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properties of the material.7-9 Structural characterizations of Li-X have shown that Li+ prefer the cation sites SI′ and SII.21,27 It is not until those preferred locations are occupied that the Li cation begins to occupy the less energetically favorable SIII locations. The adsorbate-zeolite interactions correspond only to those between the adsorbing gas and the surface oxygen and charge compensating cations. Since the alumina and silica groups are in a tetrahedral structure, the aluminum and silicon atoms of the zeolite framework are obscured by the oxygen atoms.28 Interactions between the adsorbate and the silicon and aluminum atoms are shielded and are therefore neglected.3 In faujasite zeolites, the cations in the beta-cages and the double 6-ring (D6R, the hexagonal prism) are sterically inaccessible to nitrogen; and so only the supercage cations (i.e., those in SII and SIII) are available to interact with nitrogen gas. However, the electric field around these supercage cations are partially shielded by the surrounding oxygen atoms. Because of this shielding, the electrostatic and induction interactions are expected to be lower than that of an isolated ion. Further, the dispersion forces acting on the molecule will be higher since adsorbate molecules also interact with oxygen atoms of the zeolite.28 Because of the small size of the lithium cation, it can sit crystallographically very low in the face of the single six-ring (SR6, the SII position) allowing the electric field generated to be nearly completely shielded by the surrounding framework oxygen. This explains why one must exchange in excess of 64 lithium cations into the X zeolite before there is any increase in the N2 adsorptive capacity; only the SIII Li+ cations interact with the N2 molecules. Because the SIII cations are in a high energy, low coordination environment they immediate coordinate with water when it is available. And so, the only cations available 139PDF Image | PSA USING SUPERIOR ADSORBENTS
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