LOW-TEMPERATURE HYDROGEN ADSORPTION ON SODIUM FORMS OF FAUJASITES - BAROMETRIC MEASUREMENTS AND DRIFT SPECTRA

The barometric study of hydrogen adsorption at 77 K on sodium forms of faujasites with different SI:AI ratios in the framework indicates tha t the number of hydrogen molecules adsorbed per sodium ion is largest for the most basic NaX zeolite with an Si:AI ratio in the framework eq ual to 1.05, viz. about one H-2 molecule per sodium cation in the larg e cavity (at high pressure). In contrast, the hydrogen adsorption on N aY with Si:Al=2.4 is lowest (about 0.5 hydrogen molecules per sodium i on). This is well consistent with the stronger perturbation of II-II s tretching vibrations in DRIFT spectra of hydrogen adsorbed on more bas ic zeolites. These results suggest a substantial contribution to the a dsorption bond of molecular hydrogen from the interaction with basic o xygen anions of the zeolite framework. Thus, in faujasites the adsorpt ion sites are most likely presented by acid-base pairs which contain s odium ions at S-II or S-III 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 m ay reveal more detailed features of such sites than are available from X-ray diffraction or inelastic neutron scattering data. (C) 1998 Else vier Science B.V. All rights reserved.