Study On Catalytic Oxidation Of Toluene With Ozone Over δ-MnO₂ Molecular Sieve

At present,the serious impacts of volatile organic compounds（VOCs）on people’s health and production and life have aroused widespread concern.The management of VOCs has become an important part of air pollution control.Catalytic ozonation is widely used in the removal of VOCs due to its low reaction temperature and low energy consumption.It is a promising method for removing VOCs.Toluene is a typical pollutant in industrial VOCs,selected as the target pollutant for this study.In this paper,a series ofδ-MnO₂/molecular sieve catalysts were prepared by hydrothermal method.The toluene adsorption amount,catalytic ozone oxidation toluene removal rate,ozone degradation rate and mineralization rate were used to investigate the optimal activity carrier and optimal.Screening of the loading and the catalyst XRD,SEM and BET characterization were carried out.The results show that the addition ofδ-MnO₂can significantly improve the activity of molecular sieves for the ozonation of toluene.The optimal catalyst is 3.0%δ-MnO₂/USY catalyst.The larger specific surface area,increased pore volume,special surface morphology,and active components of a particular crystalline form are the reasons for the strongest toluene treatment capacity of the medium loading catalyst.At room temperature,an increase in the concentration ratio of ozone to toluene is beneficial to the removal of toluene,but too high a concentration of ozone causes a decrease in the ability of the catalyst to decompose ozone.The addition of water vapor is not conducive to the adsorption of toluene,but is advantageous for the mineralization of toluene.When the initial concentration of toluene and the space velocity increase,the conversion of toluene is lowered.Considering the optimal conditions for the catalyst,the ratio of toluene to ozone concentration is 1:7,the relative humidity is 70%,the initial concentration of toluene is 20 ppm,and the space velocity is 5000 h^-1.At this time,the toluene removal rate reaches 97.3%,and the ozone degradation rate 100%,the mineralization rate reached 28.0%.After the catalyst was subjected to in-situ regeneration of ozone for 1 h,the toluene removal rate of the catalyst was maintained at 80%or more after 5 cycles.When formaldehyde is selected as the target pollutant of another VOCs,the active formaldehyde is more easily mineralized under the same conditions than toluene.Combined with in-situ infrared and GC-MS results,under oxygen conditions,toluene relied on oxygen and catalytic activity on the catalyst to oxidize to phenol,benzyl alcohol,benzoic acid,but neither the activity test nor the in situ infrared results the generation of CO₂ were observed.Under ozone conditions,toluene catalyzed ozone oxidation on the catalyst to phenol,benzyl alcohol,benzaldehyde,benzoic acid,benzyl formate,benzyl acetate and benzyl benzoate,accompanied by the formation of CO₂.The catalyst was subjected to in-situ regeneration of ozone,and a richer intermediate product was observed,and it was found that benzoic acid was difficult to be degraded.Based on the above experimental results,the mechanisms of oxidizing toluene byδ-MnO₂/USY were proposed from the viewpoint of chemical bond energy.