Fischer-Tropsch (F-T)synthesis involves the catalytic reductive oligomerization of carbon monoxide in the presence of hydrogen as follows:
where n is an integer and (CH2)n represents hydrocarbons (paraffinic and olefinic), with an enthalpy of reaction of −165 kJ/mole (P. F. Schubert et al. (2001) “Studies in Surface Science and Catalysis 136: natural Gas Conversion VI,”. E. Iglesia et al. (eds.) Elsevier, Amsterdam p 459.) Generally, the process converts a mixture of CO and molecular hydrogen into a mixture of hydrocarbons, including saturated hydrocarbons and olefins. Oxygenated hydrocarbons, such as alcohols, and sometimes aromatics may also be formed in a F-T process. More specifically, products of F-T processes can include gaseous, liquid, heavy oil, and wax products which can be further upgraded to various fuels (gasoline, jet fuel, diesel fuel, etc.) and other value-added hydrocarbon products, particularly liquid hydrocarbons.
A mixture of CO and hydrogen for the F-T process can be generated in a number of ways, and in particular can be synthesis gas “syngas.” Syngas is available from a variety of sources and can, for example, be prepared by coal, biomass, or waste gasification processes or can be produced from methane, for example by steam reforming or partial oxidation. The ratio of H2 to CO in syngas from a given source may vary significantly (e.g., from about 0.3 up to 3 or higher) and a given F-T catalyst may be sensitive to that ratio (i.e., exhibit increased or decreased activity or changes in selectivity dependent upon that ratio.)
While a number of metals demonstrate F-T activity, only four metals: Fe, Co, Ru, and Ni are regarded in the art as usefully active. The utility of F-T catalysts is decreased if they exhibit high methanation activity. High levels of catalytic methane formation from CO and hydrogen by a catalyst as in the reaction:
decreases the utility of that catalyst for formation of higher hydrocarbons. For example, the utility of Ni on conventional metal oxide supports as an F-T catalyst is decreased by its high methanation activity. In contrast, Ni supported on some molecular sieves such as zeolite Y does not exhibit high methanation activity making it a more useful F-T catalyst.
The mechanism of the F-T catalytic reaction is dependent upon the catalyst employed. Some metals may dissociatively adsorb CO and others associatively adsorb this molecule. The tendency for dissociative adsorption increases on going from right to left on the Periodic Table. For example, iron dissociatively chemisorbs CO and, as a consequence, the active phase of the catalyst is not metal, but is generally regarded as a metal carbide phase (C. N. Satterfield (1991) “Heterogeneous Catalysis in Industrial Practice,” McGraw-Hill, New York p. 432.) In contrast, cobalt associatively adsorbs CO, with hydrogenation of the adsorbed CO to CHx fragments. The olefin reabsorption and CO hydrogenation models have been used to describe the F-T system and explain many of the features that have been observed. (E. Iglesia et al. (1993) in “Computer Aided Design of Catalysts,” E. R. Becker, et al. (eds) Marcel Dekker, New York p. 199.)
Features of established catalytic systems for F-T synthesis are summarized hereafter. Iron and cobalt-based systems have generally received more attention and certain of these systems have been commercialized. Iron and cobalt based systems have their origins with Fischer and Tropsch who developed alkalized iron turning- and Kieselguhr-supported cobalt/magnesia (or thoria) catalysts (See: F. Fischer and H. Tropsch (1923) Brennst. Chem. 4:276 and F. Fischer and H. Tropsch (1925) German Patent 484,337) Iron-based and cobalt-based catalysts each have advantages for use in appropriate circumstances. Iron-based catalysts are typically employed, for example, where H2:CO ratios are low (e.g., 0.5 to 1.5) as might be found in coal or biomass systems. In contrast, cobalt-based catalysts are more appropriately employed when H2:CO ratios are high (>1.5) which would typically be found in natural gas-to-hydrocarbon liquids applications.
