甲烷的生成及对木质素二聚体热解过程的影响

Formation mechanism of methane and its effect on the lignin pyrolysis process

为了深入探究甲烷的生成及其对木质素热解过程的影响,选取1-甲氧基-2-苯乙氧基苯(o-OCH_(3)-PPE)为β-O-4型木质素二聚体模型化合物,采用密度泛函理论计算模拟其热解反应过程.研究结果表明,生成甲烷的关键是特征官能团甲氧基发生O—CH_(3)键均裂反应生成甲基自由基,随后甲基自由基将与木质素二聚体通过双分子供氢反应而非单分子加氢反应生成甲烷.木质素二聚体烷基侧链的C_(α)位、C_(β)位及特征官能团甲氧基通过双分子反应为甲基自由基提供H原子使其稳定,而甲基自由基也会活化木质素二聚体使其变成活性更强的木质素自由基,促进其发生后续的热解反应.

In order to deeply explore the formation mechanism of methane and its effect on the lignin pyrolysis process,1-methoxy-2-phenethoxybenze-ne(o-OCH_3-PPE)was selected as aβ-O-4 type lignin dimer model compound,and its pyrolysis process was theoretically simulated by employing the density functional theory(DFT)method at M06-2X level with 6-31+G(d,p)basis set.The results indicate that the key of CH_(4)formation is the homolytic cleavage of the O—CH_3 bond of the methoxyl characteristic functional group to generate the methyl radical.Then the methyl radical will be converted to CH_(4) through the bimolecular reaction instead of the unimolecular hydrogenation reaction.The C_(α)and C_(β)positions of the alkyl side chain and the characteristic functional group methoxyl of the lignin dimer model compound will provide H for the methyl radical to generate CH_(4) through the bimolecular reactions.Meanwhile,the lignin dimer model compound will become the more active free lignin radical after losing H to undergo the subsequent pyrolysis reactions.The free lignin radical formed by losing H at the C_(α)and C_(β)positions of the alkyl side chain of the lignin dimer model compound has much lower reaction energy barriers than the lignin dimer model compound for further homolytic cleavage of the C_(β)—O bond and C_(α)—C_βbond.After losing H at the characteristic functional group methoxyl of the lignin dimer model compound,a series of reactions will further occur to realize the evolution from the methoxyl group to the aldehyde group.In general,the formation of CH_(4) can promote the subsequent pyrolysis process of the lignin dimer model compound and the formation of pyrolytic products such as styrene,2-methoxybenzaldehyde,2-methoxyphenol,toluene and o-hydroxybenzaldehyde.As an important gaseous product during the pyrolysis process of lignin,the study of the CH_(4) formation mechanism is helpful to understand the pyrolysis mechanism of lignin comprehensively and provide a theoretical basis for the development of new pyrolysis technologies of lignin.