Heterogeneous Fischer Tropsch synthesis

Comparing heterogeneous Fischer-Tropsch synthesis with homogeneous olefin hydroformylation can be seen as a source for understanding catalytic principles, particularly because the selectivity is complex and therefore highly informative. Reliable analytical techniques must be readily available. 

FIGURE 9.26 Fischer-Tropsch synthesis vs. Oxo synthesis on cobalt catalysts. The thermodynamically controlled shift from heterogeneous to homogeneous catalysis. 

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). 

The data available for heterogeneous Fischer-Tropsch catalysts indicate that with cobalt-based catalysts the rate of the water gas-shift reaction is very slow under the synthesis conditions (5). Thus, water is formed together with the hydrocarbon products [Eq. (14)]. The iron-based catalysts show some shift activity, but even with these catalysts, considerable quantities of water are produced. 

The mechanisms proposed over the last 50 years for the Fischer-Tropsch synthesis, principally on the basis of studies using heterogeneous catalyst systems, may be divided into three main classes (a) metal-carbide mechanisms (b) hydroxyl carbene, =CH(OH), condensation mechanisms and (c) CO insertion mechanisms. 

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). 

There has been considerable recent research interest in the activation of carbon monoxide en route to more complex organic molecules. Among the various reactions that have been investigated and/or newly discovered, the transition metal catalyzed reduction of CO to hydrocarbons (Fischer-Tropsch synthesis) has enjoyed particular attention (l- ). Whereas most of the successful efforts in this area have been directed toward the development of heterogeneous catalysts, there are relatively few homogeneous systems. Among these, two are based on clusters (10,11) and others are stoichiometric in metal (12-17). In this report we detail the synthesis and catalytic chemistry of polystyrene ( ) supported... 

Reasons for interest in the catalyzed reactions of NO, CO, and COz are many and varied. Nitric oxide, for example, is an odd electron, hetero-nuclear diatomic which is the parent member of the environmentally hazardous oxides of nitrogen. Its decomposition and reduction reactions, which occur only in the presence of catalysts, provide a stimulus to research in nitrosyl chemistry. From a different perspective, the catalyzed reactions of CO and COz have attracted attention because of the need to develop hydrocarbon sources that are alternatives to petroleum. Carbon dioxide is one of the most abundant sources of carbon available, but its utilization will require a cheap and plentiful source of hydrogen for reduction, and the development of catalysts that will permit reduction to take place under mild conditions. The use of carbon monoxide in the development of alternative hydrocarbon sources is better defined at this time, being directly linked to coal utilization. The conversion of coal to substitute natural gas (SNG), hydrocarbons, and organic chemicals is based on the hydrogen reduction of CO via methanation and the Fischer-Tropsch synthesis. Notable successes using heterogeneous catalysts have been achieved in this area, but most mechanistic proposals remain unproven, and overall efficiencies can still be improved. 

According to the International Union of Pure and Applied Chemistry (IUPAC O)) the turnover frequency of a catalytic reac tion is defined as the number of molecules reacting per active site in unit time. The term active sites is applied to those sites for adsorption which are effective sites for a particular heterogeneous catalytic reaction. Because it is often impossible to measure the amount of active sites, some indirect method is needed to express the rate data in terms of turnover frequencies In some cases a realistic measure of the number of active sites may be the number of molecules of some compound that can be adsorbed on the catalyst. This measure is frequently used in the literature of the Fischer-Tropsch synthesis, where the amount of adsorption sites is determined by carbon monoxide adsorption on the reduced catalyst. However, it is questionable whether the number of adsorption sites on the reduced catalyst is really an indication of the number of sites on the catalyst active during the synthesis, because the metallic phase of the Fischer-Tropsch catalysts is often carbided or oxidized during the process. 

