Structure-Performance Relationship Of Fe-based Catalysts For Synthesis Of Lower Olefins Directly From Syngas

Lower olefins are raw materials in chemical industry,being conventionally produced from non-renewable petroleum resources.Syngas chemistry,or to say,C1 chemistry,direct conversion of syngas to light olefins,which is an important direction for the clean and efficient utilization of coal resources,opens a new path to produce olefins from non-petroleum routes.It is of great significance in safegard of national energy and to achieving sustainable development of the national economy.However,the selectivity to lower olefins and stability of catayst currently haven't met the industrial requirements,and the mechanism of Fischer-Tropsch-to-olefins(FTO)reaction remains unclear over Fe-based catalysts;this dissertation proposes approaches for the development of Fe-based catalysts based on metal-support interactions and the role of promoters.Two high-efficiency FTO catalyst systems are developed.One is the modified multi-walled carbon nanotubes supported Fe/MWCNTs,and the Fe-CNT interaction is modulated by the CNT surface modification;the other is the bimetallic FeMn composite catalysts,particularly,the preparation and reaction conditions are optimized thoroughly.In combination of a variety of in situ/operando techniques,the dynamic structural evolution of the active sites of Fe catalysts during calcination-activation-reaction,and structure-performance relationship were systematically investigated.Lastly,based on the intrinsic kinetics fitted to an empirical power-law model,the influence of Mn on the FTO mechanisms was discussed.Fe/MWCNTs catalysts were prepared using N-doped,O-functionalized and high-temperature annealed multi-walled carbon nanotubes.The product distribution could be controlled through the size effect of Fe nanoparticles(NPs),the nano-iron catalyst with a particle size of 7.0 nm was prepared through the optimization of preparation parameters.The effect of surface modification of MWCNTs on the performance of Fe/MWCNTs catalyzed the FTO reaction was studied.A variety of in situ/operando techniques were employed to establish the structure-performance relationship.The defect-rich Fe/HT-CNT exhibited the highest selectivity of 49.1%and 72.4%to C2-C4 and C2-C7 olefins after high-temperature annealing,respectively.Results showed that electronic effect significantly influenced the adsorption properties of olefinic intermediates.Defect-rich HT-CNT had an electron-withdrawing effect on Fe NPs,while N,O-functionalized carbon tubes exhibited electron-donating effects on Fe.In addition,the Fe-CNT interaction significantly affects the formation of the active phase ?-Fe2.2C/?-Fe2C and the stability of the catalyst.To further improve the lower-olefin selectivity and catalyst stability,a precipitated bimetallic FeMn composite catalyst system was designed using the promoting effect of Mn.The preparation process parameters such as the Fe/Mn ratio and the reaction operating conditions were optimized.FeMn(4-1)possesses the highest steady-state selectivity of 60.6%to C2-C4 olefins under FTO conditions(260?,2 MPa).The lower-olefin yield increases with the addition of appropriate amount of Mn.Combined with in situ/operando techniques,the dynamic structural evolution of FeMn catalysts during calcination-activation-reaction was analyzed,the structure-performance relationship was established,and the promotion effect of Mn promoters on precipitated Fe catalysts was clarified.Owing to the larger ionic radius and the lower surface energy of Mn,Mn induces lattice distortion,reduces the crystallinity and the crystallite size of microspheres.Mn stabilizes Fe1-xMnxO and suppresses the phase transformation of Fe carbides,resulting in the increase of the ?-Fe2C/?-Fe5C2 ratio.In addition,an appropriate amount of Mn could enhance the surface alkalinity,promotes CO adsorption and dissociation,concomitantly weakens the hydrogenation of intermediates,ultimately improves the olefin selectivity.Aiming at the ambiguous mechanism affected by promotion effect of Mn on Fe catalysts for FTO,the intrinsic kinetics have been carried out on unpromoted and Mn-promoted Fe/SiO2 catalysts.The dependences of product(paraffins,olefins and CO2,etc.)formation rates and partial pressures of H2(PH2=0.18-1.44 MPa,Pco=0.36 MPa),partial pressures of CO(PH2=0.6 MPa,Pco=0.15-1.2 MPa),the temperature ranged from 260 to 320 ? were studied over Fe20/SiO2 and Fe20-Mn1.0/SiO2 catalysts for FTO(260?,2MPa),and the activation energy and the reaction order of CO/H2 were fitted to the empirical power-law model.The addition of appropriate amount of Mn leads to a decrease in the activation energy and in the reaction order with respect to CO for hydrocarbon formation.Meanwhile,the activation energy for CO2 formation is increased while the activation energy for olefins production is significantly decreased.The enhanced adsorption of CO and surface carbon species facilitates the carbon-carbon coupling,hinders the adsorption and dissociation of H2,weakens the olefin re-adsorption and secondary hydrogenation,consequently promoting the formation of lower olefins.