Fischer-Tropsch chemistry

The basic transformations of Fischer-Tropsch synthesis may be generally summarized as in Eqs. (3.8) and (3.9)  

These indicate two different ways of carbon monoxide conversion both processes are highly exothermic. Iron catalyzes the transformation according to Eq. (3.8), 

Other secondary reactions taking place under operating conditions are the Bou-douard reaction [Eq. (3.10)], coke deposition [Eq. (3.11)], and carbide formation [Eq. (3.12)]  

Rhodium is a unique metal since it can catalyze several transformations.222,223 It is an active methanation catalyst and yields saturated hydrocarbons on an inert support. Methanol is the main product in the presence of rhodium on Mg(OH)2. Transition-metal oxides as supports or promoters shift the selectivity toward the formation of C2 and higher oxygenates. 

Iron catalysts used in Fischer-Tropsch synthesis are very sensitive to conditions of their preparation and pretreatment. Metallic iron exhibits very low activity. Under Fischer-Tropsch reaction conditions, however, it is slowly transformed into an active catalyst. For example, iron used in medium-pressure synthesis required an activation process of several weeks at atmospheric pressure to obtain optimum activity and stability.188 During this activation period, called carburization, phase 

In a subsequent publication, Meutterties reports that under flow conditions, at 1 atm pressure and 170-180 C, Cj-Cs hydrocarbons may be detected, but now isobutane and propane are the major products (116). Hydrogen chloride is a coproduct, but no methanol or CH3CI could be detected. An iridium carbonyl chloride species was suggested as the enduring catalyst precursor, basis ir data and by comparison with the formally analogous BBr3-0s3(C0)12 system. 

In view of the unique features of this homogeneous Fischer-Tropsch catalysis) Collman and coworkers have reexamined this melt system in both a single pass flow reactor and in a continuous recycle apparatus (117). These researchers conclude that the Ir4(C0)i2 precatalyst in molten AlCl3-NaCl at 175 C and 1 atm pressure of H2/CO (3 1) produces methane, ethane and chloromethane (eq. 30) as the major carbon-containing products. In addition, a stoichiometric amount of methane is formed at the onset of catalysis. 

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Propanol has been manufactured by hydroformylation of ethylene (qv) (see Oxo process) followed by hydrogenation of propionaldehyde or propanal and as a by-product of vapor-phase oxidation of propane (see Hydrocarbon oxidation). Celanese operated the only commercial vapor-phase oxidation faciUty at Bishop, Texas. Since this faciUty was shut down ia 1973 (5,6), hydroformylation or 0x0 technology has been the principal process for commercial manufacture of 1-propanol ia the United States and Europe. Sasol ia South Africa makes 1-propanol by Fischer-Tropsch chemistry (7). Some attempts have been made to hydrate propylene ia an anti-Markovnikoff fashion to produce 1-propanol (8—10). However, these attempts have not been commercially successful. 

Graphite compounds have been described as catalysts for ammonia synthesis from nitrogen and hydrogen (14, Pll), for Fischer-Tropsch chemistry M13, R14), for paraffin isomerization iR15), and for Friedel-Crafts chemistry (07). 

This reaction is significant in terms of Fischer-Tropsch chemistry, because it represents the first well-characterized system in which a coordinated carbonyl is reduced by molecular hydrogen. Furthermore, complex 11 could be viewed as a precursor to ethylene glycol which, as previously indicated, is a highly desirable product from the reaction between carbon monoxide and hydrogen. 

The sustained elevated price of crude oil seen in 2005 has led to increased interest in synthetic fuels. Synthetic fuels have been produced for more than 80 years through processes known as Fischer-Tropsch chemistry. Carbon monoxide is a basic feedstock in these processes. Franz Fischer (1852-1932) and Hans Tropsch (1889-1935) produced liquid hydrocarbons in the 1920s by reacting carbon monoxide (produced from natural gas) with hydrogen using metal catalysts such as iron and cobalt. Germany and Japan produced synthetic fuels during World War II. Low crude oil prices dictated little interest in synthetic fuels after the war,... 

The revival of interest in Fischer-Tropsch chemistry in the 1970s resulted in new observations that eventually led to the formulation of a modified carbide mechanism, the most widely accepted mechanism at present.202-204,206,214 Most experimental evidence indicates that carbon-carbon bonds are formed through the interaction of oxygen-free, hydrogen-deficient carbon species.206 Ample evidence shows that carbon monoxide undergoes dissociative adsorption on certain metals to form carbon and adsorbed oxygen ... 

