Preparation And Performance Of Hierarchical RuCe/ZSM-5 Catalyst For Catalytic Oxidation Of Chlorobenzene

Volatile organic compound are widely used in petrochemical,transportation,medicine,printing and other industries.They are also important intermediates in the chemical process.Among them,chlorinated volatile organic compounds（CVOCs）are usually discharged into the environment in the form of waste gas,which brings harm to the natural environment and human society.CVOCs not only has large industrial emissions,but also has the characteristics of strong biological toxicity and difficult degradation in the air.Therefore,it has become one of the difficult VOCs to deal with.With the increasing attention of the state to environmental problems and the increasingly strict corresponding atmospheric control standards,the treatment of CVOCs has become an important link of environmental treatment.Catalytic oxidation has the characteristics of wide application range,low ignition temperature,good catalytic effect and no secondary pollution.It is considered to be an efficient and economical CVOCs treatment technology.The core of catalytic oxidation method is catalyst,which can oxidize and decompose CVOCs into non-toxic or less-toxic end products such as CO₂,H₂O,Cl₂and HCl.However,traditional industrial catalysts are prone to chlorine poisoning when treating CVOCs,resulting in catalyst deactivation.Although a large number of studies have been carried out on CVOCs catalytic oxidation materials at home and abroad,the effect of catalyst pore size on CVOCs catalytic oxidation is relatively small.In addition,the mechanism of pore size in multistage pores affecting the distribution and formation of by-products in CVOCs catalytic oxidation needs to be explored.In this paper,a series of hierarchical Ru Ce/ZSM-5 catalysts were prepared,and the pore size of the catalyst was improved and optimized to improve the activity,selectivity and chlorine resistance of the catalyst for the catalytic oxidation of chlorobenzene.Firstly,hierarchical ZSM-5 was prepared by in-situ hydrothermal crystallization with tetraethyl silicate as silicon source and sodium metaaluminate as aluminum source.Ruthenium and cerium oxides were loaded on the hierarchical ZSM-5 as carrier.The effects of ruthenium and cerium oxide loading,calcination temperature and carrier silicon aluminum ratio on the catalytic oxidation of chlorobenzene were investigated.The results showed that the loading of ruthenium was 0.8 wt.%,the loading of cerium was 10wt.%and the calcination temperature was 350℃,The theoretical molar ratio of the dispersion is 1Si O₂:0.02Al₂O₃:0.15TPAOH:0.04SA:40H₂O,that is,the prepared multi-stage porous Ru Ce/ZSM-5 has the best catalytic activity for the oxidation of chlorobenzene.When the reaction space velocity is10000 h^-1and the concentration of chlorobenzene is 500 ppm,the conversion of chlorobenzene reaches 90%at 247℃.Secondly,a series of hierarchical RuCe/ZSM-5 were prepared by adjusting the ratio of sodium alginate to silica.The effect of hierarchical pore on its catalytic oxidation of chlorobenzene was investigated.The results showed that when the molar ratio of silica to mesoporous template was 0.04 and the amount of sodium alginate was 0.41g（the ratio of dispersion was 1Si O₂:0.02Al₂O₃:0.15TPAOH:0.04SA:40H₂O）,the catalyst had the best catalytic oxidation activity and mineralization rate of chlorobenzene,When the space velocity is 10000 h^-1,the concentration of chlorobenzene is 500 ppm,the conversion of chlorobenzene reaches 90%at 247℃.At the same time,in the 48h chlorine resistance experiment with reaction temperature of 325℃,space velocity of 10000 h^-1and chlorobenzene concentration of 500 ppm,the hierarchical Ru Ce/ZSM-5 catalyst showed superior chlorine resistance,and maintained 100%chlorobenzene conversion and 86.7%mineralization during the experiment.Finally,through N₂adsorption desorption,X-ray photoelectron spectroscopy,O₂temperature programmed desorption and other characterization methods,it is concluded that when 1Si O₂:0.02Al₂O₃:0.15TPAOH:0.04SA:40H₂O,the hierarchical Ru Ce/ZSM-5 catalyst has a certain micro mesoporous structure,its microporous pore size is concentrated at 0.7 nm,and the mesoporous pore size is distributed at 2.3 nm and 16.2 nm.Compared with the traditional microporous ZSM-5 catalyst,the expanded micropores and centralized and unified mesopores make the Ru O_xand Ce O_xparticles more dispersed on the surface of the ZSM-5,and expose higher concentrations of Ce⁴⁺,so as to improve the activity and mineralization of catalytic oxidation of chlorobenzene.Moreover,it can be seen from NH₃temperature programmed desorption and pyridine infrared spectroscopy that the hierarchical structure of the catalyst does not affect the distribution of Br（?）nsted acid sites of the catalyst,but increases the density of Lewis acid sites,so as to improve the oxidation capacity of the catalyst.At the same time,the hierarchical structure reduces the diffusion resistance of chlorobenzene in the catalyst.The HCl removed at the Br（?）nsted acid site and the Cl₂removed at the Lewis acid site can quickly diffuse into the gas phase,so as to inhibit the chlorine poisoning the active component and improve the anti chlorine toxicity of the catalyst.