Advantages of iron-based catalyst are generally lower cost, relative insensitivity to reaction conditions, such as H2:CO ratio and the presence of low levels of sulfide impurities; and their ability to function at low H2:CO ratios. Cobalt-based catalysts are considerably more expensive and can show strong sensitivity to H2:CO ratio. However, cobalt-based catalysts, in general, exhibit higher overall activity and increased mechanical strength compared to iron-based catalysts. Additionally, under like conditions, cobalt-based catalysts tend to produce a distribution of hydrocarbons that is more heavily weighted toward high carbon number species (i.e., give a higher chain growth probability, α) than the hydrocarbon distribution produced using iron-based catalysts. Iron-based F-T catalysts can also produce copious quantities of carbon dioxide in the F-T process, which is generally detrimental, because they can exhibit activity as an internal forward water-gas-shift catalyst.
The utility of F-T synthesis catalysts is significantly decreased if there is significant conversion of CO to CO2 rather than to value-added hydrocarbon and oxygenated hydrocarbon products. Preferred F-T synthesis catalysts exhibit low CO2 production particularly in combination with high CO conversion rates. An F-T catalyst may exhibit product selectivity for a desirable product composition. For example, certain F-T catalysts exhibit selectivity for production of higher molecular weight hydrocarbons, for production of low molecular weight hydrocarbons (e.g., low molecular weight olefins), for production of product with higher or lower amounts of olefinic materials, or for the production of product with higher or lower amounts of oxygenated materials (e.g., alcohols). Product selectivity of an F-T catalyst can be a significant factor in the utility of that catalyst for a given application.
The design and selection of F-T catalysts for low CO2 production and selective applications should consider the following composition related issues: whether the mechanism of adsorption of CO to the catalyst is associative or dissociative; the relative strengths of adsorption of CO and hydrogen to the catalyst; the redox activity or ease of reducibility of metals which is related to their activity for water-gas-shift reaction; the presence of strong or elastic bonds between component atoms of the catalysts support (if any), leading to larger elastic constants and higher mechanical strength of the catalyst support or support framework. Additionally, F-T synthesis can strongly depend on catalyst morphology, e.g., pore size, pore size distribution and particle size of the catalyst, because of the influence of such parameters on mass transport, and on the coupling between mass transport and chemical reaction. An additional factor that can be considered is catalyst acidity, because this property is expected to strongly influence hydrocracking activity by a catalyst, which is preferably to be avoided.
F-T synthesis is a process that involves an unusually high degree of interaction between reaction chemistry, catalyst properties, mass transport, reactor design, overall system integration, and economics. Catalyst selection and reactor design for a given F-T application depends upon all of these factors. For example, a cobalt catalyst may be appropriate for natural gas conversion to liquid hydrocarbons where its advantages in increased activity and longevity are beneficial. Because cobalt catalysts are sensitive to feed composition (i.e., H2:CO ratio), the F-T system employing the cobalt catalyst may then require a water-gas-shift module upstream of the F-T catalyst when low hydrogen content sources (such as coal) are used to ensure that the appropriate H2:CO ratio is provided. In contrast, unsupported baseline precipitated iron catalysts may not be amenable for use in slurry phase or fluidized bed reactors because of their lower mechanical strength and correspondingly high attrition rate, but would be preferred in other applications because of their relative insensitivity to H2:CO ratio and their product selectivity characteristics (i.e., greater selectivity for production of C5 or higher products at low H2:CO ratios). There is a continuing need in the art for catalysts for F-T synthesis which exhibit one or more beneficial characteristics for a given application, given H2/CO feedstock, given reactor design, and/or a desired product composition.
A large number of F-T catalysts are known in the art. The description that follows provides a brief summary of such catalysts.
U.S. Pat. No. 6,602,921 (Manzer) reports catalysts useful in a F-T process for producing hydrocarbons which comprise certain metals, i. e. cobalt, iron, nickel, or ruthenium and combinations thereof, in addition to silver supported on a catalyst support, such as alumina, zirconia, sulfated zirconia, tungsten oxide-doped zirconia, MCM-41, zeolites, clays, titania or silica. Catalytic activity is reported to be increased by the addition of silver.
U.S. Pat. No. 6,537,945 (Singleton et al.) reports an F-T catalyst comprising a gamma-alumina support doped with lanthanum oxide and barium oxide for increased thermal stability in a slurry bubble column reaction system and containing cobalt promoted with ruthenium as the active catalyst component. Catalyst activities are expressed in g-HC/Kg-cat/hr. (grams of hydrocarbon produced per gram of catalyst per hour), a measure of hydrocarbon productivity.
