Highly Cooperative CO2 Adsorption via a Cation Crowding Mechanism on a Cesium‐Exchanged Phillipsite Zeolite

Abstract An understanding of the CO 2 adsorption mechanisms on small‐pore zeolites is of practical importance in the development of more efficient adsorbents for the separation of CO 2 from N 2 or CH 4 . Here we report that the CO 2 isotherms at 25–75 °C on cesium‐exchanged phillipsite zeolite with a Si/Al ratio of 2.5 (Cs‐PHI‐2.5) are characterized by a rectilinear step shape: limited uptake at low CO 2 pressure (P CO2 ) is followed by highly cooperative uptake at a critical pressure, above which adsorption rapidly approaches capacity (2.0 mmol g −1 ). Structural analysis reveals that this isotherm behavior is attributed to the high concentration and large size of Cs + ions in dehydrated Cs‐PHI‐2.5. This results in Cs + cation crowding and subsequent dispersal at a critical loading of CO 2 , which allows the PHI framework to relax to its wide pore form and enables its pores to fill with CO 2 over a very narrow range of P CO2 . Such a highly cooperative phenomenon has not been observed for other zeolites.