A metal-organic framework for C2H2/CO2 separation under highly humid conditions: Balanced hydrophilicity/hydrophobicity

Abstract

Metal-organic frameworks (MOFs) intended for C2H2/CO2 separation should possess high C2H2/CO2 adsorption selectivity and large C2H2 adsorption capacity. Here we report the scalable synthesis of a cage-interconnected microporous MOF (JNU-2). Dynamic column breakthrough experiments on JNU-2 manifest a C2H2-capture capacity of 3.08 mmol g−1 for an equimolar C2H2/CO2 mixture at 298 K and 1 bar, exceeding the benchmark materials such as UTSA-74a and FeNi-M’MOF. Meanwhile, the selectivity factor was estimated to be ca. 1.5 for JNU-2 and consistent with the increasing flow rate. Theoretical calculations confirm that the surface of the JNU-2 framework enables preferential binding of C2H2 over CO2via multiple C–H···O, C–H···N, and π-complexation interactions. More importantly, JNU-2 retains its C2H2/CO2 separation capacity under highly humid conditions. Although the equilibrium adsorption of H2O is much higher than those of C2H2 and CO2, kinetic studies reveal that the diffusion of C2H2 and CO2 in JNU-2 is more than two orders of magnitude faster than that of H2O. A balanced hydrophobicity/hydrophilicity of the MOF framework surface might be the key to achieving large adsorption capacity for nonpolar gas molecule while minimizing the competition of water vapor adsorption.