Study Of Theoretical Calculation Of Ethylene Dimerization Reaction On Ga(Al)/HZSM-5 Zeolite

We studied the reaction mechanisms of ethylene dimerization on Ga/HZSM-5 and Al/HZSM-5 zeolite catalysts by theoretical calculations. Ethylene dimerization may proceed along two different pathways: either a stepwise or a concerted mechanism, and both these contain a butoxide intermediate. This produces 1-butylene that is adsorbed on the recovered Br?nsted acid sites.The calculations were performed using the hydride ONIOM2 (B3LYP/6-31G (d, p):UFF) method based on the two-layered 76T cluster model. The influences of zeolite acidity and zeolite confinement on the reaction energetics were investigated. The contents of the paper are listed as follow:1. We studied the adsorption effect of NH3 and C2H4 on Ga/HZSM-5 and Al/HZSM-5 zeolites by ONIOM2 method, made the contrast of the acidity between Ga/HZSM-5 and Al/HZSM-5, the results show that the adsorption energy of NH3 and C2H4 on Ga/HZSM-5 is lower than on Al/HZSM-5, indicating that the Ga/HZSM-5's acidity is weaker.2. We investigated the reaction mechanisms of ethylene dimerization on Ga/HZSM-5 and Al/HZSM-5 zeolite catalysts by theoretical calculation by ONIOM2 method, made the contrast of the stepwise, concerted and dehydration reaction mechanism between Ga/HZSM-5 and Al/HZSM-5, the results show that the activation energy in stepwise mechanism and concerted mechanism on Ga/HZSM-5 are higher than on Al/HZSM-5, indicating that the lower acidity of zeolites is unfavorable for ethylene dimmerization. For two kinds of zeolites, the activation energies of the deprotonation of surface butoxide are almost same, and which are obviously higher than any step of the whole reaction. It is suggested that the deprotonation is the rate-determining step for the whole catalytic reaction.3. By contrast of the reaction energetics of ethylene dimerization on 11T and 76T zeolites obtained with DFT and ONIOM2 method, we studied the effect of confinement on the reaction mechanisms. The results show that the vander Waals interaction increases the stability of the reactant species by the order of adsorption: complex > alkoxide intermediate > transition state.