Study On Catalytic Decomposition Of Methyl Mercaptan By Rare Earth Metal Modified HZSM-5 Molecular Sieve

Methyl mercaptan(CH3SH)is a representative acidic odorous compound.Because of its toxicity,irritation and corrosivity,it has become one of the environmental problems to be solved urgently.There are many ways to remove methyl mercaptan.Catalytic decomposition has the advantages of producing low energy consumption,high removal rate and non-secondary pollution,which has been developed into an economically and effective process of removing methyl mercaptan.In this paper,a series of modified HZSM-5 zeolites with the different rare earth(La,Ce,Pr,Nd,Sm,Eu,Gd,Er,Y)were prepared by an incipient-wetness impregnation method,and the dependance of corresponding activity for decomposing CH3SH on the different reaction temperatures were investigated.Study the stability,deactivation and regeneration of the catalyst by choosing one of the catalysts modified HZSM-5 by rare earth metal that has the best catalytic activity as well as further exploring better rare earth metal loading.The physical and chemical properties of the synthesized catalysts were characterized by XRD,BET,H2-TPR,NH3-TPD and CO2-TPD.The main results summarized as follows:(1)The catalytic activity of the different rare earth metals modified HZSM-5 zeolites for the catalytic decomposition of methyl mercaptan were greatly enhanced.The order of catalytic activity:La/HZSM-5>Sm/HZSM-5>Nd/HZSM-5>Gd/HZSM-5>Y/HZSM-5>Eu/HZSM-5>Er/HZSM-5>Pr/HZSM-5>Ce/HZSM-5>HZSM-5.The results showed that doping of rare earth would not destroy the framework structure of HZSM-5 zeolites,while the microporous and mesoporous were formed on ZSM-5 zeolites after treated by rare earth,which should be in favor of the catalytic decomposition of CH3SH.Furthermore,the introduction of rare earth could effectively regulate the acidity and basicity property,which may affect the catalytic activity of decomposition of CH3SH.(2)The effects of the different contents of La and Nd modified HZSM-5 zeolites in the catalytic activity of decomposition of CH3SH were studied.The results showed that the 13%of La and Nd contents will be the most beneficial to catalytic activity of decomposition of CH3SH.The modification catalysts were characterized by XRD,BET,NH3-TPD and CO2-TPD.The results showed that doping of the different contents of La and Nd would not destroy the framework structure and high dispersion of La and Nd species on the surface of HZSM-5 zeolites.Obviously,the introduction of La and Nd species leads to the decrease of surface area and microporous volume.These results may demonstrate that La and Nd species particles located on HZSM-5 partially blocked the HZSM-5 microchannels and reduced the accessibility of micropores and surface area.So,La and Nd modification with excessive loading(17%or more)led to the decrease in the catalytic conversion of methyl mercaptan.Also,the introduction of the 13%contents of La and Nd could effectively regulate the acidity and basicity property.The more favorable catalytic activity for methyl mercaptan of the La and Nd modified HZSM-5 zeolites could be attributed to the appropriate amount of weak acid sites and high amount of basic sites.(3)The catalytic lifetime test of the 13%La/HZSM-5 and 13%Nd/HZSM-5 zeolites for the catalytic decomposition of methyl mercaptan were studied.The results showed that the 13%La/HZSM-5 were selected for an 110h durability test.The CH3SH conversion of 13%Nd/HZSM-5 maintains above 90%for 70h and then declines to 60%at TOS of 80h.As known,the deactivation of the catalysts due to coke formation is inevitable because of the reaction conditions,the coverage of active sites and the channel blockage caused by carbon deposit result in the inaccessible inner acid sites with time on stream.The 13%La/HZSM-5 and 13%Nd/HZSM-5 zeolites leaching the strong acid sites can decrease the rate of coke formation and enhance the stability of catalyst.The catalyst can be repeatedly regeneration and the regenerated catalyst in the catalytic decomposition of methyl mercaptan effect is still maintained the original good activity and stability.The catalysts has a wide application in industry.