Fischer-Tropsch catalysts components

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. 

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. 

The electron microscope offers a unique approach for measuring individual nano-sized volumes which may be catalytically active as opposed to the averaging method employed by spectroscopic techniques. It is just this ability of being able to observe and measure directly small crystallites or nano-volumes of a catalyst support that sets the microscope apart from other analyses. There have been many studies reported in the literature over the past fifteen years which emphasize the use of analytical and transmission electron microscopy in the characterization of catalysts. Reviews (1-5) of these studies emphasize the relationship between the structure of the site and catalytic activity and selectivity. Most commercial catalysts do not readily permit such clear distinction of physical properties with performance. The importance of establishing the proximity of elements, elemental distribution and component particle size is often overlooked as vital information in the design and evaluation of catalysts. For example, this interactive approach was successfully used in the development of a Fischer-Tropsch catalyst (6). Although some measurements on commercial catalysts can be made routinely with a STEM, there are complex catalysts which require... 

Fig. 5 Olefin/paraffin ratio as a function of TOS for C2-C6 components for 12% Co/Si02. Step changes in olefin/paraffin ratio are due to increased GHSV at 23 h, as 20% water addition at 50 h, 33% water addition at 70 h and back to the dry feed at 95 h. H2/CO = 2.1, P - 20 bar, T = 483 K.19 Reprinted from Journal of Catalysis, Vol. 231, S. Storsaeter, 0. Borg, E. A. Blekkan and A. Holmen, Study of the effect of water on Fischer-Tropsch synthesis over supported cobalt catalysts, pp. 405 119. Copyright (2005), with permission from Elsevier. 

Fischer-Tropsch synthesis catalysts when applied in the hydrogenation of C02 perform rather poorly, yielding only small amounts of C2+ hydrocarbons. The development of new catalysts, therefore, is necessary for the production of higher hydrocarbons through the direct hydrogenation of C02. Effective hybrid or composite catalyst systems may be used for this purpose by combining C02 hydrogenation catalysts with an acidic component. Two basic combinations have been studied. 

The intrinsic nature of tungsten carbide catalyst in CO-H2 reactions is to form hydrocarbons. This property can be modified by oxidic promoters as for the case of noble metals like Pt or Rh or by the presence of carbon vacancies at the surface. To increase the production of alcohols in the Fischer-Tropsch reaction, the catalyst should be bifunctional, with oxidic and carbidic components as in the case of WC on Ti02. Overcarburization of WC on supports like Si02 or Zr02 where the W-O-metal interaction is weak leads to C/W ratios close to unity and does not result in alcohol formation. 

Another technologically important reaction is the Fischer-Tropsch synthesis, with iron oxide being one of the components of some catalysts. A detailed understanding of the complex mechanism of this reaction can be obtained by studying the chemisorption of simple molecules on well-characterized surfaces by means of advanced surface-sensitive spectroscopic techniques. A few investigations of the interaction of small molecules (such as CO, CO2, H2O, O2, H2, and NO) (520-522) and organic molecules on iron oxide surfaces (523-527) have been carried out. 

Fe203 catalyst, catalyst component Fischer-Tropsch reactions, major component of catalyst for ethylbenzene reaction to styrene... 

Where the Fischer-Tropsch process has been used on an industrial scale, iron or cobalt are the essential catalyst components. Technical catalysts also contain oxidic promoters, such as alumina and potassium oxide. Ruthenium and nickel are most attractive for academic research since they produce the simplest product packages. Nickel is used for methanation (production of substitute natural gas and removal of carbon monoxide impurities from hydrogen). 

The synthesis of light alkenes by modification of the Fischer-Tropsch process is recognised as an important route to high value fuel components (refs. 1,2). The essential requirement of the F-T catalyst is high selectivity towards alkenes, suppression of methane and resistance to coking under conditions which favour the formation of hydrogen deficient products. 

The Co-MgO-ZSM-5 catalyst was found to be suitable for the synthesis of liquid hydrocarbons from the products of the air gasification of wood and agricultural plants, its activity and liquid hydrocarbon selectivity being higher than those of classical Fischer-Tropsch cobalt catalysts without zeolitic component. ... 

The Fischer-Tropsch Hynthesis, i.e., the catalytic reduction of carbon monoxide bj" hydrogen to liquid, gaseous, and solid hydrocarbons and oxygenated organic compounds, has recently been reported to operate efficiently tinder siiperatmospheric pressure, with catalysts which contain iron as their basic component. IMore recently catalysts of this type are being used in a very fine dispersion which permits one to operate with them in a fluid bed. ... 

We now encounter a semantic problem of considerable size. It has been recognised for a very long time that the activity of metal catalysts can be helped by the presence of quite small amounts of substances that of themselves have no or little activity. This concept first achieved prominence in the development of iron catalysts for ammonia catalysts, and of iron and cobalt catalysts for Fischer-Tropsch synthesis, and the term promoter was applied to these substances. They were of two kinds (i) structural promoters such as alumina, which acted as grain stabilisers and prevented metal particle sintering and (ii) electronic promoters such as potassium that entered the metallic phase and actually enhanced its activity. In these cases the metal is the major component, so that the catalyst is a promoted metal rather than a supported metal. 

Simultaneously, the synthesis of ammonia from the elements was also tested over RE-TM intermetallic compounds by the same group (42). Here, 36 intermetallic compounds of rare earth elements and the transition metals Fe, Co, and Ru were evaluated. In the case of the ammonia synthesis, the rare earth component is transformed to the corresponding nitride and the transition metal is finely divided thereon. Some of the compounds showed an even higher specific activity than commercial catalysts used at that time. Later, the catalysts based on the intermetallic compound CeNis-j Cox (x = 0-5) were oxidized in a controlled way prior to the catalytic characterization in the Fischer-Tropsch reaction (43). The reasons to oxidize the compound before use were twofold. On the one hand, the material can be handled in air afterwards, whereas the intermetallic compound itself is not stable in air. Second, the oxidation can be better controlled and reproduced if it is not performed in situ. A review on the early work on these catalysts is available by Wallace (44). 

FejOs Catalyst for Fischer-Tropsch synthesis catalyst component for the ethylbenzene dehydrogenation catalyst... 

This case occurs rarely. It usually happens at the cases of considerably small catalyst particles, less external mass transfer coefficient and relative large reaction rate constant. This will happen when the porousless netlike materials are used for catalyst as platinum-net for ammonia oxidation, or thin layer distribution of active component at external surface of catalysts. Besides, it is commonly considered that Fischer-Tropsch synthesis reaction to produce hydrocarbons is also controlled by external diffusion on iron catalysts. 

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