Fischer–Tropsch synthesis over cobalt catalysts supported on zirconia-modified alumina

The effect of adding zirconia to the alumina support on supported cobalt Fischer–Tropsch catalysts has been studied. At 5 bar and H₂:CO ratio 9:1 zirconia addition to the support leads to a significant increase in both activity and selectivity to higher hydrocarbons as compared to the unmodified catalysts. Reducibility and cobalt dispersion on the other hand are not improved by the presence of zirconia compared to the unmodified catalysts. SSITKA measurements have been performed in order to determine the intrinsic activity per active site. At constant temperature, zirconia-modified and unmodified catalysts showed basically the same intrinsic activity. Similar results were obtained with a noble metal (Pt) promoted catalyst. The promoting effect appears to be mainly due to coverage effects rather than a change in the intrinsic activity of the active sites. The turnover frequencies were found to be independent of pressure but strongly temperature dependent. However, the increase in turnover frequency did not account for the entire increase in reaction rate with temperature. This indicates that also the coverage of reactive intermediates increases with increasing temperature.     

Construction of the Fischer–Tropsch regime with cobalt catalysts

Changes of activity and selectivity during the initial phases of Fischer–Tropsch (FT) synthesis have been measured with three promoted cobalt catalysts. It is shown that the FT regime is formed in situ in a slow process lasting several days. A “construction” of the “true FT catalyst” is therefore assumed. Taking into account complementary investigations, this construction is assigned to the segregation of the catalyst surface caused by strong CO chemisorption. This process would be accompanied by an increase of the number of active sites and their disproportionation into sites of higher and lower coordinations, which would exhibit different catalytic properties. The observed initial activity and selectivity changes are well to be explained with this concept.     

Fischer–Tropsch synthesis over Co–SiO2 catalysts prepared by the sol–gel method

Fischer–Tropsch synthesis was carried out in slurry phase over uniformly dispersed Co–SiO₂ catalysts prepared by the sol–gel method. When 0.01–1 wt.% of noble metals were added to the Co–SiO₂ catalysts, a high and stable catalytic activity was obtained over 60 h of the reaction at 503 K and 1 MPa. The addition of noble metals increased the reducibility of surface Co on the catalysts, without changing the particle size of Co metal significantly. High dispersion of metallic Co species stabilized on SiO₂ was responsible for stable activity. The uniform pore size of the catalysts was enlarged by varying the preparation conditions and by adding organic compounds such as N,N-dimethylformamide and formamide. Increased pore size resulted in decrease in CO conversion and selectivity for CO₂, a byproduct, and an increase in the olefin/paraffin ratio of the products. By modifying the surface of wide pore silica with Co–SiO₂ prepared by the sol–gel method, a bimodal pore structured catalyst was prepared. The bimodal catalyst showed high catalytic performance with reducing the amount of the expensive sol–gel Co–SiO₂.     

Fischer–Tropsch synthesis over Re-promoted Co supported on Al2O3, SiO2 and TiO2: Effect of water

The activity and selectivity of rhenium promoted cobalt Fischer–Tropsch catalysts supported on Al₂O₃, TiO₂ and SiO₂ have been studied in a fixed-bed reactor at 483 K and 20 bar. Exposure of the catalysts to water added to the feed deactivates the Al₂O₃ supported catalyst, while the activity of the TiO₂ and SiO₂ supported catalysts increased. However, at high concentrations of water both the SiO₂ and TiO₂ supported catalyst deactivated. Common for all catalysts was an increase in C₅₊ selectivity and a decrease in the CH₄ selectivity by increasing the water partial pressure. The catalysts have been characterized by scanning transmission electron microscope (STEM), BET, H₂ chemisorption and X-ray diffraction (XRD).     

Silica supported cobalt Fischer–Tropsch catalysts: effect of pore diameter of support

The influence of the effect of average pore diameter of silica support on the physical and chemical properties of supported cobalt catalysts and their performance in the Fischer–Tropsch synthesis was investigated. Silicas with different mean pore diameter (20, 40, 60, 100 and 150 Å) were impregnated with cobalt nitrate to produce catalysts containing 20 wt.% cobalt. The metal crystallite size and degree of reduction was found to increase with increasing pore diameter of the support for supports with an average pore diameter larger or equal to 40 Å, and hence the dispersion decreased. In impregnated catalysts, the metal crystallites seems to appear in clusters on the support. With increasing average pore diameter, the size of these clusters increases. In the Fischer–Tropsch synthesis, the 100 Å supported catalyst proved to be the most active and selective catalyst for hydrocarbon formation. The C₅₊ and methane selectivity passed through a maximum and minimum at the 100 Å supported catalyst, respectively, which can be explained quantitatively using the reactant transport model proposed by Iglesia et al.     

