Effects of Zr and K Promoters on Precipitated Iron-Based Catalysts for Fischer–Tropsch Synthesis

Abstract  

The effects of Zr and K promoters on the structure, adsorption, reduction, carburization and catalytic behavior of precipitated iron-based Fischer–Tropsch synthesis (FTS) catalysts were investigated. The catalysts were characterized by N₂ physisorption, temperature-programmed reduction/desorption (TPR/TPD) and M?ssbauer effect spectroscopy (MES) techniques. As revealed by N₂ physisorption, Zr and/or K promoted catalysts showed lower surface area than Fe/SiO₂ catalyst. Zr promoter inhibited the reduction and carburization because of the interaction between Fe and Zr in Fe–Zr/SiO₂ catalysts. K promoter enhanced the reduction in CO and apparently facilitated the CO adsorption, thus promoted the carburization, but it retarded the reduction in H₂ and severely suppressed the H₂ adsorption. Compared with the singly promoted catalysts, the doubly promoted catalyst had the highest FTS activity. In addition, both Zr and K promoters suppressed the formation of methane and shifted the production distribution to heavy hydrocarbons.     

Negative Effects of CO<Subscript>2</Subscript> in the Feed Stream on the Catalytic Performance of Precipitated Iron-Based Catalysts for Fischer–Tropsch Synthesis

Abstract  

Fischer–Tropsch synthesis was carried out over precipitated iron-based catalysts with different amounts of CO₂ in the feed stream while maintaining both total reaction pressure (1.5 MPa) and partial pressure of H₂ + CO (0.75 MPa) using an inert balance gas, N₂. The CO₂ in the feed stream decreased the rate of hydrocarbon formation, but it had no significant influence on the carbon number distribution of hydrocarbons. The CO₂ in the feed stream also suppressed CO₂ formation, decreasing both CO conversion and CO₂ selectivity. We attribute the decreased reaction rate to the partial competition in the adsorption behavior between CO and CO₂ as revealed in the temperature-programmed desorption.     

Brief Review of Precipitated Iron-Based Catalysts for Low-Temperature Fischer–Tropsch Synthesis

Topics in Catalysis - Fischer–Tropsch synthesis (FTS) is a promising way to produce clean liquid fuels and high value-added chemicals from low-value carbon-containing resources such as coal,...     

Effects of Zn, Cu, and K Promoters on the Structure and on the Reduction, Carburization, and Catalytic Behavior of Iron-Based Fischer–Tropsch Synthesis Catalysts

Zn, K, and Cu effects on the structure and surface area and on the reduction, carburization, and catalytic behavior of Fe–Zn and Fe oxides used as precursors to Fischer–Tropsch synthesis (FTS) catalysts, were examined using X-ray diffraction, kinetic studies of their reactions with H₂ or CO, and FTS reaction rate measurements. Fe₂O₃ precursors initially reduce to Fe₃O₄ and then to metallic Fe (in H₂) or to a mixture of Fe_2.5C and Fe₃C (in CO). Zn, present as ZnFe₂O₄, increases the surface area of precipitated oxide precursors by inhibiting sintering during thermal treatment and during activation in H₂/CO reactant mixtures, leading to higher FTS rates than on ZnO-free precursors. ZnFe₂O₄ species do not reduce to active FTS structures, but lead instead to the loss of active components; as a result, maximum FTS rates are achieved at intermediate Zn/Fe atomic ratios. Cu increases the rate of Fe₂O₃ reduction to Fe₃O₄ by providing H₂ dissociation sites. Potassium increases CO activation rates and increases the rate of carburization of Fe₃O₄. In this manner, Cu and K promote the nucleation of oxygen-deficient FeO _x species involved as intermediate inorganic structures in reduction and carburization of Fe₂O₃ and decrease the ultimate size of the Fe oxide and carbide structures formed during activation in synthesis gas. As a result, Cu and K increase FTS rates on catalysts formed from Fe–Zn oxide precursors. Cu increases CH₄ and the paraffin content in FTS products, but the additional presence of K inhibits these effects. Potassium titrates residual acid and hydrogenation sites and increases the olefin content and molecular weight of FTS products. K increases the rate of secondary water–gas shift reactions, while Cu increases the relative rate of oxygen removal as CO₂ instead of water after CO is dissociated in FTS elementary steps. Through these two different mechanisms, K and Cu both increase CO₂ selectivities during FTS reactions on catalysts based on Fe–Zn oxide precursors.     

Effect of the Mesostructuration of the Beta Zeolite Support on the Properties of Cobalt Catalysts for Fischer–Tropsch Synthesis

The effect of mesostructuration of beta zeolite and of metal loading on the properties of cobalt-based catalysts for Fischer–Tropsch synthesis was studied in this work. The most active catalyst was the mesostructured beta zeolite-supported cobalt (10%), which also showed a low selectivity to methane and the lowest olefin/paraffin ratio.     

