Study On The Catalytic Decomposition Of Methyl Mercaptan By Rare Earth (La) Modified HZSM-5 And Cr Based Catalysts

As a special volatile organic sulfur compound?VOCs?,methyl mercaptan can not only accumulate organic aerosols,but also lead to photochemical smog of the atmosphere.And it can be converted into sulfates through a series of chemical processes,promoting the formation of PM2.5 and haze.Therefore,it is of great significance to study the treatment of methyl mercaptan.Catalytic degradation is the most effective way to remove methyl mercaptan,but it is not enough to meet the requirements of practical application.In this paper,a series of La modified HZSM-5and Cr base catalysts were developed and studied to catalytic decomposition of methyl mercaptan.a series of characterization was adopted and the structure performance,physical and chemical properties are analyzed in detail.The mechanism of catalyst reaction and inactivation was analyzed.In order to provide certain theoretical guidance of catalytic decomposition CH₃SH in follow-up study.The main results are summarized as follows:1)Lanthanum?La?doped HZSM-5 and parent HZSM-5 catalysts were prepared and the role of La on the structure and the catalytic performance of HZSM-5 catalyst was estimated in detail for catalytic decomposition of methyl mercaptan?CH3SH?.Activity results show that the incorporation of La species largely shifts the temperature of complete CH3SH conversion from 600?to 450?and La doped HZSM-5 exhibits five-fold long term stability than that for parent HZSM-5.XRD,N₂adsorption-desorption,FT-IR,XPS,CO₂-TPD,NH₃-TPD,Raman,O₂-TPO,TGA and CH₃SH-TPD?characterizations?were conducted and?the corresponding?results show that the excellent performances of La doped HZSM-5 should be assigned to the synergistic effect of its tunable acid-base properties,the active surface oxygen species as well as the formed CO₂ and surface lanthanum oxy-carbonates layer.The rapid deactivation for parent HZSM-5 was assigned to the formation of deposited coke irrespective of deposited sulfur species.The nature,amount and type of deposited coke were investigated in detailed.In addition,a facile and rapid regeneration method was used to regenerate the spent La doped HZSM-5 catalyst and no significant deactivation was observed even after three cycles of deactivation-regeneration.2)Methyl mercaptan?CH₃SH?was studied over a series of Cr_?x?-Al₂O₃?x=1.0,2.5,5.0,7.5 and 10?catalysts,synthesized via incipient wetness impregnation.The catalysts were characterized by XRD,N₂ adsorption-desorption,H₂-TPR,NH₃-TPD,FTIR,Raman,UV-vis,XPS and TG measurements.Depending on Cr loading,Cr was found to be present two chemical valence as Cr³⁺and Cr⁶⁺species which were detected by UV-vis and XPS studies.Their catalytic behaviors toward CH₃SH decomposition reactivity were investigated.The results suggested that Cr doped Al₂O₃catalysts displayed improved catalytic activity,the reaction temperature for the complete conversion of CH₃SH decreased,and Cr_?7.5?-Al₂O₃ catalyst exhibited higher activity than other loadings.CH₃SH can be almost completely converted over Cr_?7.5?-Al₂O₃ catalyst at nearly 375 ^o C compared to pure Al₂O₃ at 450 ^oC.Besides,there is no obvious deactivation during the 30 h on stream.The improvement in catalytic performance can be explained by the important role of the Cr⁶⁺species in the reaction,Cr³⁺was found to be ineffective in the degradation of CH₃SH.Furthermore,comparisons between spent Al₂O₃ and spent Cr_?7.5?-Al₂O₃ catalysts were done using TG,Raman,FTIR,XPS and NH₃-TPD techniques.3)chromium?Cr?doped SiO₂ catalysts was synthesized via incipient wetness impregnation,the parent SiO₂ catalyst has little effect on CH₃SH.However,when Cr was added to Si O₂,the catalytic decomposition activity of CH₃SH was significantly increased,and the 5wt%loading show the best decomposition activity,CH₃SH can be almost completely converted over Cr_?5?-SiO₂ catalyst at about 400?.In addition,the Cr_?5?-Si O₂ catalyst exhibited excellent stability without significant deactivation even after 150h time-on-stream test,XRD,N₂ adsorption-desorption,H₂-TPR,UV-vis were conducted and the results show that the Cr⁶⁺and Cr³⁺were responsible for the improved catalytic activity.