Thermodynamic analysis of framework deformation in Na,Cs-RHO zeolite upon CO2 adsorption

Abstract

Fully dehydrated and partially sodium-cesium containing RHO zeolite (Na,Cs-RHO) shows a genuine inflection in the CO₂ isotherms in the temperature range 293–333 K that can be attributed to a sorbate-induced framework deformation from an acentric (A) to a centric (C) phase due to a partial cation rearrangement. This peculiar sorption pattern can be captured by the formulation of thermodynamic isotherms, providing a direct enthalpic and entropic signature of the CO₂ adsorption–desorption process during deformation. Using this formulation, the energy barrier between the acentric and centric phases for CO₂ adsorption–desorption was estimated in the range 4.7–9.6 J g⁻¹ of solid (15–32 kJ mol⁻¹), reflecting a higher CO₂ affinity for the acentric phase, whereas the elastic energy involved during framework distortion was estimated in the range 6–12 J g⁻¹ of solid (19–39 kJ mol⁻¹) with a relative maximum at 303 K and showing a dominant entropic contribution.