Adsorption of ethylene, benzene, and ethylbenzene over faujasite zeolites investigated by the ONIOM method

The performance of the ONIOM (Our-own-N-layered Integrated molecular Orbital + molecular Mechanics) approach utilizing 10 combinations of two-layer ONIOM2 schemes has been tested for various sizes of faujasite clusters containing up to 84T tetrahedral atoms and the complexes they form with ethylene, benzene, and ethylbenzene molecules. Interaction energies of the adsorbates with a 3T bare quantum cluster are calculated to be -8.14, -7.48, and -7.76 kcal/mol at B3LYP/6-31G(d,p) level of theory, respectively. The long-range effects of the extended structure of zeolite were found to differentiate the stability of adsorption complexes that cannot be drawn from the typical 3T quantum cluster. The interaction energies of ethylene, benzene, and ethylbenzene molecules on the more realistic cluster, 84T, using ONIOM2(B3LYP/6-311++G(d,p):UFF) scheme are predicted to be -8.75, -15.17, and -21.08 kcal/mol, respectively, which compare well with the experimental estimates of -9.1, -15.3, and -19.6 kcal/mol, respectively. This finding clearly demonstrates that the interaction between adsorbate and acidic zeolites does not depend only on the Brphinsted group center but also on the lattice framework surrounding the adsorption site. The results obtained in this study suggest that the ONIOM approach, when carefully calibrated, is a computationally efficient and accurate method for studying adsorption of aromatics on zeolites. (C) 2003 Elsevier Inc. All rights reserved.