Fischer-Tropsch reaction, important

The Fischer-Tropsch reaction is essentially that of Eq. XVIII-54 and is of great importance partly by itself and also as part of a coupled set of processes whereby steam or oxygen plus coal or coke is transformed into methane, olefins, alcohols, and gasolines. The first step is to produce a mixture of CO and H2 (called water-gas or synthesis gas ) by the high-temperature treatment of coal or coke with steam. The water-gas shift reaction CO + H2O = CO2 + H2 is then used to adjust the CO/H2 ratio for the feed to the Fischer-Tropsch or synthesis reactor. This last process was disclosed in 1913 and was extensively developed around 1925 by Fischer and Tropsch [268]. 

Catalysis. Beitel et al. (1997) have employed RAIRS to study in situ the co-adsorption behaviour of CO and hydrogen on single-crystal cobalt (0001) catalysts at pressures up to 300mbar and temperatures between 298 and 490 K. The behaviour of these adsorbates is of considerable importance in relation to their commercial importance as catalysts for the Fischer-Tropsch reaction in the... 

The Fischer-Tropsch reaction has now been known for almost 70 years and is of great importance partly for itself and also as part of a coupled set of processes whereby steam or oxygen plus coal or coke is transformed into methane, alkenes, alcohols, and gasolines. According to Eqs.I-IV in the most... 

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. 

Even if hydrides of transition metals play an important role as intermediates in catalytic processes such as hydrogenation and hydro-formylation of alkenes and the Fischer-Tropsch reaction,71 in order to follow with the bioinorganic subject, we refer to the interconversion process ... 

The insertion of ligated CO into metal-carbon -bonds (or rather the migration of an alkyl group to a coordinated CO) is a key step in a variety of synthetic and catalytic important processes, e.g., in hydroformylation (145), the Fischer-Tropsch reaction (146) and the synthesis of acetic acid from methanol (147). 

Both the Fischer-Tropsch reaction and the Mobil process enable one to convert synthesis gas into hydrocarbons. Since synthesis gas may be obtained from coal, we have in effect a means of converting coal u> gasoline. Geimany moved its Panzer Korps in World War II with synthetic fuels made from (he Fischer-Tropsch reaction, and improved technological developments have enhanced the attractiveness of the process. South African Synthetic Oil Limiied fSASOLJ currently operates several modern Fischer-Tropsch plants. Many organometallic chemists refer to both the Fischer-Tropsch and Mobil processes as political processes 1 2 because they are heavily subsidized by countries that find it important to be independent of foreign oil. 

The rapid development of the chemistry of transition metal complexes containing terminal carbene (A) or carbyne (B) ligands (7) has been followed more recently by much research centered on bridged methylene compounds (C) (2). The importance of /t-methylidyne complexes, whether in recently established binuclear examples (D), the well-known trinuclear derivatives (E), or the unusual complexes (F), has also become apparent. All are based on one-carbon (C,) fragments, and considerable interest is centered on their possible significance as models for intermediates in surface-catalyzed reactions between carbon monoxide and hydrogen (Fischer -Tropsch reactions) and related processes. These topics have been extensively ... 

Some mechanistic information is available on ruthenium-based homogeneous Fischer-Tropsch reactions. By in situ IR spectroscopy, in the absence of any promoter, only Ru(CO)5 is observed. An important difference between the cobalt and the rhodium system on the one hand and ruthenium on the other is that in the latter case no ethylene glycol or higher alcohols are obtained. In other words, in the catalytic cycle the hydroxymethyl route is avoided. 

These mixed metal systems have also been tested with the transient method for catalytic activity in the Fischer-Tropsch reaction. We would like to remark here that the nature of the cation, anion, and zeolite are all important factors in the Fischer-Tropsch reactions that we have studied. Further details of these catalytic studies can be found elsewhere (23). We do observe here, however, that some catalysts that are completely reduced to the metallic state are not necessarily the most active catalysts. Also, even though the Mossbauer experiments suggest that 400°C is sufficient for complete reduction, higher activation temperatures can increase the activity and selectivity of these reactions. We have also observed that the cation definitely changes the product distribution and the activity. 

Observations that may support this mechanism have been reported specifically, these include the disappearance of IR bands of adsorbed CO during the Fischer-Tropsch reaction (17) and during alkene hydroformy-lation, consistent with CO insertion between metal atoms and alkyl ligands (18). It is important to realize that to insert CO into metal alkyl groups, the alkyl group has to be attached to surface metal sites with empty d-orbitals (19). 