Transition metal clusters are not only of interest in an academic sense, but also in a practical one. This is due to their importance in both homogeneous and heterogeneous catalysis. Small and large transition metal clusters provide manifold opportunities to study important elementary processes in catalysis. So, clarification of such famous catalytic processes as the Fischer-Tropsch-synthesis and others can be expected by synthesizing and investigating various clusters. 

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 Fischer-Tropsch synthesis is the reductive oligomerization of carbon monoxide over heterogeneous catalysts (Eq. 11.1) [1, 5-7]. 

Although the Fischer-Tropsch synthesis is a textbook example of heterogeneous catalysis, its treatment in this book is justified for a number of reasons. 

Heterogeneous gas-liquid-solid, e.g., catalytic Fischer-Tropsch synthesis of hydrocarbons from CO and H2... 

Fischer-Tropsch synthesis is the heterogeneously-catalysed formation of hydrocarbons from CO and water. It can be seen as the inverse of the synthesis gas preparation. It is the heart of the respawned gas-to-liquids processes developed by big petrochemical firms in the 9O s. 

A great many reactions in physics and chemistry proceed via chain mechanisms. This large family of mechanisms includes free radical and ionic polymerization, Fischer Tropsch synthesis, gas phase pyrolysis of hydrocarbons, and catalytic cracking. Nuclear reactions, of both the power generating and the explosive kind, are also chain processes. Notice that chemical chain reactions can be catalytic or non-catalytic, homogeneous or heterogeneous. One is almost tempted to say that chain reactions are the preferred route of conversion in nature. 

The same metals function as catalysts for heterogeneous hydrogenation reactions and for Fischer-Tropsch synthesis. 

AH of the catalyst is available in a homogeneous system, while only part of the catalyst is available in a heterogeneous system. The steric effect of the surface may play a role in the determination of product distribution. Thus, while the Fischer-Tropsch synthesis may proceed via a hydro-formylation type of reaction, the presence of a surface will affect the distribution of isomers in the product. 

Fischer-Tropsch Synthesis. Mechanism Studies Using Isotopes 3 Heterogeneity of the Catalyst Surface... 

Much of the justification for the extensive study of transition metal cluster chemistry is embedded in the assumption that reactions of metal clusters are realistic structural models for reactions at metal surfaces in such processes as heterogeneous catalysis (9,10,11). For example, the metal carbonyl clusters, Ir4(CO)i2 and Os3(CO)i2, were demonstrated to be effective homogeneous catalysts for methanation (12). Additionally, Demitras and Muetterties (13) have found Ir4(CO)i2 to be a homogeneous catalyst in the Fischer-Tropsch synthesis of aliphatic hydrocarbons. Homogeneous catalysis of the water gas shift reaction by metal carbonyl clusters (e.g., Ru3(CO)i2) in alkaline solution has been reported by Laine, Rinker, and Ford (14), and more recently by Pettit s group (15). Nevertheless, mononuclear metal carbonyls (e.g., Fe(CO)s and the group VIb metal hexacarbonyls) have been demonstrated to have considerable activity above 120°C as soluble catalysts for Reaction 2 (16),... 

Hydroformylation, the addition of synthesis gas (CO and H2) to alkenes, is one of the most important syngas-related reactions [1]. The hydroformylation reaction was firstly discovered on a heterogeneous Fischer-Tropsch (F-T) catalyst [2]. Oxygenates, such as alcohols and esters, can be used as clean engine and vehicle fuels due to their high efficiency and low emission. The... 

The versatility of CO as a synthon also stems from its ability to undergo insertion reactions into a variety of metal-heteroatom bonds. The migratory insertion of CO into transition metal-hydride bonds, while thermodynamically unfavorable, generates metal-formyl complexes M-C(0)H (Equation (19)), a few examples of which have been isolated independently. This reaction is assumed to be a key step in both the homogeneous and heterogeneous catalytic hydrogenation (i.e., reduction) of CO, including the Fischer-Tropsch synthesis of hydrocarbons and... 

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