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

Abstract

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 (OH ) are the dominant reactive species formed from H 2O 2 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 OH . However, significant differences were found in the kinetic deuterium isotope effect (KDIE) for methanol oxidation between the heterogeneous Fenton-like reaction and genuine OH 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 OH 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 OH . In addition, Fe-zeolites catalyzed the epoxidation of cyclohexene by H 2O 2. 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.