Experimental Research On VOCs Adsorption And Catalytic Oxidation Based On Modified Molecular Sieve

Various types of volatile organic pollutants emitted from industrial sources are generally present in the air,which poses a serious threat to the atmospheric environment and human health.It is of great significance to develop a practical and efficient volatile organics prevention and control system.At present,there are a variety of methods for treating volatile organic compounds,but there are still many limitations.On the one hand,the exhaust gas emitted by industrial sources has a high humidity,but most researches on volatile organic compounds do not consider the impact of humidity on the prevention and control efficiency.On the other hand,some methods of treating volatile organic compounds have the problems of waste of resources and easy to cause secondary pollution in practical applications,and further optimization is still necessary.Through the hydrophobic modification and catalytic modification of molecular sieves,this thesis developed hydrophobic adsorbents and microwave-catalyzed oxidation catalysts used in humid environments respectively,and the adsorption performance and catalytic oxidation performance of toluene in dry and humid airflow were tested.First of all,this thesis used the preferred commercial NaY molecular sieve as the matrix to perform hydrothermal dealumination,acid treatment and surface silanization modification,and characterized it by XRD,FTIR,N2 adsorption and desorption and SEM to improve the framework Si/Al and change the surface functional groups to enhance hydrophobicity and adsorption capacity of the molecular sieve in humid environment.Subsequently,the modified hydrophobic molecular sieve was mixed with commercial hydrophobic activated carbon to improve the adsorption capacity of the adsorbents bed and at the same time enhance the regeneration efficiency under microwave heating.Finally,a commercial USY molecular sieve was used as a carrier to carry Cu-Mn-Ce composite oxides for catalytic modification Compared with the catalysis under traditional electric heating conditions,the microwave catalytic oxidation of toluene in dry and humid environments was carried out,exploring the advantages of microwave catalytic oxidation technologyThe results showed that before the hydrophobic modification,the saturated adsorption capacity of NaY for toluene dropped from 111 mg/g in a dry environment to 8 mg/g due to the competitive adsorption of water molecules and VOC molecules.After hydrothermal dealumination,acid treatment and surface silanization modification,the adsorption capacity of the molecular sieve in wet condition was increased to 38 mg/g,41 mg/g and 46 mg/g,respectively,and the competitive adsorption phenomenon was significantly improved.Through a series of characterization methods,it was found that hydrothermal dealumination caused unit cell shrinkage and framework collapse,and the specific surface area and pore volume were drastically reduced;subsequent acid treatment with HCl effectively cleaned the residual aluminum ions in the pores,and restored the adsorption site to a certain extent.The silane modification with trimethylchlorosilane(TMCS)successfully grafted the hydrophobic methyl group(-CH3)to the surface of the molecular sieve to form a layer of nano-level hydrophobic structure,analyzing the influence of modification on the structure of molecular sieveSubsequently,a microwave regeneration test was performed using the hydrophobic molecular sieve prepared above,and it was found that after the adsorption saturation was completed at 50%relative humidity,the efficiency of desorption by microwave heating was higher than that in dry condition.After 30 minutes of microwave desorption,the adsorption capacity of the molecular sieve in the wet state remained basically unchanged during the five adsorption cycles.In addition,after mixing commercial hydrophobic activated carbon with the hydrophobic molecular sieve,it can have better dynamic adsorption and microwave desorption performance,and its adsorption capacity under dry and wet conditions can be increased to 186 mg/g and 95 mg/g,respectively.After 3 cycles of microwave desorption,the mixed adsorbents can maintain almost the same adsorption capacity,showing good reusability.Finally,the commercial USY molecular sieve was catalytically modified by supporting Cu,Mn and Ce oxides.The prepared Cu-Mn-Ce-1-1-1/USY catalyst was first tested through the conventional catalytic oxidation.The corresponding temperature of T90 in the dry condition is 298.8?,but due to the competitive adsorption of water molecules and toluene molecules,the T90 in the wet condition rises to 327.5?.When heated at 800W,500W and 200W microwave power,the final temperature of the catalyst is 213?,188? and 159? respectively.The microwave catalytic oxidation efficiency in dry condition at the corresponding temperature is 65%,54%and 36%,which is significantly higher than that of electric heating.In addition,by adding a stable absorbing medium SiC,the temperature of the mixed bed was increased to 251?,238? and 226?,and the corresponding toluene conversion rate in dry condition was 93%,94%and 94%respectively.Although water vapor still inhibits the performance of microwave catalytic oxidation,this technology achieves higher catalytic oxidation efficiency of VOCs under the condition of lower electric heating temperature,and has application prospects in the degradation of volatile organic pollutants in low-humidity industrial environments.