分子筛催化的戊烯骨架异构反应机理

Reaction Mechanism of Pentene Skeletal Isomerization on Zeolites

在密度泛函理论中的B3LYP/6-31G(d,p)水平上研究了分子筛催化戊烯骨架异构的微观作用机制,分别对各个基元反应进行了内禀反应坐标(IRC)解析。结果表明:戊烯的骨架异构存在2种反应机理:烷氧基中间体机理和类甲基环丙烷中间体机理。而烷氧基中间体机理又包括2个反应途径,1个是甲基迁移,另1个是乙基迁移。因此,整个异构反应存在3个反应途径。甲基迁移机理和乙基迁移机理都含有3个基元步骤,其中速控步骤分别是甲基迁移和乙基迁移,对应的活化能分别为206.17和207.31 kJ·mol-1,二者几乎相等,表明2个反应途径从能量的角度来说是互相竞争的。类甲基环丙烷中间体机理分2步,碳链扭转和甲基的迁移,反应中间体具有高离子性和高能量的物种。反应速控步骤是碳链扭转反应,其活化能是147.93 kJ·mol-1,明显低于甲基迁移和乙基迁移2个反应途径的能垒,意味着类甲基环丙烷中间体的反应途径更容易发生。

The microcosmic interaction mechanism of pentene skeletal isomerization on zeolites was studied by the density functional theory at the B3LYP/6-31G（d,p） level.The reaction trajectories were determined by the intrinsic reaction coordinate（IRC） methods.The results indicate that the skeletal isomerization of pentene can proceed by two kinds of mechanisms：the alkoxide intermediate mechanism and methylcyclopropane-like intermediate mechanism.The alkoxide intermediate mechanism involves two reaction pathways：methyl shift and ethyl shift.Accordingly,the overall skeletal isomerization of pentene has three reaction pathways.Both the methyl and ethyl shift mechanisms consist of three elementary steps.The rate determining steps are the shift of the methyl group and the shift of the ethyl group,respectively.The corresponding activation barriers are nearly equivalent（206.17 and 207.31 kJ·mol-1,respectively）,indicating that two reaction pathways compete between each other.The methylcyclopropane-like intermediate mechanism includes two elementary steps：the torsion of the carbon chain and the methyl shift.This intermediate has highly ionic character and is a high energy species.The rate determining step is the torsion of the carbon chain,and its activation barrier is 147.93 kJ·mol^-1.This value obviously is lower than those of the methyl and ethyl shift process,implying that the methylcyclopropane-like intermediate pathway occurs more easily.