Fischer-Tropsch synthesis hydrogenation

Fischer-Tropsch Synthesis The best-known technology for producing hydrocarbons from synthesis gas is the Fischer-Tropsch synthesis. This technology was first demonstrated in Germany in 1902 by Sabatier and Senderens when they hydrogenated carbon monoxide (CO) to methane, using a nickel catalyst. In 1926 Fischer and Tropsch were awarded a patent for the discovery of a catalytic technique to convert synthesis gas to liquid hydrocarbons similar to petroleum. 

Many chemicals are produced from synthesis gas. This is a consequence of the high reactivity associated with hydrogen and carhon monoxide gases, the two constituents of synthesis gas. The reactivity of this mixture was demonstrated during World War II, when it was used to produce alternative hydrocarbon fuels using Fischer Tropsch technology. The synthesis gas mixture was produced then hy gasifying coal. Fischer Tropsch synthesis of hydrocarbons is discussed in Chapter 4. 

The catalytic hydrogenation of fatty oils, the desulfurization of liquid petroleum fractions by catalytic hydrogenation, Fischer-Tropsch-type synthesis in slurry reactors, and the manufacture of calcium bisulfite acid are familiar examples of this type of process, for which the term gas-liquid-particle process will be used in the following. 

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. 

Table 1.6 Examples of energy- and environment-related systems with metal NPs in ILs Fischer-Tropsch synthesis, fuel cells, and hydrogen generation/storage. 

This process can be tailored to maximize the hydrogen production while capturing C02, or the ratio between CO and H2 in the product stream can be adjusted for different applications. For example, the optimal ratio of H2 to CO is 2 for the Fischer-Tropsch synthesis. 

Fischer-Tropsch synthesis can be regarded as a surface polymerization reaction since monomer units are produced from the reagents hydrogen and carbon monoxide in situ on the surface of the catalyst. Hence, a variety of hydrocarbons (mainly n-paraffines) are formed from hydrogen and carbon monoxide by successive addition of C, units to hydrocarbon chains on the catalyst surface (Equation 12.1). Additionally, carbon dioxide (Equation 12.3) and steam (Equations 12.1 and 12.2) are produced C02 affects the reaction just a little, whereas H20 shows a strong inhibiting effect on the reaction rate when iron catalysts are used. 

The Fischer-Tropsch synthesis, which may be broadly defined as the reductive polymerization of carbon monoxide, can be schematically represented as shown in Eq. (1). The CHO products in Eq. (1) are any organic molecules containing carbon, hydrogen, and oxygen which are stable under the reaction conditions employed in the synthesis. With most heterogeneous catalysts the primary products of the reaction are straight-chain alkanes, while the secondary products include branched-chain hydrocarbons, alkenes, alcohols, aldehydes, and carboxylic acids. The distribution of the various products depends on both the type of catalyst and the reaction conditions employed (4). 

Methane is always one of the products of any Fischer-Tropsch synthesis and its production from CO may be schematically represented as the formal addition of three moles of hydrogen to a metal carbonyl giving,... 

Even outside the context of the Fischer-Tropsch synthesis, the water gas-shift reaction has considerable commercial importance. In addition to being used to increase the hydrogen content of synthesis gas, it is... 

Subsequent studies have failed to support the carbide theory, and it is now generally accepted that carbides of the type proposed by Craxford play little or no part in the Fischer-Tropsch synthesis (86, 87). It has, however, recently been suggested, by analogy with the mechanism proposed for the Haber synthesis of ammonia, that carbides formed by dissociative absorption of carbon monoxide would be expected to be readily hydrogenated and could therefore be of importance in Fischer-Tropsch synthesis over heterogeneous catalyst (88). 

Use of molten salts as solvent allows easy separation of organic products by distillation (376), and in this way PtCl2 with tetraalkylammonium salts of SnCl3 and GeCl3 has been used to selectively hydrogenate 1,5,9-cyclododecatriene to cyclododecene the salts in this case act as both solvent and ligand (377). A molten salt medium has been used in a homogeneously catalyzed Fischer-Tropsch synthesis (see Section VI,B). 

Iron or Cobalt Hydrogenation of CO and C02 to form hydrocarbons (Fischer-Tropsch synthesis)... 

As catalysis proceeds at the surface, a catalyst should preferably consist of small particles with a high fraction of surface atoms. This is often achieved by dispersing particles on porous supports such as silica, alumina, titania or carbon (see Fig. 1.2). Unsupported catalysts are also in use. The iron catalysts for ammonia synthesis and CO hydrogenation (the Fischer-Tropsch synthesis) or the mixed metal oxide catalysts for production of acrylonitrile from propylene and ammonia form examples. 

Figure 6.2 X-ray diffraction identifies phases in a manganese-promoted iron Fischer-Tropsch catalyst after reduction (middle) and after CO hydrogenation or Fischer-Tropsch synthesis bottom). The spectra show that Mn is present as slightly distorted MnO (see the MnO reference measurement at the top), and that bcc iron (peak at 2 6 = 57.0°) converts to iron carbides (peaks around 55°) during the Fischer-Tropsch reaction (from van Dijk et al. [7]).

The Fischer-Tropsch synthesis follows a polymerization mechanism where a Q unit is added to the growing chain. A simplified representation of the reaction network is shown in Fig. 1, where the key points are termination by either H-abstraction to give a-olefins or by hydrogenation to give w-paraffins. 

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