木质素单体模化物热解过程的理论分析

Theoretical Analysis on Pyrolysis Processes of Monomeric Model Compounds of Lignin

为了深入研究木质素的热解机理,选用香草醛、香草醇和香草酸作为木质素单体模化物模拟热解过程.运用密度泛函理论探讨不同化学连接键的解离顺序,并设计了4条反应路径(Path 1—Path 4):Path 1和Path 2都生成愈创木酚,但中间过程不同,Path 3和Path 4则分别以优先过程生成邻苯二酚和苯酚.结果表明:模化物的侧链甲基会优先解离生成CH_4,C_1取代基的解离顺序取决于其吸电子能力;在热解过程中,C_1取代基不易直接解离,而是会优先发生外界参与下的解离;3种目标产物(愈创木酚、邻苯二酚和苯酚)都是可能的,且会优先生成邻苯二酚,其次是苯酚,较难生成愈创木酚;从动力学角度分析,Path 2和Path 4要优于Path 1和Path 3进行;生成同种目标产物时总是香草醇最易进行,香草醛次之,最难进行的是香草酸.

In order to deeply investigate the pyrolysis mechanism of lignin, its pyrolysis process is simulated with vanillin, vanillic alcohol and vanillic acid being selected as monomeric model compounds of lignin. In the investi- gation, the density functional theory is employed to analyze the dissociation sequences of chemical connection bonds, and four reaction paths, namely, Path 1 - Path 4, are designed. Both Path 1 and Path 2 generate guaiacol but their intermediate processes are not the same, while Path 3 and Path 4 generate catechol and phenol with priority processes respectively. The results show that （1） the methyl in the side chains of the model compounds first disso- ciates with CH4 being released, and the dissociation sequences of the substituents on C1 depend on their ability to withdraw electrons; （2） in the pyrolysis process, it is difficult for the substituents on C1 to dissociate directly, but prior dissociation occurs instead with the help of outside factors; （3） it is possible for the target products of three kinds to be obtained in the priority sequence of catechol, phenol and guaiacol; （4） from the view of dynamics, Path 2 and Path 4 are superior to Path 1 and Path 3 ; and （5） whenever the target products of the same kind are made, it is always the easiest job for vanillic alcohol, and then vanillin, whereas vanillic acid is always the most difficult matter to be dealt with.