Study On Rare-Earth Metal Promoted Iron-Based Catalysts For Fischer-Tropsch Synthesis Of Light Olefins

Fischer-Tropsch synthesis(FTS)is an important chemical process to produce low-carbon olefins and clean liquid fuels from coal,natural gas,biomass and other carbonaceous raw materials by catalytic reaction.One of its key technologies is the research and development of catalysts.A series of iron-based catalysts with different rare earth metal promoters(La,Ce,Nd,Sm,Eu)were prepared by coprecipitation method.Characterization methods were used to investigate these catalysts,such as X-ray diffraction(XRD),N2 physical adsorption,H₂ temperature programmed reduction(H₂-TPR),CO/H₂/CO₂ temperature programmed desorption(CO/H₂/CO₂-TPD),scanning electron microscopy(SEM),transmission electron microscopy(TEM)and X-ray photoelectron spectroscopy(XPS),Mossbauer spectroscopy(MES)and thermogravimetric analysis(TGA).The effects of rare earth metal promoter,the content of Sm promoter and the influences of catalyst particle size on the structure,reductive carbonization behavior,bulk composition and FTS reaction performance of Fe based catalysts were discussed.The unpromoted iron-based catalyst and iron-based catalysts with molar ratio of 100Fe:10X(X=Mn?Cu?Si)were prepared by coprecipitation method.The effects of three promoters on FTS performance of Fe based catalyst were investigated.The results show that the addition of Si promoter increases the specific surface area of the catalyst,but inhibits the CO chemisorption.Cu and Mn could enhance the CO chemical adsorption.Compared with Cu,Mn could better improve the electron density of Fe surface and promote the formation of Fe₅C₂.According to the FTS result,FeMn show the highest CO conversion and C_2?4⁼selectivity.The FTS performance of FeMn was further investigated under the conditions of temperature 280?340?,pressure 1.0?2.5 MPa,space velocity 4000?20000 ml·g^-1·h^-1 and H₂/CO=2.It was found that the optimum reaction conditions were temperature 300?,pressure 1.0 MPa and space velocity 12000 ml·g^-1·h^-1.Under these conditions,FeMn exhibited CO conversion of 61.7%and C_2?4⁼selectivity of 25.3%.FeMn catalysts promoted by five rare-earth metals(La,Ce,Nd,Sm and Eu)for Fischer-Tropsch synthesis(FTS)were prepared by the coprecipitation method.The results show that the interaction between rare earth metal and Fe was related to their electronegativity.For Eu,Sm and Nd,with the decrease of the electronegativity,their electron donating ability increases,which could enhance the chemical adsorption of CO and the formation of Fe₅C₂.While the electronegativity of Ce and La were too low and the existence of composite oxide Fe2-xRxO3 were found in two samples,which was not conductive to the FTS.The sample FeMnNd achieved the strongest surface alkalinity and the weakest chemical adsorption of hydrogen.Based on the results of FTS performance,the FeMnNd catalyst exhibited the highest CO conversion of 74.9%,a C_2?4⁼ selectivity of 36.0%,an O/P ratio of 3.96,and the lowest C5+ selectivity of 34.3%.FeMnSm catalysts with different content of Sm promoter were prepared by coprecipitation method.The modification effect of Sm content on FeMn catalyst was studied and discussed.It is found that compared whit the un-promoted catalyst,when Sm was added to a proper content,the catalyst showed a larger BET surface area.The presence of Sm could increase the surface charge density of iron which enhance the Fe-C bond and promote the CO chemical adsorption.What's more,Sm could promote the formation of Fe₅C₂,which is the active phase of FTS.In addition,Sm could also enhance the basicity of catalysts and suppressed H₂ adsorption capacity which inhibited the hydrogenation reactionn.However,too much Sm would block the pores of the catalyst and decreased the adsorption of feed gas,which is not conducive to the formation of Fe₅C₂.The results of catalyst performance revealed that when Sm/Fe=1%(molar ratio),the catalyst showed 70.4%CO conversion and 32.6%C_2?4⁼ selectivity under the conditions of temperature 300??1.0MPa?12000 ml·g^-1·h^-1?and H₂/CO=2.Nano FeMnSm catalysts with different particle sizes were prepared by coprecipitation method and the effects of particle size on the catalysts FTS performance were investigated.The size of catalyst can be controlled by solution concentration.Compared with the 83.2 nm,the catalyst with particle size of 60.6 nm showed unconspicuous agglomeration.60.6 nm catalyst was also provided with higher CO chemisorption ability,the Fe₅C₂ formation and lower adsorption of H.When the particle size continued to decrease to 38.3 nm,MnFe2O4 appeared in the catalyst phase,which hindered the reduction of catalysts and the adsorption of feed gas.Overall,the sample FeMnSm-600 showed the highest C_2?4⁼ selectivity of 33.0%at the highest CO conversion of 78.7%during the reaction conditions as 300?,1.0 MPa,12000 ml·g^-1·h^-1,H₂/CO=2.The catalysts with microsphere type named Fe₂O₃ and core-shell structure named Fe₂O₃@SiO₂ were prepared by solvothermal method.And the effects of preparation method on the performance of the catalyst to FTS were discussed.The results showed that compared with the coprecipitation and impregnation methods,the catalyst prepared by solvothermal method showed higher crystallinity of Fe and stronger chemical adsorption capacity of CO,which were helpful to the formation of Fe₅C₂.The SiO₂ shell could effectively protect the active components and inhibite the occurrence of carbon deposition,which significantly improved the stability of the catalysts.