Metalloporphyrin/ZnO solids as selective catalysts for oxidation reaction assisted by light

Cationic manganese (MnP - [Mn(T4MPyP)]5+) and anionic iron (FeP - [Fe(TDFSPP)]3–) metalloporphyrins in conjunction with zinc oxide of different morphologies, were synthesized. After characterization by different instrumental techniques, the metalloporphyrins were immobilized on the obtained zinc oxide and the catalysts were evaluated regarding the oxidation of cyclohexane, using iodosylbenzene (PhIO) as model oxidant. In the absence of light, the activities of FeP/ZnO were higher than those obtained by neat ZnO and MnP/ZnO. Furthermore, under the reaction conditions of 1:100:1000 catalyst/oxidant/substrate, with acetonitrile as solvent, for 1 h at 25 °C without any radiation source, the reactions were selective for ketone. The same occurred for the neat FeP under homogeneous conditions. In contrast, when the reaction proportions were changed to 1:20:1000 catalyst/oxidant/substrate and when the catalyst amount was increased 10 times, the selectivity was for alcohol. Under irradiation with a dichroic halogen lamp (50 W), all the systems containing FeP showed higher selectivity for alcohol, while when MnP was used, increments like those with FeP were not observed, probably due to the structural differences between FeP and MnP. This FeP to MnP replacement led to spectral differences that might have been responsible for the inability of MnP to interact efficiently with the radiation emitted by the dichroic halogen lamp as well as promoting the higher amount of ketone produced in comparison with the FeP/ZnO system.