Understanding Carbon Dioxide Adsorption on Univalent Cation Forms of the Flexible Zeolite Rho at Conditions Relevant to Carbon Capture from Flue Gases

Magdalena M. Lozinska, Enzo Mangano, John P. S. Mowat, Ashley M. Shepherd, Russell F. Howe, Stephen P. Thompson, Julia E. Parker, Stefano Brandani, Paul A. Wright

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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 I 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 alpha-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.

Original languageEnglish
Pages (from-to)17628-17642
Number of pages15
JournalJournal of the American Chemical Society
Issue number42
Publication statusPublished - 24 Oct 2012

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