Fischer-Tropsch synthesis labeled

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. 

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. 

In principle also the possibility should be considered that CO is dissociated, as in Fischer-Tropsch synthesis of hydrocarbons, and thereafter partially hydrogenated. A metal carbene would then produce, upon addition of H2O, a molecule of methanol. However, one would expect that with a metal like Rh, which can dissociate CO, the isotopically labelled atoms from C 0 would be scrambled with C 0 atoms in the methanol product but this has not been found.With metals which dissociate CO even more reluctantly, like Pd and Cu, this mechanism is even less likely than with Rh. This does not however exclude the possibility that higher alcohols can be formed by H2O addition to a carbene-like intermediate. 

By the example of various catalytic reactions (ethylene epoxidation over Ag, deNOx with methane on Co-ZSM-5, Fischer-Tropsch synthesis over Co-based systems), we show how the reaction mechanism was revealed and the concentrations of key intermediates and reaction rate coefficients were estimated using different isotope labels ( 0, etc.). A single instance... 

Any mechanistic proposal must comply with the following observations. (1) The Fischer-Tropsch hydrocarbon synthesis follows the formalism of polymerization kinetics with a Schulz-Flory distribution of the molecular weights. (2) a-Olefins and alcohols occur as the primary products. (3) The aliphatic final products are formed consecutively by hydrogenation of the olefins according to " C-labeling experiments [4 f, 30 b]. (4) Chain termination processes do not deactivate the catalyst centers because the chain-growth velocity stays constant for weeks. 

The possible steps of Fischer-Tropsch (FT) reaction and its catalysts (Fe, Co, Ru, Ni) represent a very complicated systemThe catalysts usually need a formation or self-organisation , meaning that the full activity will only be reached after a certain period. This means that for Fe-based catalysts, a part of the initial Fe oxide is transformed into iron carbide. This was investigated as early as 1948 by the tracer method.A fused iron catalyst was carbided with The synthesis product from CO/H2 = 1 1 reactant contained 10-15% labelled molecules, almost independently of the reaction conditions, even in repeated runs, indicating the minor role of carbide incorporation into hydrocarbons. The formation of a Fe-Al-Cu catalyst at 523 K and various H2/CO ratios required 100 to 2000 minutes. The yield of retained carbon decreased gradually, while the FT yield increased more abruptly after this period. [ 1... 

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