Cation Gating and Relocation during the Highly Selective Trapdoor Adsorption of CO2 on Univalent Cation Forms of Zeolite Rho

Adsorption of CO2 and CH4 has been measured on the Na-, K-, and Cs-forms of zeolite Rho (0 9 bar; 283 333 K). Although CH4 is excluded, CO2 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 CH4 adsorption; however, in the presence of CO2, the cations can move enough to permit adsorption (several angstroms). Determination of time-averaged cation positions during CO2 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 (I4̅3m, Im3̅m) 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 I4̅3m symmetry, over the pressure range of 0 1 bar. The final cation distributions at high PCO2 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 CO2 adsorption. The authors thank the EPSRC for financial support to M.M.L and PAW. (No. EP/G062129/1, Innovative Gas Separations for Carbon Capture and postgraduate awards for J.P.S.M. and M.M.L.), and F.R., S.V., M.P., and J.L.J. thank the Spanish Government (Nos. MAT2012-38567-C02-01 and CTQ2010-17988/PPQ Consolider Ingenio No. 2010-Multicat CSD-2009-00050 and Severo Ochoa No. SEV-2012-0267). We thank Professor Chiu C. Tang and Claire Murray for assistance at the I-II Beam line at the Diamond Light Source (DLS) and DLS for beamtime (Award No. EE9027). Lozinska, MM.; Mowat, JPS.; Wright, PA.; Thompson, SP.; Jorda Moret, JL.; Palomino Roca, M.; Valencia Valencia, S.... (2014). Cation Gating and Relocation during the Highly Selective Trapdoor Adsorption of CO2 on Univalent Cation Forms of Zeolite Rho. Chemistry of Materials. 26(6):2052-2061. https://doi.org/10.1021/cm404028f