Understanding Water Enhancement of CO₂ Adsorption in Zeolite Cs–RHO

Structural characterization of humid CO2 adsorbed in Cs–RHO zeolite was achieved with computationally guided Rietveld refinement, and elucidates the extraordinary enhancement in CO2 adsorption under wet compared with dry conditions in this zeolite. Our data encompass Rietveld refinement, IR spectroscopy, and molecular simulations, and demonstrate a cooperative effect of water (pulling Cs+ cations) and CO2 (pushing Cs+ cations) in translocating Cs+ cations away from initial positions in the center of the double eight-membered ring (D8R). This translocation is crucial for unblocking the small-pore RHO framework for CO2 transport as well as exposing thermodynamically controlled selective sites that can adsorb CO2 under our humid conditions. Our data emphasize the essentialness of cooperativity in that neither water nor CO2 achieve this unblocking on their own at 5% relative humidity and 30 °C. These results also demonstrate the importance of multidentate interactions between CO2 and cations through the D8R, as well as framework oxygen atoms of the D8R, as a key motif in water-resilient CO2 bonding sites in zeolites, along with additional, weaker interactions with other cations in the alpha cage.