Fischer–Tropsch Synthesis on SiC-Supported Cobalt Catalysts

Seven SiC supports provided by SICAT with different surface areas and pore volumes were impregnated with 12,5 wt% Co. H₂-chemisorption, N₂-adsorption, temperature programmed reduction and Fischer–Tropsch synthesis in a fixed-bed reactor at 483 K, 20 bar and H₂/CO = 2.1 were performed in order to characterize and test the samples. The performances were compared with well characterized Co/Al₂O₃ and Co-Re/Al₂O₃ reference catalysts. The selectivity towards heavier hydrocarbons (C₅₊) was found to be moderately higher for the SiC supported catalysts while the site-time yields was 20 to 66 % lower than the Co-Re/Al₂O₃ catalyst. Elemental analysis showed the presence of several impurities in the SiC material. Alkali and alkaline earth elements, such as Na, K and Ca, are all known to lower the catalytic activity and also to influence the selectivity. It is proposed that these impurities in addition to sulfur and phosphorus known to be present in SiC, are responsible for the significantly lower catalytic activity of the SiC supported catalysts.     

Co/SiO2 Sol–Gel Catalysts for Fischer–Tropsch Synthesis

Six catalysts with a different cobalt content were prepared following the sol–gel method. The samples were tested by the Fischer–Tropsch synthesis performed both at low and high temperature and pressure. All the catalytic performances are well correlated with the characterization results which highlighted the good metal distribution and the presence of a large amount of metallic cobalt (but not the whole) on the surface of both 10 and 30% Co catalysts. As a result, high CO conversions were observed with a particular sensitivity to the reaction pressure and also the limitation of the produced hydrocarbons fraction to C₆ or C₉ at low or high pressure, respectively.     

Promotion Effects of Platinum and Ruthenium on Carbon Nanotube Supported Cobalt Catalysts for Fischer–Tropsch Synthesis

Abstract  

Carbon nanotube supported nano-size monometallic and noble metal (Pt and Ru) promoted cobalt catalysts were prepared by incipient wetness impregnation (IWI) using solution of cobalt nitrate and characterized by nitrogen adsorption isotherm, X-ray diffraction (XRD), temperature programmed reduction, in situ magnetic method and TEM. Analysis of the magnetization and H₂-TPR data suggested promotion with platinum and ruthenium significantly decreased the cobalt species reduction temperature. TEM and XRD results showed that the presence of noble metal promoters had no significant effect on the size of cobalt for carbon naotube as catalytic support. Promotion of cobalt carbon nanotube-supported catalysts with small amounts of Pt and Ru resulted in slight increase in Fischer–Tropsch cobalt time yield. The Pt and Ru promoted cobalt catalyst exhibited carbon monoxide conversion of 37.1 and 31.4, respectively. C₅+ hydrocarbon selectivity was attained at 80.0%. The Pt promoted cobalt supported on carbon nanotube yielded better catalytic stability than that of the monometallic cobalt catalyst.     

Cobalt Particle Size Effects in the Fischer–Tropsch Synthesis and in the Hydrogenation of CO2 Studied with Nanoparticle Model Catalysts on Silica

We have investigated the effect of cobalt nanoparticle size in Fischer–Tropsch synthesis (CO/H₂) and have compared it to data obtained for carbon dioxide hydrogenation (CO₂/H₂) using model catalysts produced by colloidal methods. Both reactions demonstrated size dependence, in which we observed an increase of the turnover frequency with increasing average particle size. In both case, a maximum activity was found for cobalt particles around 10–11 nm in size. Regarding the selectivity, no size-dependent effect has been observed for the CO₂ hydrogenation, whereas CO hydrogenation selectivity depends both on the temperature and on the size of the particles. The hydrogenation of CO₂ produces mainly methane and carbon monoxide for all sizes and temperatures. The Fischer–Tropsch reaction exhibited small changes in the selectivity at low temperature (below 250 °C) while at high temperatures we observed an increase in chain growth with the increase of the size of cobalt particles. At 250 °C, large crystallites exhibit a higher selectivity to olefin than to the paraffin equivalents, indicating a decrease in the hydrogenation activity.     

Fischer–Tropsch principles of co-hydrogenation on iron catalysts

The temporal changes of product composition together with changes of the catalyst in composition and structure have been investigated for Fischer–Tropsch synthesis with an alkalized precipitated iron catalyst at 250°C, 1 MPa, using a special synthesis gas with a molar H₂/CO₂-ratio of three. It was observed that the steady state of synthesis developed in processes of self-organization during several episodes with individual kinetic regimes. Thetrue FT catalyst apparently was constructed at reaction conditions under complete consumption of -iron and formation of iron carbide (Fe₅C₂). The magnetite phase disappeared partially and a new unknown (probably FeO_x) phase was formed. It has been concluded from the data of chain growth and branching probability that during self-organization only the number of sites increased but their nature remained unchanged. Strong spatial constraints appear to apply at the sites. On iron catalysts, the FT sites are very stable, invariant against changes in reaction conditions, in contrast to FT synthesis on cobalt. There the sites show a dynamic behavior.