An ab initio periodic study of acidic chabazite as a candidate for dihydrogen storage

A theoretical B3LYP study, adopting a polarized double-zeta quality Gaussian basis set, was performed to characterize 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 of the acidic proton, needed as charge balancer, were identified. With the optimal structures, the energy of formation and the anharmonic O-H stretching frequency were calculated with the latter being in good agreement with the experimental data. The B3LYP optimal position of H-2 physisorbed at the acidic Bronsted sites of chabazite (Si/Al = 11/1 and 5/1) brings about an interaction energy definitely smaller than that derived from infrared spectroscopy, because of the known deficiencies of this functional to cope with dispersive interactions. The latter was included by means of an ONIOM-like procedure that combines periodic B3LYP energy with results at the MP2 level on selected clusters cut out of the chabazite framework. Adsorption of two H2 molecules for Si/Al = 5/1 chabazite showed a complete independence of each Bronsted site, and neither through-space nor intrastructure polarization effects are present. Within the periodic B3LYP approach shifts in both O-H and H-H anharmonic frequencies were also computed and compared with unperturbed values and with the available experimental results.