Optimization of the pretreatment procedure in the design of cobalt silica supported Fischer–Tropsch catalysts

The effect of cobalt precursor, catalyst pretreatment and promotion with ruthenium and rhenium on the formation of cobalt metal nanoparticles and catalytic performance of supported Fischer–Tropsch (FT) catalysts was studied using a combination of techniques (DSC–TGA, UV–vis spectroscopy, XPS, XRD, EXAFS–XANES, in situ magnetization measurements, propene chemisorption and catalytic measurements). The cobalt promoted and unpromoted catalysts were prepared by aqueous co-impregnation using cobalt nitrate or acetate, ruthenium nitrosyl nitrate or perrhenic acid. In both unpromoted and Ru and Re-promoted cobalt catalysts after impregnation and drying, cobalt is present mainly in octahedrally coordinated complexes. The repartition of cobalt species between Co₃O₄ and cobalt silicate depends essentially on the exothermicity of precursor decomposition. Cobalt nitrate precursor, with an endothermic decomposition, favors Co₃O₄ crystallites. Lower temperature of cobalt nitrate decomposition and catalyst calcination generally leads to higher dispersion of supported cobalt oxide. Cobalt acetate precursor, with an exothermic decomposition, favors cobalt silicate. By optimizing the conditions of cobalt acetate decomposition, the fraction of cobalt silicate can be decreased favoring a more reducible Co₃O₄ phase. For the catalysts prepared from cobalt nitrate, promotion with ruthenium increases the cobalt dispersion, while maintaining high reducibility. For the catalyst prepared via low temperature decomposition of cobalt acetate, addition of ruthenium increases the fraction of Co₃O₄ crystalline phase and decreases the concentration of barely reducible cobalt silicate. The Fischer–Tropsch reaction rates over unpromoted and promoted cobalt catalysts were found to be primarily a function of the number of cobalt metal sites, which are generated by the reduction of Co₃O₄ crystallites.     

Effect of boron source on the catalyst reducibility and Fischer–Tropsch synthesis activity of Co/TiO2 catalysts

A series of Co/B/TiO₂ (B=ammoniumborate, boric acid, o-carborane, 0.01–1.5 wt.% B) catalysts were synthesized. The addition of boron decreased the reducibility of the Co as determined from temperature-programmed reduction studies and H₂ reduction/O₂back titration studies. This in turn decreased the FT activity but not the turnover frequency of the Co catalyst.     

Novel utilization of MCM-22 molecular sieves as supports of cobalt catalysts in the Fischer–Tropsch synthesis

Cobalt catalysts (2–10 wt% Co) supported on silica-rich MCM-22 zeolites have been prepared by impregnation with aqueous Co(NO₃)₂ solutions. The catalysts are characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD), nitrogen adsorption, solid state nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The catalytic properties for the Fischer–Tropsch synthesis (FTS) at 280 °C, 12.5 bar and H₂/CO = 2 are evaluated. The catalysts supported on MCM-22 exhibit the highest selectivity to long-chain (C₅₊) hydrocarbons when MCM-22 supports are synthesized with the appropriate Si/Al ratio.     

Oxidation of cobalt based Fischer–Tropsch catalysts as a deactivation mechanism

The oxidation of supported cobalt based slurry bed Fischer–Tropsch catalysts by means of water was studied. Water is one of the Fischer–Tropsch reaction products and can probably cause oxidation and deactivation of a reduced cobalt catalyst. Model experiments using Mössbauer emission spectroscopy and thermogravimetry as well as realistic Fischer–Tropsch synthesis runs were performed. It was demonstrated that Mössbauer emission spectroscopy can successfully be applied to the investigation of high cobalt loading Fischer–Tropsch catalysts. Strong indications were found that oxidation of reduced cobalt catalysts occurs under realistic Fischer–Tropsch conditions. Mössbauer emission spectroscopy and thermogravimetry results showed that the oxidation depends on the P_H2/P_H2O ratio, and that oxidation proceeds to less than complete extents under certain conditions. The formation of both reducible and less reducible cobalt oxide species was observed, and the relative ratio between these species depends on the severity of the oxidation conditions.     

Comparison of preparation methods for carbon nanotubes supported iron Fischer–Tropsch catalysts

Carbon nanotubes supported iron catalysts were prepared by incipient wetness, deposition/precipitation using K₂CO₃, and deposition/precipitation using urea. The incipient wetness method and the deposition/precipitation technique using urea yielded highly dispersed Fe³⁺ on the carbon nanotubes support. The deposition/precipitation technique using K₂CO₃ also yielded larger Fe₂O₃-crystallites. After reduction the three catalysts had similar metal surface areas. Nevertheless, the activity of these catalysts in the Fischer–Tropsch synthesis differed significantly with the catalyst prepared by incipient wetness being the most active one. It is speculated that the differences in the performance of the catalysts might be attributed to the different crystallite size distributions, which would result in a variation in the amount of the different phases present in the catalyst under reaction conditions. The selectivity in the Fischer–Tropsch synthesis over the three catalysts seems to be independent of the method of preparation.     

