Study On Cleavage Mechanism And Subsequent Rearrangement Behaviors Of Lignin Crucial Inter-linkage In Pyrolysis Process

Lignin,as the only natural macromolecular polymer with repeated aromatic constituent,is a potential renewable feedstock for a sustainable future carbon economy.Pyrolysis-based technologies,such as fast pyrolysis and gasification,are promising methods for converting lignin into value-added biochemicals and high-grade biofuels.However,the in-depth understanding of lignin pyrolysis mechanism is the most important technical barrier for pyrolysis mechanism research,owing to its complex chemical structure.The unsatisfactory product selectivity and degradation efficiency results in undergoing a long-term exploration process for efficient volarization of lignin.Therefore,it is necessary to study the molecular cleavage mechanisms of crucial lignin inter-linkage and subsequent rearrangement behaviors during lignin pyrolysis.A better understanding of the molecular mechanisms involved in lignin pyrolysis may guide the development of more efficient and reliable systems for the aforementioned liquor fuels production.In this paper,we firstly studied the the chemical structure evolution and in situ observed crucial intermediate radical in enzymatic hydrolysis/mild acidolysis lignin pyrolysis;Then,a comprehensive study was conducted on the pyrolysis mechanism of?-O-4?linkage,focusing on the induced deconstruction effect of the side-chain substituents and the effect of different structural environment to reaction paths;Finally,the degradation mechanism and coking/re-polymerization behaviors of biphenyl linkage during lignin pyrolysis were respectively investigated.Combinating with the state-of-the-art analytical techniques,these in-depth analyses will undoubtedly provide theoretical guidance for the directional depolymerization of lignin to prepare high-quality liquid fuel.The separation and purification method of"enzymatic hydrolysis/mild acidolysis"was adopted to isolate EMAL lignin from biomass plants in a relatively complete way.The pyrolysis behavior of such lignin in isothermal and non-isothermal conditions was respectively studied.An in situ project by using EPR high-temperature cavity for the observation of generated radical in lignin pyrolysis,has been reported.The results showed that the originate lignin structure had significant influence on the pyrolytic products and the radical process.Compared with hardwood and non-wood lignin,softwood lignin tented to produce more amount of radical,due to the higher content of guaiacyl type subunits and phenylcoumaran linkage.This directly leads to the formation of simpler structures in the secondary pyrolysis reaction from primary pyrolysis products.Corresponding with the two mainly pyrolytic reaction stage according the activation energy,the radical reaction of lignin pyrolysis was divided into three stages:radical inducing stage,main reacting stage and quenching stage.After the pyrolysis process,a large amount of stable radicals were generated,whose spin concentration was lower than the maximum spin concentration during the reaction.A series of PPE type?-O-4 dimers with different substituents on their side-chains were firstly synthesized,and their thermostability and pyrolytic products formation rules were subsequently studied.In addition,the pyrolytic radicals of these?-O-4 dimers were in situ observed by using EPR high-temperature cavity.Based on the abovementioned studies,the induced effect of side-chain substituents on the?-O-4?linkage degradation during pyrolysis process was investigated in molecular level.The results showed that the pyrolytsis reactivity of PPE dimer obviously decreased after transformed into PSE.When C_?-or C_?-hydrogen atom of PPE side-chain were substituted by oxygen-containing groups,the reactivity of the?-O-4 linkage was significantly enhanced.Without?-substitution,?-hydroxylation is more helpful than?-carbonylation for improving the pyrolysis reactivity of the?-O-4 linkage,while?-carbonylation is more conducive to directional depolymerization and product enrichment.Under?-hydroxymethylation condition,?-carbonylation is more conducive to the C_?–O bond cleavage than?-hydroxylation,and the hydroxymethyl on the C_?position easily peelled off to form formaldehyde.In general,hydroxylation can promote the pyrolysis radical reaction of the?-O-4?linkage and the radical reaction reactivity became more intense with the intensification of hydroxylation;?-carbonylation induced radical formation,leading to radical reactions occur at relatively lower temperatures.The pyrolysis behaviors of?-O-4 linkages in different chemical structural conditions,including phenolic dimeric condition coupled with G&H sub-unit and liner G-or H-type polymeric condition,were comprehensive investigated.The pyrolysis experiments of G/H coupled?-O-4 dimer demonstrated that,a large number of?-O-4 dimer containing C_?=C_?bond were generated through concerted reactions and radical induced reactions.The DFT calculation results of the G-O-G type dimer showed that the C_?–O homolysis and the Maccoll elimination are the dominant primary pyrolysis paths.In summary,radical paths and concerted paths coexist in pyrolysis process.The concerted cleavage is the dominant reaction at lower temperature,while the radical reaction is the dominant reaction at higher temperature;and such conclusion was also proved by radical in situ observation results by using EPR HT-cavity.The spin trapping experiments showed that a large number of hydrogen radicals were involved in the pyrolysis of?-O-4 dimer.In polymeric state,the main thermogravimetric range of the?-O-4 structure was 200-500°C;the high-similarity thermostability between?-O-4 polymer and lignin macromolecule indicates that the significant amount of?-O-4linkage cleavage are one of the main causes of thermo weight-loss of liner ligno-molecule fragment.Compared with G-type oligemer,H-type?-O-4 oligemer has lower reactivity,probable due to PSE derived dimer formation.Studies on pyrolysis of dehydrodivanillin showed that biphenyl linkage pyrolysis generated small molecular gases,including H₂O,CH₄,CO,through concerted dehydration,demethylation of methoxyl and decarbonylation reactions.The degradation paths of biphenyl dimers are divided into two types,namely,the C₅–C_5?bond cleavage paths and non-C₅–C_5?cleavage paths.The DFT calculation illustrated that the biphenyl bond cleavage which induced by free radicals is the dominant reaction path in pyrolysis process,which only needs to cross the energy barrier of 7.10 kcal/mol.In condition of non-C₅-C₅?bond cleavage,the methyl groups peeling off which caused by the ArO-CH₃ bond homolysis is the dominant reaction,which is also the most possible reaction in the primary pyrolysis.The in situ observation of pyrolytic radicals showed that both organic stable radicals and short-lived radicals were generated during the biphenyl dimers pyrolysis.The stable radicals are mainly carbon-center radical,and short-lived radicals attributable to benzyl and methyl radical.The drawing conclusion of radical research well verify the analysis of pyrolysis products and supposed mechanism.In order to clarify the effect of biphenyl linkage on coking/repolymerization behavior during lignin pyrolysis,the alkali lignins from softwood and hardwood were also studied as comparative samples.The organic composition and chemical structure of the residual char which generated at 400-800°C pyrolysis condition,were analyzed by elemental analysis,FTIR and solid state NMR,etc.;and the stable radicals of char were quantitatively analyzed via EPR.The results showed that the high-BDE biphenyl linkage within lignin make it easier to repolymerize and coke.With the pyrolysis temperature increase,the oxygen-containing group of char decrease,and the black carbon structure becomes more simple and aromatic.The volatiles generated in the pyrolysis process from biphenyl structure are prone to re-polymerize.This condensed structure contains a large number of unpaired electron,whose Spin concentration was positively correlated with the pyrolysis temperature.Due to higher biphenyl linkage content,softwood alkali lignin generated more radicals in pyrolytic char.