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

The strong affinity of water to zeolite adsorbents has made adsorption of CO 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 HO and Cs cations on Cs-RHO zeolite, which actually facilitates the equilibrium adsorption of CO under humid conditions. Our data demonstrate that, at a relative humidity of 5%, Cs-RHO adsorbs 3-fold higher amounts of CO relative to dry conditions, at a temperature of 30 °C and CO 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 CO 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 3̅ symmetry after HO saturation of Cs-RHO. A mixed-cation exchanged NaCs-RHO exhibits similar phase transitions after humid CO adsorption; however, we found no evidence of cooperativity between Cs and Na cations in adsorption, in single-component HO and CO adsorption. We hypothesize based on previous Rietveld refinements of CO adsorption in Cs-RHO zeolite that the observed phase change is related to solvation of extra-framework Cs cations by HO. 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 CO can interact with Cs at S8R sites to access the α-cage.