Study On The Influence Of Oxygen-containing Functional Groups On The Thermochemical Deconstruction Mechanism Of Lignin Model Compounds

Lignin,as a biopolymer that remains the only renewable source of aromatics,is considered as an ideal raw material for partly replacing petroleum-derived and obtaining high value-added phenolic chemicals.The thermochemical depolymerization of lignin that its structure was selectively modified,has gained wide spread attention due to the advantages for improving the selectivity of target product and reducing the secondary reactions of active intermediates.This paper makes a research about thermochemical characteristics ofβ-O-4type lignin which benzyl alcohol was oxidized.High purityβ-O-4 lignin compounds with C_α=O or C_α-OH groups were first synthesized and depolymerized under different conditions for obtaining the influence of C_α=O or C_α-OH groups on conversion pathways and products distribution,and then the secondary reactions of typical monomers were studied.Research results can provide theoretical basis for exploring a new method and theory of lignin depolymerization,and it is of great significance for the development of high value utilization of lignin resources.Lignin model dimers and polymers with different structural feature were successfully synthesized,and their NMR characterization data indicated their high purity.Among,pyrolysis experiments of four guaiacyl lignin dimers showed that the cleavage ratio of intermolecular Cβ-O and Cα-Cβlinkages was significantly different.Generally,the presence of C_α-OH and C_β-CH₂OH group facilitated cleavage of the C_β-O bond and favored formation of guaiacol.However,oxidation of the C_α-OH into C_α=O group weakened the C_α-C_βbond,and favored the cleavage of C_α-C_βbond,leading to the increase of 2-methoxy-benzaldehyde.When the C_α-OH or C_β-CH₂OH group existed in dimers,in addition to break the C_β-O和Cα-Cβbond,new dimers that were intermediates within Cα=Cβor Cα=O group during the pyrolysis process were formed via intermolecular elimination reactions of C_α-OH or Cβ-CH₂OH group,which further decomposed into monomer phenolics under high pyrolysis temperature and made pathways more complicated.Four dimeric model compounds with different side chains were investigated using high temperature high pressure autoclave,and obtained products including to new dimers and monomeric products were detected by qualitative and quantitative analyses.Experimental results shown that the reaction pathways of dimers had a significant difference due to the effect of benzylic alcohol（Cα-OH）and benzylic ketone（Cα=O）.The depolymerization pathway of C_α=O type dimers（M1）was relatively simple,and the C_α-C_βand C_β-O bonds tended to directly cleave and formed aromatic monomers,such as guaiacol,2-methoxy-benzaldehyde,vanillin and acetovanillone.C_α-OH type dimers（M2）can form new dimer via dehydration and dehydrogenation of C_α-OH,and it was the prominent dehydration that resulted in the formation of Cα=Cβ,meanwhile,the main products included guaiacol and4-ethyl-guaiacol.When the-C_γH₂OH group was introduced at the C_βposition,C_α=O type dimers（M3）could break the C_β-C_γbond and also occurred the dehydration reaction,and the main products were guaiacol,2-methoxy-ethoxybenzene and acetovanillone.In the presence of-C_γH₂OH group on the C_βposition,C_α-OH type dimer（M4）could convert into M3 via dehydrogenation of C_α-OH,and also convert into M3 via both dehydrogenation of C_α-OH and cleavage of Cβ-Cγbond,and detected products were guaiacol and vanillon.Theseβ-O-4 polymer models with different groups（C_α-OH and C_α=O）on C_αposition were depolymerized in closed ampoule reactor system and interrogated using in-situ FTIR.The results of U type pyrolysis system showed that oxidation of the C_α-OH to C_α=O group rose the yield of phenolic monomers almost twice that of initial G-type polymer,and the maximum can be 27.9wt%.Nevertheless,as for H-type polymer,oxidation of the C_α-OH to C_α=O group lead to decreases in phenolic monomers yield by^half.The in-situ FTIR indicated that oxidation of the C_α-OH to C_α=O accelerated cleavage of interunit linkages and shortened the progress of depolymerization,and suggested the main reason of the difference of total phenol yield is the different reactivity of intermediate products.In addition,several important transformation pathways or broken bond mechanism are proposed that clearly explain the pyrolytic behavior of pre-oxidized polymers.Secondary reactions of six monomeric aromatics（including 4-hydroxy-acetophenone,4-hydroxy-3,5-dimethoxy-acetophenone,4-hydroxy-benzaldehyde,4-hydroxy-3-methoxy-benzaldehyde and 4-hydroxy-3,5-dimethoxy-benzaldehyde）were studied,and the result showed that the same configuration acetophenone were more stable than that of benzaldehyde;Adding the amount of-OCH₃ group made the reactivity of monomeric aromatics decline.The polymerization was more competitive under all the test conditions,and it was the most important reason of monomer yield decline.Coke from the pyrolysis of H,G and S type of acetophenone contained a large number of-COCH₃ functional groups,and did not react under the high temperature condition.During the pyrolysis of three type benzaldehyde monomers,no-CHO group was detected in char,suggesting the removal of-CHO group before polymerization.Both-OCH₃ and-CHO group on the benzene ring showed high reactivity during pyrolysis.As for G and S type acetophenone,the active functional group is mainly-OCH₃,where radical induced rearrangement of aryl-OCH₃ group into aryl-CH₃ group firstly happened and as the major side-chain-conversion reaction.In the presence of-CHO group on the p-position of aryl-OH,the removal of-CHO group was major side-chain-conversion reaction with accompanied by the formation of syringe,guaiacol and phenol during the pyrolysis process.Adding the amount of-OCH₃ group was beneficial for promotion the breakage of aryl-CHO bond.