Alkylation of benzene with ethylene over faujasite zeolite investigated by the ONIOM method

The alkylation of benzene with ethylene over faujasite zeolite has been investigated using an 84T cluster of faujasite zeolite serving as a nanometer-sized chemical reactor modeled by the ONIOM3 (MP2/6-311++G(d,p):HF/6-31G(d):UFF) method, which gives accurate adsorption energies for the reactants and the product, indicating the accuracy of the model in representing interactions between the adsorbates and the zeolite. The computed adsorption energies are -8.73, -13.91, and -20.11 kcal/mol, which compared well with experimentally reported values of -9.0, - 14.0, and -20.4 kcal/mol for ethylene, benzene, and ethylbenzene, respectively. Stepwise and concerted mechanisms of the alkylation reaction are considered. For the stepwise mechanism, the alkylation starts with the protonation of the adsorbed ethylene by an acidic zeolite proton leading to the formation of the ethoxide intermediate and, subsequently, the ethoxide reacts with a benzene molecule forming an ethylbenzene product. The computed activation energies are 30.06 and 38.18 kcal/mol for the first and second step, respectively. For the concerted mechanism, the alkylation of benzene takes place in a single reaction step without prior ethoxide formation. The concerted mechanism has an activation energy of 33.41 kcal/mol which is in between the two energy barriers of the stepwise mechanism. (C) 2004 Elsevier Inc. All rights reserved.