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 P_CO2 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.