Indications of the reactive species in a heterogeneous Fenton-like reaction using Fe-containing zeolites Original Research Article

Heterogeneous Fenton-like reactions have potential applications in wastewater treatment. Recently, the use of zeolites containing iron species as catalysts in Fenton-like processes has gained increasing interest. While it has been assumed in the literature that hydroxyl radicals (OHradical dot) are the dominant reactive species formed from H2O2 in the presence of Fe-zeolites, no direct evidence for this hypothesis exists so far. This work aims at the elucidation of the type of reactive species involved in the heterogeneous Fenton reaction using the Fe-containing zeolites Fe-ZSM5 and Fe-Beta. Electron paramagnetic resonance (EPR) measurements using dimethyl pyrroline-N-oxide (DMPO) as spin trap were carried out; the formation of the complex DMPO-OH was observed, which is often ascribed to a reaction with OHradical dot. However, significant differences were found in the kinetic deuterium isotope effect (KDIE) for methanol oxidation between the heterogeneous Fenton-like reaction and genuine OHradical dot reactions initiated by the homogeneous Fenton reaction. The KDIE for methanol oxidation in the heterogeneous reaction increased from 1.33 to 1.41 with increasing concentration of the OHradical dot scavenger bicarbonate, whereas the KDIE in the classical Fenton reaction is 1.07 and unaffected by the presence of quenchers. The significantly different values are a clear indication for the participation of additional oxidants beside OHradical dot. In addition, Fe-zeolites catalyzed the epoxidation of cyclohexene by H2O2. This type of reaction suggests the participation of a high-valent oxoiron complex (ferryl) in the wet peroxide oxidation of organic compounds catalyzed by Fe-zeolites.