Selective Adsorption of Oxygen over Argon in Alkaline-Earth-Metal Cation-Exchanged Zeolite X

The separation of argon and oxygen from their gaseous mixture by adsorption is challenging because of the closeness of their molecular properties and, hence, adsorption behavior. In the present study, the potential of zeolite X with strontium as the extraframework cation as an oxygen-selective adsorbent for the separation and purification of argon is discussed. Equilibrium adsorption isotherms of oxygen and argon on calcium, strontium, and barium cation-exchanged zeolite X were measured at 288 and 303 K. The equilibrium adsorption capacity and selectivity for oxygen over argon were observed to be higher in zeolite X exchanged with alkaline-earth-metal cations than in NaX. Among these alkaline-earth-metal cation-exchanged zeolite X adsorbents, SrX showed the highest adsorption selectivity, in the range of 2.0 at 50 mmHg to 1.8 at 760 mmHg for oxygen over argon at 303 K. The increased selectivity for oxygen over argon in SrX is discussed in terms of the size, location, and effective partial charges on the extraframework cations present in the zeolite cavity and the subsequent electrostatic interactions between the adsorbed molecules and the extraframework cations. Strontium cation-exchanged zeolite X is also shown to be useful for the chromatographic separation and analysis of argon/oxygen mixtures.