Once we have deduced adequate kinetics expressions, we use the results of quantum-chemical computations to estimate the values of the relevant parameters. We make extensive use of resulfs of Hu and coworkers (31-33), who published a series of compufafional papers characterizing the Fischer-Tropsch reaction. This approach enables us to understand structure sensitivity and, more important, helps us to identify, for each mechanism, the catalyst structure that favors fhis mechanism. 

According to the Sachtler-Biloen mechanism, the Fischer-Tropsch reaction is initiated through CO adsorption followed by CO dissociation. Experimental evidence for the involvement of an oxygen-free intermediate exists it was observed that predeposited C is incorporated into the product during Fischer-Tropsch synthesis when CO was included in the feed gas (3). It is important to distinguish whether during the Fischer-Tropsch s)mthesis CO dissociation is strictly monomolecular or instead involves a reaction with Hads to produce an intermediate "HCO" formyl species that in a subsequent reaction decomposes to "CH" and Oads-Another question is how the rates of CO dissociation, chain growth, and termination depend on the catalyst surface structure. Thus, it is essential to know the relative values of the rate constants for these three elementary reactions. 

Iron is also an important Fischer-Tropsch catalyst, but in the active state, it is present as a carbide (70,71) which is characterized by a unique chemistry that we do not discuss. Computations concerning the Fischer-Tropsch reaction on iron were performed by Bromfield et al. (72) and by Lo and Ziegler (73), who investigated the chain-growth reaction. 

In this chapter, we have discussed the molecular transformations that are the basis of the Fischer-Tropsch reaction. The discussion is limited to the initial elementary steps of the reaction. Many important additional... 

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. 

Pettit [44] has stressed the importance of bridge melhylene ligands un transition metal surfaces- Similarities in product formation with CO/H] and surest a common mechanism for both processes, with methylene possibly being formed by carbide hydrogenation in Fischer-Tropsch reactions. 

Another system in this class of mechanisms that was also modeled mathematically is the technically important Fischer-Tropsch process on Fe catalysts investigated by Caldwell (217,218). Oscillation had been previously observed in this system by Tsotsis and co-workers (279). Caldwell reduced the mechanism of the Fischer-Tropsch reaction to three reactions ... 

Readsorption and secondary reactions of the intiailly produced ct-olefins is an important pathway in Fischer-Tropsch reactions on Fe single crystals Dwyer, D.J., and Somorjai, G.A., J. Catal., 56, 249, (1979). Schulz, H., and Achtsnit,... 

Again in a brief paper with considerable impact, van Barneveld and Ponec have considered many aspects of the Fischer-Tropsch reaction. On Ni and Cu/Ni they find that to explain many known features of the reaction they must dismiss polymerization of—CH2— groups on neighbouring sites and repeated CO insertion into M—H bonds. They favour CO dissociation to C + O followed by C + xH CH c and CH c + COads —> (CH,CO) then CH,CO + wH- -H20 + CHjcCH 2- The CH ,CH 2 may add further CO, or add hydrogen to give a desorbable hydrocarbon product. The role of adsorbed CHx seems of paramount importance in their mechanism. 

In a long and detailed paper on CO/H2 and CO2/H2 reactions on polycrystalline Rh, Sexton and Somorjai used a UHV-AES apparatus designed to allow sample scrutiny at low pressure yet to permit high-pressure (700 torr) reactions. They established good correlation of turn-over numbers between their results and results obtained on supported catalysts for the Fischer-Tropsch reaction. AES established that C was present on reactive surfaces yet this C (1 to 2 monolayers) did not influence rates of reaction or product distribution for high-pressure runs. It is however interesting to note that the most important influences on catalysis reported in this paper were found to be subsurface C and O, neither detectable by AES. The reactions studied at 250—300 °C showed that CO/H2 produced mainly Ci but also some C2, C3, and C4 hydrocarbons, whereas CO2/H2 produced CH4 exclusively. 

M. Green, S. J. Porter, and F. G. A. Stone, J. Chem. Soc., Dalton Trans., 1983, 513. 83Bridging carbynes are important ligands in metal cluster compounds (Chapter 13) and may play a role in surface-catalyzed reactions. Methine (C-H), bound to more than one metal atom, is considered to be involved in intermediate stages of the Fischer-Tropsch reaction, which is used to convert basic hydrocarbon starting materials, such as those from coal, into gasoline and diesel fuel (M. J. Overett, R. O. Hill, and J. R. Moss, Coord. Chem. Rev., 2000, 206-207, 581). 

