Study On Catalytic Decomposition Of HZSM-5 To Methyl Mercaptan Modified By Different Series Of Metal Promoters

Volatile organic compounds?VOCs?are a class of toxic and hazardous chemicals that would cause serious environmental pollution when discharged directly into the environment.Methanthiol?CH₃SH?,as one of the VOCs,would seriously threaten the health of the human body and environmental safety if methanthiol was emitted directly into the air.Methanthiol had attracted lots of attention because of its wide sources,strong toxicity and large hazardous properties.Therefore,it was very urgent to seek a efficient method to treat methanthiol.Catalytic decomposition as an efficient,fast and almost pollution-free method has been extensively studied in the degradation of methanthiol.Through the action of the catalyst,methanthiol could be decomposed into small molecules such as H2S and CH₄ without adding additional substances.The by-products H₂S could be removed by the present mature H₂S treatment process.Therefore,the catalytic decomposition method was considered as a highly efficient and safe treatment method for methanthiol.In this paper,the HZSM-5 zeolite molecular sieves was modified by selecting different series metals?alkali metals,transition metals and rare earth metals?as additives,and used HZSM-5 zeolite molecular sieves in the catalytic decomposition of methanthiol.Through the relevant characterization of the modified catalysts,the influence of the additives on the physicochemical properties of the HZSM-5 zeolite molecular sieves and the relationship between the catalysts and the catalytic activity and stability of the methyl mercaptan were explored,which can provide reference for the subsequent catalytic treatment of methanthiol.The main findings are as follows:The alkali metal Na was used as a modification,which was supported on the HZSM-5 zeolite molecular sieve by impregnation.The n%Na-HZSM-5 with different loadings was prepared by changing the content of the metal promoter precursor.The effects of HZSM-5 zeolite molecular sieves with different Na loading on the catalytic activity and stability of methanthiol were investigated by experiments.The study indicated that the addition of Na significantly changed the catalytic activity and stability of the catalyst to methanthiol.3%-Na/HZSM-5 catalyst showed the highest catalytic activity,and it could achieve complete catalytic conversion of methanthiol at 475?,because of the addition of Na which could reduce the strong acid sites on the catalyst surface and increases the catalyst alkaline,thereby increasing the catalyst's adsorption and activation of the acid gas methanthiol further enhance the activity of the catalyst.With the increase loading of Na,the silicon and aluminum in the catalyst framework would elute,which would destroy the structure of the catalyst and affect its catalytic decomposition activity and stability of methanthiol.Experiments indicated that proper Na addition could reduce the accumulation of carbon and sulfur in the catalyst during the reaction,and thus further increase the stability of the catalyst.Because the addition of Na eliminated the strong acid sites on the catalyst surface,which could reduce the surface carbon and accumulation of sulfur.Selecting transition metal Cr as a modification aid and modifying the HZSM-5 by impregnation.Through adjusting the content of the Cr loadings in n%Cr-HZSM-5catalysts to study the effects of the catalytic decomposition of methyl mercaptan.The study indicated that the addition of Cr improves the catalytic activity and stability of the catalyst.This was due to the addition of Cr which improved the redox performance of the catalyst.In addition,the increase in the stability of the catalyst was attributed to the fact that Cr could provide oxidation performance and partially accumulates the surface of the catalyst.Carbon oxidation reduced the formation of carbon deposits.When the Cr loading was less than 2%,the Cr species existed on the catalyst surface in a highly dispersed form.As the Cr loading gradually increased,some Cr species began to enter the interior of the HZSM-5 channel,and when the Cr loadings up to 5%,the pores were blocked due to the presence of Cr species in the pores,which leaded to a reduction in the redox performance of the catalyst,so the catalytic activity of the catalyst could not be further increased.Using Na,La,and Cr as the modified metal to compare the effects of different series of alkali metal modification on the catalytic decomposition of methanthiol and modify the HZSM-5 zeolite molecular sieve by equal volume impregnation method at same time.Different metal modified catalysts 5%n-HZSM-5?n=Na,La and Cr?were used in the catalytic decomposition of methanthiol.Through a series of characterization,the effects of different series of metal modified HZSM-5 pairs were explored to methanethiol catalytic decomposition process.The results showed that under high Na loading,the framework of HZSM-5 zeolite was destroyed during the impregnation of Na,resulting in the dissolution of some of the silicon and aluminum in the framework,which affected the activity and stability of the catalyst.Cr-modified HZSM-5 zeolite molecular sieve could provide redox properties for the catalyst,and the presence of redox properties on the catalyst surface further promoted the catalytic decomposition of methanthiol.When La modified HZSM-5 zeolite molecular sieves were used to adjust the acidity and alkalinity of the catalyst surface,a suitable acid-base characteristic was created by changing the acidic and basic contents of the catalyst surface,so as to achieve the purpose of improving the catalytic activity of the catalyst.