The Study Of Ethylene Protonation On HZSM-5 Zeolite With Neighboring Br(?)nsted Acid Sites

In the present work, the reaction mechanism of ethylene protonation on the different acid sites and neighboring acid sites in H-ZSM-5 zeolite has been investigated by using theoretical calculations. A 20T cluster model was adopted to mimic the Br?nsted acid site in ZSM-5 zeolite and the calculations were carried out by the B3LYP/6-31G(d,p) method. Then a 126T model was adopted to mimic the the Br?nsted acid site including the pore of H-ZSM-5 zeolite and calculated by ONIOM2(B3LYP/6-31G(d,p):UFF) method. The influence of acid site location and the neighboring acid sites, as well as the confinement on the ethylene protonation wre discussed. The results are summarized as follows:(1) Based on the 20T cluster model and by B3LYP/6-31G(d,p) method, the effect of different acid sites at Al6, Al9, Al12 and neighboring acid sites of Al6-(Al9), Al6-(Al6)å'Œ(Al6)-Al9 on ethylene protonation over H-ZSM-5 zeolite was investigated. The results indicated that at the isolated acid sitesthe adsorption energies of ethylene has the order of Al9>Al6>Al12, in agreement with the zeolite acidity. However the activation energy of the ethylene protonation is increased as the same order, revealing that the strong acidity is not stands for the protonation reaction. According to the natural bond orbital analysis, if the transition state is more resemble to the carbocation, the activation energy is higher, whereas, if the transition state is more resemble to the surface alkoxide, the activation energy is lower. The results also indicated that the next-nearest-neighboring acid sites led to obvious decrease in acid strength and adsorption energies but slight decrease in activation energies, while the next-next-nearest-neighboring acid sites had a very slender effect on acid strength and the affection on ethylene protonation could be neglected. The activation energies and reaction energies had no inevitable correlation with the acid strength but virtually depended on the location and chemical environment of the acid sites in zeolite. If the transition-state structure is much resemble to the carbenium ion, the activation energy would be higher.(2) ONIOM2(B3LYP/6-31G(d,p):UFF) method is used to investigated the ethylene protonation based on the 126T cluster model and the results were compared with the data from 20T models. By using this approach the effect of zeolite confinement on the reaction was investigated. The results indicated that there is stronger van der Waals interaction between the zeolite framework and the reactants, which is favorable for the adsorption of ethylene on the Br?nsted acid sites and the stability of the surface ethoxide intermediate. However it is not for the transition state. The confinement effect is varied for different acid sites. According to the result from 126T, Al6 site is most favorable for ethylene protonation.