Lower toxicity route in catalytic wet oxidation of phenol at basic pH by using bicarbonate media

Abstract

Catalytic oxidation of phenol in aqueous solution has been studied by using bicarbonate as buffer reagent to keep the pH in the interval 7–8 (pH 8B) with a commercial catalyst based on copper. The oxidation route obtained at pH 8B is different and less toxic than that obtained at acid pH because of the dihydroxybenzenes are not formed at basic pH. To determine the phenol oxidation route several runs were carried out with phenol or the intermediates detected at pH 8B (p-hydroxybenzoic, pyruvic, oxalic, acetic and formic acids, and traces of tetrahydroxy-p-benzoquinone), under a temperature of 140°C and oxygen pressure of 16bar. A basket stirred tank reactor (BSTR) and an up-flow fixed bed reactor (FBR) were used depending on the reactant employed. A radical mechanism on the catalyst surface has been elucidated to explain the oxidation route found. All the intermediates detected at pH 8B are more than one order of magnitude less toxic than phenol and copper was not leached from the catalyst at this pH. Moreover, higher mineralization than that obtained at acidic conditions was found at pH 8B.