Article pubs.acs.org/cm
Cation Gating and Relocation during the Highly Selective “Trapdoor” Adsorption of CO2 on Univalent Cation Forms of Zeolite Rho Magdalena M. Lozinska,† John P. S. Mowat,†,∇ Paul A. Wright,*,† Stephen P. Thompson,‡ Jose L. Jorda,§ Miguel Palomino,§ Susana Valencia,§ and Fernando Rey§ †
EaStCHEM School of Chemistry, University of St. Andrews, Purdie Building, North Haugh, St. Andrews, Fife KY16 9ST, Scotland Diamond Light Source Ltd, Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 0DE, United Kingdom § Instituto de Tecnología Química (UPV-CSIC), Universidad Politécnica de Valencia, Consejo Superior de Investigaciones Científicas, Avenida de los Naranjos s/n, 46022 Valencia, Spain ‡
S Supporting Information *
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 timeaveraged 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 (I43̅ m, Im3m ̅ ) 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.
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INTRODUCTION The selective adsorption of CO2 from biogas and natural gas, as well as pre-combustion and post-combustion gases in power stations, is of great potential importance, both in increasing the calorific value of fuels and in carbon capture. Among the different porous solids that have been investigated as potential CO2 adsorbents, most work has focused on porous carbons, commercial zeolites, and metal organic frameworks, each family of which has advantages and disadvantages, in terms of their cost and ease of manufacture and their uptake, selectivity, and stability.1−4 Among these families, zeolites are already synthesized and applied in large-scale industrial processes, are readily modifiable by established methods, and can be prepared to have good stability to exposure to water and other gasstream impurities, and so are attractive candidate sorbents. Their uptake of CO2 is strongly influenced by framework structure and composition as well as the composition and location of extra-framework cations.5 Zeolite A (and related zeolites with the LTA framework), ZK-5, chabazite, and faujasitic zeolites have been shown6−12 to have good working capacities that can be modified according to the chosen process conditions by changes in framework and extra-framework © 2014 American Chemical Society
cation compositions, but they have relatively low selectivities to CO2 over CH4 and N2. Recently, some of us showed that zeolite Rho (topology type RHO), prepared in a Na,Cs-form with a framework Si/Al ratio of 4.5, showed both good uptake of CO2 and also high CO2/CH4 selectivity over a wide pressure range.13 The reason for the remarkable selectivity of Na,Cs-Rho was investigated in our subsequent investigations of CO2 adsorption at a partiasl pressure of CO2 (PCO2) value of
