Wet oxidation of phenol by hydrogen peroxide: the key role of pH on the catalytic behaviour of Fe-ZSM-5

Abstract

This paper presents an original approach to the treatment of phenolic aqueous wastes using H2O2 with Fe-ZSM-5 as a heterogeneous catalyst. The Fe-ZSM-5 zeolite with MFI structure allows a total elimination of phenol and a significant total organic carbon (TOC) removal at 90°C and atmospheric pressure in a batch reactor. The studies with this Fe-ZSM-5 catalyst deal mainly with the influence of external diffusion and pH on the oxidation process. It appears that the phenol degradation and the toxicity reduction, on the one hand, and the leaching-off of Fe(III) ions in the solution, on the other hand, depend strongly on the pH. In these conditions, an optimum value exists at pH = 5. At higher pH values, the system becomes less effective. At lower values, a significant leaching-off will induce homogeneous catalysis and the degradation of the catalyst. Generally speaking, when this heterogeneous catalyst is used in the presence of H2O2, the reaction rates depend on the degree of hydroxylation of the aromatic compound in contact with the catalyst. So, a chain radical mechanism could be envisaged.