A computational study of ethyl chloride conversion reactions catalyzed by acidic zeolites

The interaction of ethyl chloride with two model zeolite clusters, each including one hydroxyl Brönsted acid site, has been investigated using density functional theory (DFT) and MM methods. The first model contains three tetrahedral zeolite units (T3) and has been studied at the B3LYP(full) level in combination with the 6-311G(d,p) and the cc-pVTZ basis sets. The second model is represented by a 12-membered ring (T12) and has been investigated with the help of the two-layered ONIOM (B3LYP/6-311G(d,p): UFF) methodology. The reliability of the latter method was checked against the optimization of the adsorption complex in the T12 cluster at the B3LYP(full)/6-311G(d,p) level. The calculations show that the reaction mechanism involves two competing channels, a direct E2 type dehydrohalogenation channel and a SN2 type pathway through the intermediate formation of an alkoxy species, the latter channel presenting a lower activation energy than the former. The overall potential energy surface becomes more attractive when using the more realistic T12 framework and the calculated difference in the activation energy barrier heights between the two channels increases from ∼1 kcal mol−1 in the T3 framework to ∼4 kcal mol−1 in T12 model. Finally, the effect of zeolite acidity on the reaction barrier is investigated by increasing the length of a terminal Si-H bond in the T3 model. The results indicate the distinct reduction of the activation barrier as the acidity increases.