Catalytic wet oxidation of p-coumaric acid: Partial oxidation intermediates, reaction pathways and catalyst leaching

The catalytic oxidation of p-coumaric acid, a compound representative of the polyphenolic fraction typically found in olive processing and wine-distillery wastewaters, has been investigated using various homogeneous and heterogeneous catalysts. Experiments have been performed with homogeneous Fe2+, Cu2+, Zn2+ and Co2+ ions at pH = 1, and with metal oxide catalysts in suspension at pH 3.5, 7 and 12. Additional uncatalyzed experiments have been performed and the results are compared to those of the catalyzed runs. The temperature was 403 K and the oxygen partial pressure was 2.8 MPa in all runs. The distribution of the reaction intermediates was determined, using HPLC and GCMS as the main analytical techniques, and reaction pathways are speculated. It was found that the use of catalysts could increase the rate of destruction of p-coumaric acid compared to the uncatalyzed reaction, while the distribution of the intermediate compounds was strongly dependent on the pH of the solution. A CuO . ZnO-Al2O3 heterogeneous catalyst was found to be effective for the oxidation of p-coumaric acid although leaching of dissolved metals to the solution was found to occur. The stability of the heterogeneous catalysts was investigated by measuring the extent of metal leaching into the solution. The results are discussed with respect to the impact of various conditions (catalyst, pH) on the oxidation of p-coumaric acid and compared to those of the uncatalyzed reaction, studied in previous work.