Effect of catalyst composition and preparation conditions on catalytic properties of unsupported manganese oxides for benzene oxidation with ozone

Catalytic oxidation of benzene in gas phase with ozone was carried out at 70°C over manganese oxides and manganese-based mixed oxides to investigate the effect of catalyst composition and preparation conditions on their catalytic properties. The oxides were prepared by an evaporation-to-dryness method and a co-precipitation method from metal nitrate precursors, followed by calcination at 400-900°C. As for manganese monoxides, benzene oxidation rate normalized by catalyst surface area, product distribution and ozone/benzene decomposition ratio were almost independent of the preparation method and calcination temperature. Perovskite-type mixed oxides, LaMnO₃ and La_0.8Sr_0.2MnO₃ showed much lower activity and lower efficiency for ozone utilization in benzene oxidation than manganese monoxide and La sites promoted the accumulation of less-reactive byproduct compounds on the catalysts. In the case of manganese-based mixed oxides that contained Fe, Co, Ni, and Cu, benzene oxidation activity, CO₂ selectivity and ozone/benzene decomposition ratio depended on the catalyst composition and preparation conditions. Among the mixed oxide catalysts, the Co-Mn mixed oxide prepared by the evaporation-to dryness method and calcined at 400°C was the most effective for benzene oxidation from the standpoint of CO₂ selectivity and efficiency for ozone utilization.