Study On Fischer-tropsch Synthesis Using Fe@C Catalysts Obtained By Melting Method

Fischer-Tropsch synthesis is an important method to convert syngas（H₂/CO）into clean liquid fuels and high-valuable chemicals,which can effectively solve the current problem of shortage of petroleum resources and energy structure transformation.Iron-based Fischer-Tropsch synthesis catalysts have been widely used in Fischer-Tropsch synthesis due to their low price,rich contents,high activity and low selectivity of CH₄.However,there are some problems,such as high selectivity of CO₂ due to high activity of water-gas-shift reaction,easy deposition of carbon and oxidation,etc.The complex phase changes of iron-based catalysts in Fischer-Tropsch synthesis is leads to uncertainty of structure-activity relationship.Therefore,it is very important to know the effect of pretreatment conditions on the phase and structure of iron-based catalysts.In order to improve the catalytic activity and selectivity of the target product,the phase and structure of the catalyst were adjusted by changing the type of precursor and synthesis method.As-synthesized Fe@C catalysts prepared by melting method were investigated in this thesis.The effects of calcination temperature and pretreatment atmosphere on catalyst structure and catalytic performance were investigated,and the difference of catalytic performance between different phases of iron-based Fischer-Tropsch synthesis catalysts was further studied.The physical and chemical properties of the catalysts were analyzed by X-ray Diffraction（XRD）、X-ray Photoelectron Spectroscopy（XPS）、Nitrogen Physisorption（BET）、Raman Spectroscopy（Raman）、Thermogravimetirc（TG）、Transmission Electron Microscopy（TEM）and⁵⁷Fe Mossbauer Spectrometry(⁵⁷Fe MBS).The results are as follows:（1）Fe@C catalyst was prepared by melting method,the effects of calcination at different temperatures on the structure and catalytic performance of the catalyst were investigated.The results showed that the specific surface area of the catalyst was decreased at higher calcination temperature,but it has no obvious effect on the reaction activity.At the same time,different calcination temperature will affect the surface carbon species of catalyst,mainly the surface carbon and iron carbide.The catalyst was characterized by ⁵⁷Fe MBS.The results showed that the content ofχ-Fe₅C₂was the highest at 1100℃.The Fischer-Tropsch synthesis catalytic performance of a series of catalysts was tested.It was found that the catalyst roasted at 1100℃had the best catalytic performance,with the best CO conversion,the lowest CH₄selectivity and the highest C₅₊selectivity.The optimum reaction conditions were T=270℃,P=1.0 MPa,H₂/CO=1and space velocity=2 SL·g^-1·h^-1.（2）The best Fe@C catalysts prepared by（1）were pretreated in different atmospheres（H₂,CO）.The results showed that the change of pretreatment atmosphere had no obvious effect on the pore structure of catalyst.According to the performance test of the catalyst,the catalyst pretreated with CO for 3 h at 280℃has the highest activity.The activity of the catalysts pretreated with CO was stable and the selectivity of heavy hydrocarbons was high in 120 h.The catalysts pretreated with H₂ showed high selectivity to CH₄ and tended to produce gaseous hydrocarbons in the120 h stability test.Considering the activity,selectivity and reaction temperature,the optimum pretreatment conditions were 280℃and CO treatment for 3 h under atmospheric pressure.