Squarate-Calcium Metal–Organic Framework for Molecular Sieving of CO2 from Flue Gas with High Water Vapor Resistance

Abstract

While molecular sieving is a desirable approach for extracting CO2 from 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 CO2 and 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 CO2 based on the very high isosteric heat of 47.7 kJ/mol. The strong affinity between CO2 and Ca(C4O4)(H2O) was further explained with density functional theory calculations, which exhibited that CO2 was bound tightly via hydrogen bonds between the oxygen atoms in CO2 and the hydrogen atoms in H2O, as well as π–π interactions between CO2 and the aromatic ligand in the framework. Furthermore, the cyclic adsorption/regeneration experiments under 100% relative humidity, 15 vol % CO2, and balance N2 demonstrated that Ca(C4O4)(H2O) is a promising adsorbent for CO2 removal in practical applications due to its excellent tolerance to water vapor and recyclability.