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

Abstract

Adsorption of CO 2 and CH 4 has been measured on the Na-, K-, and Cs-forms of zeolite Rho (0−9 bar; 283−333 K). Although CH 4 is excluded, CO 2 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 CH 4 adsorption; however, in the presence of CO 2 , the cations can move enough to permit adsorption (several angstroms). Determination of timeaveraged cation positions during CO 2 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 (I4̅ 3m, Im3̅ m) 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 I4̅ 3m symmetry, over the pressure range of 0−1 bar. The final cation distributions at high P CO 2 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 CO 2 adsorption.

Cation Control of Molecular Sieving by Flexible Li-Containing Zeolite Rho

“…22,23 The slow kinetics observed for the Na-rich end of the solid solution series can be attributed to the presence of Na + cations in 8R window sites at levels that require access to cages in the structure via occupied windows, with resulting need for cation movement. In situ synchrotron PXRD ( Figure S2.11, also Table S2 different from what is seen with Li-Rho.…”

“…[14][15][16] Recent computational studies have demonstrated that extraframework cations can move in the presence of CO 2 , 17 making cation gating effects possible. [18][19][20][21][22][23][24] In the postulated 'trapdoor' mechanism, cations occupying window sites permit the passage of molecules such as CO 2 or H 2 O that interact strongly with cations but block the windows to molecules that interact more weakly (such as CH 4 or N 2 ). These 'trapdoor' 3 zeolites adsorb CO 2 from mixtures of CH 4 or N 2 with very high selectivity.…”

“…These 'trapdoor' 3 zeolites adsorb CO 2 from mixtures of CH 4 or N 2 with very high selectivity. Zeolites A 18,19 and chabazite 20,21 with particular cation compositions have been suggested to demonstrate such behaviour, as have certain forms of zeolite Rho [22][23][24] and structures related to Rho. 25,26 In zeolite Rho, 27 large lta cages are linked by double 8-ring structural units, which act as windows and are also the favoured sites for large univalent cations such as Na + , K + and Cs + (8-ring refers to a ring of 8 framework metal atoms and 8 oxygen atoms).…”

“…23 However, the kinetics of adsorption and desorption in this Na-Rho are not fast enough for pressure swing adsorption applications, where timescales less than a few minutes are required. As a consequence, alternative approaches to high selectivity must be found that also permit rapid adsorption/desorption cycles.…”

Structural changes of synthetic paulingite (Na,H-ECR-18) upon dehydration and CO2 adsorption

“…Typically, moderate selectivity for CO 2 over N 2 or CH 4 can be achieved over cationic zeolites such as Na-X [4] or Na-A [5], which result from the quadrupole moment of CO 2 and its polarisability. Recently, however, small pore zeolites of specific structures and compositions have been found to show very high selectivity for CO 2 over less polar gases such as CH 4 and N 2 due to a molecular 'trapdoor' effect that makes them strong candidates as sorbents for carbon capture [6][7][8][9][10][11][12].…”