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Adsorption of CO2andCH4has been measuredon the Na-, K-, and Cs-forms of zeolite Rho (0–9 bar; 283–333K). Although CH4is excluded, CO2is readilytaken up, although the uptake at low pressures decreases strongly,in the order Na+> K+> Cs+. Structuralstudies by powder X-ray diffraction (PXRD) suggest that cations inintercage window sites block CH4adsorption; however, inthe presence of CO2, the cations can move enough to permitadsorption (several angstroms). Determination of time-averaged cationpositions during CO2adsorption at 298 K by Rietveld refinementagainst PXRD data shows that (i) in Na-Rho, there is a small relaxationof Na+cations within single eight-ring (S8R) sites, (ii)in Cs-Rho, D8R cations move to S8R sites (remaining within windows)and two phases of Cs-Rho (I4̅3m, Im3̅m) are present overa wide pressure range, and (iii) in K-Rho, there is relocation ofsome K+cations from window sites to cage sites and twophases coexist, each with I4̅3msymmetry, over the pressure range of 0–1 bar. The final cationdistributions at high PCO2aresimilar for Na-, K-, and Cs-Rho, and adsorption in each case is onlypossible by “trapdoor”-type cation gating. Complementarystudies on K-chabazite (Si/Al = 3) also show changes in time-averagedcation location during CO2adsorption. [ABSTRACT FROM AUTHOR]

A series of univalent cation forms of zeolite Rho (M9.8Al9.8Si38.2O96, M = H, Li, Na, K, NH4, Cs) and ultrastabilized zeolite Rho (US-Rho) have been prepared. Their CO2 adsorption behavior has been measured at 298 K and up to 1 bar and related to the structures of the dehydrated forms determined by Rietveld refinement and, for H-Rho and US-Rho, by solid state NMR. Additionally, CO2 adsorption properties of the H-form of the silicoalumino-phosphate with the RHO topology and univalent cation forms of the zeolite ZK-5 were measured for comparison. The highest uptakes at 0.1 bar, 298 K for both Rho and ZK-5 were obtained on the Li-forms (Li-Rho, 3.4 mmol g-1; Li-ZK-5, 4.7 mmol g-1). H- and US-Rho had relatively low uptakes under these conditions: extra-framework Al species do not interact strongly with CO2. Forms of zeolite Rho in which cations occupy window sites between α-cages show hysteresis in their CO2 isotherms, the magnitude of which (Na+,NH4+ < K+ < Cs+) correlates with the tendency for cations to occupy double eight-membered ring sites rather than single eight-membered ring sites. Hysteresis is not observed for zeolites where cations do not occupy the intercage windows. In situ synchrotron X-ray diffraction of the CO2 adsorption on Na-Rho at 298 K identifies the adsorption sites. The framework structure of Na-Rho "breathes" as CO2 is adsorbed and desorbed and its desorption kinetics from Na-Rho at 308 K have been quantified by the Zero Length Column chromatographic technique. Na-Rho shows much higher CO2/C2H6 selectivity than Na-ZK-5, as determined by single component adsorption, indicating that whereas CO2 can diffuse readily through windows containing Na+ cations, ethane cannot. [ABSTRACT FROM AUTHOR]

Mixed-linker zeolitic imidazolate frameworks (ZIFs) with the sodalite ( sod ) topology type and based on ZIF-7 have been prepared by direct synthesis from the mixtures of benzimidazole (BzIm) and 4,5-dichloroimidazole (dcIm). Incorporation of dcIm into the ZIF-7 structure gives ZIF-7/COK-17 hybrids with rhombohedral symmetry that do not show the "open-to-closed form" structural transition upon solvent removal exhibited by ZIF-7. They show Type I isotherms for low molecular weight gases and high affinity for CO 2 even at low partial pressures. Synthesis under mild conditions gives ZIF nanoparticles (250-400 nm) suitable for incorporation into mixed matrix membranes (MMMs): these were prepared with both glassy (Matrimid) and rubbery (PEBAX 1657) polymers. Permeation tests at 298 K and 1.2 bar reveal that the incorporation of Zn(BzIm 0.55 dcIm 0.45 ) 2 nanoparticles at up to ca. 12 wt % gives defect-free membranes with enhanced CO 2 permeability in both polymer matrices, with retention of selectivity (Matrimid) or with an enhancement in selectivity that is most pronounced for the smaller nanoparticles (PEBAX). The membrane with the best performance exhibits a selectivity of ca. 200 for CO 2 /N 2 (a 4-fold increase compared to the pure polymer) and a CO 2 permeability of 64 Barrer. At the relatively low loadings investigated, the MMMs' performance obeys the Maxwell model, and the intrinsic property of diffusivity of the ZIFs can be extracted as a result.