Investigation of the generation of hydroxyl radicals and their oxidative role in the presence of heterogeneous copper catalysts

The presence and impact of hydroxyl radicals generated via the catalytic decomposition of H2O2 over heterogeneous copper catalysts were investigated by using two detection methods, an electron spin resonance-spin trapping method and a chemical probe method. Detection of the (5,5-dimethyl-1-pyrroline-N-oxide)–OH adduct signal and formation of 4-chlorocatechol during the oxidation of a 4-chlorophenol substrate demonstrated that the three heterogeneous copper catalysts employed here (CuO, Cu/Al2O3 and CuO · ZnO/Al2O3) were capable of generating hydroxyl radicals in combination with H2O2. The oxidative mechanism of the hydroxyl radical in the presence of heterogeneous copper catalysts is discussed with regard to the further oxidation of the (5,5-dimethyl-1-pyrroline-N-oxide)–OH adduct and hydroxylated products of 4-chlorophenol oxidation. Interestingly, integration of the 5,5-dimethyl-1-pyrroline-N-oxide–OH adduct signal could not be used to reliably measure the total amount of hydroxyl radicals generated as a result of oxidative attack on the adduct. This may be as a result of locally higher hydroxyl radical concentrations in the presence of a heterogeneous catalyst leading to further unwanted oxidation of the (5,5-dimethyl-1-pyrroline-N-oxide)–OH.