Thermodynamics of H2O and CO2 Absorption and Guest-Induced Phase Transitions in Zeolite RHO

Gas absorption calorimetry at low pressures has been employed to probe the interaction of water and carbon dioxide with several ion-exchanged zeolite RHO samples. It reveals that guest-induced framework flexibility transitions from a less hydrated (initially anhydrous) acentric form to the more hydrated centric phase during water absorption. The differential enthalpy of absorption as a function of water loading directly identifies the strengths of different interactions along with possible water binding mechanisms. Interactions with CO2 are weaker, and no phase transition is seen except in Li,H-RHO. The most negative initial enthalpies have been obtained in both water and CO2 absorption for Cd and Cs-RHO. However, Li,H-RHO shows the strongest capture ability for CO2 despite a less exothermic initial enthalpy of absorption. The derived differential enthalpy, chemical potential, and entropy elucidate the thermodynamic behavior of multiple interactions of small guest molecules (H2O and CO2) and ion-exchanged flexible zeolite frameworks.