Drift spectra of adsorbed dihydrogen as a molecular probe for alkaline metal ions in faujasites

DRIFT spectra of dihydrogen adsorbed at 77 K on alkaline-metal forms of X and Y zeolites demonstrate the unique properties of this molecular probe. The spectra provide an information on perturbation of the H-H stretching frequency, on oscillation of hydrogen relative to adsorption sites, and on hindered rotation of adsorbed molecules. The obtained results demonstrate that the perturbed ii-Il stretching frequency enables to discriminating S(II) and S(III) sites inside the zeolite supercages. In addition, for NaY zeolite, the spectral data allow one to draw more detailed conclusions about local distribution of aluminum atoms in the six-membered rings at S(II) sites. The obtained results also show that the hindered rotation of adsorbed molecules is most sensitive to the nature of cations and to their environment. Finally, the frequency of adsorbed hydrogen oscillations relative to sodium cations is consistent with the diffusion coefficient inside the zeolite micropores previously estimated for NaX by the field gradient NMR. (C) 1999 Elsevier Science B.V. All rights reserved.