Functionalization of MOFs via a mixed-ligand strategy: enhanced CO₂ uptake by pore surface modification

A new Zn(ii) metal-organic framework (MOF) [Me2NH2][Zn2(BDPP)(HTZ)]·4DMF (1) (H4BDPP = 3,5-bis(3,5-dicarboxylphenyl)pyridine, HTZ = 1H-tetrazole) has been constructed under solvothermal conditions by using a mixed-ligand strategy. Structural analysis demonstrates that 1 is a 3D framework based on four kinds of secondary building units (SBUs), which presents a rare structure constructed from quaternary SBUs and shows an uncommon (3,3,4,6)-connected topology. In particular, 1 contains two shapes of 1D open channels with suitable pore sizes, high porosity, and a highly polar pore system decorated with uncoordinated N atoms and carboxylic O atoms, providing a good environment for selective adsorption of CO2. Inspired by the structure of 1 and reticular chemistry, 5-amino-1H-tetrazole (ATZ) was used to replace 1H-tetrazole to enhance CO2 sorption capacity by pore surface modification; as a result, an amino-functionalized MOF, [Me2NH2][Zn2(BDPP)(ATZ)]·4DMF (1-NH2) was successfully built. 1-NH2 exhibits multipoint interactions between the CO2 molecules and the framework, resulting in better CO2 uptake and selectivity for CO2 over CH4 than 1.