Fischer-Tropsch, ruthenium catalyst

Iglesia, E., Soled, S. L., Fiato, R. A., and Via, G. H. 1993. Bimetallic synergy in cobalt-ruthenium Fischer-Tropsch synthesis catalysts. J. Catal. 143 345-68. 

Catalysts. The methanation of CO and C02 is catalyzed by metals of Group VIII, by molybdenum (Group VI), and by silver (Group I). These catalysts were identified by Fischer, Tropsch, and Dilthey (18) who studied the methanation properties of various metals at temperatures up to 800°C. They found that methanation activity varied with the metal as follows ruthenium > iridium > rhodium > nickel > cobalt > osmium > platinum > iron > molybdenum > palladium > silver. 

Mossbauer spectroscopy is a specialist characterization tool in catalysis. Nevertheless, it has yielded essential information on a number of important catalysts, such as the iron catalyst for ammonia and Fischer-Tropsch synthesis, as well as the CoMoS hydrotreating catalyst. Mossbauer spectroscopy provides the oxidation state, the internal magnetic field, and the lattice symmetry of a limited number of elements such as iron, cobalt, tin, iridium, ruthenium, antimony, platinum and gold, and can be applied in situ. 

Ruthenium is a known active catalyst for the hydrogenation of carbon monoxide to hydrocarbons (the Fischer-Tropsch synthesis). It was shown that on rathenized electrodes, methane can form in the electroreduction of carbon dioxide as weU. At temperatures of 45 to 80°C in acidihed solutions of Na2S04 (pH 3 to 4), faradaic yields for methane formation up to 40% were reported. On a molybdenium electrode in a similar solution, a yield of 50% for methanol formation was observed, but the yield dropped sharply during electrolysis, due to progressive poisoning of the electrode. 

Li, J.L., Jacobs, G., Das, T.K., and Davis, B.H. 2002. Fischer-Tropsch synthesis Effect of water on the catalytic properties of a ruthenium promoted Co/Ti02 catalyst. Appl. Catal. 233 255-62. 

Claeys, M., and van Steen, E. 2002. On the effect of water during Fischer-Tropsch synthesis with a ruthenium catalyst. Catal. Today 71 419-27. 

Tavasoli, A., Malek Abbaslou, R. M., and Dalai, A. K. 2008. Deactivation behavior of ruthenium promoted Co/a-Al203 catalysts in Fischer-Tropsch synthesis. Appl. Catal. A 346 58-64. 

Kogelbauer, A., Goodwin, Jr., J.G., and Oukaci, R. 1996. Ruthenium promotion of Co/A1203 Fischer-Tropsch catalysts. J. Catal. 160 125-33. 

Fischer-Tropsch synthesis could be "tailored by the use of iron, cobalt and ruthenium carbonyl complexes deposited on faujasite Y-type zeolite as starting materials for the preparation of catalysts. Short chain hydrocarbons, i.e. in the C-j-Cq range are obtained. It appears that the formation and the stabilization of small metallic aggregates into the zeolite supercage are the prerequisite to induce a chain length limitation in the hydrocondensation of carbon monoxide. However, the control of this selectivity through either a definite particle size of the metal or a shape selectivity of the zeolite is still a matter of speculation. Further work is needed to solve this dilemna. 

The incorporation of a ZSM-5 class zeolite into a ruthenium Fischer-Tropsch catalyst promotes aromatics formation and reduces the molecular weight of the hydrocarbons produced. These composite catalysts can produce a high octane aromatic gasoline in good yield in a single step directly from synthesis gas. 

The conversion of iron catalysts into iron carbide under Fischer-Tropsch conditions is well known and has been the subject of several studies [20-23], A fundamentally intriguing question is why the active iron Fischer-Tropsch catalyst consists of iron carbide, while cobalt, nickel and ruthenium are active as a metal. Figure 5.9 (left) shows how metallic iron particles convert to carbides in a mixture of CO and H2 at 515 K. After 0.5 and 1.1 h of reaction, the sharp six-line pattern of metallic iron is still clearly visible in addition to the complicated carbide spectra, but after 2.5 h the metallic iron has disappeared. At short reaction times, a rather broad spectral component appears - better visible in carburization experiments at lower temperatures - indicated as FexC. The eventually remaining pattern can be understood as the combination of two different carbides -Fe2.2C and %-Fe5C2. 

Methane is the principal gas found with coal and oil deposits and is a major fuel and chemical used is the petrochemical industry. Slightly less than 20% of the worlds energy needs are supplied by natural gas. The United States get about 30% of its energy needs from natural gas. Methane can be synthesized industrially through several processes such as the Sabatier method, Fischer Tropsch process, and steam reforming. The Sabatier process, named for Frenchman Paul Sabatier (1854—1941), the 1912 Nobel Prize winner in chemistry from France, involves the reaction of carbon dioxide and hydrogen with a nickel or ruthenium metal catalyst C02 + 4H2 —> CH4 + 2H20. 

