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

Abstract

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.