Three stable dinuclear [M2(OH)0.5(NO3)0.5(RCOO)2(RN)4] (M = Cu, Ni) based metal–organic frameworks with high CO2 adsorption and selective separation for O2/N2 and C3H8/CH4

Abstract

Three highly porous MOFs with dinuclear [M₂(OH)_0.5(NO₃)_0.5(RCOO)₂(RN)₄] SBUs (secondary building units), [Cu₄(L₁)₄(OH)(NO₃)]·2(NO₃)·4DMF (compound Cu-L₁, HL₁ = 5-imidazol-1-yl-nicotinic acid, DMF = N,N-dimethylformamide), [Ni₄(L₁)₄(OH)(NO₃)₃(DMF)₂]·3DMF·4H₂O (compound Ni-L₁) and [Ni₄(L₂)₄(OH)(NO₃)₃(DMF)₂]·3DMF (compound Ni-L₂, HL₂ = 5-[1,2,4]triazol-1-yl-nicotinic acid), have been solvothermally constructed using a heterofunctional ligand motif with carboxylate and azolate groups. All three compounds show high thermal and chemical stability, not only in water but also in acidic and basic solutions (pH = 2–11). Due to the presence of accessible open metal sites (OMSs), compounds Cu-L₁, Ni-L₁, and Ni-L₂ exhibit significant ability to capture CO₂ (114.8, 88.3 and 93.7 cm³ g⁻¹, respectively, under 1 bar at 273 K). Interestingly, gas adsorption results show that all three compounds exhibit high selectivity toward O₂ over N₂ at 77 K and C₃H₈ over CH₄ at 298 K, which indicates that the three materials have potential for gas separation applications.