A series of univalent cation forms of zeolite Rho (M(9.8)Al(9.8)Si(38.2)O(96), M = H, Li, Na, K, NH(4), Cs) and ultrastabilized zeolite Rho (US-Rho) have been prepared. Their CO(2) 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, CO(2) 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 CO(2). Forms of zeolite Rho in which cations occupy window sites between α-cages show hysteresis in their CO(2) isotherms, the magnitude of which (Na(+),NH(4)(+) < 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 CO(2) adsorption on Na-Rho at 298 K identifies the adsorption sites. The framework structure of Na-Rho "breathes" as CO(2) 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 CO(2)/C(2)H(6) selectivity than Na-ZK-5, as determined by single component adsorption, indicating that whereas CO(2) can diffuse readily through windows containing Na(+) cations, ethane cannot.

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Adsorption of CO 2 and CH 4 has been measured on the Na-, K-, and Cs-forms of zeolite Rho (0−9 bar; 283−333 K). Although CH 4 is excluded, CO 2 is readily taken up, although the uptake at low pressures decreases strongly, in the order Na + > K + > Cs + . Structural studies by powder X-ray diffraction (PXRD) suggest that cations in intercage window sites block CH 4 adsorption; however, in the presence of CO 2 , the cations can move enough to permit adsorption (several angstroms). Determination of timeaveraged cation positions during CO 2 adsorption at 298 K by Rietveld refinement against PXRD data shows that (i) in Na-Rho, there is a small relaxation of 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 (I4̅ 3m, Im3̅ m) are present over a wide pressure range, and (iii) in K-Rho, there is relocation of some K + cations from window sites to cage sites and two phases coexist, each with I4̅ 3m symmetry, over the pressure range of 0−1 bar. The final cation distributions at high P CO 2 are similar for Na-, K-, and Cs-Rho, and adsorption in each case is only possible by "trapdoor"-type cation gating. Complementary studies on K-chabazite (Si/Al = 3) also show changes in time-averaged cation location during CO 2 adsorption.

Ab initio molecular dynamics (AIMD) simulations have been used to predict structural transitions of the breathing metal-organic framework (MOF) MIL-53(Sc) in response to changes in temperature over the range 100-623 K and adsorption of CO2 at 0-0.9 bar at 196 K. The method has for the first time been shown to predict successfully both temperature-dependent structural changes and the structural response to variable sorbate uptake of a flexible MOF. AIMD employing dispersion-corrected density functional theory accurately simulated the experimentally observed closure of MIL-53(Sc) upon solvent removal and the transition of the empty MOF from the closed-pore phase to the very-narrow-pore phase (symmetry change from P2(1)/c to C2/c) with increasing temperature, indicating that it can directly take into account entropic as well as enthalpic effects. We also used AIMD simulations to mimic the CO2 adsorption of MIL-53(Sc) in silico by allowing the MIL-53(Sc) framework to evolve freely in response to CO2 loadings corresponding to the two steps in the experimental adsorption isotherm. The resulting structures enabled the structure determination of the two CO2-containing intermediate and large-pore phases observed by experimental synchrotron X-ray diffraction studies with increasing CO2 pressure; this would not have been possible for the intermediate structure via conventional methods because of diffraction peak broadening. Furthermore, the strong and anisotropic peak broadening observed for the intermediate structure could be explained in terms of fluctuations of the framework predicted by the AIMD simulations. Fundamental insights from the molecular-level interactions further revealed the origin of the breathing of MIL-53(Sc) upon temperature variation and CO2 adsorption. These simulations illustrate the power of the AIMD method for the prediction and understanding of the behavior of flexible microporous solids.

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