Study On Pyrolysis Characteristics And Product Formation Mechanism Of Guaiacyl Lignin

Lignin is an abundant,yet highly underutilized renewable aromatic resource in nature.According to the type of plants,lignins are roughly classified into three major groups,namely softwood,hardwood and herbaceous lignin.Improving high added-value utilization of lignin in the lignocellulose biomass through directional pyrolysis technology is one of the hotspots in current research.The pyrolysis mechanism of lignin still needs further studies due to the complex and stable structure of lignin.In this study,guaiacyl lignin（G-lignin）is used as the main research object.Different types of milled wood lignins（MWLs）were prepared by using the Bj?rkman method,and the main pyrolysis process of different types of lignin were characterized and evaluated under different temperatures.It was found that the pyrolysis products of G-MWL were selectively enriched in low-temperature process.On this basis,thermal characteristics and product formation mechanism during pyrolysis of G-lignin were systematically analyzed by using different scales of G-lignin model compounds and various analytical techniques.Meanwhile,the pyrolysis selectivity of G-MWL was improved by directionally modification of lignin by laccase.The main research conclusions are presented in the following sections.The structures and pyrolysis charateristics of MWLs isolated from ginkgo（soft wood）,poplar（hardwood）,and wheat straw（herbaceous biomass）were systematically investigated in this study.Compared to the MWLs from wheat straw and poplar,G-MWL from ginkgo contained relatively lessβ-O-4 aryl ether linkages and moreβ-5phenylcoumarans,and showed better pyrolysis selectivity at low temperature.The main bio-oil products from the pyrolysis of G-MWL was monophenols（96.61%of the total bio-oil yield）at 400°C,of which guaiacols accounted for 92.38%and five major G-types of compounds（creosol,2-methoxy-4-vinylphenol,vanillin,isoeugenol,and apocynin）accounted for 67.7%,indicating that low pyrolysis temperature is beneficial for the selective enrichment of guaiacol phenolic compounds.The composition and distribution of G-MWL pyrolysis products were further investigated by Py-GC×GC-TOF/MS.On this basis,the changes of volatile functional groups and the mechanism of low-temperature selective enrichment of guaiacols（G）during lignin pyrolysis process were analyzed by in-situ FTIR combined with 2D-PCIS.The results from the study of the changes of the functional groups during pyrolysis showed that the CO bonds in the methoxy group（-OCH₃）of G lignin unit in G-MWL was broken to form free methyl radical（·CH₃）,while H radical is beneficial for the removal of methyl radical to forms a new phenolic hydroxyl group.In addition,the free methyl radical（·CH₃）can directly attack the CH bond of the macromolecular aromatic hydrocarbon and forms the CH₄ gas with the detached H atoms.The conversion of products during the process of pyrolysis and the mechanism of low-temperature enrichment of G-compounds showed that initially favored the formation of an abundant guaiacol phenols were formed in the initial stage（400°C）of G-MWL pyrolysis by breaking the C-O bond in the alcoholic or phenolic hydroxyl.Due to the poor thermal stability of the phenolic compounds,with the increase of pyrolysis temperature the subsequent reactions included substitution reactions of aromatic hydrocarbon and removal reactions of non-conjugated C=O occurred,forming conjugated C=O small molecules,which can be further converted to phenols and other aromatic hydrocarbons.Therefore,guaiacols（G）enriched greatly and showed high selectivity at low temperature conditions.Meanwhile,the density functional theory（DFT）based on quantum mechanics was used to verify the conversion mechanism of the main pyrolysis products of G-MWL.According to the three main bonds cleavage（i.e.β-O-4,α-O-4 and 5-5’）and product formation mechanisms in G-MWL,a series of G-lignin model compounds were synthesized,and their structures were characterized by ¹H-NMR.These synthesized lignin model compounds included 5-5’biphenyl,phenolic/non-phenolicβ-O-4 dimers,trimers containingβ-O-4 and 5-5’,and dehydrogenation polymer models of lignin（G-DHPs）.The breaking mechanism of the main bonds and the characteristics of pyrolysis products in G-lignin were studied by pyrolysis of G-lignin model compounds at different scales.The results showed that the breaks of the main G-lignin bonds and product formation mainly followed three pathways:（1）the cleavage of theα-O-4 chemical bonds.3-（1-hydroxyethyl）-4-hydroxyphenylethanol was firstly produced,followed to form 2-hydroxy-5-methylacetophenone by C-C bond cleavages and dehydrogenation reactions,and then form p-hydroxy toluene via decarboxylation reactions;（2）the cleavage of 5-5’bonds.2-methoxyphenol was firstly formed,which was further converted to catechol by demethylation,and finally formed phenol and benzene,with the increase of temperature;（3）the cleavage ofβ-O-4 bonds.The main product was coniferyl alcohol,which was then further converted to 2-methoxy phenol,etc.The directional modification of lignin by laccase derived from white rot fungi（Physisporinus vitreus）can enhance the pyrolysis selectivity of G-MWL and the enrichment of guaiacols（G-compounds）in pyrolysis products.Fixed-bed pyrolysis of G-MWL showed that the oxidative modification of lignin by laccase resulted in the increase of G-unit content and the changes of the distribution of gases,bio-oil and biochar in pyrolysis products.Especially for the bio-oil,the ration of the guaiacol phenolic compound/total yield G/T is improved under low pyrolysis temperature conditions after laccase treatment increased,and the yields of guaiacol,vanillin and coniferaldehyde were increased by 93.11%,92.16%and 12.44%,respectively.The pyrolysis process and low-temperature selectivity of G compounds conformed to the transformation pathway and mechanism of G-lignin pyrolysis products.This study systematically elucidated the pyrolysis characteristics of G-lignin,and clarified the improvement of low temperature pyrolysis selectivity by laccase modification of G-MWL.The formation mechanism of G-lignin pyrolysis products was synergistically clarified by using lignin model compounds and various analysis techniques from different dimensions.The results provided theoretical support for the pyrolysis of softwood lignin,and also provided guidance for high-value and directional utilization of industrial lignin.