Thermal Catalytic Oxidation Of Toluene Over Sepiolite Supported Manganese Oxides

Volatile organic compounds?VOCs?are one of the main atmospheric pollutants,as well as important precursors of PM_2.5 and ozone,which are very harmful to the atmospheric environment and human health.In recent years,significant progress has been made in the control of SO₂ and nitrogen oxides in China,but VOCs emissions are still increasing,so the management of VOCs is imminent.Thermal catalytic oxidation technology is one of the most economical and effective methods for treating VOCs pollution due to its good treatment effect and no secondary pollution to the environment.The key issue is to develop a catalyst with low catalytic reaction temperature,high activity and good stability.Manganese oxide is a very promising catalyst for degrading VOCs due to its variable valence state,good oxygen storage capacity,oxygen mobility and good catalytic performance.However,for simple manganese oxide,the lower specific surface area and poorer thermal stability limit its catalytic activity.So it is necessary to support it on a carrier with a suitable specific surface area to improve its thermal stability and active site dispersibility,thereby improving its catalytic activity.As a typical VOCs,toluene accounts for the largest proportion of industrial VOCs emissions.In this paper,a supported manganese oxide catalyst was prepared using Xiangtan's characteristic mineral sepiolite as a carrier,and industrially discharged toluene was the target VOCs pollutant.The relationship between catalyst structure,physical and chemical properties and its catalytic oxidation of toluene.The main results of the research are as follows:?1?Study on catalyst preparation conditions.The sepiolite-supported manganese oxide catalyst was prepared by impregnation method and co-precipitation method using acid-modified sepiolite as the carrier,and the effect of different preparation methods on the catalytic performance was investigated.The results show that the CP-Mn/Sepiolite catalyst prepared by the co-precipitation method has a higher specific surface area and better activity for thermal catalytic oxidation of toluene.On the basis of the above,co-precipitation method was used to prepare sepiolite supported cobalt-manganese,copper-manganese and nickel-manganese composite oxide catalysts respectively,and the effect of doping different metals on the catalytic performance of supported manganese oxide was investigated.We found that the synergistic effect between cobalt and manganese oxide is the best.In order to investigate the effect of different calcination temperatures on catalysts,a series of sepiolite-loaded cobalt-manganese composite oxides for thermal catalytic oxidation of toluene were prepared by co-precipitation under different calcination temperatures?350?,450?,550?,and 650??The results show that the catalyst calcined at550?has the best catalytic activity.?2?Study on the synergistic effect of cobalt-manganese composite components.A series of sepiolite-supported cobalt-manganese composite oxide catalysts(Co_xMn_100-xO_y/Sepiolite)were prepared under optimal preparation conditions,where x=3,10,15 and 20.The results show that different cobalt doping levels have an effect on the morphology,structure,reducing ability and surface species composition of cobalt-manganese composite oxides.When the cobalt doping amount is 15%,the Co₁₅Mn₈₅O_y/Sepiolite catalyst has the best activity for thermal oxidation of toluene(T₅₀=160?,T₉₀=193?).Analysis of SEM,TEM,XRD,XPS,Raman and H₂-TPR shows that the doping of cobalt changes the lattice of manganese oxide,and as the content of cobalt increases,the content of Mn⁴⁺gradually increases.And high valence state Mn species are induced by synergism effects of Co???-O octahedrite transferred from Co???-O tetrahedron and re-dox ability between Co³⁺and Mn²⁺.Therefore,cobalt doping can increase the content of Mn⁴⁺and accelerate the generation of oxygen vacancies,thereby inducing the transfer of surface lattice oxygen from the bulk phase to the surface.The redox cycle of Mn⁴⁺/Mn³⁺allows continuous transfer of lattice oxygen and improves the catalytic activity of the catalyst.In addition,Co_xMn_100-xO_y/Sepiolite catalyst also has a high reutilization rate and stable performance.?3?Catalyst kinetics research.Using the most active Co₁₅Mn₈₅O_y/Sepiolite as catalyst,the kinetic behavior of catalytic oxidation of toluene was carried out.The results show that the MVK kinetic model is suitable for the kinetic process of toluene catalyzed by Co₁₅Mn₈₅O_y/Sepiolite catalyst.The conversion rate and reaction rate of toluene catalyzed by the catalyst both increased with the increase of temperature,and the reduction activation energy of the catalyst surface was calculated to be 25.63k J/mol and the surface oxidation activation energy was 27.39 k J/mol.