Fischer methane formation

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. 

In reality, not only the main reaction (the Fischer-Tropsch reaction) leading to the formation of higher hydrocarbons (Equation 12.1), but also methane formation (Equation 12.2) and the water-gas shift reaction (Equation 12.3) have to be considered. The rate equations for these three reactions on a commercial Fe-catalyst were determined by Popp8 and Raak2 and summarized by Jess et al.9 However, to simplify matters, just the Fischer-Tropsch reaction forms the basis of the approach presented here ... 

Fischer-Tropsch process. The methane selectivity is t)q)ically 30-60 mol%. This significant selectivity for methane formation implies that the rate of hydrogenation of the "Ci" species is of the same order of magnitude as the rate of chain growth. The Fischer-Tropsch process is economical only if a major fraction of the carbon in the synthesis gas is converted to long-chain hydrocarbons and not more than about 10% is converted to methane (41). 

These observations regarding the stability of the methyl species are in line with conclusions about the frustration of methane formation in Fischer-Tropsch catalysis 40) and also the inference that the slow step in CO methanation is the reaction between methyl groups and hydrogen 41), as Fig. 8c illustrates a resistance of the methyl groups to hydrogenation. 

Indirect evidence of the mechanism of methane formation was reported in the early part of the twentieth century (Sohngen, 1910), and in the 1930s (Stephenson and Strickland, 1931, 1933 Fischer, Lieske, and Winzer, 1931, 1932). Sohngen found that enrichment cultures can couple the oxidation of hydrogen with the reduction of carbon dioxide according to ... 

Fischer R, Thauer RK (1988) Methane formation from acetyl phosphate in cell extracts of Metha-nosarcina barkeri. Dependence of the reaction on coenzyme A. FEBS Lett 228 249-253 Fisher JR (1966) Bacterial leaching of Elliot Lake uranium ore. Transactions 69 167-171 Freitag A, Bock E (1990) Energy conservation in Nitrobacter. FEMS Microbiol Lett 66 157-162... 

Cobalt precipitation catalysts. The development of cobalt catalysts (Fischer and Koch, 12) was similar to the development of the corresponding nickel catalysts. In the case of cobalt, however, it was easier to prevent extensive methane formation. The lOOCo 18Th02 100 kieselguhr catalyst became the so-called standard cobalt catalyst. 

A variety of catalysts can be used for the Fischer-Tropsch process, but the most common are the transition metals cobalt, iron, and ruthenium. Nickel can also be used but tends to favor methane formation (methanation). 

Numerical Modeling of Transient Isotope Responses On the first step, the authors analyzed in detail five possible heterogeneous methanation models based on two gas phase (CO, CH4) and three surface components (COads, Ca,ads, and Cp,ads) that follow from qualitative analysis of CO labeling data [19,21]. These models contained either a buffer step or parallel routes of methane formation. The homogeneous model having one type of methane intermediate was also considered. A methanation reaction was modeled separately from the entire set of reactions included in the Fischer-Tropsch ... 

In order to produce methanol the catalyst should only dissociate the hydrogen but leave the carbon monoxide intact. Metals such as copper (in practice promoted with ZnO) and palladium as well as several alloys based on noble group VIII metals fulfill these requirements. Iron, cobalt, nickel, and ruthenium, on the other hand, are active for the production of hydrocarbons, because in contrast to copper, these metals easily dissociate CO. Nickel is a selective catalyst for methane formation. Carbidic carbon formed on the surface of the catalyst is hydrogenated to methane. The oxygen atoms from dissociated CO react with CO to CO2 or with H-atoms to water. The conversion of CO and H2 to higher hydrocarbons (on Fe, Co, and Ru) is called the Fischer-Tropsch reaction. The Fischer-Tropsch process provides a way to produce liquid fuels from coal or natural gas. 

Methane Formation versus Fischer-Tropsch Kinetics... 

Fischer-Tropsch Process. The Hterature on the hydrogenation of carbon monoxide dates back to 1902 when the synthesis of methane from synthesis gas over a nickel catalyst was reported (17). In 1923, F. Fischer and H. Tropsch reported the formation of a mixture of organic compounds they called synthol by reaction of synthesis gas over alkalized iron turnings at 10—15 MPa (99—150 atm) and 400—450°C (18). This mixture contained mostly oxygenated compounds, but also contained a small amount of alkanes and alkenes. Further study of the reaction at 0.7 MPa (6.9 atm) revealed that low pressure favored olefinic and paraffinic hydrocarbons and minimized oxygenates, but at this pressure the reaction rate was very low. Because of their pioneering work on catalytic hydrocarbon synthesis, this class of reactions became known as the Fischer-Tropsch (FT) synthesis. 

The catalytic partial oxidation of methane for the production of synthesis gas is an interesting alternative to steam reforming which is currently practiced in industry [1]. Significant research efforts have been exerted worldwide in recent years to develop a viable process based on the partial oxidation route [2-9]. This process would offer many advantages over steam reforming, namely (a) the formation of a suitable H2/CO ratio for use in the Fischer-Tropsch synthesis network, (b) the requirement of less energy input due to its exothermic nature, (c) high activity and selectivity for synthesis gas formation. 

As a check to confirm that no extraneous non-polymer-attached catalytic species were present, the following experiment was performed. Polystyrene without attached cyclopentadiene was exposed to Co2(C0)e, extracted using a Soxhlet extractor and dried in vacuo in exactly the same manner as was used to synthesize 5. When used under the above Fischer-Tropsch reaction conditions, these treated, white polystyrene beads did not discolor, release any detectable species into solution, cause a CO/H2 pressure drop, or result in the formation of any detectable amounts of methane. These observations argue against the presence of small amounts of occluded Co2(C0)e or C04 (CO) 12 which could conceivably have been active or precursors to active species. It should be noted that the above clusters were reported to be essentially inactive under Fischer-Tropsch conditions (140°C, toluene, 1.5 atm., 3/1 H2/CO, three days) leading to mere traces of methane (11). The lack of products under our conditions also indicates that, at least in the absence of resin-bound CpCo(C0)2 or its derivatives, the polystyrene support did not degrade. 

Supported Fe-Mn Fischer-Tropsch Catalysts. A much more limited number of studies have dealt with supported Mn-promoted Fe F-T catalysts. In this respect, it is worthwhile to mention the work of Xu et al These authors added MnO to a Fe/silicalite catalyst and observed an enhanced selectivity towards light olefins. Meanwhile the yields for methane as well as for CO2 formation were almost unaffected by MnO addition. Moreover, the conversion of CO was also insensitive to the addition of the MnO promoter. 

Group II The activity drops more than the Ni surface concentration (Fig. 13), i.e., at least about 20 times. However, for several reactions this drop is two or more orders of magnitude. The reactions included in this group are methanation and Fischer-Tropsch synthesis, isomerization, de-hydrocyclization or hydrogenolysis of alkanes, ether formation from alcohols, metathesis of alkylamines, and possibly other reactions. 

Metal molybdates421 and cobalt-thoria-kieselguhr422 also catalyze the formation of hydrocarbons. It is believed, however, that methanol is simply a source of synthesis gas via dissociation and the actual reaction leading to hydrocarbon formation is a Fischer-Tropsch reaction. Alumina is a selective dehydration catalyst, yielding dimethyl ether at 300-350°C, but small quantities of methane and C2 hydrocarbons423 424 are formed above 350°C. Heteropoly acids and salts exhibit high activity in the conversion of methanol and dimethyl ether.425-428 Acidity was found to determine activity,427 130 while hydrocarbon product distribution was affected by several experimental variables.428-432... 

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