U.S. Pat. No. 6,515,035 reports a catalyst comprising at least one metal from Groups 8, 9 or 10 of the Periodic Table (IUPAC) impregnated on a modified alumina support that is pre-reduced using at least one reducing compound, for example selected from the group formed by hydrogen, carbon monoxide and formic acid, optionally mixed with an inert gas, for example nitrogen, in a reducing compound/(reducing compound+inert gas) mole ratio in the range of 0.001:1 to 1:1. The reduction may be carried out in the liquid phase with the catalyst suspended in an inert liquid phase.
U.S. Pat. No. 6,313,062 relates to the preparation of an F-T catalyst comprising a Group VII noble metal and an “Iron Group metal” (preferably cobalt) on a support. The catalysts are reported to have “high activity.”
U.S. Pat. Nos. 6,271,432 and 6,191,066 relate to F-T catalysts which are non-promoted cobalt catalysts supported on alumina which is preferably doped with titanium. U.S. Pat. No. 6,262,132 relates to a method for producing an attrition-resistant catalyst which employs an attrition-resistant support that is a titanium-doped gamma alumina. U.S. Pat. No. 6,100,304 relates to F-T catalysts which are certain supported palladium promoted cobalt catalysts. U.S. Pat. No. 5,939,350 relates to processes and catalysts for F-T synthesis in a slurry bubble column reactor (SBCR). The patent reports the use of certain non-promoted cobalt catalysts and certain palladium-promoted cobalt catalysts in the SBCR.
U.S. Pat. No. 5,856,365 relates to the preparation of a catalyst useful for conversion of synthesis gas wherein the catalyst comprises cobalt, ruthenium and either scandium or yttrium on an inert support. The support can be selected from at least one oxide of at least one of Si, Ti, Al, Zr, Zn, Mg, and Sn.
U.S. Pat. No. 5,728,918 reports a catalyst comprising cobalt on a support, used for conversion of synthesis gas with an H2:CO ratio of 1-3, preferably 1.8-2.2, to C5+ hydrocarbons at a pressure of 1-100 bar and at a temperature of 150-300° C., at a typical gas hourly space velocity of 1000-6000 v/hr/v. This catalyst is reported to be regenerated by contacting it with a gas containing carbon monoxide and less than 30% hydrogen, at a temperature more than 10° C. above Fischer-Tropsch conditions and in the range 100-500° C., and at a pressure of 0.5-10 bar, for air, at least 10 minutes, preferably 1-12 hours. The contact time period depends on temperature and gas hourly space velocity. This patent also reports an activation procedure, which may include a first step of contacting the catalyst with a gas containing molecular oxygen, preferably air at 200-600° C., at atmospheric pressure, for more than 30 minutes, and preferably for 1-48 hours.
U.S. Pat. Nos. 5,292,705 and 5,389,690, both to Mitchell, describe a fresh, previously reduced hydrocarbon synthesis catalyst activated by contact with hydrogen at elevated temperatures and pressures and in the presence of liquid hydrocarbons, preferably, sufficient to immerse the catalyst therein.
U.S. Pat. No. 5,292,705 relates to a method for activating a hydrocarbon synthesis catalyst by treating a reduced, “essentially fresh” (unused) catalyst with hydrogen in the presence of hydrocarbon containing liquids. The method is exemplified with a titania/alumina supported Co-Re catalyst.
