The Reaction Mechanisms Of Ethylene Aromatization In H-ZSM-5 Zeolite: From C4 To C6 Intermediates

In the present work, the reaction mechanism of C4 intermediate to benzene product during the ethylene aromatization on the Br?nsted acid site in H-ZSM-5 zeolite has been investigated by using theoretical calculations. A 78T cluster model was adopted to mimic the Br?nsted acid site located at the channel intersection of ZSM-5 zeolite. All calculations were carried out by the ONIOM2(B3LYP/6-31G(d,p):UFF) method, one of the widespread QM/MM hybrid methods. The main results are summarized as follows:(1) The n-butoxide covalently bonded to the zeolite framework oxygen was transformed into n-butene through deprontonation, and then reacted with ethylene to form the 3-methyl-pentane alkoxide intermediate, which then formed methylcyclopentane through cyclization and deprotonation. The methylcyclopentane released a hydrogen molecule by aid of zeolite acidic proton and formed the unstable methyl-cyclopentane carbonium, which then generated the cyclohexane carbonium through reconfiguration. The calculated activation energies were 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, respectively.(2) The C6 intermediate of cyclohexane carbonium preceded a series of dehydroproton and dehydrogenation reaction with zeolite, lead to the end product benzene. The zeolite acid site is the catalytic active center, which acted repeatedly as the proton acceptor or donor. The neighboring basic framework oxygen atom was also involved in the reaction process. In this process, the activation energies of deprotonation were 41.8, 49.03 and 8.31 kJ/mol, respectively, and the activation energies of dehydrogenation were 205.95 and 264.97 kJ/mol, respectively.(3) With different methods, we have investigated the van der Waals interaction and the electrostatic effect between the framework atoms and the reactant molecules, and compared their contribution on the nature of confinement of zeolite pore. The mechanism of zeolite shape selectivity for the ethylene aromatization was discussed.