Structure and adsorption of a basic probe molecule on H-ZSM-5 nanostructured zeolite: an embedded ONIOM study.

The adsorption properties of pyridine on H-ZSM-5 zeolites have been investigated by cluster calculations with the ONIOM scheme and with an embedded-ONIOM scheme. The active site has been modeled with cluster sizes of up to 46 tetrahedra. Two different types of pyridine adsorption complexes on the zeolite models are found. If Zeolite is modeled by a small 3T quantum cluster, the adsorption energy of the hydrogen-bonded pyridine complex (Py-Hz), is found to be -18.5kcal/mol. When a larger cluster or the ONIOM models are employed, the optimized geometries show the formation of pyrdinium cation [PyH(+)] bound as an ion-pair complex [PyH(+)][Z(-)]. The calculated energy of formation for this ion-pair complex is -36.8kcal/mol in the ONIOM (B3LYP/6-31G(d,p):UFF) model. Both values are considerably lower than the experimentally estimated heat of adsorption of pyridine in ZSM-5 zeolite of -47.6kcal/mol. Inclusion of the electrostatic effects of the zeolite crystal lattice via the embedded ONIOM model increases the adsorption energy to -44.4kcal/mol. Performing the quantum-chemical treatment at the MP2/6-31G(d,p) level instead of the B3LYP/6-31G(d,p) leads to a slightly lower adsorption energy to -45.9kcal/mol. These data suggest that the embedded ONIOM scheme provides an accurate method of studying the interaction of small organic molecules with zeolites.