Abstract: The ONIOM2 (B3LYP/6-31G(d,p):UFF) method based on the 78T cluster model was used to study the reaction mechanism of C4 to C6 intermediates during the ethylene aromatization over HZSM-5 zeolite. The catalytic mechanism of acidic zeolite and the effect of zeolite pore size on the shape selectivity for the products was discussed. The results indicated that the n-butoxide, which is the intermediate product of ethylene dimmerization, reacted with ethylene to form n-hexane alkoxide, but it was difficult to carry out further cyclization because of the restriction of the pore size of ZSM-5 zeolite. However, along the stepwise pathways, n-butoxide was transformed into n-butene through deprontonation and then reacted with ethylene to form 3-methylpentane alkoxide intermediate, which then formed methylcyclopentane through cyclization and deprotonation. The methylcyclopentane released a hydrogen molecule by the aid of zeolite acidic proton and formed the unstable methyl-cyclopentane carbonium, which then generated the cyclohexane carbonium through reconfiguration. The calculated activation energy was 158.42 kJ/mol for n-butoxide deprontonation, 130.71 kJ/mol for the oligomerization of 1-butene and ethylene, and 122.06 kJ/mol for the cyclization of 3-methylpentane alkoxide. As a result of the confinement of zeolite pore, the five-member ringed me-thylcyclopentane was formed as a crucial intermediate.

Key words: density functional theory, ethylene, aromatization, HZSM-5 zeolite, reaction mechanism, confinement effect