Entropic effects and isosteric heats of nitrogen and carbon dioxide adsorption on chabazite zeolites

Equilibrium adsorption isotherms for CO2 and N-2 were measured on a fully exchanged potassium chabazite (KCHA) zeolite, and partially exchanged sodium-chabazite (NaCHA) and lithium-chabazite (LiCHA) zeolites. This work presents integral thermodynamic functions and isosteric heats of N-2 and CO2 adsorption equilibria on a fully exchanged potassium chabazite (KCHA) zeolite, and compares these to those functions measured on sodium exchanged chabazite (NaCHA) and lithium exchanged chabazite (LiCHA) zeolites. The thermodynamic quantities were calculated from equilibrium isotherms as function of loading. Adsorption of N-2 on KCHA was considered with distinct layers in inter-crystal spaces and mesopores volume, while CO2 adsorption in KCHA cavities was more readily interpreted as fluid-like volume filling mechanism. The molecules comprising this fluid (CO2) are in the dispersion energy field of the crystals and of each other, bounded also by considerable field-gradient quadrupole energy. In all cationic forms the adsorption equilibrium loading and differential enthalpies for CO2 were considerably higher than those for N-2. The CO2 integral entropy showed a rapid decreasing trend close to the saturation capacity of the cavities, demonstrating an appreciable loss of molecule degrees of freedom. The results of this work provide useful thermodynamic information that help in understanding the adsorption of N-2 and CO2 on a fully exchanged KCHA zeolite. (C) 2009 Elsevier Inc. All rights reserved.