Cation Gating and Relocation during the Highly Selective“Trapdoor” Adsorption of CO2on UnivalentCation Forms of Zeolite Rho.

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]