Effects of the zeolite framework on the adsorption of ethylene and benzene on alkali-exchanged zeolites: an ONIOM study

The density functional theory (B3LYP/6-31G(d,p)) and our-own-N-layered integrated molecular orbital + molecular mechanics (ONIOM) approach utilizing two-layer ONIOM schemes (B3LYP/6-31G(d,p): UFF) have been employed to investigate the structures of alkali-exchanged faujasite (FAU) and ZSM-5 zeolites, and their interactions with ethylene and benzene. Inclusion of the extended zeolite framework has an effect on the structure and energetics of the adsorption complexes and leads to differentiation of different types of zeolites (ZSM-5 and FAU), which cannot be drawn from typical small quantum clusters. The ONIOM binding energies of the Li-ZSM-5 and Li-FAU zeolites that are bound to an ethylene are -16.94 and -14.27 kcal/mol, respectively, and to a benzene are -28.78 and -19.46 kcal/mol, respectively, which agrees favorably with the known adsorption trend of these two zeolites. On the other hand, the quantum cluster models yield virtually the same binding energies for both zeoliteS/C2H4 Complexes (-12.25 kcal/mol versus -12.91 kcal/mol) and even yield an unreasonable trend of adsorption energies for zeoliteS/C6H6 complexes (-11.91, -15.75 kcal/mol, for Li-ZSM-5 and Li-FAU, respectively). For the larger cation-exchanged Na-ZSM-5/C2H4 and Na-FAU/C2H4 complexes, the calculated interaction energies (-15.67 kcal/mol versus -11.83 kcal/mol) are predicted to be lower than those of smaller Li-zeolites following the conventional electrostatic trend. With the inclusion of basis set correction and the effects of the extended framework included in the ONIOM model, the interaction energy for the Na-FAU/C2H4 complex is predicted to be -8.65 kcal/mol, which can be compared favorably with the experimental data (8.8-9.6 kcal/mol) for ethylene adsorption on a Na-Y zeolite. (C) 2003 Elsevier B.V. All rights reserved.