Efforts have been made over the years to advance a unified concept of Fischer-Tropsch chemistry. The basic problem, however, is that most information comes from studies of different metals. Considering the specificity of metals, it is highly probable that different mechanisms may be operative on different metals. The numerous mechanistic proposals, therefore, may represent specific cases on specific surfaces and may be considered as extremes of a highly complex, widely varied... 

Methanol Synthesis. The transformation of synthesis gas to methanol [Eq. (3.3)] is a process of major industrial importance. From the point of view of hydrocarbon chemistry, the significance of the process is the subsequent conversion of methanol to hydrocarbons (thus allowing Fischer-Tropsch chemistry to become more selective). 

The conversion of CO + H2 (syn-gas) to hydrocarbons and oxygenates (Fischer-Tropsch chemistry)119 is of considerable industrial importance and recently the activation and fixation of carbon monoxide in homogeneous systems has been an active area for research.120,121 The early transition elements and the early actinide elements, in particular zirconium124 and thorium,125 126 supported by two pentamethylcyclopentadienyl ligands have provided a rich chemistry in the non-catalytic activation of CO. Reactions of alkyl and hydride ligands attached to the Cp2M centers with CO lead to formation of reactive tf2-acyl or -formyl compounds.125,126 These may be viewed in terms of the resonance forms (1) and (2) shown below. 

A rare example of the formation of an alkoxide ligand in a metal cluster compound is seen in the reaction between methyl fluorosulfonate and the triiron carbonyl clusters anion [Fe3(CO)9(/i3-MeCO)], which gives Fe3(CO)9(/x3-CMe)(/i3-OMe) by C—O bond cleavage.135,136 The C—O bond cleavage provides a possible model for a step in Fischer-Tropsch chemistry.121... 

As chemical "building blocks" to produce a broad range of higher-value liquid or gaseous fuels and chemicals using processes well established in today s chemical industry such as water-gas shift and Fischer-Tropsch chemistry. 

Schemes 15 and 16 summarize several papers on the synthesis and reactions of the ligands CH, CH2, and CH3 in triosmium clusters, and their formation from or transformation into C2 ligands (see the schemes for references). The CH2 ligand in Os3(CH2)(CO)u has been formed in three ways from a CO ligand of Os3(CO)12, from CH2N2, and from CH2CO (Scheme 15). The reduction of CO to CH2 relates interestingly to Fischer-Tropsch chemistry. There is good evidence that [BH(0 Pr)3] or [BHEt3] generate...

Ruthenium is known to catalyze a number of reactions, including the Fischer-Tropsch synthesis of hydrocarbons (7) and the polymerization of ethylene (2). The higher metal dispersions and the shape selectivity that a zeolite provides has led to the study of ruthenium containing zeolites as catalytic materials (3). A number of factors affect the product distribution in Fischer-Tropsch chemistry when zeolites containing ruthenium are used as the catalyst, including the location of the metal (4) and the method of introducing ruthenium into the zeolite (3). 

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. 

Selectivity control continues to be a critical issue in Fischer-Tropsch chemistry, a catalytic process that dates back more than seventy years [1]. Operating conditions can be adjusted to control selectivities but overall effects are limited [2-4]. During Fischer-Tropsch synthesis with conventional bulk iron catalysts, various phases, including metal, metal carbides and metal oxides are present at steady-state catalytic conditions [5-7]. 

The higher nuclearity allenyi clusters provide an attractive potential model for the chemistry of surface-bound Cj fragments. Such species could form from the combination of a surface-bound vinylidene and car-byne or an acetylide and methylene unit, These fragments have been identified in the heterogeneously catalyzed Fischer-Tropsch chemistry. 

By way of contrast, Fischer-Tropsch chemistry requires heterogeneous catalysts of structures close to 3 and 4, of which the surface stmctures are not precisely known, and for which therefore there is no clear molecular mechanism known [27] ... 

Otherwise, only significantly increased product selectivities (e. g., to value-added chemical feedstocks such as ethylene and propene) could make Fischer-Tropsch chemistry attractive in other countries, too [4 h, i]. 

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