The Study Of VOCs Adsorbed On HZSM-5 Zeolite And Oxidized By Palladium-based Catalysts

VOCs?Volatile Organic Compounds?containing exhausted gases are one of the main sources of atmospheric pollution.In industrial VOCs treatment technology,a combination of zeolite runner adsorption concentration technology and regenerative catalytic combustion technology?RCO?is often used.The exhaust gas passes through the zeolite molecular sieve runner for adsorption-desorption-concentration continuous process,and the concentrated exhaust gas enters the regenerative catalytic combustion device for purification.During this process,the excess heat released by the combustion of organic matter is effectively utilized,and power consumption is saved,thus the running cost is reduced.In the actual industrial production process,the water content of the exhaust gas is high,and the water and the VOCs form competitive adsorption,occupying the adsorption space of the runner to reduce the removal efficiency of VOCs.Investigating the factors affecting hydrophobicity and improving the water resistance of adsorbents are the hotspots of current research.The core of RCO is a catalyst for efficient combustion.The stability and low temperature activity of the catalyst are the key tasks for catalytic combustion of VOCs at this stage.In terms of adsorption,the relationship between silica-alumina ratio and its water resistance based on commercial HZSM-5 molecular sieve was studied in this paper.The organic gases of several different boiling points:m-xylene,n-hexane,acetone,ethanol and ethyl acetate were used to explore the adsorption law and water resistance of HZSM-5 molecular sieve,which provided reliable basis for the designing of industrial adsorption process.In terms of catalytic combustion,in this paper,the stability and low temperature activity characteristics of typical VOCs catalytic oxidation reactions were studied based on Pd-based catalysts which process high hypothermia activity and good heat resistance.The specific research contents are as follows:In this paper,the physical and chemical structures of commercial powder molecular sieves with different silicon-aluminum ratio HZSM-5 molecular sieves?23,42,80,280,882,respectively?,were investigated.The hydrophobic properties of the catalysts and their adsorption properties for acetone were also evaluated.In the study of the dynamic adsorption law of organic gases,it is found that the shape of the breakthrough curve is basically the same when the inlet concentration and flow rate were fixed.There is no significant difference in the adsorption of the sa me component VOC between silicon and aluminum.However,when the water vapor is increased,it is found that the adsorption performance is optimal when the ratio of silicon to aluminum is 280.The reason is that when the Si⁴⁺in the molecular sieve framework is replaced by Al³⁺,the skeleton is negatively charged.In order to balance the negative charge of the Al-O tetrahedron,there must be a monovalent or multivalent cation compensation in the skeleton.These cations become polar spots and have stronger adsorption effect on polar water molecules.In moisture atmosphere,the ratio of silicon to aluminum should be increased,to enhance the hydrophobicity of the molecular sieve,thus making it more favorable for the adsorption of VOCs.HZSM-5?280?molecular sieve-coated honeycomb ceramic adsorbent was prepared by coating slurry technique on HZSM-5?280?molecular sieve with excellent water resistance.The effects of different coating amounts and VOCs?acetone,n-hexane,ethyl acetate and ethanol?on the adsorption performance were investigated.It is found that the greater the polarity of the organic matter,the greater the amount of saturated adsorption.As the coating amount increases,the penetration time increases and the penetration capacity decreases.The adsorption of VOCs can only temporarily alleviate the pollution of VOCs.In order to thoroughly purify VOCs,the design and research of deep catalytic oxidation reaction were carried out in this paper.The Pd/Al₂O₃ catalysts were prepared by complexation-solvent thermal method,hydrothermal method and impregnation method.The effect of different preparation methods on the performance of the catalyst was evaluated by using m-xylene as the representative of VOCs.The results showed that the most effective catalyst Pd/Al₂O₃-com was prepared by complexingsolvothermal.X-ray photoelectron spectroscopy?XPS?was performed to characterize the physico-chemical properties of synthesized Pd/Al₂O₃ catalysts.The results indicated that the Pd element of Pd/Al₂O₃-com was mainly dispersed on the surface of the support at the reduction state Pd⁰,while the Pd element of Pd/Al₂O₃-imp and Pd/Al₂O₃-hyd catalyst was at the form of Pd²⁺with poor dispersion.Combined with the catalytic activity performance and evaluation results,a highly dispersed and reduction state form of Pd active component on the surface of the carrier resulted in a remarkable catalytic activity on m-xylene conversion.The Pd/Al₂O₃-com catalyst with these features exhibit good activity for removing VOCs,thus meet well the requirements of industrial application.In order to investigate the catalytic difference between Pd-based catalysts for hydrocarbon organics and oxygenated organic compounds,the performance of Pd/Al₂O₃-com is compared by catalytic oxidation of one-component VOC?m-xylene,n-hexane,c-hexane,acetone,ethanol,ethyl acetate?.From the molecular configuration,the Pd-based catalyst has better activity for aromatic hydrocarbon VOCs with a large?bond.The c-hexane of the cyclic structure is more easily activated than the chain-like n-hexane.For oxygenated organics,low molecular weight alcohols are most susceptible to oxidation,followed by ketones and esters.This law is not necessarily re lated to the polarity of organic molecules and the weakest C-H.The paper has 30 figures,12 tables,and 125 reference articles.