Hydroxyl Radical-Dominated Catalytic Oxidation in Neutral Condition by Axially Coordinated Iron Phthalocyanine on Mercapto-Functionalized Carbon Nanotubes

The ligands and protein surroundings are important in peroxidase processes with iron porphyrins as catalysts. Similarly, two bioinspired composite catalysts made from iron phthalocyanine with axial ligands, 4-aminopyridine and 2-aminoethanethiol, were anchored on multiwalled carbon nanotubes to degrade some pollutants to the water environment, such as 4-chlorophenol, dyes, and so on. The effect of pH and sustained catalytic stability were investigated in presence of two catalysts. Different axial ligands and carbon nanotubes that synergistically donated electrons to the central iron of iron phthalocyanine significantly improved the catalytic activity and stability during hydrogen peroxide activation. Electron paramagnetic resonance spin-trapping experiments indicated that catalytic oxidation is dominated by hydroxyl radicals in both catalytic systems, which is different from the high-valent metal-oxo generated in common biomimetic catalytic systems with iron porphyrins in the presence of the fifth ligands. The high catalytic activity and strong durability are distinct from traditional peroxide-activating catalysts of metal complexes dominated by hydroxyl radicals, where catalysts have poor stability and are self-destructive in repetitive cyclic oxidation. In our catalytic system, the axial ligand and carbon nanotubes together affect the electronic structure of the central iron in which electron-donor substituerits shift the Fe-III/II potential to more negative values, which make the activation process of hydrogen peroxide occur at neutral pH, and increase the rate of the step from Fe-III to Fe-II. However, the reaction takes place under acidic conditions, and Fe-III/Fe-II cycling occurs slowly in the traditional Fenton system with hydrogen peroxide.