CO<sub>2</sub> Desorbs Water from K-MER Zeolite under Equilibrium Control
Hwangho Lee, Dan Xie, Stacey I. Zones, Alexander Katz
Abstract
Competitive adsorption by water in zeolites is so strongly prevalent that established gravimetric techniques for quantification have assumed that humid CO 2 has no effect on preadsorbed water at the same relative humidity. Here, we demonstrate sites in small-pore zeolite K-MER, in which CO 2 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 CO 2 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 CO 2 outcompeting water in Cs-RHO. We conclude that the desorption of water by CO 2 in K-MER is driven by a highly desired site for CO 2 adsorption as opposed to an intrinsically weak binding of water to the zeolite. Our demonstration that CO 2 can outcompete water in a zeolite under wet conditions introduces new opportunities for the design of selective sites for humid CO 2 adsorption and stresses the importance of independently characterizing adsorbed water and CO 2 in these systems.