CO₂ Desorbs Water from K‑MER Zeolite under Equilibrium Control

Competitive adsorption by water in zeolites is so strongly prevalent that established gravimetric techniques for quantification have assumed that humid CO₂ has no effect on preadsorbed water at the same relative humidity. Here, we demonstrate sites in small-pore zeolite K-MER, in which CO₂ adsorption causes 20% of preabsorbed water to desorb under equilibrium control at 30 °C and 5% relative humidity. Diffuse reflectance IR spectroscopic data demonstrate that dimeric water species that are coordinated to cationic sites in K-MER zeolite are selectively displaced by CO₂ under these humid conditions. Though Cs-RHO contains more weakly bound water than K-MER, we observe a lack of dimeric water species and no evidence of CO₂ outcompeting water in Cs-RHO. We conclude that the desorption of water by CO₂ in K-MER is driven by a highly desired site for CO₂ adsorption as opposed to an intrinsically weak binding of water to the zeolite. Our demonstration that CO₂ can outcompete water in a zeolite under wet conditions introduces new opportunities for the design of selective sites for humid CO₂ adsorption and stresses the importance of independently characterizing adsorbed water and CO₂ in these systems.