It is very clear from the data above that there is virtually no difference in the performance of the HyperCat as compared to the powder catalyst. This result is extremely important in light of other efforts being made to develop structured catalysts for Fischer-Tropsch reaction, which show that the activity of the structured catalyst is always lower than the powder catalyst (d). 

Above 650 K, carbon in the nebula should be primarily in CO but, below 650 K, if equilibrium persisted, nearly all carbon would be in CFI4. The condensation temperature of methane is 50 K and if all carbon was in this form, then it could not have been efficiently incorporated into solids except perhaps at the extreme outer edges of the solar nebula. It is likely, however, that equilibrium between CO and CH4 did not occur and that CO was probably an important reservoir of carbon throughout the nebula. With abundant CO, catalytic reactions on grain surfaces could form carbonaceous coatings. It has been suggested that Fischer-Tropsch reactions, similar to the following, produced much of the carbonaceous matter in meteorites ... 

The hydroformylation or oxo reaction has been chosen for particular study for several reasons (a) The reaction was discovered by Roelen 2) in the course of an investigation of the mechanism of the Fischer-Tropsch reaction, and a study of the hydroformylation reaction could furnish information on the course of this heterogeneously catalyzed synthetic fuel process (6) hydroformylation involves the activation of hydrogen by a molecularly dispersed catalyst (c) there are few side reactions (d) the catalyst for the reaction, Co2(CO)s, is easily prepared, is relatively nontoxic, and is consequently readily available for study and (e) the reaction is of great industrial importance. 

Due to the known limitations of the world oil reserves, methane oxidation under fuel rich conditions will become increasingly important for the production of synthesis gas. which through methanol synthesis and Fischer-Tropsch reactions is the basis of many important petrochemical synthesis routes. Therefore, catalytic oxidation of methane has again become the focus of much basic and applied catalysis research in recent years. In this context, Schmidt and coworkers were able to show recently, that catalytic direct oxidation of methane over noble metal coated monoliths can yield CO and H2 with very high conversions and selectivities at the desired 1 2 CO H2 ratio (Hickman and Schmidt. 1992 and 1993a Torniainen and Schmidt. 1994 Bharadwaj and Schmidt. 1995). 

Carbon monoxide may be hydrogenated to produce either alcohols or hydrocarbons, depending on the catalysts used and the reaction conditions. Temperatures ranging from 100-400°C and pressures as high as 1,000 atm have been studied. Different catalysts yield radically different types of products. Important processes for uch reactions consist of the methanol synthesis, the higher-alcohol synthesis (or the variation termed the iso synthesis), the Fischer-Tropsch reaction (or the version called hydrocarbon synthesis), and the methanation reaction. These syntheses were discovered in the period 1920-1925, at which time the methanol and higher-alcohol syntheses developed rapidly. A brief summary of processes and conditions used for the hydrogenation of carbon monoxide is presented in Table 10-17. 

One of the earliest reactions involving the insertion of CO into a Cn-olefin molecule to produce an aldehyde with one greater Cn+i carbon number is the so-called hydroformylation or "0x0 " reaction. The 0x0 reaction is carried out over homogeneous catalysts, rhodium or cobalt carbonyls, and is an important industrial process. Recently the production of acetic acid, acetaldehyde, and glycol from CO and H2 over heterogeneous and homogeneous rhodium catalysts have been reported. Straight-chain saturated hydrocarbons are not the only molecules that may be produced in the Fischer-Tropsch reaction. There have been... 

Apparatus. The experimental unit, shown in Figure 1, was made of stainless steel and was used for the simultaneous operation of up to seven Alonized (a process which coats the stainless steel with aluminum and alumina it renders the reactor walls inert for Fischer-Tropsch reactions and, more importantly, for interaction by sulfur compounds) stainless steel reactors, all at the same temperature, pressure, H2.-CO ratio, and space velocity. The unit could be used at pressures up to 3 MPa. 

The making and breaking of C-H and M-H bonds are key steps in important metal-catalyzed reactions, such as hydrogenations, hydrosilylations, C-H activations, and Fischer-Tropsch reactions. Insertion of an alkene into a metal-hydride bond and the reverse, /7-hydride elimination, are key steps in some of these processes (see Equation (3)). Occasionally, compounds exist as an agostic complex, which may be viewed as a key intermediate in these reaction paths. Brookhart has carried out extensive DNMR studies of these processes and agostic interactions,... 

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