H-ZSM-5分子筛上的乙烯二聚反应机理的理论计算研究

Investigation on Ethylene Dimerization over H-ZSM-5 Zeolite by Theoretical Calculation

应用分子力学和量子力学联合的ONIOM2(B3LYP/6-31G(d,p):UFF)计算方法研究了H-ZSM-5分子筛上乙烯二聚反应的机理.用40T簇模型模拟ZSM-5分子筛位于孔道交叉点的酸性位,对乙烯二聚过程的分步反应和协同反应两种机理进行了考察.对于分步反应机理,乙烯分子首先通过π-氢键作用在酸性位形成稳定的吸附络合物,再进一步发生质子化并生成乙醇盐中间体,随后乙醇盐与第二个乙烯分子发生碳-碳键结合形成丁醇盐产物.第一步质子化和第二步碳链聚合的活化能分别为152.88和119.45 kJ/mol,表明乙烯质子化反应为速控步骤.对于协同反应机理,乙烯质子化、碳-碳键和碳-氧键生成同时进行,生成丁醇盐,反应的活化能为162.30 kJ/mol,略高于分步反应机理中的速控步骤.计算结果表明这两种反应机理之间存在相互竞争.

By the QM/MM hydride ONIOM2（B3LYP/6-31G（d, p） ： UFF） method, the reaction mechanism of ethylene dimerization over H-ZSM-5 zeolite was investigated. A 40T cluster model was used to simulate the acidic sites located at the intersection of channels in ZSM-5 zeolite. Both stepwise and concerted reaction path-ways were examined. For the stepwise mechanism, the ethylene molecule was firstly adsorbed at the Brǒnsted acid site via π-hydrogen bonding to form a stable complex, which then went through the protonation, leading to the formation of the ethoxide intermediate. Furthermore, the ethoxide reacted with the second ethylene molecule to form the butoxide product. The calculated activation energies for the first and second steps were 152.88 and 119.45 kJ/mol, respectively. The ethylene protonation step was confirmed as the rate-limiting step. For the concerted mechanism, the protonation, C-C and C-O bonds formation proceeded simultaneously to produce butoxide. The calculated activation energy of 162.30 kJ/mol was slightly higher than that of the rate-limiting step in the stepwise mechanism. It was proposed that there should be a competition between the two reaction mechanisms.