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

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 display high thermal and chemical stability, not only in water but also in acidic and basic solutions (pH = 2-11). Due to the existence 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 cm3g-1, respectively, under 1 bar at 273K). 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.