Fine-tuning And Mechanistic Studies Of Novel Iron-based Fischer-tropsch Catalysts For Direct Production Of Lower Olefins

Lower olefins(C2=-C4=)are widely used as key building blocks in the chemical industry.Traditionally,they are mainly produced by cracking of oil-derived hydrocarbons and dehydrogenation of alkanes.Taking into account the energy structure of "rich coal,meager oil,and deficient gas" in China,developing direct conversion of CO-rich syngas derived from coal or biomass to lower olefins,i.e.,Fischer-Tropsch-to-Olefins(FTO),is of vital strategic significance toward making fully use of the resources and ensuring the national energy security.Recently,supported Fe-based catalysts have gained a renewed interest not only for the higher water-gas shift activity enabling the direct use of CO-rich syngas without H2/CO ratio adjustment,but also for the lower cost,high mechanical stability and tolerance to contaminants.In this thesis,some unresolved issues of iron-based FTO catalysts such as the non-uniform distribution of iron species and promoters,the nature of catalyst surface and interfacial structures and the reaction mechanism are addressed by combining well-defined catalyst preparation methods,catalyst testing and characterization,density functional theory(DFT)calculations with steady-state isotopic transient kinetic analysis(SSITKA).A novel method is developed to prepare controllable Mn-and/or K-promoted iron-based composite nanocatalysts by a redox reaction between KMnO4 or K2FeO4 and carbon nanotubes(CNTs),and then the micromixing between iron species and promoters is achieved by tailoring the thermal treatments of the resultant iron-based composite nanocatalysts.Finally,the structure-performance relationship is established,and a new plausible Fischer-Tropsch synthesis mechanism is proposed.(1)Novel iron-based composite nanocatalysts(i.e.,Fe/MnK-CNTs)are developed by taking advantage of unique structural transformation of MnK-CNTs,synthesized by a redox reaction between KMnO4 and CNTs,as a function of temperature and atmosphere.That is,increasing the thermal treatment temperature results in spontaneous transformation from the coating layers to the spherical nanoparticles,and appropriate reduction treatment with the crystal phase transformation from MnO2 to MnO(regarded as an effective promoter of Fe-based catalysts for the production of lower olefins).Compared to FeMnK/CNTs catalyst prepared by traditional co-impregnation method,our developed Fe/MnK-CNTs composite nanocatalysts present small-sized and uniform iron nanoparticles,well-distributed promoters,weaker metal-support interaction and more defects on support.(2)The novel Fe/MnK-CNTs composite nanocatalysts exhibit shorter induction period as well as higher activity compared to the FeMnK/CNTs catalyst.Under similar CO conversion levels,the Fe/MnK-CNTs catalysts show higher selectivity to lower olefins and paraffin-to-olefin ratio in lower hydrocarbons.TEM and XRD measurements show that the used Fe/MnK-CNTs catalyst compared the used FeMnK/CNTs catalyst does not show obvious agglomeration,but still present uniform iron nanoparticles,and contains more active phase x-Fe5C2.These results suggest that exploiting the unique structural transformation of MnK-CNTs as a function of temperature and atmosphere is an efficient method to prepare highly efficient Fe-based FTO catalysts.(3)Manipulating the calcination temperature of the Fe/MnK-CNTs catalysts significantly change the morphology,metal-support interaction,surface defects on the support,carbidization behavior and stability.The resultant catalyst from the calcination temperature of 220 ? appeares to be the optimal one,delivering a high iron time yield of 337.2?molco·gFe-1·s-1 with 51.3%C selectivity toward C2=-C4=under the optimal reaction conditions(270 ?,2.0 MPa,30000 mL·h-1 ·gcat-1).On the other hand,there exists a trade-off between the selectivity of lower olefins and the carbidization extent with the increase of the Mn loading.In addition,lower carbidization temperature and/or higher carbon chemical potential(?c)facilitate the formation of active phase.(4)Another novel iron-based composite nanocatalyst(i.e.,FeK-OX)is prepared by a redox reaction between K2FeO4 and carbon nanotubes.The novel FeK-OX composite catalyst shows small-sized and uniform nanoparticles,unique metal-promoter interaction and more surface defects on the support compared to the FeK-IM catalyst prepared by traditional impregnation method.As consequences,this novel catalyst exhibits higher stable mass-specific reactivity with respect to the formation of lower olefins and shorter catalyst induction period.Additional potassium promoters introduced by impregnation method further endow the catalysts with higher FTY and less selectivity to methane.These results indicate that introducing additional potassium promoters to the FeK-OX composite catalyst is an efficient method to enhance the activity and suppress the formation of methane.(5)Concerning the complexity of Fischer-Tropsch synthesis reaction pathways,a multicomponent steady-state isotopic transient kinetic analysis method,dealing with total number of surface active sites,turnover frequency and the isotopic distributions of intermediates,is developed to probe into the information of C2-C6 products and thus understand the nature of iron-based FTO catalyst.Two kinds of CO activation mechanisms(i.e.,the direct and H-assisted CO dissociation)and the six corresponding CO activation pathways as well as CH4 formation on dominant surfaces of x-FesC2 and ?-Fe2C are investigated by DFT calculations.Kinetic study based on SSITKA is performed to gain a better understanding of the elementary reactions at different conditions by measuring the surface concentrations of CO and CHx intermediates.Afterwards,the preferred CO activation pathway and rate determining step are discriminated by a combination of DFT calculations and SSITKA.(6)Combining all the above results,a plausible reaction mechanism of FTO on the main active phase x-Fe5C2 of iron-based Fischer-Tropsch catalyst is proposed.That is,the resultant CHx species from CO activation react with the surface C atoms of iron carbide or the corresponding CHy species by hydrogenation to form C-C coupling products,resulting in the formation of carbon vacancy for subsequent CO activation to recover the initial iron carbide surface and thus to follow the above similar C-C coupling mechanism toward chain growth.Interestingly,the C atom of the products locating at the end of the carbon chain derives from the surface C atom of the iron carbide.