Fischer-Tropsch reactions reviews

Five of the chapters in this volume can be considered directly related to this topic. First, Edd Blekkan, 0yvind Borg, Vidar Froseth, and Anders Holmen (Norwegian University of Science and Technology, Trondheim) review recent work on the effect of water on the Fischer-Tropsch reaction. Steam is both a reactant and product in this syngas-based process, and its effect on Co- and Fe-based catalysts is important in determining the activity and selectivity of the FT process. 

In this paper, we will review the chemical behaviour of transition metal oxides which is of crucial importance for heterogeneous catalysis, adhesion and many technological applications. Among them, MgO(lOO) is the simplest surface, with a square unit-cell containing two ions with opposite charges titanium oxides represent another important class of systems used for their catalytic properties either directly as catalyst or indirectly as support for other catalysts (metals such as Ni, Rh for the Fischer-Tropsch reaction or V2O5 for the reduction of NOx) or as promotors[l]. The most stable surface for rutile is the (110) face. 

After a short review of the historical developments, the basic features of the Fischer-Tropsch reaction will be dealt with and special attention will be given to the possibilities of product selectivity control. Finally, the various mechanistic proposals known so far will be discussed in respect to recent studies of catalyst... 

The main argument in favor of this possibility is that in metal-organic complexes containing alkyl groups and carbon monoxide, cis insertion has been observed. Application of this mechanism to Fischer-Tropsch reactions is therefore essentially reasoning by analogy. Since these views have been reviewed properly, we can confine ourselves to referring to the relevant literature (20). 

Transient Isotopic Kinetic Studies of Methanation. - The Fischer-Tropsch reaction results in the formation of a wide distribution of hydrocarbons containing different numbers of carbon atoms. In contrast, the related reaction of methanation of CO/H2 mixtures involves only one product and is easier to study using isotope transient kinetic techniques. The results of the methanation reaction have a direct relevance to the Fischer-Tropsch reaction and are reviewed below. 

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). 

Maitlis has reviewed the homogeneous and heterogeneous systems and noted that the former produce mainly oxygenated products, such as alcohols and esters. Most of the recent work on the Fischer-Tropsch reaction has focused on heterogeneous catalysts. The mechanism(s) of the later ate mainly inferred from isotope labeling and product distributions. The observations and mechanistic proposals of the Sheffield group have been summarized by Maitlis. However, there is still some controversy about the heterogeneous pathways. ... 

Mossbauer spectroscopy is one of the techniques that is relatively little used in catalysis. Nevertheless, it has yielded very useful information on a number of important catalysts, such as the iron catalyst for Fischer-Tropsch and ammonia synthesis, and the cobalt-molybdenum catalyst for hydrodesulfurization reactions. The technique is limited to those elements that exhibit the Mossbauer effect. Iron, tin, iridium, ruthenium, antimony, platinum and gold are the ones relevant for catalysis. Through the Mossbauer effect in iron, one can also obtain information on the state of cobalt. Mossbauer spectroscopy provides valuable information on oxidation states, magnetic fields, lattice symmetry and lattice vibrations. Several books on Mossbauer spectroscopy [1-3] and reviews on the application of the technique on catalysts [4—8] are available. 

A recent comprehensive review of a set of elementary reactions that can be chosen to represent the Fischer Tropsch synthesis has been presented by Rofer-De Poorter (40). Such sets of elementary reactions form the starting point for our treatment as discussed in Section II. 

In a series of papers from the Zelinskil laboratory (152,153,158,160,-443) a systematic study was reported of a variety of catalysts for the reaction under discussion, including its application to synthetic (Fischer-Tropsch) gasoline. Chromia-alumina or silica gel was apparently preferred. Plate reviewed the field of Russian and non-Russian work up to 1940 (304), and Shuikin published another review in 1946 in which dehydrogenation of cyclohexanes as a source of aromatics is considered to be at least equal in value to dehydrocyclization (371). 

Interest in the Fischer-Tropsch synthesis was manifested by Russian investigators soon after the first information about it was published the synthesis was discussed at the Mendeleev Chemical Congress in 1934, and reviews were published both in book (148) and in journal (73,74,75, 82,84,146,234) form. The work has led to pilot plant studies full-scale industrial development of this process may have been achieved by 1950. There has been considerably more attention given to the performance of various catalysts and to the reaction mechanism, and, recently, to related processes, such as the hydropolymerization, mentioned above, and the hydrocondensation of olefins, rather than to such aspects of this process as preparation of synthesis gas,... 

Sulfur poisoning is a key problem in hydrocarbon synthesis from coal-derived synthesis gas. The most important hydrocarbon synthesis reactions include methanation, Fischer-Tropsch synthesis, and methanol synthesis, which occur typically on nickel, iron, or cobalt, and ZnO-Cu catalysts, respectively. Madon and Shaw (2) reviewed much of the early work dealing with effects of sulfur in Fischer-Tropsch synthesis. Only the most important conclusions of their review will be summarized here. 

Metal-support interactions have been recently reviewed by Bond (93), who drew special attention to catalysts that gave evidence for strong metal-support interactions (SMSI). This condition was first observed in 1978 by Tauster et al. (94) for Pt on titania catalysts. The catalysts seemed to lose their capacity for H2 and CO chemisorption but nevertheless retained and enhanced their activity for only two types of reaction methanation and Fischer-Tropsch synthesis. Since then a considerable number of papers devoted to SMSI studies have been published all over the world. 

In this review, we limit ourselves to the mechanisms of primary product formation, which are fimdamental to Fischer-Tropsch chemistry. Using new information mainly from computational studies, we focus on two coidlicting hypotheses regarding the key reaction steps that lead to chain growth. 

Three-phase reactors are widely used in hydroprocessing operations and for oxidation reactions. Trickle bed reactors have been widely used for hydrodesulfurization of residue oils, hydrodesulfurization, and hydrocracking of gas oils and in numerous oxidation reactions. Three-phase fluidized bed reactors are also used in coal liquefaction and in Fischer-Tropsch synthesis. It is in these and similar examples that the review presented in this monograph can most pertinently be applied. 

The benefits of nonuniform activity distributions (site density) or diffusive properties (porosity, tortuosity) within pellets on the rate of catalytic reactions were first suggested theoretically by Kasaoka and Sakata (Ml). This proposal followed the pioneering experimental work of Maatman and Prater (142). Models of nonuniform catalyst pellets were later extended to more general pellet geometries and activity profiles (143), and applied to specific catalytic reactions, such as SO2 and naphthalene oxidation (144-146). Previous experimental and theoretical studies were recently discussed in an excellent review by Lee and Aris (147). Proposed applications in Fischer-Tropsch synthesis catalysis have also been recently reported (50-55,148), but the general concepts have been widely discussed and broadly applied in automotive exhaust and selective hydrogenation catalysis (142,147,149). 

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