Ab Initio Study of the Adducts of Small Molecules with the Isolated Hydroxyl of Silica and the Brønsted Site in Zeolites: A Comparison between B3-LYP and MP2 Methods

A set of adducts of H 3 SiOH (SIL) and H 3 Si(OH)AlH 3 (BRO), minimal cluster models for the isolated hydroxyl species in silica and the Brønsted site in zeolites, respectively, have been studied by the B3-LYP method using various basis sets, including aug-cc-pVDZ and aug-cc-pVTZ. Molecules considered were H 2 (in two configurations), CO, H 2 O, and NH 3 with SIL and CO only with BRO: results were compared with similar calculations run at MP2 as well as with experimental results for amorphous silica and proton exchanged zeolites. B3-LYP results were as good as MP2, though a larger basis set dependence is seen for the former. As the interactions studied span a sizable range of energy, a number of correlations are drawn, similar to those usually drawn for experimental data. The case of SIL/NH 3, treated including anharmonicity of the OH motion, shows that SIL is definitively less acidic than the isolated hydroxyl of silica.