The same catalyst compositions used in the more important methane steam reforming [Eq. (3.1), forward reaction], may be used in methanation, too.222 All Group VIE metals, and molybdenum and silver exhibit methanation activity. Ruthenium is the most active but not very selective since it is a good Fischer-Tropsch catalyst as well. The most widely used metal is nickel usually supported on alumina or in the form of alloys272,276,277 operating in the temperature range of 300-400°C. 

Isoalkanes can also be synthesized by using two-component catalyst systems composed of a Fischer-Tropsch catalyst and an acidic catalyst. Ruthenium-exchanged alkali zeolites288 289 and a hybrid catalyst290 (a mixture of RuNaY zeolite and sulfated zirconia) allow enhanced isoalkane production. On the latter catalyst 91% isobutane in the C4 fraction and 83% isopentane in the C5 fraction were produced. The shift of selectivity toward the formation of isoalkanes is attributed to the secondary, acid-catalyzed transformations on the acidic catalyst component of primary olefinic (Fischer-Tropsch) products. 

Good evidence has been obtained that heterogeneous iron, ruthenium, cobalt, and nickel catalysts which convert synthesis gas to methane or higher alkanes (Fischer-Tropsch process) effect the initial dissociation of CO to a catalyst-bound carbide (8-13). The carbide is subsequently reduced by H2to a catalyst-bound methylidene, which under reaction conditions is either polymerized or further hydrogenated 13). This is essentially identical to the hydrocarbon synthesis mechanism advanced by Fischer and Tropsch in 1926 14). For these reactions, formyl intermediates seem all but excluded. 

Ruthenium is known to catalyze a number of reactions, including the Fischer-Tropsch synthesis of hydrocarbons (7) and the polymerization of ethylene (2). The higher metal dispersions and the shape selectivity that a zeolite provides has led to the study of ruthenium containing zeolites as catalytic materials (3). A number of factors affect the product distribution in Fischer-Tropsch chemistry when zeolites containing ruthenium are used as the catalyst, including the location of the metal (4) and the method of introducing ruthenium into the zeolite (3). 

Dautzenberg et al. (3) have determined the kinetics of the Fischer-Tropsch synthesis with ruthenium catalysts. The authors showed, that because the synthesis can be described by a consecutive mechanism, the non steady state behaviour of the catalyst can give information about the kinetics of the process. On ruthenium they found that not only the overall rate of hydrocarbon production per active site is small, but also that the rate constant of propagation is low. Hence, Dautzenberg et al. find that the low activity of Fischer-Tropsch catalysts is due to the low intrinsic activity of their sites. On the other hand, Rautavuoma (4) states that the low activity of cobalt catalysts is due to a small amount of active sites, the amount being much smaller than the number of adsorption sites measured. 

Ruthenium supported on oxides is a catalyst of various reactions. It is active in methanation reactions [e.g. 1, 2, 3], in Fischer-Tropsch synthesis [e.g. 4, 5, 6], in CO oxidation [7, 8], in the synthesis of methyl alcohol [9], 1" the redu ction of NO to nitrogen CIO] and in hydrogenolysis of ethane [11] and of butane [12]. Ru supported on carbon is supposed to replace the iron in ammonia synthesis [13]. Lately ruthenium supported on oxides is intensively investigated as a potential... 

The reactions are catalyzed by transition metals (cobalt, iron, and ruthenium) on high-surface-area silica, alumina, or zeolite supports. However, the exact chemical identity of the catalysts is unknown, and their characterization presents challenges as these transformations are carried out under very harsh reaction conditions. Typically, the Fischer-Tropsch process is operated in the temperature range of 150°C-300°C and in the pressure range of one to several tens of atmospheres [66], Thus, the entire process is costly and inefficient and even produces waste [67]. Hence, development of more economical and sustainable strategies for the gas-to-liquid conversion of methane is highly desirable. 

It is now widely accepted that the activation of CO is highly structure sensitive (II). The activation of CO on most of the transition metals has been investigated. The computational results for cobalt (6) and ruthenium (5) are of particular relevance to us because these elements in the metallic state are active for the Fischer-Tropsch reaction. These results can be compared with those obtained for rhodium (40), which selectively catalyzes the formation of alcohols from CO and H2, and for nickel (30), which is a methanation catalyst. 

These trends are consistent with observations made to characterize the chain growth of surface carbon that was deposited by methane decomposition. In a row of the periodic table, the selectivity to hydrocarbon formation was foimd to increase from right to left for example, palladium shows a lower selectivity than ruthenium 111,112). Metals such as platinum and iridium are characterized by higher selectivities for chain growth initiated from "Cl" species than other metals because of their relatively high M—C bond energies. However, platinum and iridium are unsuitable as Fischer-Tropsch catalysts because the dissociation of CO is too slow. 

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