Potential of a pH-Stable Microporous MOF for C₂H₂/C₂H₄ and C₂H₂/CO₂ Gas Separations under Ambient Conditions

Cost-effective adsorption-based C₂H₂/C₂H₄ and C₂H₂/CO₂ gas separations are extremely important in the industry. Herein, a pH-stable three-dimensional (3D) metal-organic framework (MOF), IITKGP-25, possessing exposed functional sites is presented, which facilitates such separations with excellent ideal adsorbed solution theory (IAST) selectivity (4.61 for C₂H₂/C₂H₄ and 3.93 for C₂H₂/CO₂) under ambient conditions (295 K, 100 kPa, 50:50 gas mixtures) and a moderate affinity toward C₂H₂ (26.6 kJ mol^-1). Interestingly, IITKGP-25 can maintain structural integrity in water and in aqueous acidic/alkaline (pH = 2-10) medium because of the higher coordination numbers around the metal center and the hydrophobicity of the ligand. The adsorption capacity for C₂H₂ remains unchanged for a minimum of up to five consecutive cycles and 15 days of exposure to 97% relative humidity, which are the prerequisites of an adsorbent for practical gas separation application. Density functional theory (DFT) calculations reveal that the open Cd(II) sites and carboxylate oxygen-coordinated Cd(II) corner of the triangle-shaped one-dimensional (1D) channel are the enthalpically more preferred binding sites for C₂H₂, which stabilize the adsorbed C₂H₂ through nonlocal stronger H-bonding and also pπ-dπ and CH-π interactions.