Fischer–Tropsch synthesis on monolithic catalysts with oil circulation

Monolithic catalysts made of cordierite and γ-Al₂O₃ have been prepared and tested for the Fischer–Tropsch (FT) synthesis. When operated without oil circulation, washcoated cordierite monoliths have previously been shown to be as active and selective as the corresponding powder catalyst provided that the monoliths have low washcoat loadings. Two-phase operation, i.e. with oil/product circulation during reaction, resulted in improved heat removal and temperature control, in lower apparent activity and faster deactivation, but the C₅₊ selectivity was equal to or even better than without oil circulation. The lower apparent activities obtained with oil circulation seem to be a combination of catalyst deactivation and flow-related problems in the present experimental set-up.     

Fischer–Tropsch synthesis using monolithic catalysts

Square channel cordierite monoliths have been loaded with alumina washcoat layers of various thicknesses (20–110 μm) and loaded with rhenium and cobalt resulting in a 0.1 wt.% Re/17 wt.% Co/Al₂O₃ catalyst. These monolithic catalysts have been tested in the Fischer–Tropsch synthesis in a temperature window (180–225 °C) under synthesis gas compositions ranging from stoichiometrically excess carbon monoxide to excess hydrogen (H₂/CO = 1–3). The results include data on the activity and selectivity of CoRe/Al₂O₃ monolithic catalysts for FTS under these process conditions. Washcoat layers thicker than about 50 μm appear to lead to internal diffusion limitations. Thinner washcoat layers yield, depending on the conditions, to larger amounts of -olefins than alkanes for chain lengths below 10 carbon atoms. ASF and non-ASF chain length distributions are obtained for thin washcoats, whereby the chain growth probability increases from 0.83 to 0.93. Under certain conditions the amounts of alkanes even increase with chain length. These experimental results with different diffusion lengths have been used to analyze the effects of secondary reactions on FTS selectivity.     

Structural changes and activation treatment in a Co/SiO2 catalyst for Fischer–Tropsch synthesis

We examined the effect of the activation process on the structural and morphological characteristics of a cobalt-based catalyst for Fischer–Tropsch synthesis. A 10 wt.% Co/SiO₂ catalyst prepared by wet impregnation was separately activated under H₂, CO or a H₂/CO mixture. The structural changes during activation from 298 to 773 K were studied by in situ X-ray diffraction. Catalysts were examined by SEM, TEM, XPS and in situ DRIFT-MS. The H₂/CO activation produced redispersion of cobalt particles and simultaneous carbon nanostructures formation. The catalyst showed the highest performance in the Fischer–Tropsch synthesis after the H₂/CO activation.     

Kinetics of Fischer–Tropsch synthesis on titania-supported cobalt

Rate data have been obtained for CO hydrogenation on a well-characterized 11.7% Co/TiO₂ catalyst in a differential fixed bed reactor at 20 atm, 180–240°C, and 5% conversion over a range of reactant partial pressures. The resulting kinetic parameters can be used to model precisely and accurately the kinetics of this reaction within this range of conditions. Turnover frequencies and rate constants determined from this study are in very good to excellent agreement with those obtained in previous studies of other cobalt catalysts, when the data are normalized to the same conditions of temperature and partial pressures of the reactants. Based on this comparison CO conversion and the partial pressure of product water apparently have little effect on specific rate per catalytic site. The data of this study are fitted fairly well by a simple power law expression of the form −r_CO=kP_H2^0.74P_CO^−0.24, where k=5.1×10⁻³ s⁻¹ at 200°C, P=10 atm, and H₂/CO=2/1; however, they are best fitted by a simple Langmuir–Hinshelwood (LH) rate form −r_CO=aP_H2^0.74P_CO/(1+bP_CO)² similar to that proposed by Yates and Satterfield.     

Fischer–Tropsch synthesis on monolithic catalysts of different materials

Monolithic structures made of cordierite, γ-Al₂O₃ and steel have been prepared as catalysts and tested for Fischer–Tropsch activity. The monoliths made of cordierite and steel were washcoated with a 20 wt.% Co–1 wt.% Re/γ-Al₂O₃ Fischer–Tropsch catalyst whereas the γ-Al₂O₃ monoliths were made by direct impregnation with an aqueous solution of the Co and Re salts resulting in a loading of 12 wt.% Co and 0.5 wt.% Re. The activity and selectivity of the different monoliths were compared with the corresponding powder catalysts.

