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Introduction Since their introduction in the 1950’s, synthetic zeolites have been used in numerous applications such as catalysis, ion exchange, drying, and separation by selective adsorption. Among the most commonly used in these applications (especially in the selective adsorption of gases) are the faujasite analogs (types X and Y). Faujasite zeolites are composed of silica and alumina tetrahedra which are joined together to form the truncated octahedral or sodalite structure. These sodalite units are connected with tertiary units (hexagonal prisms in faujasite) to form the structured zeolite unit cell. In these structures, the SiO2 groups are electroneutral, but the (AlO2)- groups are not, and thus introduce a negative charge to the structure which is offset by the presence of a charge compensating, non-framework cation (e.g., Na+, Li+, Ca2+). For the faujasite zeolites, the cation site designations are conventionally designated as SI (the center of the hexagonal prism), SI′ (opposite SI but located in the cubooctahedron), SII (single six-ring in the supercage), SII′ (opposite SII but inside the cubooctahedron), and SIII (near the four-ring windows in the supercage). The unit cell, including cation sites, for faujasite- type zeolites is shown in Figure 1. It is known that the extraframework cations in the zeolite are largely responsible for the adsorptive capacity of these materials.1,2 This is due primarily to van der Waals and Coulombic interactions between the charge compensating cations of the zeolite and the adsorbing gas. Because the extraframework cations so significantly influence the adsorption properties of the zeolites, numerous attempts have been made to optimize these properties by (1) increasing the number of cation sites (the cation exchange 8PDF Image | PSA USING SUPERIOR ADSORBENTS
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