β-5型木脂素气化反应机理的密度泛函理论

Density Functional Theory of Gasification Reaction of Lignan Model Compound With β-5 Linkage

在密度泛函理论的基础上,采用B3LYP/6-31+G(d,p)方法对β-5型木脂素的气化反应机理进行研究,考察温度对气化反应机理的影响。设计3条热解反应路径及相应的后继反应,并对反应路径中各驻点结构进行能量梯度全优化以及振动频率计算。热力学计算结果表明,在中低温(T<1500K)阶段,Path 1、4、5、6和7的焓变(ΔHθ)均大于0,反应是吸热的;而Path 2的ΔHθ在温度达到1500K后降为-50.6kJ/mol,反应从低温时吸热变为高温时放热;Path 3的焓值从2.1kJ/mol逐渐减小到-125.1kJ/mol,反应由吸热变为放热。当300K<T<1700K,Path 2、Path 3都能自发进行;当T>700K时,Path 1也可以自发进行,而Path 4、5、6和7均不能自发进行。动力学计算结果表明,Path 1、2、3的第1步反应活化能分别为192.5、159.0和113.0kJ/mol;第2步反应活化能分别为138.1、184.1和280.3kJ/mol,Path 1的第3步反应为79.5kJ/mol。其中Path 2为最优反应路径,即在木脂素分子中Cβ—Cα键更容易断裂。后继反应中,Path 4为最优反应路径。木脂素在热解反应的产物为甲醛、苯酚、2-羟基苯基乙醛和4-羟甲基苯酚等,主要为液体;后继反应的产物为CO、CO2和H2等小分子气体以及苯酚和甲醇。

Gasification of the lignan model compound withβ-5 linkage was theoretically investigated by employing density functional theory methods at B3 LYP/6-31 G＋（d,p）level.Three pyrolysis reaction pathways and corresponding subsequent reactions were considered.The equilibrium geometries of the reactants,transition states,intermediates and products were optimized,and the standard kinetic parameters of reaction pathways were calculated.Thermodynamics calculation results showed that enthalpy changes of Path 1,4,5,6 and 7 were greater than 0 at the low temperature（T1500 K）,and the reactions were endothermic.The enthalpy change of Path 2 decreased to-50.6 kJ/mol when temperature reached to 1500 K.The enthalpy of Path 3 decreased from 2.1 kJ/mol to-125.1 kJ/mol,and the reaction changed from endothermic to exothermic.Path 2 and Path 3 could occur spontaneously at temperature arrange from 300 Kto 1700 K.When T was larger than 700 K,the Path 1 could also occur spontaneously,but Path 4,5,6 and 7 couldn＇t.Dynamics calculation results showed that first step of Path 1,2,3,the corresponding reaction activation energies were 192.5,159.0 and 113.0 kJ/mol,respectively.The second step reaction activation energies were 138.1,184.1 and 280.3 kJ/mol,respectively.The third step of Path 1 reaction was 79.5 kJ/mol.Path 2 was the optimal reaction path,namely Cα—Cβkey fractured more easily in the lignan model compound.Path 4 was optimal in the subsequent reaction.The main products of Path 2 were（2-hydroxyphenyl）acetaldehyde and 4-（hydroxymethyl）phenol.In the subsequent reactions,the products were CO,CO_2,H_2,phenol and methanol.