The Mechanism Study Of Catalytic Process And Byproduct Selectivity For Chlorobenzene Combustion On Solid Acid Coupled Metal Oxide Catalysts

The effective control of volatile organic compounds(VOCs)emitted from petrochemical and pharmaceutical industries is one of the important ways to improve the quality of the current atmospheric environment.The chlorinated VOCs not only have high emission and toxicity,but also are with high chemical stability,which can stay in the natural world for a long time,yielding a serious impact on the environment and human health.The catalytic combustion technology can convert the chlorinated VOCs into the products such as H2O,CO2,HCl,etc.at low temperatures.It is considered as an efficient,economical and feasible method to remove the chlorinated pollutants.However,there are still many problems in the industrial application of the technology.When the traditional industrial catalyst is used to treat the chlorinated VOCs,the catalyst inclines to suffer chlorine accumulation and carbon deposition,leading to catalyst deactivation.Although tremendous efforts have been devoted to developing advanced catalytic combustion materials for chlorinated VOCs removal,studies on the reaction process,the generation of toxic by-products,and the selectivity of the oxidation are still insufficient.In addition,there are many interference factors in industrial flue gas,such as water vapor,alkali/alkaline earth metal,etc.The effects of these interferon on the catalytic reaction process of chlorinated VOCs oxidation and the mechanism of toxic by-products generation are also lacking systematic research and exploration.In this thesis,the solid acid HZSM-5 was used as a carrier to load the MnxCe1-xO2 oxides on the surface.First,the catalytic activity and the structure-activity relationship of the catalysts in catalytic oxidation of chlorobenzene were investigated.It was found that when the active phase Mn/Ce ratio was at 4:1,the oxidation efficiency of the catalyst was the highest,and the conversion rate of chlorobenzene at 240 ? could reach more than 99%.The solid acid HZSM-5,on the one hand,made the active component highly dispersed on the surface of the catalyst by ion exchange,thus improving the redox ability of the catalyst.On the other hand,the HZSM-5 as a carrier provided a large number of Bronsted acidic sites,so that chlorobenzene can be adsorbed efficiently,thus promoting the low temperature conversion of chlorobenzene.Through the analyses of the products from Mn0.8Ce0.2O2/HZSM-5 and Mn0.8Ce0.2O2 catalysts in the catalytic oxidation of chlorobenzene,it is found that the oxidation has produced a variety of chain polychlorinated byproducts on the surface of Mn0.8Ce0.2O2/HZSM-5 catalyst,such as trichloroethylene(C2HC13),tetrachloroethylene(C2Cl4)and dichlorobenzene(C6H4Cl2),which were attributed to the metal chloride,including MnOClx,CeClx and CeOClx that resulted in chloride reaction with intermediate products.The oxidation path of chlorobenzene on the surface of the Mn0.8Ce0.2O2/HZSM-5 catalyst was mainly through the nucleophilic substitution with the hydroxyl group of the Bronsted acidic sites,then produced the benzoquinone intermediates and gradually cleavage into low carbon chain organic compounds.The intermediate products of Mn0.8Ce0.2O2 catalyst were mainly benzene.It showed that the oxidation path of chlorobenzene was mainly through the dechlorination to produce benzene,which was further oxidized into CO2,H2O and HC1.Through the study of the catalytic oxidation process of Mn0.8Ce0.2O2/HZSM-5 catalyst under a high concentration of water vapor,it was found that the selectivity of the catalyst in such a high humidity environment increased significantly,and the content of polychlorinated organic compounds by-products decreased obviously.The reason was that the existence of water vapor facilitated the dissociated chlorine to desorb in the form of HCl through hydrolysis reaction,thus inhibiting the formation of metal chloride and avoiding the generation of polychlorinated intermediate product.In addition,the high concentration of water vapor also promoted the formation of H5O2+ in the solid acid HZSM-5,which increased the Lewis acidity,and further improved the oxidation efficiency of chlorobenzene over the catalyst.Through the study of the effects of potassium on the physical and chemical properties of Mno.8Ceo.2O2/HZSM-5 catalyst,the catalytic process and the selectivity of chlorobenzene oxidation,it was found that the excessive addition of K+ could neutralize the Breonsted and Lewis acidities,resulting in the deactivation of the catalyst.However,the presence of appropriate content of K+ can promote the immobilization of Cl in the form of KCI,thus inhibiting the reaction of electrophilic chlorination.Because of the hydrophilicity of KOH,the adsorption water could be effectively provided to the catalyst during the reaction process,and the hydrolysis reaction could be promoted with the HCl selectivity increased.The polychlorinated by-products in potassium loaded catalysts decreased significantly,where the dichlorobenzene and other chlorinated by-products were not detected in the gas phase and on the catalyst surface.