Preparation Of CuFe Binary Catalysts Towards Syngas Conversion To Long-chain Alcohol

Syngas conversion reaction is one of the most important industry reactions in C1 chemistry,which plays a vital role in adjustment of energy structure and production of fine chemicals.Due to the increased demand and broad applications of long-chain alcohols,the synthesis from syngas by a tandem strategy provides a facile,economical and environment-friendly approach.Among various catalyst systems,Cu-Fe binary candidate plays a crucial role in this reaction for fundamental research.However,the low selectivity and productivity for long-chain alcohols and a superficial understanding of interfacial active sites remain unsolved in the development of Cu-Fe catalysts.Therefore,in this dissertation,a new catalyst with Fe5C2-Cu interfacial structure based on the structural topological transformation of layered double hydroxides(LDHs)precursors was obtained.The resulting Fe5C2-Cu catalyst exhibits a low pressure performance at 1 MPa with a high long-chain alcohols selectivity and stability,which is comparable to the optimal level of Cu-Fe catalysts operated at much higher pressure(normally above 3 MPa).This study demonstrates a new strategy of preparation of Cu-Fe catalyst,which can be potentially applied in syngas transformation.The main research contents and results are as follows:1.Interfacial Fe5C2-Cu catalysts toward low-pressure syngas conversion to long-chain alcohols Based on the mixed metal oxides obtained from calcination ofCu4Fe1Mg4-LDH precursor,the catalyst was prepared in syngas with 50%CO2 through a two-step activation process.The resulting Fe5C2-Cu catalyst gives a total alcohol selectivity of 29.8%and long-chain alcohol selectivity of 49.1%at 1 MPa.Moreover,the long-chain alcohol productivity reaches to 0.101 g gcat-1 h-1,which is comparable to the optimal level of Cu-Fe catalysts operated at much higher pressure(3-8 MPa).A satisfactory catalyst stability is obtained:only 5%decrease in activity is observed within a 100 h catalytic evaluation test.Based on a combination study including ac-STEM and EELS/EDS mapping,it is found that Fe5C2 nanoclusters are highly dispersive on the surface of Cu nanoparticles,which is further proved by seMi-in-situ STEM and EELS.Moreover,CO-TPD measurements display a positive correlation between the yield of long-chain alcohols and the relative concentration of interfacial Fe5C2,demonstrating the Fe5C2-Cu interfacial site acts as active center toward LAS.The optimal Cu4Fe1 catalyst with abundant Fe5C2-Cu interfacial sites shows a weakened hydrogenation when pressure decreases from 3 MPa to 1 MPa,which could reduce the rate of hydrogenation for hydrocarbons chain termination and facilitate the kinetic rate matching between CO insertion and C-C coupling.Hence,a precise control over double-active-site in Cu4Fe1 catalyst accounts for the yield toward high long-chain alcohols at 1 MPa.A new interfacial Fe5C2-Cu catalyst was obtained,which achieved a low-pressure syngas conversion to long-chain alcohols.2.Promotion Effect of Ag on Syngas Transformation to Long-chain Alcohols over CuFe CatalystsWith a Cu/Fe molar ratio of 4/1,Mo,In,Ag,Cr,Ga,Mn,Nb and Re were introduced into the host matrix of LDHs precursor,respectively,for the purpose of investigating the promotion effect of catalyst promoter.Subsequently,Cu4Fe1M1 catalysts were obtained through the same activation method mentioned above;and catalytic evaluations showed that the Cu4Fe1Ag1 sample gives the best performance.By virtue of the modulation on Ag loading,the Cu4Fe1Ag0.5 sample exhibits the optimal performance with a CO conversion of 70.1%,total alcohol productivity of 0.29 g gcat-1 h-1 and long-chain alcohols productivity of 0.139 g gcat-1 h-1.Stability tests verified that no obvious decrease in catalytic behavior was found within 100 h.XRD and STEM mapping confirm the dispersion of Ag in Cu nanoparticles promotes the formation of Fe5C2-Cu interfaces.By the addition of C2H4 in reaction gas,it is found that Ag accelerates the transformation of olefin intermediates and thus enhances the CO conversion.Moreover,the optimal Cu4Fe1Ag0.5 catalyst gives a higher propanol selectivity(52.1%)than ethanol selectivity(6.6%),which indicates that Ag improves the rate of CO insertion on Cu active sites.This effect maintains a high long-chain alcohols selectivity at a high CO conversion,resulting in enhanced long-chain alcohols productivity.Therefore,this thesis provides an effective promoter(Ag)for Cu-Fe catalysts to strikingly enhance catalytic performance of synthesis of long-chain alcohols,which can be potentially used in practical applications.