Adsorption of CO2, CH4, and N2 on Gas Diameter Grade Ion-Exchange Small Pore Zeolites

We prepared several different surfaces and pore volumes on small pore zeolites by hydrothermal synthesis and ion exchange (Li+, Na+, and Ca2+) based on three basic gas diameter grade structures of zeolites: KFI (0.39 x 0.39 nm), CHA (0.38 x 0.38 nm), and LEV (0.36 x 0.48 nm), which were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and elemental analysis. The surface area and microporous volumes were calculated using the Dubinin-Radushkevitch (D-R) equation based on CO2 adsorption at 273 K. The CHA was synthesized with a lower Si/Al ratio, and the surfaces and microporous volumes were changed greatly by ion exchange. KFI had a higher Si/Al ratio, and the scope of the surface could be kept smaller. Focusing on the CO2, CH4, and N-2 adsorption isotherms at high pressure (1 MPa), we found that the samples were exchanged by Li+ and Na+ with bigger surfaces and greater adsorption volumes. The orifice diameter was very important in the sieving of CO2 and N-2 or CO2 and CH4, while Na-LEV had an excellent sieving effect because N-2 and CH4 could not diffuse into its structure like CO2. From the viewpoint of the separate adsorption equilibria: Na-zeolites had the highest data adsorption equilibrium selectivity for CO2 and N-2 or CO2 and CH4, followed by Li-zeolites, which only had a strong adsorption potential of CO2; K-zeolites with high S-CO2/N2 and S-CH4/N2, based on the strong adsorption of CO2 and CH4. We conclude that the adsorption potential order was K-zeolites > Na-zeolites > Li-zeolites, so the bigger ions had a stronger affinity.