Fischer-Tropsch Activity Research Articles

Effect of cobalt carboxylate precursor chain length on Fischer-Tröpsch cobalt/alumina catalysts

Abstract The effect of the cobalt carboxylate chain length (C2, C5 and C9) on the preparation of alumina supported cobalt catalysts has been studied by TPR, XRD and hydrogen chemisorption techniques. The activity and selectivity of the prepared catalysts have been evaluated for Fischer-Tropsch (FT) activity in a stirred basket reactor. It is shown that for catalysts with Co content of 10 wt.% the activity increases as the carboxylate chain length increases while the selectivity towards methane and light hydrocarbons decreases with the carboxylate chain length. This change relates to the effect of the chain length on the catalyst particle size. The catalyst prepared using cobalt acetate was found to present the highest metal–support interaction and the poorest performance for the Fischer-Tropsch reaction. When the metal content was increased to 15 and 20 wt.% Co, respectively, the metal–support interaction for the catalyst particles prepared from cobalt acetate significantly decreased making it a better catalyst for the FT reaction compared to the catalyst particles prepared from C5 and C9 cobalt carboxylates.

Cobalt dispersion, reducibility, and surface sites in promoted silica-supported Fischer–Tropsch catalysts

Cobalt particle size, cobalt reducibility, and metal surface sites in a series of ruthenium- and rhenium-promoted cobalt silica-supported Fischer–Tropsch catalysts were studied by X-ray diffraction, UV–vis spectroscopy, in situ X-ray absorption, in situ magnetic method, X-ray photoelectron spectroscopy, DSC-TGA thermal analysis, and propene chemisorption. The catalysts were prepared by co-impregnation; in several catalyst syntheses, sucrose was added to the impregnating solutions. Mononuclear octahedral cobalt complexes were observed in the catalysts after impregnation and drying. Cobalt repartition on silica in the impregnated and dried catalysts depended primarily on the pH of the impregnating solution. Cobalt repartition was uniform on the silica surface if the pH of the impregnating solution was higher than the point of zero charge (PZC) of silica, but was less uniform at pH below that of the PZC of silica. Cobalt dispersion proceeded during catalyst calcination in air. Decomposition of cobalt nitrate and crystallization of cobalt oxide seemed to be the crucial steps in the preparation of highly dispersed cobalt catalysts. Promotion with noble metals resulted in greater cobalt dispersion, probably due to higher concentrations of cobalt oxide crystallization sites. Addition of sucrose modified the structure of supported cobalt complexes and led to higher temperatures of crystallization of cobalt oxide and to catalysts with extremely high cobalt dispersion. In situ magnetization measurements show that promotion with Ru moderated the temperature of reduction of cobalt oxide to metal phases, whereas the effect was less significant for Re-promoted catalysts. The addition of sucrose during impregnation, although significantly enhancing cobalt dispersion, did not diminish cobalt reducibility. Due to a combination of high cobalt dispersion and reducibility, the ruthenium- and rhenium-promoted catalysts prepared using sucrose had the highest number of cobalt metal surface sites. Fischer–Tropsch reaction rates were determined principally by the number of cobalt surface sites, with high cobalt dispersion and easy reducibility resulting in more active Fischer–Tropsch catalysts.

Evolution of the bulk structure and surface species on Fe–Ce catalysts during the Fischer–Tropsch synthesis

Two Fe–Ce catalysts were prepared by wet impregnation of Ce onto iron oxyhydroxide (FeOOH) and hematite iron oxide (α-Fe2O3), respectively. Their performance in the Fischer–Tropsch (FT) synthesis was investigated and compared with that obtained with a Ce-free α-Fe2O3 catalyst. It was observed that the behavior of the different catalysts changed along the course of the FT reaction. The catalysts were tested for different periods of time, carefully passivated, recovered from the reactor and characterized by different techniques. The FT activity of the Ce-loaded and Ce-free catalysts decreased initially, but at a certain point the catalytic activity started to increase. The time needed to reach this inflection point depended on the catalyst composition, being shorter for the Ce-promoted catalysts. The catalytic activity of the Ce-free catalyst increased when the Fe3C species were transformed into χ-Fe2.5C, which are suggested to be the carbide phase present when polymerized carbon species (Cβ) are formed. The addition of Ce to the iron oxyhydroxide developed solids with a higher BET surface area. Besides, these samples displayed a higher FT activity at long time-on-stream (TOS). Moreover, Ce addition also facilitated the formation of the Cβ species previous to the evolution of Fe3C into χ-Fe2.5C, and therefore, promoted the FT synthesis reaction.

