Preparation Of Fe-Mn Catalyst By Low Temperature Coprecipitation Method And Its Catalytic Activity For Synthesis Of Light Olefins From Co Hydrogenation

Light olefins are important basic chemicals in the chemical industry.Currently,olefins are mainly produced through petroleum routes,Our country’s coal-based syngas with independent intellectual property rights to produce olefins through dimethyl ether（DMTO）has achieved an annual production capacity of 10 million tons,greatly relieving the dependence on the oil pipeline.The one-step process for producing light olefins from syngas has the advantages of short process,low energy consumption and low coal consumption,and it’s an effective way to achieve clean and efficient utilization of energy such as coal,natural gas and biomass.The basic research on the direct catalytic catalysis of olefins in one-step to improve the activity of catalysts and the selectivity of olefins which is still of great significance for the clean conversion and utilization of coal and the energy and resource security of our country.The development of high-performance catalysts is the key issue of the FTO technology route,and it is necessary to strengthen basic theoretical research such as the relationship between catalyst structure and performance.In this paper,the Fe-Mn model catalyst was prepared by low temperature coprecipitation technology for the dynamic structure change and structure-activity relationship in the life cycle of Fe-based catalysts.Compared with the conventional co-precipitation method at room temperature,the low-temperature precipitation technology can accelerate the nucleation and splitting of the crystal nucleus,which is beneficial to obtain a catalyst with a higher specific surface area and a smaller particle size.In our study,we used transmission electron microscopy（TEM）and high-resolution field emission scanning electron microscopy（SEM）to investigate the microscopic morphology of the catalyst precursor before and after calcination.X-ray diffraction（XRD）analysis of the catalyst precursor phase before and after calcination.The thermogravimetric analysis was combined with FTIR spectrometer（TG-IR）to study the phase change of the catalyst precursor during calcination.We also used N₂ physical adsorption（BET）and Raman spectroscopy（Raman）to characterize the structure of the calcined catalyst.H2/CO temperature programmed reduction（TPR）and H2/CO temperature programmed desorption（TPD）were used to study the phase change and chemisorption characteristics of the catalyst reduction process.The FTS performances of the catalysts with different mole ratios were tested in fixed bed reactor.The results show that the addition of Mn forms a(Fe_1-xMn_x)₂O₃ composite oxide structure during the calcination stage,which destroys the crystal structure of Fe oxide and makes it difficult to crystallize,resulting in a decrease in catalyst grain size and an increase in specific surface area.The addition of Mn also inhibits the agglomeration of the Fe phase during the calcination process,and functions to physically disperse the active phase;During the activation of the catalyst,and the formation of iron carbide phase during the reduction of CO atmosphere,mainlyθ-Fe3C,χ-Fe5C2 phase.Reduction of H2 atmosphere will produce Fe3O4,andα-Fe will be formed above 600°C.the addition of Mn（20%mol）can promote the reduction of the catalyst,The catalyst forms an iron carbide phase during the reduction of the CO atmosphere,which has higher catalytic activity than the reduction of the H2 atmosphere in the syngas gas to olefin reaction.The structure-activity relationship study also found that the more the amount of Mn added,the larger the specific surface area of the catalyst,but only the addition of an appropriate amount of Mn can improve the selectivity of the light olefin.The conversion frequency（TOF）of the pure Fe catalyst to form the low carbon olefin is1.6×10^-3(s^-1),and the low carbon olefin selectivity is 14.9%.The addition of 20%mol of Mn can form the most active CO adsorption sites on the surface of the Fe-based catalyst,thereby increasing the activity of the catalyst and the selectivity of the low-carbon olefin.The conversion frequency（TOF）of the low-carbon olefin is5.7×10^-3(s^-1),the selectivity of low carbon olefins is 36.3%.The addition of Mn has significant geometric effects and partial electronic effects,because after adding an appropriate amount of Mn,more FeC_X active phase can be formed on the catalyst surface,and the adsorption amount of CO per unit mass catalyst increases.And the interaction between Fe-Mn weakens the adsorption performance of the olefin product,thereby reducing the alkane selectivity and increasing the selectivity of the olefin.Excessive addition of Mn increases the catalyst specific surface,However,the exposed Fe atoms on the surface are reduced,the molar amount（FTY）of the catalytic conversion of CO on per unit mass Fe is reduced.