Study On Catalytic Ozonation Of Tolueneover Mn-based Supported Catalyst

The emission of volatile organic compounds(VOCs)is regarded as one of the major causes of atmospheric pollution.As a typical type of VOCs,toluene is widely used in solvents in the chemical industry and synthetic raw material of organic compounds containing benzene ring.However,the low boiling point of toluene makes it easy to volatilize into the atmosphere during use,which will not only cause environmental degradation but also pose a serious health threat to the organisms in the environment.Catalytic ozonation technology is widely used to degrade VOCs due to its high purification efficiency at room temperature,no secondary pollution,and energy-saving.Therefore,toluene was selected as the degradation object for catalytic ozone oxidation technology research in this study.The key to this research is to prepare the catalyst with high reactivity and strong stability that matches the reaction system.The preparation of Mn-based supported catalysts with g-C3N4 and TiO2 as supports by impregnation method.The type of Mn/CN catalyst with different Mn loadings(2.0%,4.5%,and 5.1%)was prepared at different calcination temperatures(600℃,700℃,and 800℃).Among the type of Mn/CN catalysts,4.5%Mn/CN-700 showed the best catalytic performance;among the type of Mn/TiO2 catalysts with different Mn loadings(1.8%,1.9%,2.1%,and 2.6%),the 2.1%Mn/TiO2 catalyst had the best catalytic performance.The catalytic activity of 2.1%Mn/TiO2 was higher compared to that of 4.5%Mn/CN-700.Therefore,the 2.1%Mn/TiO2 catalyst was used to investigate the effect of reaction conditions(the molar ratio of ozone/toluene,concentration of toluene,and GHSV)on catalytic performance in the experiment of catalytic ozonation of toluene.In the experiment of reaction conditions,it was found that the catalytic activity of the catalyst decreased with GHSV and the concentration of toluene increased.When the molar ratio of ozone/toluene was less than 14:1,the catalytic activity continued to increase with the molar ratio of ozone/toluene increased.When the molar ratio exceeded 14:1,the catalytic activity remained stable which indicated that the molar ratio of ozone/toluene of 14:1 is the most suitable.Under the condition of the ozone/toluene molar ratio of 14:1,choosing a lower toluene concentration and GHSV is beneficial to improve ozone utilization and CO2 selectivity.When the reaction temperature raised to 80℃ toluene and O3 is completely decomposed and the CO2 selectivity reached 100%.When the RH was 60%,the CO2 selectivity reached the highest value(80%)ndicating the moderate addition of moisture stimulated CO2 selectivity.Increasing the reaction temperature to 80℃ or increasing the RH to 60%can greatly improve the catalytic activity.According to the FTIR results,the corresponding path of toluene catalyzed by ozone oxidation is inferred.The activation of O3 and H2O at the active site to generate active free radicals for oxidation is the prerequisite for the start of the reaction.Toluene is gradually oxidized according to the oxidation path of toluene→benzyl alcohol→benzaldehyde→benzoic acid to produce by-product benzoic acid;After the phenyl group in the toluene molecule is separated from the methyl group,the phenyl group is oxidized to produce phenol,which undergoes ring-opening,hydrogenation,and oxidation to produce chain hydrocarbon groups and carboxylic acids.Finally,the product and the methyl group are oxidized to H2O and CO2.