Highly active Fischer–Tropsch synthesis Co/SiO2 catalysts prepared from microwave irradiation

Studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) as well as energy-diffusive X-ray spectroscopy (EDX), a highly active Co/SiO2 catalyst which was prepared through microwave irradiation showed more uniform and better dispersion of the Co particles within the catalyst pellets, compared to that prepared by the conventional heating method. The Fischer–Tropsch (FT) synthesis activity of the microwave-irradiated catalyst was greater than that of the conventional heating one. Not only did the catalytic activity depend on the particle distribution, but also the irradiative time. The longer irradiative time increased the catalytic activity. It was shown that the irradiative time of 14min was an optimum. It was found that cobalt distributed uniformly inside the whole catalyst pellet prepared by the microwave irradiation. On the contrary, cobalt concentration at the outer surface, with agglomerated form, was greater than that at the center or inner part of the catalyst pellet made by the conventional heating method. Higher dispersion state of the supported cobalt, as also proved by XRD, realized at the microwave irradiated catalyst determined its improved FT activity.

Improvement of promoters on the Fischer–Tropsch activity of mechanically mixed Fe catalysts

The precipitated Fe2O3 was promoted with K, Cu, Zn and Al by impregnation and mechanical mixing. Their activity for Fischer–Tropsch (FT) synthesis was tested in fixed bed reactor under the conditions of 503K and 1.6MPa syngas (H2/CO=2). The Fe catalysts co-promoted by K, Cu, Zn and Al showed increasing CO conversion with time on stream. At present, the improvement is ascribed to the increased resistance to water oxidation on active sites and the further formation of active sites during FT synthesis with the addition of K, Cu, Zn and Al to Fe catalysts. The carbon deposition is not responsible for the variety of catalytic activity in this study.

Effect of catalyst pore size on the catalytic performance of silica supported cobalt Fischer–Tropsch catalysts

A series of cobalt catalysts supported on silica with different pore size were prepared by incipient wetness impregnation method. N 2 physisorption, XRD, H 2-TPR, H 2-TPD, DRIFTS and O 2 pulse reoxidation were used to characterize the catalysts. The results showed that the pore size of the support had a significant influence on the Co 3O 4 crystallite diameter, catalyst reducibility and Fischer–Tropsch activity. The larger pore could cause the Co/SiO 2 to form larger Co 3O 4 crystallite and decreased its dispersion. Catalysts with different pore size showed different CO adsorption property. The catalysts with pore size of 6–10 nm displayed higher Fischer–Tropsch activity and higher C 5+ selectivity, due to the moderate particle size and moderate CO adsorption on the catalysts.

Deactivation of a precipitated iron Fischer–Tropsch catalyst—A pilot plant study

A pilot plant Fischer–Tropsch synthesis reaction was performed in a fixed bed reactor containing a precipitated iron catalyst. Sections were taken from the reactor and tested for Fischer–Tropsch activity in micro reactors. The catalyst sections were subjected to SEM, SIMS and XRD analysis and revealed catalyst deactivation due to (i) sulphur poisoning in the top portion of the reactor bed and (ii) magnetite formation and crystallite growth in the bottom portion of the reactor bed.

Effect of K, Mn and Cr on the Fischer–Tropsch Activity of Fe:Co/TiO2 Catalysts

A range of 5% Fe/5% Co bimetallic catalysts supported on TiO2 was promoted with 0.1%, 0.5% and 1.0% potassium, chromium and manganese respectively, and their physical properties and Fischer–Tropsch (FT) CO hydrogenation capabilities were evaluated. Low level promotion (0.1% and 0.5%) by K and Cr modestly improved the activity, decreased the selectivity to methane and increased the +C5 fraction of the FT product while the Mn promoted catalysts retarded activity and reduced selectivity.

Characteristics of Cobalt Nanoneedles in 10% Co/Aerogel Fischer−Tropsch Catalyst

The 10% Co/aerogel catalyst used in Fischer−Tropsch (FT) synthesis recently by Dunn et al. (Dunn, B. C.; Cole, P.; Covington, D.; Webster, M. C.; Pugmire, R. J.; Ernst, R. D.; Eyring, E. M.; Shah, N.; Huffman, G. P. Appl. Catal., A 2005, 278, 233) is investigated using the complimentary techniques of X-ray diffraction (XRD), transmission electron microscopy (TEM), and detailed magnetic measurements. TEM studies show that the catalyst contains Co nanoneedles with diameter d ≈ 1 nm, whereas the XRD studies show the presence of CoO and cobalt silicate hydroxide Co3(Si2O5)(OH)2, in addition to Co. Analysis of the temperature and magnetic field variation of the magnetization from 5 to 350 K shows that the measured magnetic properties are primarily due to the Co nanoneedles oriented randomly, with concentration ≈8%. The presence of the unreduced CoO and the Co silicate explains why the FT activity of this catalyst is about equal to that of the 6% Co/aerogel catalyst. Some interesting magnetic properties of the Co nanoneedles are also presented.

