Squarate-Calcium Metal-Organic Framework for Molecular Sieving of CO₂from Flue Gas with High Water Vapor Resistance

While molecular sieving is a desirable approach for extracting CO₂from flue gas, water vapor in the flue gas often poses significant difficulty in this separation process. To tackle this challenge, we targeted a squarate-calcium metal-organic framework with strong water stability, Ca(C₄O₄)(H₂O), for its pore diameter was in the middle of the kinetic diameters of CO₂and N₂. Ca(C₄O₄)(H₂O) exhibited excellent separation performance for CO₂/N₂(15/85 vol) with adsorption selectivity values of 73 at 1 bar and 298 K. This efficient separation was further confirmed by multicomponent breakthrough experiments. Additionally, Ca(C₄O₄)(H₂O) showed strong interaction with CO₂based on the very high isosteric heat of 47.7 kJ/mol. The strong affinity between CO₂and Ca(C₄O₄)(H₂O) was further explained with density functional theory calculations, which exhibited that CO₂was bound tightly via hydrogen bonds between the oxygen atoms in CO₂and the hydrogen atoms in H₂O, as well as π-π interactions between CO₂and the aromatic ligand in the framework. Furthermore, the cyclic adsorption/regeneration experiments under 100% relative humidity, 15 vol % CO₂, and balance N₂demonstrated that Ca(C₄O₄)(H₂O) is a promising adsorbent for CO₂removal in practical applications due to its excellent tolerance to water vapor and recyclability.