Route of the catalytic oxidation of phenol in aqueous phase

Catalytic oxidation of phenol in aqueous phase over a copper catalyst supplied by Engelhard (Cu-0203T) has been studied. A reaction pathway of phenol oxidation under intermediate temperature and pressure has been proposed. Temperatures employed were 140 and 160 degreesC and catalyst concentration ranged from 4 to 1550 g l(-1) of liquid phase. To achieve this wide interval of catalyst concentration values, two experimental set-ups were employed: a basket stirred tank reactor (BSTR), with the liquid phase in batch, and an integral fixed-bed reactor (FBR) with co-current up-flow of gas and liquid phases. The main intermediates detected in the phenol oxidation were ring compounds (hydroquinone, catechol, benzoquinone), which break to yield CO2 and short chain acids, mainly maleic, formic, acetic and oxalic acids, and also traces of malonic, succinic and fumaric acids. Oxalic acid was also found to form a complex with the copper which oxidizes to CO2. The only non-oxidizable intermediate under the conditions sets was acetic acid. In order to propose a phenol oxidation pathway, several runs were carried out where the main intermediates detected in the phenol oxidation were fed to the FBR under different temperatures and catalyst loadings. It was found that catechol oxidation does not yield either benzoquinone or maleic acid but oxalic acid which finally mineralized to CO2. However, benzoquinone and maleic acid are products clearly detected in the hydroquinone oxidation. Oxidation reactions of phenol and those intermediates studied take place not only on the solid surface but also in the liquid phase. (C) 2002 Elsevier Science B.V. All rights reserved.