Fe:Co/TiO 2 bimetallic catalysts for the Fischer–Tropsch reaction : Part 3: The effect of Fe:Co ratio, mixing and loading on FT product selectivity

A series of catalysts, Fe:Co/TiO 2, were made with different total metal loadings (Fe:Co wt.% loading; 1:1–5:5), with different Fe:Co ratios (at 10% metal loading) and using different synthetic procedures (impregnation versus physical mixing). The catalysts were characterised by BET, CO-chemisorption and Mössbauer spectroscopy. Their Fischer–Tropsch (FT) activity/selectivity properties were also measured. The co-impregnated catalysts revealed that the 5:5 Fe:Co catalyst showed higher activity than catalysts prepared with either 2.5:7:5 or 7.5:2.5 Fe:Co ratios. Physically mixed samples of Fe/TiO 2 and Co/TiO 2 showed different selectivities and also gave better FT activity than equivalent co-impregnated Fe:Co samples. The reducibility behaviour and Mössbauer spectra of the Fe:Co samples suggested that the catalyst activity and selectivity changes related to the Co phase, Fe–Co alloy formation and the ability of the Co to enhance the reducibility of the Fe. A relationship between reduction level and activity was observed. The bimetallic systems also showed good stability and only modest changes in selectivity with time on line.

Silica aerogel supported catalysts for Fischer–Tropsch synthesis

Cobalt catalysts supported on silica aerogel have been prepared using sol–gel chemistry followed by drying under supercritical ethanol conditions. Three different loadings of cobalt were synthesized: 2, 6, and 10% by weight. Transmission electron micrographs indicate that the metallic cobalt exists as discrete particles 50–70 nm in diameter for the 2 and 6% loadings. The 10% catalyst shows long needles of cobalt. BET and BJH measurements indicate that the catalysts retain the silica aerogel properties of high surface area (∼800 m 2/g), large pore volume (∼5 cm 3/g), and an average pore diameter in the mesoporous regime (∼25 nm). The catalysts were evaluated for Fischer–Tropsch activity in a laboratory-scale packed bed reactor. All three catalysts were active with the 10% Co catalyst achieving more than 20% CO conversion which corresponds to a rate of 1.53 g CO per g-cat per hour. The catalysts were selective for the C 10+ hydrocarbons with more than 50% of the carbon contained within this fraction. A significant portion of the C 9–C 15 hydrocarbon product was observed as 1-olefins which reflects the enhanced mass transport within the very porous aerogel support.

Surface and reactor study of the effect of zinc on titania-supported Fischer–Tropsch cobalt catalysts

A series of Co/Zn/TiO 2 catalysts have been synthesized and their Fischer–Tropsch (FT) activity studied. The effect of changing the sequence of impregnating zinc (5%) and cobalt (10%) onto the titania support as well as the effect of calcination of the catalyst in-between the impregnation steps have generated a range of new catalysts. Results obtained from XPS, AES and chemisorption (with hydrogen) techniques suggest that a ‘sandwich’ layer of zinc is formed in-between the cobalt and the titania even when the zinc was impregnated after the cobalt. The study reveals that Zn forms agglomerates on the TiO 2 surface and that the Co sits on top of the non-crystalline Zn particles. The presence of zinc was found to have both beneficial as well as detrimental effects on Fischer–Tropsch activity that was determined by the amount of Zn added and the preparation procedures used to make the Co/Zn/TiO 2 catalysts. Zinc was found to increase the dispersion of cobalt, but uncovered zinc particles were found to be active under the FT synthesis conditions forming mostly methane.

