| Abstract
| - A theoretical B3LYP study, adopting a polarized double-ζ quality Gaussian basis set, was performed tocharacterize acidic chabazite by using the periodic CRYSTAL03 program. Different Si/Al loadings (1/1, 3/1,5/1, and 11/1) were considered, and for each of them the most stable aluminum distribution and location ofthe acidic proton, needed as charge balancer, were identified. With the optimal structures, the energy offormation and the anharmonic O−H stretching frequency were calculated with the latter being in goodagreement with the experimental data. The B3LYP optimal position of H2 physisorbed at the acidic Brönstedsites of chabazite (Si/Al = 11/1 and 5/1) brings about an interaction energy definitely smaller than that derivedfrom infrared spectroscopy, because of the known deficiencies of this functional to cope with dispersiveinteractions. The latter was included by means of an ONIOM-like procedure that combines periodic B3LYPenergy with results at the MP2 level on selected clusters cut out of the chabazite framework. Adsorption oftwo H2 molecules for Si/Al = 5/1 chabazite showed a complete independence of each Brönsted site, andneither through-space nor intrastructure polarization effects are present. Within the periodic B3LYP approachshifts in both O−H and H−H anharmonic frequencies were also computed and compared with unperturbedvalues and with the available experimental results.
|
