DFT方法研究酸性沸石上苯与乙烯烷基化反应的机理

Reaction mechanism study on alkylation of benzene with ethylene over acid zeolite based on DFT method

采用B3LYP/6-31G*水平计算来研究酸性沸石上苯与乙烯的烷基化反应历程,从生成能和反应活化能角度分析并讨论了苯与乙烯的反应机理.选取4T簇模型模拟分子筛的酸性位,使用密度泛函理论对烷基化反应三种不同的反应机理(两个联合反应机理和一个分步反应机理)进行计算分析.结果表明,在联合反应机理中,乙烯的质子化和苯与乙烯间C-C键的形成同时发生;分步反应机理中,首先形成一个稳定的乙醇盐中间物种,然后与苯分子反应形成乙苯.联合机理速控步骤的活化能约为160kJ/mol,分步机理速控步骤的活化能为190.24kJ/mol,因此,酸性沸石上苯与乙烯烷基化反应机理主要以联合机理为主,但分步机理与其有一定程度的竞争。

The acid zeolite catalyzed alkylation of benzene with ethene is investigated at the B3LYP/6-31G level of calculation with a 4T-cluster representing the Brφnsted acid site of the zeolite. The reaction mechanism has been analyzed according to the formation energy and reac- tion activation energy. Three different reaction mechanisms of alkylation, two concerted mech- anism and one stepwise mechanism, have been analyzed by density functional theory. The re- sults showed that ethene protonation and C--C bond formation occur simultaneously in the concerted mechanism. The stepwise mechanism starts with the formation of a stable ethoxide intermediate which subsequently reacts with benzene to form the reaction product. The activa- tion energy for the concerted mechanism was about 160 kJ/mol, while it was 190.24 kJ/mol for the stepwise mechanism. The results showed that alkylation of benzene with ethylene occurred through both concerted mechanism and stepwise mechanism, and they were competitive to each other.