The Research Of Catalysts For Syngas To Lower Olefins

Lower olefins,including ethylene,propylene and butylene,are the key feedstock for the manufacture of plastics,drugs and cosmetics.Generally,lower olefins are produced by the thermal or catalytic cracking of naphtha or oil.However,the shortage of oil resource and environmental issues pose significant challenges for the development and commercial application of these traditional routes.Consequently,alternative routes,especially Fischer-Tropsch to Olefins（FTO）has been a promising and attractive route for the direct and single-step conversion of syngas to lower olefins,according to he energy structure characteristics of rich coal,less gas and poor oil in China.Syngas can be derived from biomass,coal and natural gas with relatively low cost.However,the products of Fischer-Tropsch synthesis（FTS）are confined by the rule of Anderson-Shultz-Flory（ASF）distribution.Therefore,the research and development of proper catalyst with high activity and selectivity to light olefins in FTO reaction is the key and has attracted increasing attention.In addition,due to the complexity of FTS,the active phase of Fe-based catalysts has been controversial.Most researchers believe that iron carbide is the active phase in FTS process.Therefore,the preparation of iron carbide with different structure by changing pretreatment condition,and the study of the relationship between iron carbide and the activity and product distribution is also an important challenge in this work.Fused iron catalyst,as the research object in this work,is researched systematically that the effect of alkali metal promoters and iron carbide with different structure on the activity of catalyst and product distribution.In addition,a new preparation method for iron-based catalyst is investigated,compared with traditional catalysts for the catalytic performance.The intrinsic properties,morphology,composition,phase structure,reduction and surface properties of the catalysts were investigated by BET,SEM,XRF,XRD,H₂（CO）-TPR,H₂（CO）-TPD,CO₂-TPD,XPS and TEM characteristics.The main conclusions are as following:1、Alkali metal promoters inhibit the reduction of fused iron catalysts,and different kinds of alkali metal promoters have different effects on the reduction properties of catalysts.The promoters,such as K₂O and Rb₂O,show easier reduction compared with Li₂O and Na₂O promoters,and exhibit higher catalytic activity.In addition,alkali metal promoters also change the surface basicity of catalysts.Li₂O,K₂O and Rb₂O can increase the surface basicity of catalyst and improve the selectivity to lower olefins.However,the catalyst with Na₂O promoters reduces the selectivity to C⁼_2-4,owing to no surface basicity of catalyst.2、The active phases of Fe₅C₂,Fe₂C and Fe₃C were obtained under different carburizing conditions,such as temperature and H₂/CO ratio.Iron carbide with different structure affects the CO conversion of catalyst and product distribution.CO conversion for iron carbide active phase is LY2（Fe₅C₂）>LY2（Fe₂C）>LY2（Fe₃C）,which is related to the specific surface area of catalyst.LY2（Fe₂C）is favor of lower olefins,while LY2（Fe₃C）promotes the formation of C₅₊,but inhibits the production of CH₄.3、The grain size ofα-Fe also affects the formation of iron carbide.α-Fe with smaller grain size is easier to form Fe₂C,butα-Fe with larger grain size is easier to form Fe₅C₂ active phase with high activity.4、γ-Fe₂O₃,iron oxalate as the catalyst precursor,was obtained via solid-state reaction at room temperature.The results indicate that uniform nanoparticles were obtained by solid-state reaction.Compared with conventional catalysts,solid-state reaction derived catalysts exhibit much higher FTO reaction activity and selectivity to lower olefins.The FTY value of is 5.62×10^-3 mol_CO·g_Fe^-1·s^-1 which is almost two times higher than that of catalyst prepared by co-precipitation(3.19×10^-3 mol_CO·g_Fe^-1·s^-1)or3 times higher than that of catalyst prepared by fusing method(1.98×10^-3mol_CO·g_Fe^-1·s^-1).The selectivity to light olefins around 40%is obtained for SR catalyst.High activity is attributed to small iron particles,high reducibility and formation of many active phases.In addition,the high selectivity to light olefins is mainly attributed to proper surface basicity and the exposure of iron carbide facets,such as（021）facet,on the catalyst surface.