Octanuclear Cobalt(II) Cluster-Based Metal–Organic Framework with Caged Structure Exhibiting the Selective Adsorption of Ethane over Ethylene

A novel metal–organic framework (MOF) of [Co 8 (OH) 4 (TCA) 4 (H 2 O) 4 ] n (abbreviation: JXNU-9) based on the unique octanuclear Co 8 (μ 3 -OH) 4 clusters linked by 4,4′,4″-nitrilotribenzoate (TCA 3– ) ligands featuring small caged structures and one-dimensional channels was prepared and characterized. JXNU-9 shows a high C 2 H 6 uptake capacity of 3.60 mmol g –1 (4.46 mmol cm –3 ) at 298 K and 1 atm with a small isosteric heat of adsorption (23.6 kJ mol –1 ) and a moderate C 2 H 6 /C 2 H 4 adsorption selectivity of 1.7, resulting in excellent C 2 H 6 /C 2 H 4 separation performance. The pore walls decorated by plenty of aromatic rings provide π-electron-cloud-surrounding environments to accommodate the large polarizable C 2 H 6 molecules. The calculations demonstrate that the rich π-systems in JXNU-9 facilitate an adsorption affinity for large C 2 H 6 molecules through multiple C–H···π interactions. Additionally, the open metal sites located in the concave pores with a close Co···Co separation (4.21 Å) in octanuclear Co 8 (μ 3 -OH) 4 clusters make the open metal sites inaccessible for the C 2 H 4 molecule with a kinetic diameter of 4.163 Å. Thus, the annihilation of open metal sites in this structure is achieved, which further facilitates the C 2 H 6 -selective C 2 H 6 /C 2 H 4 separation.