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采用密度泛函(DFT)方法对于ZSM-5分子筛上苯与乙烯的烷基化反应机理进行研究。选取了包含酸性质子的8T原子簇模型代表ZSM-5分子筛的部分结构。采用B3LYP/6-31G(d)水平从生成能与反应活化能角度对于烷基化反应发生时可能存在的2种联合机理和一种分步机理进行了计算与比较。结果表明,联合反应机理中乙烯的质子化和C-C键的形成同时发生,而分步机理开始于乙醇盐中间体的生成,随后与苯反应生成反应产物。分步机理中乙醇盐中间体生成步骤的活化能(124.55 kJ/mol)低于联合机理的反应活化能(168.98 kJ/mol和156.06 kJ/mol),而烷基化步骤的活化能(209.35 kJ/mol)高于联合机理的反应活化能,由此可以推断ZSM-5分子筛上苯与乙烯的烷基化反应时2种机理同时发生,存在竞争关系。
The mechanism of alkylation of benzene and ethylene on ZSM-5 zeolite was studied by density functional theory (DFT). The 8T cluster model containing acidic protons was chosen to represent the partial structure of the ZSM-5 zeolite. Using B3LYP / 6-31G (d) level, two kinds of combined mechanisms and one kind of staged mechanism that may exist in the alkylation reaction were calculated and compared from the angles of formation energy and reaction activation energy. The results show that the protonation of ethylene and the formation of C-C bonds occur simultaneously in the combined reaction mechanism. The stepwise mechanism begins with the formation of the ethoxide intermediate, which then reacts with benzene to form the reaction product. The activation energies (124.55 kJ / mol) of the stepwise mechanism were lower than those of the combined mechanism (168.98 kJ / mol and 156.06 kJ / mol), while the activation energies of the alkylation step (209.35 kJ / mol) is higher than the reaction activation energy of the combined mechanism. Therefore, it can be inferred that there are competing relations between the two mechanisms in the alkylation of benzene and ethylene on the ZSM-5 zeolite.