Effect of porphyrin metal center on synthesis, structure, morphology and oxygen reduction properties of porphyrin encapsulated metal organic frameworks

A series of different metal centered 5,15-Bis(4-aminophenyl)-10,20-bis(4-bromophenyl)porphyrins (PorM) where M = H-2, MnOAc, FeCl, Co or Zn were synthesized, and then encapsulated in zeolite imidazole framework-8 (ZIF-8) by typical template directed strategy to generate PorM@ZIF-8 metal organic frameworks (MOFs). These composites were characterized by UV-vis, FTIR, XRD, FESEM and HRTEM methods. Each prepared PorM@ZIF-8 MOF retained the molecular structure of porphyrin and crystal structure of ZIF-8. It is clearly shown that porphyrin centered metal ions will affect MOFs morphology. Both PorH(2)@ZIF-8 and PorZn@ZIF-8 gave a rhombic dodecahedron, PorMnOAc@ZIF-8 gave a truncated hexagonal prism-like structure, while no specific structures were obtained for PorFeCl@ZIF-8 and PorCo@ZIF-8 due to aggregation as characterized by FESEM spectrum. Oxygen reduction catalytic ability of ZIF-8, PorM and PorM@ZIF-8 were measured in alkaline condition (0.1 M KOH) with the number of electrons transferred being n = 2.20-2.60 and generating HO2- as the oxygen reduction product. The catalytic property slightly increased after the porphyrin was encapsulated, due possibly to the capacity limit, inappropriate molecular distance or the direction of encapsulated porphyrin molecular.