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Abstract:
The barometric study of hydrogen adsorption at 77 K on sodium forms of faujasites with different Si:Al ratios in the framework indicates that the number of hydrogen molecules adsorbed per sodium ion is largest for the most basic NaX zeolite with an Si:Al ratio in the framework equal to 1.05, viz. about one H2 molecule per sodium cation in the large cavity (at high pressure). In contrast, the hydrogen adsorption on NaY with Si:Al=2.4 is lowest (about 0.5 hydrogen molecules per sodium ion). This is well consistent with the stronger perturbation of H–H stretching vibrations in DRIFT spectra of hydrogen adsorbed on more basic zeolites. These results suggest a substantial contribution to the adsorption bond of molecular hydrogen from the interaction with basic oxygen anions of the zeolite framework. Thus, in faujasites the adsorption sites are most likely presented by acid–base pairs which contain sodium ions at SII or SIII sites inside the supercages of the zeolite framework and the adjacent basic oxygen anions. The results obtained also demonstrate that DRIFT spectra of adsorbed hydrogen are sensitive to the local environment of such adsorption sites. In this way they may reveal more detailed features of such sites than are available from X-ray diffraction or inelastic neutron scattering data.
