Study On The Catalytic Decomposition Of Methyl Mercaptan By Ruthenium-based Solid Solution And Rare Earth Modified HZSM-5

Methyl mercaptan,as a kind of typical volatile organic sulfur compounds(VOSCs),has a wide range of sources and strong toxicity,and it can cause serious impact on the natural environment and human health.Therefore,it is urgent to study the application of methyl mercaptan treatment technology and is of great practical significance to study the treatment of methyl mercaptan tail gas in industrial production process.Catalytic decomposition method used for the removal of methyl mercaptan is considered to be one of the most cost-effective approach,because of its high efficiency,non-secondary pollution,no additional reaction agent(such as H2.O2,etc.).CH3SH could be completely decomposed to simple and tractable inorganic carbon hydrogen compounds and sulfur compound.In this paper,a series of ceria based oxides and rare earth modified HZSM-5 zeolites were prepared and investigated for CH3SH catalytic decomposition.Based on the characterization of the catalysts,the structural characteristics of the catalysts as well as the mechanism of the reaction and deactivation were investigated in detail to provide theoretical guidance for the treatment of CH3SH.The main results are summarized as follows:(1)A series of cerium zirconium solid solutions with different Ce/Zr ratios were prepared by a citric acid complexation method with microwave-assisted.The preparation method is simple and rapid.The influence of introduction of different content Zr ions on the structural properties of the catalysts and the catalytic performance for decomposing CH3SH were investigated.The results show that synthetic cerium zirconium solid solutions all retain cubic fluorite structure.The introduction of zirconium leads to smaller grain size and lattice shrinkage of the synthesized solid solution,and the lattice parameter "a" decreases with the increase of doped zirconium.At the same time,addition of zirconium only results in the formation of species defects without producing more oxygen vacancies.Under the condition of close surface lattice oxygen content.Ce0.75Zr0.25O2 has the best specific surface area and the best reduction performance,which shows the best catalytic activity for decomposing methyl mercaptan.And the catalytic performance decreases with the introduction of zirconium.Furthermore,the product distribution of methyl mercaptan catalyzed by Ce0.75Zr0.25O2 was studied.CH3SCH3 is intermediate and can be only produced in low temperature region.(2)Heteroatom Y(Zr)with different valence states and ionic radii was doped to prepare ceria-based catalysts(Ce0.75Zr0.25O2 and Ce0.75Y0.25O2-δ).The effects of valence and ionic radius on catalytic activity and stability for CH3SH decomposition with corresponding characterization were performed to investigate deactivation mechanism with CH3SH over ceria-based catalysts and the relations between oxygen species and catalytic behaviors.The experimental results show that the surface lattice oxygen plays a key role in CH3SH catalytic conversion.Ce0.75Zr0.25O2 shows higher catalytic activity for decomposition of CH3SH due to more surface lattice oxygen cooperated with better redox property.Ce0.75Y0.25O2-δ exhibits better catalytic stability compared with CeO2 and Ce0.75Zr0.25O2.This is because that more oxygen vacancies within Ce0.75Y0.25O2-δ can promote the migration of bulk lattice oxygen to replenish surface lattice oxygen as to improve its catalytic stability.In a word,the valence difference between Ce and heteroatom would seriously affect amounts of surface lattice oxygen and bulk lattice oxygen mobility for cerium based catalysts,thus influencing catalytic activity and stability for CH3SH conversion in turn.Moreover,the radius difference of Ce and heteroatom would lead to formation of defect species that can’t promote bulk lattice oxygen mobility as to improve catalytic stability of cerium based catalysts.(3)A series of rare earth metal(Nd,Er.Y)modified HZSM-5 zeolite catalysts were prepared by traditional impregnation method to investigate the CH3SH decomposition,and the effects of rare earth modification on the physicochemical properties of the catalysts were investigated.In addition,a series of characterization methods were used to investigate the reasons of improved activity and stability of modified HZSM-5 zeolite catalysts for the catalytic decomposition of methyl mercaptan.At the same time,the deactivation and regeneration of HZSM-5 catalyst modified by rare earth metal(Nd,Er,Y)were also investigated.Results show that CH3SH can be completely converted over pure HZSM-5 zeolite above 550℃,while modified Nd can obviously improve the activity of the catalyst,and can achieve the complete 100%conversion of methyl mercaptan at the temperature of 500 ℃.Pure HZSM-5 and other popular catalysts(such as cerium based catalysts)will deactivate at about 10h.The Nd modified HZSM-5 catalyst exhibits high stability without occurring obvious deactivation after the 60h reaction time.The increased amount of basic sites on the modified HZSM-5 catalysts will be favorable for the adsorption of more CH3SH molecules.The decrease of the number of strong acid sites leads to the active site of catalyst more stable and can effectively inhibits the formation of coke on the catalysts surface.The above characteristics can all improve the activity and stability of the catalysts.In addition,the deactivated catalysts can be recycled several times in the air.so they have good prospect in industrial application.