Hydrocarbon chain growth pathway

Figure 7 Simplified scheme of hydrocarbon chain growth pathways. (S -FT chain growth site k rate constants, see text).

Higher intrapellet residence times increase the contribution of chain initiation by a-olefins to chain growth pathways. This intrapellet delay, caused by the slow diffusion of large hydrocarbons, leads to non-Flory carbon number distributions and to increasingly paraffinic long hydrocarbon chains during FT synthesis. But intrapellet residence time also depends on the effective diameter and on the physical structure (porosity and tortuosity) of the support pellets. The severity of transport restrictions and the probability that a-olefins initiate a surface chain as they diffuse out of a pellet also de-... 

Many of the chain growth pathways and transport effects described above also occur on Fe-based FT synthesis catalysts. As on Co and Ru catalysts, FT synthesis on Fe often yields non-Flory carbon number distributions of products, where the chain growth probability and the paraffin content increase with hydrocarbon chain size (38-40). These effects were previously... 

At higher Co site densities, the probability of chain termination to olefins is lower and decreases somewhat faster with hydrocarbon chain size (Fig. 13b). Thus, the heavier product obtained on materials with high site densities reflects a lower probability of chain termination to olefins, an effect that arises from the enhanced readsorption of such olefins within transport-limited pellets with high site density. In contrast, chain termination to paraffins is not influenced by Co site density (Fig. 13c). Site density does not affect primary chain growth and termination chemistry or inhibit secondary olefin hydrogenation pathways, effects that could otherwise account for the higher Cs+ selectivity observed on higher site density pellets. 

It is noted that, in a recent review by van Santen et that rationalizes the main FT reaction mechanisms (the carbide and the CO insertion pathways) using a BEP type calculation, they concluded that chain growth does not occur by CO insertion. Alternatively, recent works suggest the co-existence of two reaction mechanisms. Gaube and Klein suggested that the ASF plots obtained from analysis of FT hydrocarbon synthesis reactions arose from the participation of two incompatible mechanisms. Both involve surface allq ls, but in one case the chain growth takes place by insertion of CO, while in the other the monomer is a surface carbene (CH2). 

A number of Pseudomonas strains can accumulate PHAs from a variety of aromatic hydrocarbons. In many strains, the level of PHA accumulation is dependent on the side chain length of the phenylalkanoic acid provided for growth. The PHA accumulated from styrene and phenylacetic acid was composed of aliphatic monomers only. The PHA accumulated from any one of the phenylalkanoic acids with five carbons or more in their side chain was almost identical for all strains with the PHA composed of both aromatic and aliphatic monomers. The predominant monomers accumulated were 3-hydroxyphenylvaleric acid and 3-hydro3g henylhexanoic acid. The addition of the metabolic pathway inhibitors acrylic acid and 2-bromooctanoic acid resulted in decreased levels of PHA from phenylacetic acid, suggesting a... 

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