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

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.