Highly Cooperative CO Adsorption via a Cation Crowding Mechanism on a Cesium-Exchanged Phillipsite Zeolite.

An understanding of the CO adsorption mechanisms on small-pore zeolites is of practical importance in the development of more efficient adsorbents for the separation of CO from N or CH . Here we report that the CO 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 pressure (P ) is followed by highly cooperative uptake at a critical pressure, above which adsorption rapidly approaches capacity (2.0 mmol g ). 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 , which allows the PHI framework to relax to its wide pore form and enables its pores to fill with CO over a very narrow range of P . Such a highly cooperative phenomenon has not been observed for other zeolites.