木质素模化物紫丁香酚热解机理的量子化学研究

Studies on pyrolysis mechanism of syringol as lignin model compound by quantum chemistry

采用密度泛函理论方法 B3LYP/6-31G++(d,p),对木质素模化物紫丁香酚的热解反应机理进行了量子化学理论研究。提出了三种可能的热解反应途径,对各种反应的反应物、产物、中间体和过渡态的结构进行了能量梯度全优化。计算了各热解反应途径的标准动力学参数,分析了各种主要热解产物的形成演化机理。键离解能计算结果表明,紫丁香酚中CH3-O键的键离解能最小,各种键离解能的大小顺序为CH3-O<O-H<CH3O-Caromatic<CH2-H<HO-Caromatic<Caromatic-H。在反应路径(1)中,主要热解产物是3-甲氧基邻苯二酚,其形成反应的总能垒为366.6 kJ/mol;在反应路径(2)中主要热解产物是2-甲氧基-6-甲基苯酚,其形成反应的总能垒为474.8 kJ/mol;在反应路径(3)中形成邻甲氧基苯酚的总能垒很低,为21.4 kJ/mol,这表明,在连接甲氧基的碳原子上加氢后能够有效地降低木质素芳环模化物紫丁香酚去甲氧基反应的反应能垒。

The pyrolysis of syringol as lignin model compound was investigated using density functional theory methods at B3LYP/6-31G＋＋（d,p） level.Three possible pyrolytic pathways were proposed and the equilibrium geometries of the reactants,transition states,intermediate and products were fully optimized.The standard kinetic parameters in each reaction pathway were calculated and the formation and evolution mechanism of main pyrolysis products were analyzed.Bond dissociation energies calculation results show that the bond dissociation energy of CH3-O of syringol is the lowest and the order of all kinds of bond dissociation energy is CH3-O O-H CH3O-Caromatic CH2-H HO-Caromatic Caromatic-H.In reaction pathway 1 and 2,the main pyrolysis product is 3-methoxycatechol and 2-methoxy-6-methylphenol,respectively.The total energy barrier is 366.6 and 474.8kJ/mol in pathway 1 and 2,respectively.For reaction pathway 3,the total energy barrier of o-methoxyphenol formation is as low as 21.4kJ/mol,which shows that addition of hydrogen to the carbon atom connected with methoxyl can effectively lower the reaction energy barrier of demethoxy reaction of lignin model syringol.