Study On Zr And K Modified Iron-Based Catalysts For Fischer-Tropsch Synthesis

Due to the limited supplement and unpredictable price of crude oil, Fischer-Tropsch (F-T) synthesis converting syngas (mainly composed with CO and H2) derived from coal, natural gas or biomass to clean liquid fuels and other chemicals, has been renewedly attracted much attention in recent years. One of the key technologies for indirect coal liquefaction is to explore high performance catalysts for F-T synthesis. The iron-based catalysts are often used in commercial operations, due to their low cost, flexible operation and high water-gas-shift (WGS) activity, which helps to make up the deficit of H2 in the syngas from coal gasification.Catalysts modified with Zr and K promoters in present study were prepared by a combination of coprecipitation. spry drying and impregnation techniques. The characterization technologies of N2-physisorption, X-ray diffraction (XRD), temperature-programming reduction (TPR), temperature-programming desorption (TPD) and Mossbauer effect spectroscopy (MES) were used to study the effects of incorporation manner of Zr, Zr and K promoters, different content of Zr addition on structure, reduction and carburization behaviors phase transformation and F-T synthesis performances of iron-based catalysts. The kinetics of CO consumption on selected catalytst was investigated in a stirred slurry autoclave. Works mentioned above can provide the basic support for industrial application of iron-based catalysts for F-T synthesis.Zr modified catalysts were prepared by adding Zr to Fe/SiO2 using coprecipitation and impregation. The effects of incorporation manner of Zr on structure, reduction, carburization, phase transformation and catalytic performance were investigated. The result of N2-physicorption shows that Zr decreases the catalyst surface area and total pore volume significantly, but increases the average pore size of the catalyst. Results of H2-TPR and CO-TPR indicate that Zr inhibites the reduction of catalyst in the atmosphere of H2 and CO. After reduction and reaction, catalyst Fe/SiO2 has the highest content of iron carbides, thus it owns the highest initial activity but deactivates quickly. Though Zr inhibits the formation of active phases and decreases the activity, Zr improves the stability of the catalyst apparently. Zr modified catalyst by impregnation possesses the best stability. Zr suppresses the formation of gaseous hydrocarbons (CH4 and C2-C4), but enhances the C5+selectivity significantly.Zr was added by impregnation and a series of catalysts (Fe/SiO2. Fe/Zr/SiO2, Fe/K/SiO2 and Fe/Zr/K/SiO2) were prepared in order to investigate the effects of Zr, K. the synergy effect of Zr and K on structure, reduction, carburization. phase transformation and catalytic performance. It is found that Zr and K decrease the surface area. The interaction between Zr and Fe suppresses the reduction or carbonization of Fe/Zr/SiO2 in H2 and CO. As an alkaline promoter, K enhances the adsorption of CO and promotes the reduction and carbonization in CO and feed gas. but obviously suppresses the adsorption and reduction in H2. When Zr and K coexit in catalyst, the synergy effect of Zr and K further enhances the adsorption of CO and facilitates the reduction and carbonization of the catalyst in CO and feed gas so that Fe/Zr/K/SiO2 has the highest activity. Zr and (or) K suppresses the formation of gaseous hydrocarbons (CH4 and C2-C4), but promotes the formation of heavy products to some extent. The selectivity to gaseous hydrocarbons (CH4 and C2-C4) reaches minimum on Fe/Zr/K/SiO2. In addition, the selectivity to olefin increases with the addition of Zr and K. Zr almost has no effect on WGS reaction activity, while K improves the WGS reaction activity significantly.A group of Fe/Zr/K/SiO2 catalysts with different content of Zr were prepared for the purpose of study the Zr loading on structure, reduction, carburization, phase transformation and catalytic performance. The results show that small amount of Zr increases the BET surface area, promotes the reduction/carburization and improves the activity of the catalyst. While large amounts of Zr addition decreases the surface area, suppresses the reduction/carburization of the catalyst, decreases the activity and speeds up the catalyst deactivation. With the increasing Zr loading, the selectivity to gaseous hydrocarbons (CH4 and C2～C4) first decreases then increases, while the selectivity to C5+is on the contrary.In addition, the selectivity to olefin increases and the activity of WGS reaction decreases with the increasing Zr loading. The optimal catalyst with n(Zr):n(Fe)=0.01 has high CO conversion and high selectivity to C5+hydrocarbons.The global kinetics of the Fischer-Tropsch synthesis on selected 100Fe/1Zr/3K/12SiO2 (in atomic ratio) catalyst was investigated in a 500mL stirred slurry autoclave. In the range of 240～270℃,1.0～2.2MPa.0.3～1.0NL/(g-h) and H2/CO=0.67. kinetic data were obtained under exclusion from effects of mass transfer. Parameters of rate constant, activation energy and adsorption coefficient are estimated by the Levenberg-Maquardit algorithm. The final kinetic model has the following rate expression: rco=9.96×10×exp(?)The results of statistical tests and relative error analysis show that the final kinetic model is appropriate.