Modulation Of Active Phase And Influences On Catalytic Performance For Fischer-Tropsch Synthesis

Based on the current energy structure and natural reserves situation in China,developing the Fischer-Tropsch synthesis(FTS)technology which convert syngas to synthetic fuels and lower olefins is of great importance,especially considering the increasing domestic consumption demands.The structure and species of active phases for iron and cobalt catalysts are complex and dynamic under the FTS environment,which makes it challenging to identify the relationship between specific phase and catalytic performance.In this paper,we aimed at modulating active sites and investigating structure-performance relationship.We systematically discussed the following issues:(1)general descriptors were established to interpret the structure-performance relationship for Fe-based FTS catalysts,through the samples started from various Fe precursors and imposed to controlled reaction atmosphere;(2)a simple methanol pretreatment method is developed to modulate active sites and the modulation mechanism was elucidated;(3)the chemical environment of Fe active sites was modified by nitrogen doping through the MOF-mediated strategy,and the promotion mechanism of nitrogen doping on catalytic performance was further investigated.Starting from the various synthesized iron precursors(Fe₂O₃??-Fe₅C₂??-Fe₃C and Fe⁰),we modulated the reactive atmosphere including H₂/CO molar ratio and pressure.Then by employing various bulk characterization techniques including XRD and HRTEM combined with EELS,we found that core-shell structures were formed for all catalysts irrespective of the catalyst precursors,while the compositions within the core and shell strongly depend on the intrinsic properties of the precursors.Quasi in-situ soft X-ray absorption techniques was used to determine the surface iron composition.The surface of catalysts consists of iron oxide(s)and carbide(s).Further quantitative fitting for both the surface and bulk was conducted.It is found that the iron coordination environment mainly governs the performance during FTS.The FTY linearly correlates with the averaged bulk iron OS rather than the surface properties,and the CH₄ yield is related to the CN of the Fe-C bond.This study thus provides a general descriptor to interpret the structure-performance relationship of Fe-based FTS catalysts.Methanol was used as carbon source to modulate the reduction and carburization behavior of active sites for Co-based and Fe-based catalysts.For Co-based catalysts,XRD,chemisorption and XPS results show that methanol pretreatment is beneficial for the reduction of cobalt species;HRTEM and EELS results show the formed carbon species on cobalt phase after methanol pretreatment favors the electron donation to cobalt species.In-situ FT-IR results prove that methanol pretreatment can improve the dissociative adsorption of CO on Co active sites.For iron-based catalysts,methanol pretreatment is favorable for the formation of iron carbide active phase.TPH-MS results show that methanol pretreatment contribute to the formation of active C?species on both the Co-based and Fe-based catalysts,thus improving the CO conversion.Co-Me OH catalyst possesses superior X_CO of 17.1%compared to that of 8.7%for Co-4h,and Fe-Me OH catalyst presents increased X_CO of 9.7%in relative to that of 6.0%for Fe-4h.Under the condition of 240?,2.0 MPa and H₂/CO=2/1,the C₅⁺selectivity was found to increase from 81.8%for Co-4h catalyst to 91.9%for Co-Me OH catalyst,while the influences of methanol pretreatment on the products distribution is not obvious for Fe-based catalysts.Amino-group modification was achieved by the MOF-mediated strategy.Fe/AM53 and Fe/NM53(x)catalysts with various nitrogen amount were successfully synthesized.The electronic properties of iron carbides active phase were effectively modulated for the supported Fe-based catalysts.XRD and XPS results verified nitrogen promoter is favorable for the reduction and carburization of iron species.CO-DRIFTS and CO-TPD results evidenced nitrogen promoter plays as the electron donor to Fe species,which enhances the adsorption strength of CO on Fe species.Catalytic tests results show that the catalytic performance is dominated by both the electronic promoter effect and metal-support interaction.Meanwhile,the total content of pyrrolic N and graphitic N were found to correlate well with the FTY,and the addition of nitrogen can suppress the secondary hydrogenation ability and lead to higher O/P ratio.Moreover,proper amount of nitrogen doping(x=1)is beneficial for the enhancement of the CO uptake on Fe active sites.While excess nitrogen(x>1)doping would lead to the formation of inactive Fe Al₂O₄ species,which is unfavorable for CO conversion.The optimal FTY of 677.0×10^-6 mol _CO·g _Fe^-1·s^-1 and SC₂-C₄⁼of29.4%can be achieved on the Fe/NM53(1)catalyst under the evaluation condition of340?,2.0 MPa,H₂/CO=1/1.