Thermodynamic evidence of flexibility in H₂O and CO₂ absorption of transition metal ion exchanged zeolite LTA

Gas absorption calorimetry has been employed to probe the intercation of water and carbon dioxide with transition metal ion (TM = Mn²⁺, Fe²⁺, Co²⁺, Cu²⁺, and Zn²⁺) exchanged zeolite A samples. There appears to be a two-phase region, indicative of a guest-induced flexibility transition, separating hydrated zeolite A and its dehydrated form, both of which have variable water content in the single phase region. The differential enthalpy of absorption as a function of water loading directly identifies different strengths of multiple interactions along with possible binding mechanisms of Zn-A and Mn-A exhibiting the highest water absorption with most exothermic initial enthalpies of -125.28 ± 4.82 and -115.30 ± 2.56 kJ mol^-1. Zn-A and Mn-A also show moderately good capture ability for CO₂ with zero-coverage negative enthalpies of -55.59 ± 2.48 and -44.07 ± 1.53 kJ mol^-1. The thermodynamic information derived from differential enthalpy, chemical potential and differential entropy elucidated the multistage interactive behavior of small guest molecules (H₂O/CO₂) and ion-exchanged frameworks.