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

Abstract

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.