Impact of Copper on an Alkali Promoted Iron Fischer–Tropsch Catalyst

Copper has traditionally been added to precipitated iron Fischer–Tropsch (FT) catalysts to facilitate reduction of Fe2O3 to zero valent iron during activation [M.E. Dry, in: J.R. Anderson and M. Boudart (eds), Catalysis Science and Technology, Vol. 1 (Springer-Verlag, New York, 1981) p. 179] by lowering the reduction temperature when activating with hydrogen, carbon monoxide or syngas [R.J. O'Brien et al., Catal. Today 36 (1997) 325]. This is particularly important when activating with hydrogen because metallic iron which is formed will sinter easily if the temperature is too high; however, it is not as critical when activating with carbon monoxide or syngas because iron carbides are formed and they are not as susceptible to sintering. The effect of copper on activity and selectivity has not been studied as thoroughly as its effect on catalyst activation. Kolbel reported that copper loadings less than 0.1% (weight % relative to iron) were sufficient to produce an active catalyst and that increased copper loading had no effect on FT activity [H. Kolbel and M. Ralek, Catal. Rev. Sci. Eng. 21 (1980) 225]. It has previously been shown that copper increases the activity of precipitated iron catalysts when operating at low temperature (<250°C) [2]. Bukur and Mukesh have reported that copper increases FT activity and water gas shift activity [D.B. Bukur et al., Ind. Eng. Chem. Res. 29 (1990) 194]. In addition, they reported that copper increased the average molecular weight of the product and increased hydrogenation of alkenes and isomerization of 1-alkenes. Soled et al., have reported that promotion with copper in conjunction with potassium increased FT activity but had little effect on alkene selectivity [S.L. Soled et al., Top. Catal. 2 (1995) 193]. Water gas shift activity and FT selectivity have been shown to depend on syngas conversion [A.P. Raje and B.H. Davis, Catal. Today 36 (1997) 335]. To determine the true impact of copper on FT selectivity and water gas shift activity comparisons should be done at similar conversion. No such study has been reported for the effects of copper. Herein are reported the effects of copper on FT activity and selectivity and water gas shift activity over a wide range of syngas conversions.

Support and rhenium effects on the intrinsic site activity and methane selectivity of cobalt Fischer–Tropsch catalysts

Catalyst composition effects on site activity, active carbon coverage, and methane selectivity in the cobalt-catalyzed Fischer–Tropsch synthesis were examined using 12CO→ 13CO isotope transients at reaction steady state. All catalysts (18 total) were tested at identical conditions: 221 °C, 6.5 atm of H 2/CO/Ne=2/1/1. The intrinsic site activity of unsupported Co and Co-supported on SiO 2, TiO 2, and Al 2O 3 (Co dispersion < 10% ) varies within a relatively narrow band ( ∼ 2x maximum rate difference). Re present at a Re/Co weight ratio of 0.1 does not affect Fischer–Tropsch activity and selectivity. Since its own activity is 20 times lower than that of Co, Re does not appear to block or modify active Co sites. While Re and the above-noted three common oxide supports do not affect the Fischer–Tropsch activity and selectivity of the relatively large cobalt crystallites, some modifications of SiO 2 and Al 2O 3 supports (with MgO, Y 2O 3, ZnO, and Ce 2O 3, by coating and coprecipitation) reduce site activity by up to an order of magnitude. Interestingly, the reduced site activity does not measurably affect methane selectivity. For all catalysts, the observed site-activity differences, whether small or large, do not correlate with bulk composition, and are possibly the result of unidentified impurities introduced during catalyst preparation. These impurities affect the reactivity of both the adsorbed CO and the active carbon present on the catalyst: higher site activity correlates with higher active carbon reactivity. Although this correlation is strong, some variability in the CO/C ∗ reactivity ratio does occur. This variability appears to play a key role in methane selectivity, namely, higher values correlate with lower methane selectivity. These observations can be rationalized by our previously published simple kinetic model.

Co 0 from partial reduction of La(Co,Fe)O 3 perovskites for Fischer–Tropsch synthesis

Small Co clusters ( d<10 nm) supported over mixed La–Co–Fe perovskites were successfully synthesized. These catalysts were active for Fischer–Tropsch (FT). Depending on the Co to Fe ratios the mixed perovskite exhibited two different forms: the rhombohedral phase of LaCoO 3 is maintained for the mixed perovskite when x>0.5, the orthorhombic phase of LaFeO 3 is found for x<0.5. Interestingly only one of these structures is active for the FT reaction: the orthorhombic structure. This is most likely due to the capacity of this material to maintain its structure even with a high number of cation vacancies. These cations (mostly Co) were on purpose extracted and reduced. Magnetic measurements clearly showed their metallic nature. Rhombohedral Co–Fe mixed perovskites ( x≥0.5) cannot be used as precursors for Fischer–Tropsch catalysts: their partial reduction only consists in a complete reduction of Co 3+ into Co 2+. The partial reduction of orthorhombic perovskites ( x<0.5) leads to active Fischer–Tropsch (FT) catalysts by formation of a metal phase well dispersed on a cation-deficient perovskite. The FT activity is related to the stability of the precursor perovskite. When initially calcined at 600 °C, a maximum of 8.6 wt.% of Co 0 can be extracted from LaCo 0.40Fe 0.60O 3 (compared to only 2 wt.% after calcination at 750 °C). The catalyst is then composed of Co 0 particles of 10 nm on a stable deficient perovskite LaCo 0.05 3+Fe 0.60 3+O 2.40. Catalytic tests showed that up to 70% in the molar selectivity for hydrocarbons was obtained at 250 °C, 40% of which was composed of the C 2–C 4 fraction.

