| In the present work, the dimerization of ethylene on the Br?nsted acid site in H-ZSM-5 zeolite has been investgated by using theoretical calculations. We mainly focus on the following topics: (1) the mechanisms for C-C bond formation and chain propagation in ethylene oligomerization; (2) the effect of zeolite acid sites; (3) the effect of zeolite framework and pore structure on reaction selectivity. A 40T cluster model was adopted to mimic the Br(o|¨)nsted acid site located at the channel intersection of ZSM-5 zeolite. All calculations were carried out by the ONIOM method, one of the widespread QM/MM hybrid methods. Two different reaction mechanisms, stepwise and concerted pathways were examined. The natural bond orbital analysis was used to explore the electronic properties forπ-hydrogen bonding interaction in adsorption complexes and the molecular orbital features at the transition states.The results indicated that in the stepwise mechanism, ethylene firstly adsorbs on the acid site throughπ-hydrogen bonding and then proceeds protonation to form the ethoxide intermediate; then the intermediate reacts with the second ethylene molecule to give the surface butoxide product. In which the ethylene protonation shows the highest energy barrier and is confirmed as the rate-limiting step. In the concerted mechanism, two ethylene molecules co-adsorb on the acid site. The reaction proceeds via simultaneous protonation and formation of C-C bond to give the butoxide product. The corresponding activation energy is slightly higher than that of the rate-limiting step in the stepwise mechanism. It is proposed that there should be a competition between two reaction mechanisms. The natural bond orbital analysis revealed that the ethoxide intermediate reactes with another ethylene by a transition state of carbonium. The confinement effect of zeolite micropores plays important role in the physisorption of ethylene molelcule, which also facilitates the competition of stepwise mechanism. |
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