Ab initio study of the adducts of small molecules with the isolated hydroxyl of silica and the Bronsted site in zeolites: A comparison between B3-LYP and MP2 methods

A set of adducts of H3SiOH (SIL) and H3Si(OH)AlH3 (BRO), minimal cluster models for the isolated hydroxyl species in silica and the Bronsted 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, H2O, and NH3 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/NH3, treated including anharmonicity of the OH motion, shows that SIL is definitively less acidic than the isolated hydroxyl of silica.