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 20wt.% Co–1wt.% 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 12wt.% Co and 0.5wt.% 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 (38–53μm). Lower activities were observed with the steel monoliths probably due to experimental problems.

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.

97/05163 Effect of water partial pressure on steady state Fischer-Tropsch activity and selectivity of a promoted cobalt catalyst: Schulz, H. et al. Stud. Surf Sci. Catal., 1997, 107 (Natural Gas Conversion IV), 193–200

97/05163 Effect of water partial pressure on steady state Fischer-Tropsch activity and selectivity of a promoted cobalt catalyst: Schulz, H. et al. Stud. Surf Sci. Catal., 1997, 107 (Natural Gas Conversion IV), 193–200

Behaviour of a cyanide-derived Fe/Al 2O 3 catalyst during Fischer-Tropsch synthesis

The interaction of carbon monoxide and hydrogen with an alumina-supported iron catalyst has been studied at temperatures ranging from 300 to 523 K and at a pressure of 103 kPa. The species produced and adsorbed on the catalyst surface have been examined by infrared spectroscopy, while the chemical state of the iron within the catalyst was investigated by magnetization measurements and Mössbauer spectroscopy. At 300 K the main type of adsorbed species on the iron particle surface appeared to be molecular carbon monoxide and formate groups. The amount of the latter species increases considerably at elevated reaction temperatures up to 473 K. Changes in the infrared absorption bands of CO, and in the magnetization and Mössbauer data indicate that under the applied conditions the surface properties of the iron particles are affected by the synthesis gas, but that the bulk remains metallic. The increased Fischer-Tropsch activity at more elevated temperatures, viz. 473 and 523 K, is accompanied by significant changes in the in situ recorded infrared spectra. The formation of hydrocarbons is evident from the well developed absorption bands in the 3000-2800 cm −1 range, due to (a)symmetric stretching vibrations of CH 2 and CH 3 groups. Assignment of the various absorption bands in the 1700-1200 cm −1 wavenumber range is speculative due to (partial) coincidence of the bands and the gradual change in chemical composition of the catalyst particles. At higher temperatures the bulk of the iron particles reacts rapidly to a mixture of carbides. Initially the unstable ε-Fe 2C is formed which reacts to ε′-Fe 2.2C upon prolonged periods of time of reaction and/or increased reaction temperatures. These carbides appeared to be thermally unstable: upon heating in helium they are converted into a mixture of χ-Fe 5C 2 and metallic Fe. Interaction of the reduced iron catalysts with ethylene or methane at 523 K gives rise to absorption bands at 1555, 1345 and 1050 cm −1, which are assigned to CH x adsorbed species. Upon reaction with ethylene also a band at 2160 cm −1 was observed, which was assigned to an ethylidyne species.

95/04651 Moessbauer study of iron-carbide growth and Fischer-Tropsch activity

There is a need to establish a correlation between the Fischer-Tropsch (FT) activity of an iron-based catalyst and the catalyst phase during FT synthesis. The nature of iron phases formed during activation and FT synthesis is influenced by the gas used for activation. Moessbauer investigations of iron-based catalysts subjected to pretreatment in gas atmospheres containing mixtures of CO, H{sub 2}, and He have been carried out. Studies on UCI 1185-57 catalyst indicate that activation of the catalyst in CO leads to the formation of 100% magnetite and the magnetite formed gets rapidly converted to at least 90% of x-Fe{sub 5}C{sub 2} during activation. The x-Fe{sub 5}C{sub 2} formed during activation gets partly (= 25%) converted back to Fe{sub 3}O{sub 4} during FT synthesis and both x-Fe{sub 5}C{sub 2} and Fe{sub 3}O{sub 4} reach constant values. On the other hand, activation of the catalyst in synthesis gas leads to formation of Fe{sub 3}O{sub 4} and which is slowly converted to x-Fe{sub 5}C{sub 2} and e-Fe{sub 2.2}C during activation, and both carbide phases increase slowly during FT synthesis. FT synthesis activity is found to give rise to {approx} 70% (H2+CO) conversion in the case of CO activated catalyst as compared to {approx}more » 20% (H2+CO) conversion in the case of synthesis gas-activated catalyst.« less