Abstract: 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 pathways were examined. For the stepwise mechanism, the ethylene molecule was firstly adsorbed at the Brnsted 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-CandC-Obonds 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.

Key words: H-ZSM-5 zeolite, ethylene, dimerization, theoretical calculation