La-Eu Cuprates as Catalysts for Phenol Oxidation

Abstract

A set of six La 2− x Eu x CuO 4+δ oxides were prepared, with x ranging from 0 to 1.33. They were characterized by various techniques, including, in addition to chemical analysis, surface area and porosity measurements, XRD, SEM-EPMA, EPR, and electric resistivity measurements. All the oxides proved to be good catalysts for the oxidation of phenol with hydrogen peroxide and, under proper reaction conditions, a high selectivity to p-benzoquinone was obtained with each solid. Different selectivities to hydroquinone or to catechol were also obtained, depending on the value of x. The best catalytic performance was obtained in any case at relatively low temperatures (≤70°C). Increasing the temperature beyond 70°C led only to fast wasting of hydrogen peroxide and to faster oxidation of the products to CO 2, without any advantage on yield to the desired products. A competition exists between the reactants for adsorption on the same surface sites. Phenol is adsorbed very strongly, although it can be displaced rather easily by hydrogen peroxide. The results of the characterization analysis indicate that catalytic activity is very probably connected with the presence of Cu ions forming anisotropic quantum spin fluid (AQSF) or glass (AQSG) structures. When the catalyst becomes inactive, AQSF and AQSG disappear and strongly oxygen-deficient copper clusters form.