2025 AIChE Annual Meeting

(196h) Zeolitic Active Sties in K-MER for Applications Involving Humid CO2

Authors

Dan Xie, Braskem
Alexander Katz, University of California Berkeley
While zeolites possess high volumetric CO2 adsorption capacities under dry conditions, and weak interactions with CO2, which are preferred for energetically facile adsorption/desorption, the inevitable presence of water in post-combustion gas streams results in strong competitive adsorption, which decreases CO2 adsorption capacities and leads to creeping death in adsorption unit operations. Discovering selective CO2 adsorption sites in zeolites that can function under humid conditions remains a grand challenge for enabling practical CO2 capture. A grand challenge in the molecular design of selective CO2 binding sites is to synthesize environments in zeolites in which CO2 can thermodynamically outcompete H2O. Previous literature highlighted that small pore zeolites with 8-membered ring (8MR) secondary building units provide adequate confinement for CO2 to be energetically favored over water, resulting from strong quadrupolar interaction with CO2. In this study, we demonstrate an unprecedented selective adsorption site in small-pore zeolites with double eight-membered ring (D8R) structure, in which CO2 outcompetes water under thermodynamic control. We demonstrate this type of site in action within K+ exchanged zeolite MER (K-MER) with a combined thermogravimetric analysis (TGA) and in-situ diffused reflectance infrared Fourier transform spectroscopy (DRIFTS) system that characterizes multicomponent H2O and CO2 adsorption in the zeolite. Our results demonstrate that CO2 adsorption leads to H2O desorption under thermodynamic control, leading to the equilibrated desorption of a water dimer during CO2 under humid conditions. DRIFTS data demonstrate that this displacement of water by CO2 occurs within D8R structure of K-MER in which H2O is present as a dimer due to the isolation effect of a water dimer within the D8R. These data lead us to conclude that the D8R structure provides a unique proficient environment for humid CO2 adsorption, where CO2 can outcompete H2O adsorption. Applications of K-MER zeolite involving humid CO2 gas mixtures will also be described.