The thermodynamics of gas absorption and guest-induced flexibility in zeolite Y

The interaction of water, carbon dioxide and methane with faujasite zeolite Y has been probed by gas absorption calorimetry. The energetics of water sorption reveals guest-induced framework flexibility with transition from a less hydrated form to a more hydrated phase. The differential enthalpy of absorption directly measures strengths of guest-host interactions at different sites and possible binding mechanisms. Interactions of CO₂ and CH₄ with the faujasite framework are weaker and no phase transition is seen. The most negative enthalpies in water, CO₂ and CH₄ absorption occur near zero loading with exothermic values (relative to the vapor) of −110.59 ± 1.5 kJ/mol, −34.40 ± 0.47 kJ/mol and −21.87 ± 0.78 kJ/mol respectively. The initial entropy of CO₂ sorption, unlike that of water sorption, is not the lowest near zero loading but falls to a stable plateau of lowest level before increasing, reflecting a possible dynamic ordering process of absorbed CO₂ molecules. The entropy of CH₄ sorption follows a monotonic change upon loading. Thus the chemical potential obtained directly from the absorption isotherm and the calorimetrically measured enthalpy can be used to calculate the entropy of sorption and together provide a detailed map of thermodynamic behavior of H₂O, CO₂ and CH₄) in this flexible zeolite framework.