Cs-RHO Goes from Worst to Best as Water Enhances Equilibrium CO₂ Adsorption via Phase Change

The strong affinity of water to zeolite adsorbents has made adsorption of CO₂ from humid gas mixtures such as flue gas nearly impossible under equilibrated conditions. Here, in this manuscript, we describe a unique cooperative adsorption mechanism between H₂O and Cs⁺ cations on Cs-RHO zeolite, which actually facilitates the equilibrium adsorption of CO₂ under humid conditions. Our data demonstrate that, at a relative humidity of 5%, Cs-RHO adsorbs 3-fold higher amounts of CO₂ relative to dry conditions, at a temperature of 30 °C and CO₂ pressure of 1 bar. A comparative investigation of univalent cation-exchanged RHO zeolites with H⁺, Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺ shows an increase of equilibrium CO₂ adsorption under humid versus dry conditions to be unique to Cs-RHO. In situ powder X-ray diffraction indicates the appearance of a new phase with Im3̅m symmetry after H₂O saturation of Cs-RHO. A mixed-cation exchanged NaCs-RHO exhibits similar phase transitions after humid CO₂ adsorption; however, we found no evidence of cooperativity between Cs⁺ and Na⁺ cations in adsorption, in single-component H₂O and CO₂ adsorption. We hypothesize based on previous Rietveld refinements of CO₂ adsorption in Cs-RHO zeolite that the observed phase change is related to solvation of extra-framework Cs⁺ cations by H₂O. In the case of Cs-RHO, molecular modeling results suggest that hydration of these cations favors their migration from an original D8R position to S8R sites. We posit that this movement enables a trapdoor mechanism by which CO₂ can interact with Cs⁺ at S8R sites to access the α-cage.