Fischer-Tropsch-type catalytic reactions

The synthesis of fatty acids by a Fischer-Tropsch-type process as described in this chapter required the use of a catalyst (meteoritic iron) and a promoter. Potassium carbonate and rubidium carbonate were the only compounds evaluated which unambiguously facilitated the production of fatty acids. These catalytic combinations (meteoritic iron and potassium carbonate or rubidium carbonate) also produced substantial amounts of n-alkenes (in excess of n-alkanes) and aromatic hydrocarbons. A comprehensive study of the nonacidic oxygenated compounds produced in Fischer-Tropsch reactions (20,21) was not made. However, in the products analyzed (all promoted by potassium carbonate), long-chain alcohols and aldehydes were detected. 

Complexes of type [LyM (CH2)IIX ], where n > 1, have been shown to be useful precursors for hetero- and homobimetallic n(a.,a>) alkanediyl complexes, [LjcM(CH2)bM L, ] (where ML, is not necessarily the same as M Lj,). Such hydrocarbon-bridged binuclear compounds have been proposed as models for intermediates in the Fischer-Tropsch reaction (18,19) and other significant catalytic processes (20-23). Some [LyM (CH2)BX ] complexes are precursors to cyclic carbene complexes (Section III), whereas others have been shown to have synthetic utility in organic chemistry (24). 

The adsorption of CO is probably the most extensively investigated surface process. CO is a reactant in many catalytic processes (methanol synthesis and methanation, Fischer-Tropsch synthesis, water gas shift, CO oxidation for pollution control, etc. (1,3-5,249,250)), and CO has long been used as a probe molecule to titrate the number of exposed metal atoms and determine the types of adsorption sites in catalysts (27,251). However, even for the simplest elementary step of these reactions, CO adsorption, the relevance of surface science results for heterogeneous catalysis has been questioned (43,44). Are CO adsorbate structures produced under typical UHV conditions (i.e., by exposure of a few Langmuirs (1 L = 10 Torrs) at 100—200 K) at all representative of CO structures present under reaction conditions How good are extrapolations over 10 or more orders of magnitude in pressure Such questions are justified, because there are several scenarios that may account for differences between UHV and high-pressure conditions. Apart from pressure, attention must also be paid to the temperature. 

The goal of using solid-state electrolytic reactors is not only to generate electrical power, but also to combine this with an industrially important catalytic reaction, such as dissociation of oxygen-containing compounds like NO [40,41], quantitative oxidation of NH3 to NO [42-44], oxidation of SO2 [45], and methanol [46], ethylene epoxidation [46], or Fischer-Tropsch synthesis [47]. The cross-flow reactor used in this type of study (Fig. 10) [48,49] has a solid electrolyte consisting of yttria-doped zirconia. The plates are electrically connected in series, with a varying number of plates in parallel. The oxidant flow channels... 

In summary, cluster-derived catalysts have been widely used in various types of CO-based reactions such as Fischer-Tropsch synthesis, methanol synthesis, hydroformylation, carbonylation, and water-gas shift reactions. The catalytic performances of cluster-derived species are evaluated in terms of higher activities and selectivities for lower olefins and oxygenates in CO hydrogenation, compared with those of metal complexes in solution and conventional metal catalysts (Table XIII). 

CO and CO2 may react with H2 to yield hydrocarbons and oxygenates. Both types of products are thermodynamically favored and therefore, the route selection is catalytically controlled. Other secondary undesirable reactions may occur leading to carbon deposition and CO consumption (water gas shift reaction). Fe and Co are traditional Fischer-Tropsch catalysts while Cu and Rh lead the reaction to alcohol formation. 

Most major improvements in performance of Fischer-Tropsch and other catalysts have been achieved through advanced imaging technologies. For example, different types of electron microscopy (Florea et al. 2013 Thomas et al. 2013), X-ray, and neutron powder diffraction (Rozita et al. 2013) facilitate characterization of catalyst particles and the included pores ranging in size from micron to a few nanometers. These advanced tools lend a capability to analyze down to the level of an atom. Further, the same facilitate introduction of efficient promoters and also distribution of smaller catalytic species over greater surface areas. The last feature implies a catalyst with high activity and surface area and should allow higher rates of reaction. 

Recent reviews (31-34,36,37,51) provide a comprehensive survey of the types of heterogeneous catalytic reactions investigated at supercritical conditions including alkylation, amination, cracking, disproportionation, esterification, Fischer-Tropsch synthesis, hydrogenation, isomerization, and oxidation. Table 2 summarizes reported investigations under these classes of reaction. Some of these examples are described here to show how to systematically exploit supercritical media in heterogeneous catalysis. 

Fluidized bed reactors (FBRs) are chemical reactors in which (catalytic) particles interact with a gas stream that is fed from the bottom, such that the mixture (emulsion phase) behaves as a fluid. This type of reactors is often used in the chemical and process industries, where they have gained their popularity due to their excellent heat and mass transfer characteristics. FBRs are used for instance for gas-phase polymerization reactions for polyolefin production (polyethylene, polypropylene), chemical looping combustion or reforming processes, and gas-phase Fischer—Tropsch synthesis. 

In spite of such catalytic effects of the metal cluster, the rate of this reaction remains very low. Nevertheless, this reaction is an example of a very interesting type of homogeneous catalysis. Here the activation of carbon monoxide appears to be achieved by interaction of both the carbon and oxygen atoms with the metal cluster atoms in a similar way to the CO-chemisorption on metals in heterogeneous Fischer-Tropsch processes. 

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