U.S. Pat. No. 5,248,701 (Soled et al.) relates to a copper-promoted cobalt manganese spinel catalyst useful in F-T synthesis. The spinel is described as having the formula: Co3-xMnxO4, where x is from 0.5 to about 1.2. The patent also reports the formation of this cobalt-manganese mixed metal catalyst in an acidic aqueous solution containing alpha-hydroxy carboxylic acids rather than in alkaline solution to provide the spinel with high BET surface area of at least 5 m2/g. The copper promoter is reported to be preferably present in the catalyst from about 0.1 to about 5 gram atom %. While a relatively high level of CO conversion is reported for the catalysts tested, a significant amount of the CO was converted to carbon dioxide. This patent provides a brief description of several additional U.S. patents which relate to iron-cobalt spinel catalysts for F-T synthesis, including U.S. Pat. Nos. 4,518,707; 4,584,323 and 4,607,020, which relate to iron-cobalt spinels for selective conversion of syngas to alpha-olefins and where the spinels may be reduced and carburized, copper-promoted or carbided in situ, U.S. Pat. Nos. 4,537,867; 4,544,672 and 4,544,674, which relate to iron-cobalt spinels containing low levels of cobalt for selective conversion of syngas to low molecular weight (C2-C6) olefins and where the spinel may be promoted or reduced and carbided.
U.S. Pat. No. 5,140,050 (Mauldin et al.) relates to catalysts useful in F-T synthesis which are prepared on binder-containing titania supports. Catalysts are prepared by dispersing catalytic amounts of metal or metals, on the support. Preferred metals are cobalt or a combination of cobalt with another metal.
U.S. Pat. No. 5,145,876 (Shutt) reports a F-T catalyst that is ruthenium metal supported on a high surface area support and which is promoted employing bromine moieties. The preferred support is gamma-alumina. The specific source of ruthenium and bromine for catalyst preparation is disclosed as an aqueous solution of ruthenium bromide, such as ruthenium tribromide.
U.S. Pat. No. 4,861,802 (McCann) reports a modified F-T process utilizing certain perovskite catalysts for the synthesis of low molecular weight olefinic hydrocarbons. The catalyst comprise a perovskite oxide of the empirical formula AB1-a Fea O3, wherein A is selected from cations of the alkali metals, alkaline earth metals, lanthanides, Th or U, B is selected from cations of certain transition metal or main group elements (Al, Ga, In, TI, Ge, Sn, Pb, Sc, Y, Ti, Zr, HF, V, Nb, Ta, Cr, Mo, W, Mn, Tc, Re, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, or Hg) and a is within the range 0.01 to 1. The perovskites tested exhibited at most about 8% conversion of CO with a large amount of the converted CO going to CO2.
U.S. Pat. No. 4,738,948, issued on Apr. 19, 1988, describes a catalyst comprising cobalt and ruthenium at an atomic ratio of 10-400, on a refractory carrier, such as titania or silica. The catalyst is used for conversion of synthesis gas with an H2:CO ratio of 0.5-10, preferably 0.5-4, to C5-C40 hydrocarbons at a pressure of 80-600 psig and at a temperature of 160-300° C., at a gas hourly space velocity of 100-5000 v/hr/v. This catalyst is reported to be regenerated by contacting it with hydrogen gas at 150-300° C., preferably 190-260° C., for 8-10 hours.
U.S. Pat. No. 4,595,703 reports a catalyst comprising cobalt or thoria promoted cobalt on a titania support, used for conversion of synthesis gas with an H2:CO ratio of 0.5-4, preferably 2-3, to C10+ hydrocarbons at a pressure of preferably 80-600 psig, and at a temperature of 160-290° C., at a gas hourly space velocity of 100-5000 v/hr/v. This catalyst is reported to be regenerated by contacting it with hydrogen gas, or a gas which is inert or non-reactive at stripping conditions such as nitrogen, carbon monoxide, or methane, at a temperature substantially the same as Fisher-Tropsch conditions. If it is necessary to remove coke deposits from the catalyst, the catalyst can be contacted with a dilute oxygen-containing gas, at oxygen partial pressure of at least 0.1 psig, at 300-550° C., for a time sufficient to remove coke deposits, followed by contact with a reducing gas containing hydrogen, at a temperature of 200-575° C. and at a pressure of 1-40 atmospheres, for 0.5-24 hours.
U.S. Pat. No. 4,585,798 reports a catalyst comprising cobalt and ruthenium in an atomic ratio greater than about 200:1 and, preferably, a promoter, such as a Group IIIB or IVB metal oxide, on an alumina support, used for conversion of synthesis gas to hydrocarbons at a pressure of preferably 1-100 atmospheres and at a temperature of 160-350° C., at a gas hourly space velocity less than 20,000 v/hr/v, preferably 100-5000 v/hr/v, especially 1000-2500 v/hr/v, which is reported to be activated prior to use by reduction with hydrogen gas, followed by oxidation with diluted air, followed by further reduction with hydrogen gas.
