Chain growth, Fischer-Tropsch synthesis

Mendes FMT, Perez CAC, Noronha FB, Souza CDD, Cesar DV, Freund HJ, Schmal M. Fischer-Tropsch synthesis on anchored Co/Nb205/Al203 catalysts the nature of the surface and the effect on chain growth Fischer-Tropsch synthesis on anchored Co/Nb205/Al203 catalysts the nature of the surface and the effect on chain growth. J Phys Chem B. 2006 110 9155-63. 

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. 

N. O. Elbashir and C. B. Roberts, Enhanced Incorporation of a-Olefins in the Fischer-Tropsch synthesis Chain-Growth Process over an Alumina-Supported Cobalt Catalyst in Near-Critical and Supercritical Hexane Media, Ind. Eng. Chem. Res., 2005, 44, 505-521. 

Fischer-Tropsch synthesis, 28 80, 97, 103, 30 166-168, 34 18, 37 147, 39 221-296 activation energy and kinetics, 39 276 added olefin reactions, 39 251-253 bed residence time effects on chain growth probability and product functionality, 39 246-250... 

A new mechanism to interpret alkene formation in Fischer-Tropsch synthesis has been presented 499-501 There is a general agreement that hydrocarbon formation proceeds according to the modified carbene mechanism. Specifically, CO decomposes to form surface carbide and then undergoes hydrogenation to form surface methine (=CH), methylene (=CH2), methyl and, finally, methane. Linear hydrocarbons are formed in a stepwise polymerization of methylene species. When chain growth is terminated by p-hydride elimination [Eq. (3.61)], 1-alkenes may be formed,502 which is also called the alkyl mechanism ... 

At the Mellon Institute he applied l4C tracers to examine the behavior of intermediates in Fischer-Tropsch synthesis over iron catalysts. By adding small amounts of radioactively labeled compounds to the CO/H2 synthesis gas mixtures, he was able to prove that some of these compounds (e.g., small alcohols) are involved in the initiation step of the chain growth process that leads to larger hydrocarbon products. It was during this era that his associates first placed a catalytic reactor into the carrier gas stream of a gas chromatograph and developed the microcatalytic pulse reactor, which is now a standard piece of equipment for mechanistic studies with labeled molecules. While at Mellon Institute Emmett began editing his comprehensive set of seven volumes called Catalysis, which he continued at Hopkins. 

It will be interesting to see whether further reports with more precise analytical data than those heretofore offered (89,90) will be forthcoming. Evidence ruling out the possibility of toluene formation by chain growth in Fischer-Tropsch synthesis followed by dehydrocyclization would also be necessary for a confirmation of the methylation of benzene as suggested by Eidus (89). 

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. 

Fig. 1. Chain growth and termination and secondary reactions in Fischer-Tropsch synthesis on Co and Ru catalysts.

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. 

Fig. 31. Fischer-Tropsch synthesis over Ru/Ti02. Model for chain growth (after 199).

Figure 10 Fischer-Tropsch synthesis with Ru/Na. Effect of catalyst potential (C/wr) on the probability of hydrocarbon 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. 

There is at present much interest in the use of solid ruthenium catalysts for Fischer-Tropsch synthesis.24 It has been found that the maximum chain growth in the synthesis reaction is strongly affected by the size of the ruthenium particles. The smaller the particles, the lower the molecular weight of the products. Specifically it has been found that the maximum petroleum production should result if the crystallite size can be controlled in the range of 3-4 nm. 

FIGURE 12.3 Growth of paraffin chains in the Fischer-Tropsch synthesis and Schulz-Flory kinetics. 

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

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... 

Fischer-Tropsch synthesis (FTS) has been widely studied during the past 80 years due to its significance in indirectly converting coal/natural gas to transportation fuels. However, one of big challenges is how to control hydrocarbon chain growth during the FTS reaction [1], It has been proposed that proper process conditions or some kinds of porous supports can be used to restrict chain propagation [1,2]. 

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