文摘
Adsorption of CO<sub>2sub> and CH<sub>4sub> has been measured on the Na-, K-, and Cs-forms of zeolite Rho (0鈥? bar; 283鈥?33 K). Although CH<sub>4sub> is excluded, CO<sub>2sub> is readily taken up, although the uptake at low pressures decreases strongly, in the order Na<sup>+sup> > K<sup>+sup> > Cs<sup>+sup>. Structural studies by powder X-ray diffraction (PXRD) suggest that cations in intercage window sites block CH<sub>4sub> adsorption; however, in the presence of CO<sub>2sub>, the cations can move enough to permit adsorption (several angstroms). Determination of time-averaged cation positions during CO<sub>2sub> adsorption at 298 K by Rietveld refinement against PXRD data shows that (i) in Na-Rho, there is a small relaxation of Na<sup>+sup> 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<sup>+sup> cations from window sites to cage sites and two phases coexist, each with I4̅3m symmetry, over the pressure range of 0鈥? bar. The final cation distributions at high P<sub>CO<sub>2sub>sub> are similar for Na-, K-, and Cs-Rho, and adsorption in each case is only possible by 鈥渢rapdoor鈥?type cation gating. Complementary studies on K-chabazite (Si/Al = 3) also show changes in time-averaged cation location during CO<sub>2sub> adsorption.