Study On The Preparation And Catalytic Performance Of K/Mg-Fe-M Catalysts For CO Hydrogenation

Direct synthesis of light olefins from syngas via Fischer-Tropsch Synthesis(FTS)has the advantages of shorter process,lower consumption of energy and water,which has become a new process of non-oil route to produce olefins.However,due to the complexity and diversity of the CO hydrogenation,it has the wide range of product distribution.The low selectivity of light olefins ruled by the Anderson-Schulz-Flory(A-S-F)distribution function and the limitation of thermodynamics and kinetics and the high selectivity of CH4 and CO2 by-products have restricted the application of this process into an industrial scale.In addition,secondary reactions such as hydrogenation,polymerization and disproportionation of primary olefins also limit the selectivity of olefins.Therefore,the product distribution and the selectivity towards light olefins need further improvement during CO hydrogenation.MgFe-HTLcs(Hydrotalcite-Like Compounds,HTLcs)has the advantages of typical layered structure,high specific surface area,good hydrothermal stability,adjustable surface acid-base and uniform dispersion of active components.In this work,we propose MgFeM-HTLcs(M:Zn,Al,Mn)materials as catalyst precursor or catalysts used in CO hydrogenation to produce low olefins,which can meet the reaction required structure,surface alkaline and other factors.MgFeM-HTLcs materials were prepared by introducing a certain amount of Zn,Al and Mn promoter into MgFe-HTLcs structure by co-precipitation and coprecipitation-hydrothermal methods.Finally,all the samples were impregnated with K2CO3.The textural properties,bulk structures,reduction behavior and surface composition of the catalysts were characterized by the methods of XRD?TG?SEM?N2 adsorption-desorption?H2-TPR and XPS.The relationship between the catalytic performance in CO hydrogenation and structure of the catalysts was investigated.Studies have shown that Mg/Fe/Zn,Mg/Fe/Al,Mg/Fe/Mn precursors prepared by the methods of co-precipitation and coprecipitation-hydrothermal,possess the characteristics of typical layered structure,high specific surface area,uniform mesoporous distribution and good thermal stability.The addition of alkaline additives Mg and Mn,and amphoteric Al can modulate basicity of the catalysts compared with K/Mg-Fe-HTLcs samples.Furthermore,the addition of Mg and Mn inhibit the secondary hydrogenation of olefins,which then increase the selectivity of light olefins.The formation of ZnFe2O4 with lower Zn content can promote the dispersion of the active components of the catalysts.Which then enhances the weight content of C2=-C4= and inhibits the formation of heavy hydrocarbons.The product distribution is significantly improved.For all the samples,the C2=-C4= weight content of 48.56 wt%,O/P value of 5.15 and the C5+ weight content of 13.93 wt%were obtained over K/2Mg-2Fe-lZn sample,respectively.All the catalysts showed good stability.The hydrotalcite-like structure was recovered with larger average pore size after calcination of MgFeAl-HTLcs and then impregnation with K2C03.During CO hydrogenation,the C2=-C4= weight content was increased and the content of C5+ hydrocarbons significantly decreased compared with that of the K/MgFe-HTLcs sample.The product distribution is significantly improved.The C2=-C4= weight content of 42.62 wt%and an O/P value of 2.87 over K/1.5Mg-0.2Fe-0.8Al-H catalyst were obtained,respectively.