Study On Structure-activity Relationship Of Fused Iron Catalyst For Producing Light Olefinsvia Fischer-tropsch Synthesis

Light olefins?including ethylene,propylene and butane?are key building-block chemicals.At present,light olefins are traditionally produced from steam cracking of naphtha.With increasing demands and oil crises,it is imperative to develop new non-oil-based processes to prepare light olefins.Currently,there is a renewed interest in producing lower olefins from alternative feedstocks such as coal,natural gas,or biomass of the carbon resource,producing light olefins via Fischer Tropsch synthesis?FTO?.It has been attracting increasing attention for it is simple process.The key technology of this process is to prepare the catalyst with highest selectivity to light olefins.In fact,the traditional Fischer-Tropsch synthesis catalysts can be used for the process of FTO.In particular,the fused iron catalyst has been proved to be an effective Fischer-Tropsch synthesis catalyst.A large number of literature reports,in the Fischer-Tropsch synthesis reaction conditions,the evolution of active phase of iron-based catalyst of is very complex.The mainly active phases of iron-based FTO are iron,iron oxide and iron carbide?Fe₂C,Fe₅C₂,Fe₃C and Fe₇C₃?.The different iron carbide phases and the different crystal surface exposure of the same iron carbide will affect the activity and selectivity of Fischer-Tropsch synthesis reaction.There is still a lot of controversy over the determination of the really active phase of the iron-based Fischer-Tropsch synthesis.Therefore,the exact identification of the iron carbide active phases is still a great challenge in the field of heterogeneous catalysis.In this paper,the fused iron catalysts were as the research object,discussing the effects of different promoters,carbonization pretreatment,in-situ nano iron carbide decorating of fused iron catalyst on the surface phases,microstructure of catalysts after FTO,and the FTO activity and selectivity.The phase,microstructure and physical-chemical properties of the catalysts were systematically characterized by means of XRD,BET,SEM,TEM,XPS,Raman and H₂-TPR/TPSR.At the same time,the existence morphology and structure-activity relationship of the active phase of the fused iron catalysts in the FTO reaction were discussed.The main results are as follows:?1?Study on the addition of different kinds of catalyst promoters in the fused iron catalysts had significant effects on the active phase and the performance of the catalysts for FTO.?The additions of alkali metal promoters into fused iron catalysts.Under FTO conditions,the mainly phase was Fe₅C₂,which could greatly improve the CO conversion,reducing the selectivity of methane,increasing the selectivity of C₅₊,and increase ratio of the light olefins.?The additions of alkaline earth metal promoters into fused iron catalysts.Under FTO conditions,the mainly phase were?-Fe and Fe₃O₄,a small amount of Fe₅C₂,which could increase the selectivity of light hydrocarbons,reducing the selectivity of C₅₊hydrocarbons,and the CO conversion was no obvious effects.?The additions of transition metal promoters into fused iron catalysts.Under FTO conditions,the mainly phases were changed.For example,the addition of Mn improve the formation of Fe₅C₂,which could increase the CO conversion and reduce the selectivity of CH₄;The addition of V led to the existence of Fe₃C,which could increase the selectivity of light olefins,greatly reducing the selectivity of C₅₊,and reduce the CO conversion.?2?Study on the different reduction carbonization atmosphere effects on the active phase and the reaction performance of the catalysts for FTO synthesis.?After complete reduction in pure H₂,and then cabonization and FTO synthesis were under the conditions of P=2.0Mpa,t=320?,H₂/CO=1.5,GHSV=11400h^-1,the main phase of the product was Fe₅C₂,and the ratio of light olefins and alkane was 2.2,the selectivity of light olefins was 24.0%,the selectivity of methane was 12.4%.?The catalyst was carbonization pretreatment with CO(normal pressure,2500h^-1,250?for 6h)or H₂/CO=2(normal pressure,2500h^-1,310?for 6h)atmosphere after reduced in H₂,and then FTO synthesis was under the conditions of P=2.0Mpa,t=320?,H₂/CO=1.5,GHSV=11400h^-1,the main phases were Fe₅C₂ and Fe₃C,and the diffraction peak of Fe₃C was stronger.The ratio of light olefins and alkane was 3.3 and 3.7,the selectivity of methane was 8.0 and 8.4,the C₅₊selectivity was 69.9%.?The catalyst was carbonization pretreatment with CO/H₂=4(normal pressure,2500h^-1,200?,250?,275?for12h,respectively)atmosphere after reduced in H₂,and then FTO synthesis was under the conditions of P=2.0MPa,t=320?,H₂/CO=1.5,GHSV=11400h^-1,the main phase of the catalyst is Fe₂C,the ratio of light olefins and alkane was 2.0.?The main phases of the fused iron catalyst were iron oxide and a small amount of?-Fe in the H₂ atmosphere at 300?for 6h.and then cabonization and FTO synthesis were under the conditions of 1MPa,340,H₂/CO=1.5 and GHSV=11400h-1,the main phases were the mixture of Fe₃C,Fe₇C₃,Fe₃O₄,and FeO.The phase of Fe₇C₃ and a certain amount of carbon nanofibers on the fused iron catalyst surface when reacting 24h,and then the selectivity of light olefins and alkane were 54%and 12.9,respectively.Under the conditions of 1MPa,340,H₂/CO=1.5 and GHSV=11400h-1,the iron carbide compounds were constantly changing with the time on stream,the transformation from Fe₃C to Fe₇C₃,meanwhile the CO conversion was increased,and the selectivity of methane and light olefins were decreased.?4?A preliminary attempt was made to stabilize the iron carbide phase to get high selectivity of light olefins.Via simple mixing and heat treatment of iron oxalate and fused iron,iron carbides over fused iron catalysts for FTO were obtained with suitable particles.The results show that different carbonization temperature and different content of iron oxalate had significant effect on the species,distribution and content of nano iron carbides.After 350?carbonization,the mainly iron carbide was Fe₅C₂.After 435?carbonization,the mainly iron carbide was Fe₃C,the selectivity of light olefins and the ratio of light olefins with light alkane were 28.6%and14.0,respectively.As the content of iron oxalate increased,the phases of Fe₃C and Fe₅C₂ increased on the catalyst surface.After FTO reaction,the transformation from Fe₃C and Fe₅C₂ to Fe₇C₃ was on the catalyst surface.The stronger the diffraction peak of Fe₇C₃ phase,the higher the ratio of light olefins with light alkane and the lower the C₅₊selectivity.