Fischer-Tropsch carbon chain growth

Oxygenates (linear alcohols, aldehydes, esters and ketones) 

If petroleum is cheap and readily available, the FT process is not commercially viable and in the 1960s, many industrial plants were closed. In South Africa, the Sasol process continues to use H2 and CO as feedstocks. Changes in the availability of oil reserves affect the views of industry as regards its feedstocks, and research interest in the FT reaction continues to be high. 

It has also been suggested that vinylic species are involved in FT chain growth and that combination of surface-bound CH and CH2 units to give CH=CH2 may be followed by successive incorporation of CH2 units alternating with alkene isomerization as shown in scheme 26.29. Release of 

Without a catalyst, the reaction between N2 and H2 occurs only slowly, since the activation barrier for the dissociation of N2 and H2 in the gas phase is very high. In the presence of a suitable catalyst such as Fe, dissociation of N2 and H2 to give adsorbed atoms is facile, with the energy released by the formation of M—N and M—H bonds more than offsetting the energy required for N=N and H—H fission. The adsorbates then readily combine to form NH3 which desorbs from the surface. The rate-determining step is the dissociative adsorption of N2 (equation 26.30) the notation (ad) refers to an adsorbed atom. 

Dihydrogen is similarly adsorbed (equation 26.26), and the surface reaction continues as shown in scheme 26.31 with gaseous NH3 finally being released activation barriers for each step are relatively low. 

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The authors conducted an experiment (now regarded as classical) in Fischer-Tropsch catalysis that supports this initiation mechanism (3,4). Using isotopes, they demonstrated that the carbon chain-growth reaction can occur from Ci species generated by the dissociation of CO. As shown below, this hypothesis implies that the rate of CO dissociation should be fast and should not control the overall Fischer-Tropsch reaction. 

Limitation of Metal Particle Size to Carbon Chain Growth in Fischer-Tropsch Synthesis... 

Aiming at explaining the phenomenon, Nijs and Jacobs did lots of work, and many interesting results were obtained [3,12]. They postulated that the simple chain growth scheme occurs,but that the chains are terminated at certain carbon length that is proportional to the size of metal crystallite in the catalysts.Later in 1992,Y.Yang et al [13], based on the restriction of the dimension of the metal crystallites to the carbon chain growth, established a new Fischer-Tropsch product distribution formulation, which extended ASF model This new model has two... 

The carbon number distribution of Fischer-Tropsch products on both cobalt and iron catalysts can be clearly represented by superposition of two Anderson-Schulz-Flory (ASF) distributions characterized by two chain growth probabilities and the mass or molar fraction of products assigned to one of these distributions.7 10 In particular, this bimodal-type distribution is pronounced for iron catalysts promoted with alkali (e.g., K2C03). Comparing product distributions obtained on alkali-promoted and -unpromoted iron catalysts has shown that the distribution characterized by the lower growth probability a, is not affected by the promoter, while the growth probability a2 and the mass fraction f2 are considerably increased by addition of alkali.9 This is... 

In Fischer-Tropsch synthesis the readsorption and incorporation of 1-alkenes, alcohols, and aldehydes and their subsequent chain growth play an important role on product distribution. Therefore, it is very useful to study these reactions in the presence of co-fed 13C- or 14 C-labeled compounds in an effort to obtain data helpful to elucidate the reaction mechanism. It has been shown that co-feeding of CF12N2, which dissociates toward CF12 and N2 on the catalyst surface, has led to the sound interpretation that the bimodal carbon number distribution is caused by superposition of two incompatible mechanisms. The distribution characterized by the lower growth probability is assigned to the CH2 insertion mechanism. 

A new approach to develop a molecular mechanism for Fischer-Tropsch catalysis based on the use of [Fe2Co(CN)6] and [Fe(HCN)2]3 precursor complexes has been disclosed.509 The former produced mainly liquid aliphatic hydrocarbons, whereas the latter gave waxy aliphatic products. Results acquired by various techniques were interpreted to imply that chain growth proceeds via the insertion of CO into an established metal-carbon bond, that is, a C, catalytic insertion mechanism is operative. It follows that C2 insertion is an unlikely possibility. 

Hydrocarbon distributions in the Fischer-Tropsch (FT) synthesis on Ru, Co, and Fe catalysts often do not obey simple Flory kinetics. Flory plots are curved and the chain growth parameter a increases with increasing carbon number until it reaches an asymptotic value. a-Olefin/n-paraffin ratios on all three types of catalysts decrease asymptotically to zero as carbon number increases. These data are consistent with diffusion-enhanced readsorption of a-olefins within catalyst particles. Diffusion limitations within liquid-filled catalyst particles slow down the removal of a-olefins. This increases the residence time and the fugacity of a-olefins within catalyst pores, enhances their probability of readsorption and chain initiation, and leads to the formation of heavier and more paraffinic products. Structural catalyst properties, such as pellet size, porosity, and site density, and the kinetics of readsorption, chain termination and growth, determine the extent of a-olefin readsorption within catalyst particles and control FT selectivity. 

