The Origin of Selective Adsorption of CO₂ on Merlinoite Zeolites

The CO₂ adsorption behavior at 25-75 °C and 0-1.0 bar of various alkali cation-exchanged forms of merlinoite (framework type MER) zeolites with Si/Al=2.3 and 3.8 is described. The adsorption isotherms at 25 °C on the Na⁺ , K⁺ , Rb⁺ , and Cs⁺ forms of MER zeolite with Si/Al=2.3 are characterized by a clear step, the CO₂ pressure of which differs notably according to the type of their extraframework cations. Structural analysis shows that CO₂ adsorption on the former three zeolites includes the relocation of gating cations with high site occupancy and the remarkable concomitant structural breathing. We define this unusual adsorption phenomenon as a cooperative cation gating-breathing mechanism. The overall results suggest that the actual mechanism of selective CO₂ adsorption on intermediate-silica small-pore zeolites can change from cation gating to cooperative cation gating-breathing to breathing, depending on a combination of their topological and compositional flexibilities.