Design Of Active Phase For Fe,Co-based Catalysts And Performance In Fischer-Tropsch Synthesis

Fischer-Tropsch Synthesis?FTS?is an important catalytic process to produce long-chain hydrocarbons production from syngas.It is an important pathway to obtain valuable chemical products and liquid transportation hydrocarbon fuels from alternative carbon sources.Fe-based and Co-based catalysts have been widely used in industry for their excellent performance and cost-effective price.Clearly understanding of the structure-activity relationship of catalyst is the key to further enhance catalytic performance and productivity,which depends on deeply understanding the active phase nature of active phase.Fe-based catalysts have advantages of low cost and flexible conditions,while with complex active phase,poor catalytic activity and stability.For Co-based catalyst,obtaining higher active hexagonal closed packed cobalt by phase controlling is a significant and challenging topic.In addition,FTS products have a wide distribution which is limited by the ASF?Anderson-Schulz-Flory?distribution.Hence,it was also one of important factors that restricts the efficient conversion of syngas.In this thesis,research work was conducted on the vital issue of active phase and product selectivity for Fischer-Tropsch synthesis catalysts,the main conclusions are as follows:?1?Series of Fe₃C@C catalysts with different loading were synthesized by a melt approach.Characterization date show that Fe₃C@C catalysts possess number of advantages such as highly dispersion of active phase,suitable functional groups,abundant pore structure and special encapsulation structure,which are responsible for the excellent performance in Fischer-Tropsch synthesis reaction.The FTY value of55-Fe₃C@C was as high as 1250?mol_CO·g_Fe^-1·s^-1,and had no deactivation within 600h.By investigating carbon structure of Fe₃C@C catalysts at different reaction stages,it is found that the carbon in the framework of Fe₃C@C catalyst would be consumed continuously in high temperature Fischer-Tropsch synthesis,leading to the changes of catalyst structure and then affecting the catalytic activity.?2?A new“buffer layer model”was proposed to explain the uncontrollability of iron carbide in Fischer-Tropsch synthesis.The stabilizing effect of graphite layer on the iron carbide was studied by combining with theoretical calculations and experiment.It is found that the graphite layer can effectively prevent the formation of the“buffer layer”,thereby preventing the tranformation of metastable and highly active?-Fe₂C at high temperature,further to stabilize iron carbide phase.First principle calculations of the surface-normalized carbon absorption energy(?_abs)rationalize the feasible formation of?-Fe₂C from the carburization of?-Fe,especially by chemical bond formation between graphitic C and Fe atoms.The strong interfacial interactions contribute to the high stability and activity of the?-Fe₂C@graphite catalyst?3?With the use of carbon nanofiber as catalyst support,a new approach was developed to regulate the crystal form of Co.This was accomplished by controlling the crystal type of its oxide precursor CoO,which was found to be highly dependent to the final crystal form of reduced Co.The obtained Co/carbon nanofibres catalysts were characterized by in-situ XRD and TEM,and it was revealed that the formation of pure-phase CoO nanocrystals were responsible for the final pure-phase Co formation.Compared to the conventional reduction-carburization-reduction?RCR?process,this method allows for a much higher dispersion of Co particle with higher activity by avoiding agglomeration of the nanoparticles before or after reduction.Equally important,under FTS conditions,no catalyst deactivation and phase transformation were observed for 400 hours.?4?A Fischer-Tropsch synthesis/Olefin Metathesis bifunctional catalyst was developed for tuning product distributions in Fischer-Tropsch synthesis.It is found that Olefin Metathesis active sites can effectively prevent the secondary adsorption of olefins product on Fischer-Tropsch synthesis active sites,then improve conversion rate and decrease CH₄ selectivity.Furthermore,Olefin Metathesis active sites effectively convert ethylene and C₇₊olefins into C₄-C₆ olefins,and break the ASF law.Performances of metathesis component have an great important impact on products selecivity.When?zeolite was used as the support metathesis catalysts,C₄ component and iso-butane selectivities are as high as 43%and 34.7%,respectively.Both he acidity and pore structure of the zeolite can greatly affect the product distribution.