Carbon chain growth

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. 

Keister and co-workers isolated a cluster containing a hydrocarbyl fragment of type N, isomeric with the allenyl ligand (50). The cluster was identified as an intermediate in the carbon chain growth reactions of [Ru3H3(CO)9(/i3-CSEt)]. However the carbon-carbon bond lengths for... 

Fukuzawa, K. Osamura, Y. Schaefer III, H.F. Are neutral-neutral reactions effective for the carbon-chain growth of cyanopolyynes and polyacetylenes in interstellar space Astrophys. J. 1998, 505. 278-285. 

Fig. 7. Scheme of carbon chain growth to form linear alcohols, branched alcohols, and methyl esters over alkali-promoted Cu/ZnO-based catalysts. 

Methylation is an important reaction in causing carbon chain growth during the conversion of MeOH to gasoline over the zeolite catalyst H-ZSM-5. The nature of the methylating agent is uncertain, the choice lying between (i) carbenium ion CH3, (ii) carbene CHg > and (iii) methyl oxonium ion. 

Alkyl migrations are not confined to the formation of acyl groups, and, for example, alkene insertion involves the conversion of a coordinated alkene to a cr-bonded alkyl group. Equation 23.37 shows the migration of an H atom related alkyl migrations occur and result in carbon chain growth. 

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 equation (8) is based on the two ideas, (a) The negative deviation is due to the limitation of the particle size to carbon chain growth, (b) A simple linear relationship exists between the metal particle size and maximum molecule chain length formed on it. So.the results confirm the viewpoint that for FT synthesis on the catalysts with highly dispersion,the negative deviation is the result of cut-off effect of the small particle or crystallites size.The results also confirm the assumption that the maximum product chain length depends on the particle size linearly. 

Compared with that in the conventional supercritical phase reaction, the carbon chain growth was accelerated by the addition of alkenes with long carbon chains into the accompanying fluid. The essential prerequisite for this process is the rapid diffusion of these heavy added alkenes inside the catalyst pores to reach the metal sites, as well as effective diffusion of the heavy products produced from the interior active sites to the outer catalyst surface. Both diffusion processes are readily achieved in the supercritical phase. 

The added 1-alkenes reach the metal sites aided by the SCF and adsorb onto the active sites as alkyl radicals to initiate carbon chain growth the resulting chains are indistinguishable from other carbon chains formed directly from synthesis gas. These new alkyl radicals consume additional methylene units to initiate new carbon chain propagation processes. Thus the selectivity for methane, which is formed mainly from methylene hydrogenation, decreases. CO adsorption and cleavage of CO to carbide on the metal site, as well as hydrogenation of carbide to methylene species, are both accelerated. This is attributed to increased consumption of the adsorbed methylene species. Experimentally, the CO conversion increased with addition of 1-alkene. This acceleration may contribute to the suppressed CO2 selectivity as well in the alkene-added reaction, as CO2 is the byproduct from CO in the water-gas shift reaction. 

---