Wet air oxidation of aqueous solutions of maleic acid over Ru/CeO2 catalysts

The catalytic batch wet air oxidation of maleic acid over 5 wt.% Ru/CeO2 catalysts was investigated at temperatures of 160 and 200°C and an oxygen partial pressure of 20 bar. Additional uncatalysed experiments were carried out and the results are compared to those of the catalytic runs. The concentration profiles of liquid phase reaction intermediates were followed by means of high-performance liquid chromatography, while the concentration of carbon dioxide formed due to total oxidation reactions was followed by means of gas chromatography. It was found that, at the conditions under consideration, maleic acid is highly reactive and susceptible to both thermal and oxidative degradation. Catalysts were capable of substantially increasing the rates of the corresponding uncatalysed reactions with respect to both maleic acid and total organic carbon removal. In this respect, a high surface area (125 m2 g−1) ruthenium supported on ceria catalyst was found to be more effective than a low surface area (38 m2 g−1) catalyst. Maleic acid is converted through a series of reactions to form lower molecular mass acids which eventually decompose to carbon dioxide. Of these acids, acetic acid was the major compound formed and was relatively refractory at the conditions under consideration. An experimentally derived reaction network is proposed for the oxidation of maleic acid. However, complementary information provided by a computational approach capable of predicting possible reaction pathways was employed and found useful in refining the experimental reaction network.