Investigation of the possibility of low pressure encapsulation of carbon dioxide in potassium chabazite (KCHA) and sodium chabazite (NaCHA) zeolites

Adsorption and desorption equilibrium isotherms of CO2 on lithium chabazite (LiCHA), sodium chabazite (NaCHA) and potassium chabazite (KCHA) zeolites were measured at 273 K up to 103 kPa using a volumetric method. The effect of cation type, and hence the structure of the chabazite cavities on the adsorption behavior was revealed through the analysis of isotherm branches. Low pressure hysteresis loops were observed on NaCHA and KCHA demonstrated by residuals of 0.37 and 0.57 molecule cavity(-1) at pressures of 0.04 and 0.09 kPa, respectively. Hysteresis loops commenced at pressures of 0.86 kPa on NaCHA and 1.05 kPa on KCHA. The earlier appearance of the hysteresis loop on KCHA over that on NaCHA suggested a higher extent of blockage of the 8-ring window aperture by K+ cations. Low pressure hysteresis loops in molecular sieves zeolites reflect the intriguing possibility of encapsulation. A quadrupolar interaction potential was used in the formulation of an encapsulation model utilizing the statistical theory of the radial distribution function (rdf) and the theory of a perfect 3D lattice gas. The model was validated with published literature data using the Lennard-Jones potential. However, both models underestimated the number of CO2 molecules in the cavities of the chabazite. Including the interaction terms of CO2-CO2 and CO2-host cavity pairs may improve the prediction of the model. The cavity's dimensions and portals can be carefully designed to achieve greater selectivities in gas separation and stabilities in gas storage. (C) 2009 Elsevier Inc. All rights reserved.