DRIFTS and Raman Investigation of N2 and O2 Adsorption on Zeolites at Ambient Temperature

Abstract

Diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) and Raman spectroscopy were used to examine N2 and O2 adsorption on cation-exchanged (K, Na, Sr, Ca, and Li) low silica X (LSX) zeolites. IR and Raman absorption bands were observed for the molecular vibration of adsorbed N2 and O2 at room temperature and atmospheric pressure. The intensity (in Kubelka-Munk units) of the IR band increased with N2 pressure and mirrored the adsorption isotherm for N2. Both O2 and N2 displayed a similar dependence of the molecular vibrational frequency on cation charge density, which suggests that both gases are interacting directly with the cations. The vibrational frequencies for adsorbed N2 and O2 were more sensitive to the cation charge density than to framework Al content. Infrared studies of N2 and O2 on mixed cation forms of LSX show that N2 interaction was localized near individual cations within the sorption cavity of the zeolite. Thus, adsorbed N2 can be used to probe accessibility of specific cations within the zeolite framework. The spectroscopic data are consistent with the theory that the stronger interaction of N2 over O2 is caused by the stronger influence of the electric field with the larger quadrupole of N2.