Oxydation des phenols par le peroxyde d'hydrogene en milieu aqueux en presence de fer supporte sur alumine

The aim of this work was to study, in order to specify its reactional mechanism, the oxidation by hydrogen peroxide of phenols in an aqueous medium in the presence of various heterogeneous catalysts and in particular alumina supported iron. In previous works we studied the action of the Fenton reactant (H2O2 + Fe2+) on phenols and on organic acids and compared this system to the catalytic oxidation on a solid catalyst (iron/alumina, iron-copper/alumina) by hydrogen peroxide. The structure of the supported metal was determined by Mossbauer Spectroscopy thanks to which we were able to establish on the one hand the relationships between the structure and the mode of preparation of the catalyst and on the other the modifications of the catalyst surfaces after an oxidation reaction of organic compounds in an aqueous medium. The results of this work showed that the catalytic oxidation of phenol is very weak. It depends on the mode of preparation of the catalyst and the nature of the supported metals, the mode of thermic treatment of the catalyst and the reaction temperature and, above all, on the presence of polyhydroxybenzenes at the beginning of the reaction. Contrarily to phenol, polyhydroxybenzenes are readily degraded in heterogeneous catalysis by hydrogen peroxide. The reaction rate is a function on the one hand of the catalytic properties and on the other of the reaction conditions (pH, temperature, presence of bicarbonates in the solution …). In general the reaction rate in heterogeneous catalysis always seems to be a function of the degree of hydroxylation of the organic compounds in contact with the catalyst in the presence of H2O2. Two types of oxidation mechanisms in heterogeneous catalysis can be envisaged: • -radical mechanism: the radicals formed by the decomposition of H2O2 on the active sites of the catalyst react with the organic compounds; • -non-radical mechanism (Hamilton's reaction). The radical pathway is the result of reaction of hydroxyl radicals with organic compounds. The resulting by-products of this first attack (mainly carbonyl compounds) are subjected to a second radical oxidation or play the part of reductor of oxidized sites. As for the non-radical mechanism, many pathways can occur and among them are the: • -reaction between oxidizing entities and adsorbed organic molecules on the catalyst area; • -oxidation reaction by an oxidative complex of pyrocatecol or hydroquinone (Hamilton reaction); • -formation of inorganic peroxo-compounds adsorbed to the catalyst area. The proposed reactional scheme allows us to explain the whole of the obtained results and particularly the difference in the catalytic activity between phenol and dihydroxybenzene.