FT-IR Studies of the Interaction between Zeolitic Hydroxyl Groups and Small Molecules. 3. Adsorption of Oxygen, Argon, Nitrogen, and Xenon on H−ZSM-5 at Low Temperatures

Abstract

Adsorption of O2, Ar, N2, and Xe on H−ZSM-5 at low temperatures has been studied by transmission Fourier transform infrared spectroscopy. These molecules form 1:1 hydrogen-bonded complexes with the acidic OH groups in H−ZSM-5, which is characterized by the downward shift and the broadening of the ν(OH) band due to the acidic OH groups. The accompanying behavior of the adsorbed species has been examined simultaneously for the case of the O2 and N2 adsorption; their absorption bands have been observed at 1550 cm-1 for the dioxygen species adsorbed on Brønsted acid sites (BAS) and at 2334 and 2352 cm-1 for the dinitrogen species on BAS and Lewis acid sites (LAS), respectively. The broadening of the ν(OH) band caused by Xe is much larger than those by the other gases, which is tentatively associated with the large polarizability of Xe. The degrees of the shift of the acidic-OH band caused by the adsorption of these molecules were systematically in the order of H−ZSM-5 > H−MOR > H−Y. Using these data, the correlation between the shift and the acidity of the zeolites has been discussed.