Synthesis Of MnO_x-Cr₂O₃ And Its Catalytic Oxidation Of Benzene

In recent years,the control of volatile organic compounds（V OCs）has become the focus of air pollution control.VOCs are one of the main factors that cause haze weather,and they have great harm to human body.When the human body inhales VOCs,it will damage the nervous system,hematopoietic organs,liver and bone,and will cause nervous system obstacle,liver obstacle and even cancer.The degradation of VOCs by catalytic oxidation is one of the most effective methods at present.In the aspect of catalyst selection,manganese oxide is rich in structure and oxidation state,and has outstanding redox performance,but poor thermal stability.Chromium oxide has strong thermal stability and good antitoxicity to organic oxidation,but its catalytic activity is low.At the same time,the metal oxides prepared by metal organic framework（MOF）pyrolysis show unique physical and chemical properties,showing great advantages in active sites and specific surface area.Therefore,this paper will explore the physical and chemical properties of Cr₂O₃ prepared from MIL-101（Cr）,in order to improve its catalytic activity,and combine it with MnO_x to complement each other.Finally,SiO₂ is doped to further protect the concentration of high valent chromium species in the catalyst,improve the catalytic activity of the catalyst and reduce the leakage of Cr.The catalytic efficiency of the composite catalyst for VOCs（benzene as an example）is investigated,and the structure-activity relationship of the catalyst is explored.The main achievements are as follows:（1）Preparation of Cr₂O₃ from MIL-101（Cr）and its catalytic performance for benzeneUsing chromium nitrate,terephthalic acid and hydrofluoric acid as raw materials,MIL-101（Cr）is prepared by hydrothermal method,and then CraO3 is obtained by pyrolysis method.The effect of HF content on the specific surface area of MIL-101（Cr）is investigated.The results show that the sample with the highest specific surface area（3447 m2·g-1）can be prepared by adding 125 μL of 40%HF in the preparation of MIL-101（Cr）,which is helpful to the dispersion of the load in the subsequent preparation.Cr₂O₃ obtained from pyrolysis of MIL-101（Cr）has higher specific surface area（89.3 m2·g-1）,pore volume（0.549 cm3·g-1）,high lattice oxygen concentration on surface.The catalytic performance of the catalyst for 1000 ppm benzene（T90%＝280℃,SV=60000 mL·g-1·h-1）is much better than commercial Cr₂O₃,which can be used as excellent carrier（2）Preparation of MnO_x-Cr₂O₃ catalyst and its catalytic performance for benzene Using MIL-101（Cr）,potassium permanganate and hydrogen peroxide as raw materials,MnO_x-Cr₂O₃ is prepared by template method,first loading MnO_x on MIL-101（Cr）,and then pyrolysis.The effects of pyrolysis temperature and loading amount of MnO_x on the performance of the catalyst are investigated.The results show that the skeleton structure of MIL-101（Cr）is not damaged during the process of loading MnO,.MnO_x load is distributed well in the pore of MIL-101（Cr）.The results of the comparison of the performances of the catalysts prepared at different pyrolysis temperatures show that 500℃ is more suitable than 400℃.The specific surface area and pore volume of MnO_x-Cr₂O₃ are large,and have abundant oxygen vacancy,good oxidation and oxygen mobility,and significant catalytic effect.The catalyst with a load of 20%MnO_x has higher surface lattice oxygen concentration,and the best catalytic activity is T90%=265℃,1000 ppm benzene,and SV=60000 mL·g-1·h-1.The catalyst with 5%load has higher Mn3+concentration to improve the catalytic performance.The catalytic activity is close to 20%MnO_x-Cr₂O₃.At the same time,the performance of the catalyst with low benzene concentration and high mass space velocity is investigated.The results show that the loading amount of MnO_xhas no significant effect on the catalytic performance of the catalyst in a certain range at low benzene concentration,but it is still better than Cr₂O₃.（3）Preparation of MnO_x-SiO₂-Cr₂O₃ catalyst and its catalytic performance for benzeneSiO₂ is prepared on MIL-101（Cr）by sol-gel method,and then MnO,is loaded Finally,MnO_x-SiO₂-Cr₂O₃ is prepared by pyrolysis at 500℃.The presence of SiO₂ reduces the surface lattice oxygen concentration of the catalyst,and the catalytic activity of SiO₂-Cr₂O₃ is not ideal.After the MnO_x loading,SiO₂ protects Mn3+from over oUidation during pyrolysis,which increases the concentration of Mn3+ in the catalyst with 5%MnO_x load,and help the skeleton not shrink too much during the pyrolysis of MIL-101（Cr）,and increases the specific surface area of the catalyst.At the same time,the addition of SiO₂ makes the existence of high valence chromium species more stable and enhances the oxidation of catalyst.The catalytic activity of 5%MnO_x-SiO₂-Cr₂O₃ is significantly increased at 1000 ppm benzene concentration and SV=60000 mL·g-1·h-1,T90%=256 ℃.However,excessive MnO_x will make Cr₂O₃ surface covered by a large amount of MnO_x,the original structure of the catalyst is destroyed,and the catalytic activity of MnO_x-SiO₂-Cr₂O₃ with 20%load decreases.