cleavage,these reactive radicals are stabilized by capturing their own hydrogen on the one hand,and polymerizing to form macromolecular products（or solid products）on the other hand.HZSM5 can significantly promote the cleavage conversion of the carbon-carbon linkage model（1,2-diphenylethane）,and the product yield of benzene,toluene and ethylbenzene could reach 76.4%,16.9%and 16.9%,respectively,in the presence of DHA.However,it could not promote the depolymerization of enzymatic hydrolysis lignin to generate more monomers.The reason may be that the catalyst pore size and substrate size cannot be matched effectively as well as the active intermediates produced by cleavage cannot be stabilized by hydrogen in time.