Anchoring Highly Unsaturated Nickel(II) Sites into a Metal-Organic Framework for Simultaneous High C₂H₂ Adsorption...

Separation of acetylene (C₂H₂) from carbon dioxide (CO₂) remains a challenge to achieve both high C₂H₂ uptake and selectivity for a single material. Porous materials with open metal sites (OMSs) have been widely employed to separate various gas mixtures; however, routine OMSs often show the limitation in C₂H₂/CO₂ separation due to the comparable binding affinity with the two gases. Herein, we report a new strategy of anchoring highly unsaturated nickel(II) sites into a large-pore MOF (Ni²⁺@NOTT-101-(COOH)₂) for simultaneously improving C₂H₂ adsorption and selectivity. The coordination geometry of the anchored Ni²⁺ ion was accurately determined by SCXRD studies, featuring a two-coordination model with highly unsaturated OMSs after the activation. This highly unsaturated Ni²⁺ ion can provide additional binding sites to improve C₂H₂ adsorption and also enable highly selective binding of C₂H₂ over CO₂ through the specific and strong π- complexation interactions, as revealed by gas-loaded SCXRD studies and theoretical simulations. This metalated MOF thus exhibits both significantly enhanced C₂H₂ uptake (201.4 cm³ g^-1) and C₂H₂/CO₂ selectivity (25.7) than the pristine NOTT-101-(COOH)₂ (148.0 cm³ g^-1 and 3.8) at 298 K and 1 bar, making an unprecedented balance between C₂H₂ adsorption and selectivity to overcome the trade-off challenge. Breakthrough experiments on equimolar C₂H₂/CO₂ mixtures afford both the top-tier dynamic selectivity (14.4) and C₂H₂ productivity of 114.5 L kg^-1 (>99.5% purity) at ambient conditions.