U.S. Pat. No. 4,544,671 (Soled et al.) relates to F-T synthesis catalysts which are slurried high surface area iron-cobalt spinels and to a slurry process employing the catalysts. The catalysts are certain unsupported Group IA or IIA metal salt-promoted Fe—Co spinels. The patent reports certain high surface area (100-200 m2/g) Fe—Co spinels prepared in an acid aqueous solution containing alpha-hydroxy carboxylic acids. The iron to cobalt atomic ratio of the metals in the spinel is reported to be 4:1 or above. The catalysts are reported to be useful in slurry processes for selectively producing high amount of C2 to C20 alpha-olefin materials.
U.S. Pat. No. 4,151,190 reports a catalyst comprising at least one of a sulfide, oxide, or metal of Mo, W, Re, Ru, Ni, or Pt, at least one of a hydroxide, oxide, or salt of Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, or Th, and a support, used for conversion of synthesis gas with an H2:CO ratio of 0.25-4.0, preferably 0.5-1.5, to C2-C4 hydrocarbons at a pressure of 15-2000 psia and a temperature of 250-500° C. at a typical gas hourly space velocity of 300 v/hr/v. This catalyst is reported regenerated by contacting it with hydrogen gas at 500-600° C. for 16 hours.
Moser et al. (Moser, W. R., Lennhoff, J. D., Cnossen, J. E., Fraska, K., Schoonover J. W., and Rozak, J. R. (1996) in “Advanced Catalysts and Nanostructured Materials; Modern Synthetic Methods” (Moser, ed.) Academic, San Diego, Calif., p. 540) reports the synthesis and testing of certain perovskite compositions by spray pyrolysis or high temperature aerosol decomposition (HTAD). The materials synthesized have the general stoichiometry: La(1-x)MxFeO3, where M is Ca2+ or Sr2+; La(1-x)LnxFeO3, where Ln is certain unidentified lanthanide ions; and La(1-x)Srx CoO3 where in each case x ranges between 0.0 to 1.0, as well as certain unidentified perkovskites in the LaCoO3 and LaFeO3 series “having two additional ions modifying both the A- and B-sites simultaneously.” All of the perovskites tested were reported to be catalytically activity for F-T synthesis. However little detail of the catalytic activity of the catalysts as a function of structure was provided. CO consumption rate (normalized to unit surface area) for the La(1-x)CaxFeO3 series as a function x from 0.0 to 0.8 was illustrated in FIG. 3 of the reference to increase with increasing x value (i.e. increasing Ca content). It was also reported that the surface areas of the perovskites could be varied from 10 to 50 m2/g and that the perovskites prepared had little to no micropore structure. Perovskites prepared by classical synthesis by high temperature fusion techniques were reported to result in materials having much lower surface areas (0.1-1 m2/g) compared to HTAD materials and in materials in which the perovskite could not be obtained free from separate phases of the component metal oxides.
Goldwasser et al. J. Mol. Catalysis A: Chemical 193(2003) 227-236 report a series of iron perovskite oxide materials containing K or Mn or both as precursors for conversion of syngas to low molecular weight alkenes the having the general composition:La1-xKxMnyFe1-yO3where 0≦x, y≦0.2, specifically La Fe O3, La0.9K0.1FeO2.9, LaO0.8K0.2FeO2.9, LaFe0.8Mn0.2O3, and La0.9K0.1Fe0.9Mn0.1O2.9. The active catalysts are prepared by reduction of the raw catalyst. Prior to assessing catalytic activity the perovskite oxides were subjected to reduction at 450° C. in a flow of H2 (30 mL/min/g catalyst) for 16 h followed by exposure to CO (10 mL/min/g catalyst) for 8 h at 150° C. and for carburated catalysts at 350° C. for 14 h. The activity of the perovskite-based catalysts was reported to seem to be directly related to the formation of Hatgg carbides. For conversion of a mixture where H2/CO=2, SCO2(%) (CO2 selectivity) of 30 or more was observed.
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