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

Abstract

A novel metal-organic framework (MOF) of [Co8(OH)4(TCA)4(H2O)4]n (abbreviation: JXNU-9) based on the unique octanuclear Co8(μ3-OH)4 clusters linked by 4,4',4″-nitrilotribenzoate (TCA3-) ligands featuring small caged structures and one-dimensional channels was prepared and characterized. JXNU-9 shows a high C2H6 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 C2H6/C2H4 adsorption selectivity of 1.7, resulting in excellent C2H6/C2H4 separation performance. The pore walls decorated by plenty of aromatic rings provide π-electron-cloud-surrounding environments to accommodate the large polarizable C2H6 molecules. The calculations demonstrate that the rich π-systems in JXNU-9 facilitate an adsorption affinity for large C2H6 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 Co8(μ3-OH)4 clusters make the open metal sites inaccessible for the C2H4 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 C2H6-selective C2H6/C2H4 separation.