Effectiveness of toluene mineralization by gas-phase oxidation over Co(II)/SiO₂ catalyst with ozone

The results of experimental study on effectiveness of gas-phase total oxidation of toluene towards carbon dioxide and water with the aid of ozone over Co(II)/SiO₂ catalyst are presented in this work. The main objective of the work was to determine ozone demand necessary for total mineralization of toluene at the temperature range of 40-100°C chosen to minimize catalyst poisoning by water. Complete mineralization of toluene was possible if sufficient ozone/toluene ratio was maintained in the gas supplied to the reactor. For ozone/toluene molar ratios less than 20 the extent of toluene mineralization increased with temperature up to a plateau starting at approximately 60°C, which was caused by ozone shortage. Stoichiometry of the total oxidation of toluene with ozone indicates that only one oxygen atom in the ozone molecule is used for the oxidation of toluene, to achieve complete mineralization. Experimentally determined ozone/toluene ratio (20-25) necessary for the total oxidation of toluene was larger than the theoretical one mostly due to ozone losses resulting from its 'unproductive' decomposition. At the range of lower values of mineralization rate, the toluene oxidation proceeds according to a more efficient mechanism, indicating less ozone demand being between 6 and 18 moles of ozone per mole of toluene. A possible mechanism of toluene oxidation was suggested. The mechanism involves the formation of •OH radicals, which may explain the effectiveness of Co(II)/SiO₂ catalyst in combination with ozone for the oxidation of toluene and other aromatic VOCs in a low-temperature process.