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.