Water vapor adsorption on small pore ion-exchanged zeolites

Abstract

Ion exchange is the reversible exchange of ions in which there is no significant change in the solid structure. Zeolites are aluminosilicates with a defined structure, including cavities occupied by cations and water molecules, both with great freedom of movement, which makes cation exchange possible. In this study, small-pore zeolites chabazite (CHA) and clinoptilolite (CLI) were ion-exchanged with potassium. Then, the samples were characterized by N₂ isotherms at 77 K, CO₂ adsorption microcalorimetry at 298 K, and water vapor isotherms at 313 K. A mathematical model was applied to evaluate the adsorption kinetics for water vapor uptakes. Textural analysis showed that the ion exchange with potassium decreased the porosity of both zeolites, but CO₂ microcalorimetric data showed that these samples had higher CO₂ adsorption enthalpy, indicating a greater sorbate-sorbent interaction as compared to the pristine zeolites. Uptake rate curves suggest water diffusion is not appreciably altered after ion exchange. Interestingly, despite the larger size of K⁺ cations as compared to Na⁺, effective diffusion time constant is on order of magnitude larger for the potassium-loaded CLI very likely due to the leaching of other contaminants upon ion-exchange.