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A systematic study has been carried out to investigate the impact of potassium promoter on the performance of a precipitated iron-manganese catalyst for Fischer–Tropsch synthesis (FTS). Characterization technologies of N2 physisorption, X-ray diffraction (XRD), Mössbauer effect spectroscopy (MES) and H2 thermal gravimetric analysis (H2-TGA) were used to study the effect of potassium on the textural properties, bulk phase composition and reduction behavior. FTS reaction test was performed in a fixed bed reactor. The results of characterization showed that the addition of potassium leads to the relatively large crystallite size of α-Fe2O3 and inhibits the reduction of catalyst. The carbonization of the catalyst is enhanced with the increase in both the potassium content and the reaction temperature. A maximum in FTS and water-gas shift (WGS) activity is noted upon increasing K content (0.7 wt.% K), followed by a sharp decline in activity at the potassium level in excess of the maximum. It is found that potassium is an effective promoter to suppress the hydrogenation function of the catalyst. The selectivity to olefins is promoted and the formation of methane and light hydrocarbons is restrained with the increasing potassium level. The selectivity to oxygenates shows a rapid and monotonic decrease with the increase of potassium loading and passes through a minimum at potassium loading of 0.7 wt.%. After the point, it increases slowly with further increasing in potassium content. At the same time, increasing reaction temperature results in a monotonic decrease in the weight percent of oxygenates over the un-promoted and potassium-promoted catalysts. [Copyright &y& Elsevier]

Abstract: Two model spherical iron catalysts (100Fe/0Al2O3 and 100Fe/15Al2O3) with free Cu and K promoters were prepared by the combination of co-precipitation and spray drying method for the application of slurry Fischer–Tropsch synthesis (FTS). The effect of Fe–Al2O3 interaction on the reduction/carburization behavior in H2/CO/syngas, surface basicity and the change of phase structure were comparatively studied by means of H2 or CO temperature-programmed reduction (TPR), CO2 temperature-programmed desorption (TPD) and Mössbauer effect spectroscopy (MES). The results showed that the catalyst incorporated with Al2O3 exhibits a strong Fe–Al2O3 interaction, which obviously weakens the surface basicity, stabilizes the FeO phase and inhibits the reduction of iron catalyst in H2 or syngas. Furthermore, Fe–Al2O3 interaction also restrains the carburization of iron catalyst in CO or syngas. In slurry FTS process, it was found that the strong Fe–Al2O3 interaction decreases the FTS activity and suppresses the water gas shift (WGS) reaction, but can stabilize the active sites of iron catalyst and improve its run stability. Due to the strong Fe–Al2O3 interaction, the weak surface basicity on the catalyst incorporated with Al2O3 greatly decreases the selectivity of heavy hydrocarbon products. [Copyright &y& Elsevier]

Abstract: An industrial iron-based catalyst (100Fe/5Cu/6K/16SiO2, by weight) was characterized after reduction at different temperatures and after Fischer–Tropsch synthesis (FTS) in a stirred tank slurry reactor (STSR). The BET surface area and pore volume of the catalyst decreases with increasing reduction temperature, and the contrary trend was found for pore size. The iron phase compositions of catalysts reduced with syngas were strongly dependent on pretreatment conditions employed. Pretreatment with syngas at lower temperature prevents iron catalyst activation. Carburization was intensified with the increase in reduction temperature. The formation of iron carbides in reduced catalyst was necessary for obtaining stable high FTS activity. The relationship between the amount of CO2 in tail gas during activation and the Fe3+ (spm) content in the reduced catalyst was observed. The rapid carburization at high reduction temperature resulted in the formation of a superparamagnetic Fe3+ core and an iron carbide layer of the reduced catalyst. FTS activity decreased with the increase in the reduction temperature, but the stability distinctly improved. It was found that the working catalyst loss in the heavier waxy products resulted in higher deactivation rate of the catalyst reduced at lower temperature. With the increase in the reduction temperature, the product distribution shifted towards the lower molecular weight products. [Copyright &y& Elsevier]

Abstract: The effect of adding SiO2 to a precipitated iron-based Fischer–Tropsch synthesis (FTS) catalyst was investigated using N2 physical adsorption, H2 differential thermogravimetric analysis, temperature-programmed reduction/desorption (TPR/TPD) and Mössbauer spectroscopy. The FTS performances of the catalysts with or without SiO2 were compared in a fixed bed reactor. The characterization results indicated that SiO2 facilitates the high dispersion of Fe2O3 and significantly influences the Fe/Cu and Fe/K contacts, which play an important role in the surface basicity, reduction and carburization behaviors, as well as the FTS performances. The incorporation of SiO2 enhances the Fe/Cu contact, further enlarges the H2 adsorption and promotes the reduction of Fe2O3 →FeO x , while the transformation of FeO x →Fe is suppressed probably due to the strong Fe–SiO2 interaction. SiO2 indirectly weakens the surface basicity and severely suppresses the carburization and CO adsorption of the catalyst. In the FTS reaction, it was found that SiO2 decreases the FTS initial activity but improves the catalyst stability. Due to the lower surface basicity than the catalyst without SiO2, the catalyst incorporated with SiO2 has higher selectivity to light hydrocarbons and methane and decreased selectivity to the olefins and heavy hydrocarbons. [Copyright &y& Elsevier]

Abstract: The metal–silica interaction and catalytic behavior of Cu-promoted Fe–Mn–K/SiO2 catalysts were investigated by temperature-programmed reduction/desorption (TPR/TPD), differential thermogravimetric analysis, in situ diffuse reflectance infrared Fourier transform analysis, and Mössbauer spectroscopy. The Fischer–Tropsch synthesis (FTS) performance of the catalysts with or without copper was studied in a slurry-phase continuously stirred tank reactor. The characterization results indicate that several kinds of metal oxide–silica interactions are present on Fe–Mn–K/SiO2 catalysts with or without copper, which include iron–silica, copper–silica, and potassium–silica interactions. In addition to the well-known effect of Cu promoter on easing the reduction of iron-based FTS catalysts, it is found that Cu promoter can increase the rate of carburization, but does not vary the extent of carburization during the steady-state FTS reaction. The basicity of the Cu and K co-promoted catalyst is greatly enhanced, as demonstrated by CO2-TPD results. In the FTS reaction, Cu improves the rate of catalyst activation and shortens the induction period, whereas the addition of Cu has no apparent influence on the steady-state activity of the catalyst. Promotion of Cu strongly affects hydrocarbon selectivity. The product distribution shifts to heavy hydrocarbons, and the olefin/paraffin ratio is enhanced on the catalyst due to the indirect enhancement of surface basicity by the copper promotion effect. [Copyright &y& Elsevier]