针叶木木质素单体模型化合物热解研究进展

Research advances in pyrolysis of softwood lignin-based monomers

木质素目前是唯一可持续生产芳香基化合物的可再生资源。然而,当前绝大多数的木质素未能得到有效利用。热解可以将木质素快速转化为生物炭、生物油和生物气等产物并实现其资源化和高值化利用的有效途径。愈创木基单元是针叶木木质素的主要组成单元,且其结构中的甲氧基和酚羟基等官能团在木质素中广泛存在,因此作为模型化合物被广泛应用。愈创木酚类化合物直接热解产物以苯酚类和邻苯二酚类化合物为主,且热解温度对其热解过程具有一定影响,提高热解温度提高转化率且产生少量芳烃和更多的烯烃,且愈创木基结构单元的C4取代基官能团对愈创木酚直接热解的影响较小。分子筛由于具有独特结构和酸性位点,是催化裂解愈创木酚脱氧制备芳烃和单酚的有效催化剂。催化热解反应条件(如热解温度、重时空速和原料分压等)对催化热解产物具有重要影响;且在热解过程中添加氢供体可以显著提高愈创木酚脱氧率并降低催化剂的积碳。热解机理方面,愈创木酚基化合物直接热解反应主要反应途经是自由基反应,且结构单元中的甲氧基与焦炭形成具有直接关系。初步热解产物邻苯二酚及其衍生的邻羟基苯醌是形成气体产物的重要中间体。与直接热解不同的是,愈创木酚催化热解的主要反应机理是“烃池机理”。该研究通过对愈创木酚类化合物直接热解研究、催化热解研究和反应机理等方面进行总结和综述,期望加深对木质素热解过程的理解,为木质素热解产物的调控提供理论指导。

Lignin is the second largest natural polymer material in lignocellulose-based biomass components,just behind cellulose and the only sustainable source to produce renewable aromatic compounds.However,lignin is always treated as the cause of serious environmental problems,as it was burned under low temperature and discharged arbitrarily without effectively utilization.Pyrolysis technology offers an effective way for fast conversion of lignin into biochar,biooil and biogas products,to realize its high-value utilization and valorization.In recent years,Guaiacyl structural units,the main component structures of softwood lignin and gramineous lignin,are widely used as model compounds for the understanding of decomposition and coking mechanisms in lignin pyrolysis,not to mention their functional groups,such as methoxy and phenolic hydroxyl groups in their structures widely exist in lignin.This review summarized the pyrolysis process,impact factors,product distribution,and catalyst deactivation mechanism,during guaiacol-based model compounds pyrolysis process.During the direct pyrolysis process of guaiacol compounds,the main products mainly including phenols and catechol compounds,and pyrolysis temperature showed a certain influence on the guaiacol compounds conversion and products distributions.Increasing the pyrolysis temperature can increase the conversion rate,while lead to produce more olefins and a small number of aromatics.Moreover,the C4 substituent functional group of guaiacol-type compounds(e.g.vanillin,vanillic acid and vanillyl alcohol)also affects the pyrolysis product distributions.In catalytic pyrolysis,most previous studies focused on the catalytic pyrolysis of guaiacol,in which aromatic hydrocarbons and phenols compounds served as the main products,Zeolites,especially HZSM-5 based catalysts dominated.The unique structure and acidic sites of zeolite-based catalysts are the main active sites for guaiacol conversion and products formation during catalytic pyrolysis process.The addition of hydrogen donors can significantly increase the deoxygenation rate of guaiacol,while,reduce the carbon deposition of the catalyst.The impact factors,such as pyrolysis temperature,Weight Hourly Space Velocity(WHSV),and guaiacol partial pressure,strongly affect the catalytic pyrolysis of guaiacol.Increasing the pyrolysis temperature can enhance the coke formation on the catalyst,and promote the production of aromatic hydrocarbons and olefins,whereas,increasing the WHSV and guaiacol partial pressure can inhibit the coke formation on the catalyst,and reduce the efficiency of deoxygenation,leading to more phenolic compounds production,and guaiacol partial pressure.The deactivation of the catalyst is mainly resulted from the loss of active sites,and the blockage of the channel caused by carbon deposition of its surface area.In the pyrolysis mechanism,the pyrolysis of guaiacol-based compounds is mainly a free radical reaction.Catechol is mainly generated through the homolytic cleavage of O-CH3 bond,when phenol is mainly produced through the demethoxylation pathways,which was promoted by the H-atom and CH3-radical.Catechol and its derivatived o-hydroxybenzoquinone are the key intermediates during the production of gas.The aromatic hydrocarbons formation during the catalytic pyrolysis is mainly through the hydrocarbon pools pathways.At first,Guaiacol participates in the pyrolysis reaction to form the intermediates,such as phenol,catechol,then exists as the intermediates to form a hydrocarbon pool inside the catalyst,and finally converts into aromatics and olefins.This critical review can be necessary to further deepen the understanding of the lignin pyrolysis process,and thereby to provide some theoretical guidance for the regulation of lignin pyrolysis products.