Higher washcoat loadings resulted in decreased C₅₊ selectivity and olefin/paraffin ratios due to increased transport limitations. The impregnated γ-Al₂O₃ monoliths also showed similar C₅₊ selectivities as powder catalysts of small particle size (38–53 μm). Lower activities were observed with the steel monoliths probably due to experimental problems.     

Transient studies of the elementary steps of Fischer–Tropsch synthesis

The pulse transient method has been used to study the kinetics of several key steps of Fischer–Tropsch (FT) synthesis over cobalt supported catalysts. These elementary steps involve chemisorption of hydrogen and propene, and chemisorption and hydrogenation of carbon monoxide. It is found that at the conditions of Fischer–Tropsch synthesis, hydrogen chemisorption is reversible and quasi-equilibrated, while carbon monoxide adsorption is generally irreversible. Chemisorption of propene on cobalt metal sites results in its rapid autohydrogenation to propane and simultaneous formation of C_xH_y surface species.

The transient response curves produced during hydrogenation of carbon monoxide pulses in a flow of hydrogen have been analyzed using the modified Kobayashi model, which involves irreversible chemisorption and dissociation of carbon monoxide, quasi-equilibrated adsorption of hydrogen and reversible adsorption of water. The kinetic analysis suggests that oxygen-containing species are probably the most abundant surface intermediates. Desorption of water from the catalysts seems to be much slower than hydrogenation of surface carbon species.     

C-tracer study of the Fischer–Tropsch synthesis: another interpretation

The ¹³C-tracer results from the introduction of ¹³C₂H₄ into syngas prior to conversion with a rhodium catalyst have been used to support a surface vinyl mechanism for Fischer–Tropsch synthesis. The results were first interpreted by a mechanism that involved a decrease in ¹³C species on the surface as the carbon number increased. This model is shown to be incorrect. Considering only the ¹³C-labeled products, the data are consistent with earlier tracer studies showing that the added ¹³C₂H₄ initiates chains.     

Attrition studies with catalysts and supports for slurry phase Fischer–Tropsch synthesis

Attrition properties of several oxide supports and precipitated iron-based F–T catalyst (100Fe/3Cu/4K/16SiO₂ in parts by weight) were evaluated using ultrasound irradiation test and stirred tank slurry reactor (STSR) test under non-reactive conditions. Attrition by fracture and erosion of the iron-based catalyst was small in both types of tests and its overall attrition strength was better than that of the alumina and silica supports, which were evaluated under the same conditions. Also, attrition studies with four iron-based F–T catalysts were conducted under reaction conditions in the STSR. Catalyst of similar composition, as that used in non-reactive studies, prepared by spray drying technique had the highest attrition strength among all catalysts tested.     

Development of Fe Fischer–Tropsch catalysts for slurry bubble column reactors

The effect of two binder systems — a silica-based system and a silica–kaolin–clay–phosphate-based system — on a doubly promoted Fischer–Tropsch (FT) synthesis iron catalyst (100Fe/5Cu/4.2K) was studied. The catalysts were prepared by coprecipitation, followed by binder addition and spray drying at 270°C in a 1 m diameter, 2 m tall spray dryer. The binder silica content was varied from 0 to 20 wt.%. A catalyst with 12 wt.% binder silica was found to have the highest attrition resistance. The FT activity and selectivity of this catalyst are better than a Ruhrchemie catalyst at 270°C and 1.48 MPa. The addition of precipitated silica or kaolin to catalysts containing 10–12 wt.% binder silica decreases attrition resistance and increases methane selectivity. Based on the experience gained, a catalyst has been successfully spray dried in 500 g quantity. This catalyst showed 95% CO conversion over 125 h of testing at 270°C, 1.48 MPa, and 2 NL/g-cat/h and had less than 4% methane selectivity. Its attrition resistance was one of the highest among the catalysts tested.     

Passivation of a Co–Ru/γ-Al2O3 Fischer–Tropsch catalyst

Passivation of highly dispersed metal catalysts after reduction is necessary prior to exposure to air due to the exothermicity of metal oxidation. This exothermicity can result in a significant increase in temperature of the catalyst resulting in catalyst degradation and a potential fire hazard. This paper reports the results of a study of passivation of Ru-promoted Co/alumina. Passivations using CO and CO+H₂ mixtures were compared to the standard method of passivation using small concentrations of O₂. Passivation by CO+H₂ resulted in a lower temperature rise upon exposure to air than oxygen passivation. Passivation using CO/H₂=10 resulted in a catalyst whose catalytic activity for CO hydrogenation was able to be recovered after exposure to air by re-reduction similar to after oxygen passivation. CO passivation yielded a catalyst that was not able to be as well recovered upon re-reduction, probably due to the formation of graphitic carbon. Exposure of the CO/H₂ passivated catalyst to air for at least 90 min actually made it easier to recover the original activity upon re-reduction. This is probably related to the oxidation of the carbidic passivation layer during air exposure.