pH-Induced Two Co (II) Metal–Organic Frameworks with Different Topologies: Magnetism and CO₂/CH₄ Separation

The self-assembly process of the metal–organic framework (MOF) is affected by many factors, so the construction of the MOF with unique structure and properties is still a great challenge. Herein, based on 3,5-bis (2,5-dicarboxylic phenyl) benzoic acid (H5L), two novel 3D porous Co-MOFs, {[Co3(HL)2(μ2-H2O)2]·2NH3·4CH3CN·4H2O}n (Co-MOF 1) and {[Co2.75(L)(H2O)3.5(μ3-OH)]·0.5NH3·3CH3CN·3H2O}n (Co-MOF 2), were designed and constructed by changing the pH under solvothermal condition. Co-MOF 1 displays a 2-node (4, 8)-c flu topological network with the symbol of {4·126·1284}{46}2, while Co-MOF 2 exhibits a 3-n (4,4,6)-c 4,4,6T17 (MOF.ttd) topology type with the symbol of {4·284}{4·462}4{4·76·882}2. Meanwhile, the magnetic test results of Co-MOFs show that Co-MOFs have antiferromagnetic interaction between Co (II) ions. Furthermore, the adsorption selectivities (Sads) of Co-MOFs for 0.05/0.95 CO2/CH4 binary mixture are 27.1 (Co-MOF 1) and 38.0 (Co-MOF 2) at 298 K, respectively, and the high adsorption performance of Co-MOFs for CO2 was attributed to the smaller dynamic radius, larger quadrupole moment, and higher polarizability of CO2, the strong adsorption heat of Co-MOFs for CO2, and the strong interaction force between Co-MOFs and CO2. In addition, the action sites and bond energy of the hydrogen bonds between CO2 and Co-MOF 2 (O–H···O) are obviously better than those between CO2 and Co-MOF 1 (C–H···O) simulated by Grand canonical Monte Carlo simulations.