FIGURE 10 The Fischer-Tropsch window. A schematic representation of the selectivity for several product groups as a function of the metal-carbon interaction (expressed by the adsorption energy of a carbon atom). Lines represent four reaction pathways CH4 via methanation, C + C conversion to graphene, C Hj, + CHy conversion leading to chain growth, and CO dissociation. 

Fischer-Tropsch process. The methane selectivity is t)q)ically 30-60 mol%. This significant selectivity for methane formation implies that the rate of hydrogenation of the "Ci" species is of the same order of magnitude as the rate of chain growth. The Fischer-Tropsch process is economical only if a major fraction of the carbon in the synthesis gas is converted to long-chain hydrocarbons and not more than about 10% is converted to methane (41). 

These trends are consistent with observations made to characterize the chain growth of surface carbon that was deposited by methane decomposition. In a row of the periodic table, the selectivity to hydrocarbon formation was foimd to increase from right to left for example, palladium shows a lower selectivity than ruthenium 111,112). Metals such as platinum and iridium are characterized by higher selectivities for chain growth initiated from "Cl" species than other metals because of their relatively high M—C bond energies. However, platinum and iridium are unsuitable as Fischer-Tropsch catalysts because the dissociation of CO is too slow. 

Chain growth during the Fischer-Tropsch synthesis is controlled by surface polymerization kinetics that place severe restrictions on our ability to alter the resulting carbon number distribution. Intrinsic chain growth kinetics are not influenced strongly by the identity of the support or by the size of the metal crystallites in supported Co and Ru catalysts. Transport-limited reactant arival and product removal, however, depend on support and metal site density and affect the relative rates of primary and secondary reactions and the FT synthesis selectivity. 

Catalysis by Metal Ousters in Zeolites. There is an increasing interest in the use of metal clusters stabilized in zeolites. One objective of such work is to utilize the shape and size constraints inherent in these support materials to effect greater selectivities in typical metal-catalysed reactions. Much work has been concerned with carbon monoxide hydrogenation, and although the detailed nature of the supported metals so obtained is not well understood, there is clear evidence of chain limitation in the Fischer-Tropsch process with both RuY zeolites and with HY and NaY zeolites containing Fe3(CO)22- In the former case there is a drastic decline in chain-growth probability beyond C5- or C10-hydrocarbons depending upon the particle size of the ruthenium metal. 

A simple picture of Fischer Tropsch chain growth is given in Fig. 5. The molecule chain of n carbon atoms (surface species "Sp ") is prolonged to a chain with n+1 carbon atoms (surface species "Spn+1") the carbon atoms n,... 

The first stage, Heavy Paraffin Synthesis (HPS), converts hydrogen and carbon monoxide into heavy paraffins by the Fischer-Tropsch process. The product distribution is in accordance with Schultz-Flory polymerization kinetics, which is characterized by, the probability of chain growth. 

Liquid-phase Fischer-Tropsch synthesis has been investigated using a slurry-bed reactor. The catalytic activity of ultrafine particles (UFP) composed of Fe was shown to be greater than that of a precipitated Fe catalyst. The difference was interpreted as caused by the different nature of surface structure between these catalysts, whether porous or not. The obtained carbon number distributions over alkali-promoted Fe UFP catalysts were simulated by a superposition of two Flory type distributions. It is ascertained that the surface of alkali-promoted UFP catalysts possesses promoted and unpromoted sites exhibiting different chain growth probabilities. 

Investigations into these topics are presented in this volume. Iron, nickel, copper, cobalt, and rhodium are among the metals studied as Fischer-Tropsch catalysts results are reported over several alloys as well as single-crystal and doped metals. Ruthenium zeolites and even meteo-ritic iron have been used to catalyze carbon monoxide hydrogenation, and these findings are also included. One chapter discusses the prediction of product distribution using a computer to simulate Fischer-Tropsch chain growth. 

Developments in the Fischer-Tropsch synthesiis at the Bureau of Mines from 19 5 to I960 include a simple mechanism for chain growth and the use of iron nitrides as catalysts. The chain-growth schene can predict the carbon-number and isomer distributions for products from most catalysts with reasonable accuracy using only 2 adjustable parameters. Iron nitrides are active, durable catalysts that produce high yields of alcohols and no wax. During the synthesis, the nitrides are converted to carbonitrides. 

In research on the Fischer-Tropsch synthesis, FTS, at the Bureau, mechanisms of the growth of the carbon chain and the use of iron nitrides as catalysts were developments that were not anticipated by previous German work. Herington (2) in 1946 was the first to consider chain growth in FTS. He defined a probability 3 that the chain will desorb rather than grow at the surface, where... 

The Fischer-Tropsch reaction of carbon monoxide with hydrogen can be described, in a simplified way, according to equation (8.9). Chain growth proceeds at the surface of cobalt- and/or iron-based catalyst systems, e.g. Fe-, Fe/Co-, Fe/Co-Spinel-, Co/Mn-Spinel- or Cu-doped Co-catalysts. Iron catalysts, on the one hand, are cheaper than cobalt catalysts and more flexible as well as resistant with respect to syngas composition and quality. Cobalt catalysts, on the other hand, exhibit best performances at a H2 CO ratio of... 

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