Hydrocarbon synthesis, effect

McDonald, M.A., Storm, D.A., and Boudart, M. 1986. Hydrocarbon synthesis from carbon monoxide-hydrogen on supported iron Effect of particle size and interstitials. J. Catal. 102 386 -00. 

R. Madon and E. Iglesia, Hydrogen and CO intrapellet diffusion effects in ruthenium-catalyzed hydrocarbon synthesis, J. Catal., 1994, 149, 428 137. 

CO pressure, secondary reactions and hydrocarbon synthesis selectivity effects, 39 257-260... 

Good evidence has been obtained that heterogeneous iron, ruthenium, cobalt, and nickel catalysts which convert synthesis gas to methane or higher alkanes (Fischer-Tropsch process) effect the initial dissociation of CO to a catalyst-bound carbide (8-13). The carbide is subsequently reduced by H2to a catalyst-bound methylidene, which under reaction conditions is either polymerized or further hydrogenated 13). This is essentially identical to the hydrocarbon synthesis mechanism advanced by Fischer and Tropsch in 1926 14). For these reactions, formyl intermediates seem all but excluded. 

Sulfur poisoning is a key problem in hydrocarbon synthesis from coal-derived synthesis gas. The most important hydrocarbon synthesis reactions include methanation, Fischer-Tropsch synthesis, and methanol synthesis, which occur typically on nickel, iron, or cobalt, and ZnO-Cu catalysts, respectively. Madon and Shaw (2) reviewed much of the early work dealing with effects of sulfur in Fischer-Tropsch synthesis. Only the most important conclusions of their review will be summarized here. 

F. Effect of CO Pressure on Secondary Reactions AND Hydrocarbon Synthesis Selectivity... 

The effects of compositions and reaction conditions on product distribution were investigated over various metal promoted Cu-based catalysts to improve the performance for synthesis of hydrocarbons. The formation of carbon monoxide was suppressed and the formation of hydrocarbons increased with the increase in the amount of Fe. The synergetic effect between copper and iron was required for hydrocarbon synthesis. 

Figure 2 shows the effect of method of Fe addition on product distributions. Cu0-Zn0/Ti02 (cat. A) was active for methanol synthesis, but it was not effective for the synthesis of hydrocarbons. This indicates that Cu species alone is not enough to produce hydrocarbons. On the contrary, Fe-based catalysts are known as hydrocarbon synthesis catalysts from CO, that is, Fischer-Tropsch reaction. However, Fe/TiOj catalyst (cat. C) showed poor... 

For the Fischer-Tropsch hydrocarbon synthesis (FTS) it has been shown that alkali modifiers are effective in improving a catalyst s behavior (refs. 11-16). For FTS higher-hydrocarbon production rates drop upon alkali addition but that of methane drops more dramatically, resulting in improved selectivity for ihe desired higher-hydrocarbon... 

At first, the effect of only one additive on the hydrocarbon synthesis was investigated (Tab.l). When the total hydrocarbon yield was related to the CO+H content in the reaction feed it decreased with increasing CO2 concentration but was not practically influenced by the N2 concentration. CO may be suggested to act as an inhibitor while N2 only dilutes the gaseous mixture. Also the chain growth factor depends on the feed gas composition. The addition of 18% CO2 caused its decrease from 0.86 to 0.80 while a 16% N2 admixture increased its value to 0.90. These variations are in agreement with the respective roles of both additives. N2 causes an increase of... 

The fact that fused iron catalysts of the synthetic ammonia type were successively used in many investigations of hydrocarbon synthesis for both fluidized and fixed catalyst bed operations is of interest in different respects. Due to this fact it is possible to make use of the valuable experience obtained during development work of the ammonia synthesis (73). This applies to the reduction, the tendency to oxidize, and the effect of promoters and poisons, and to a certain extent also to questions regarding the reaction mechanism. 

The work of Sanders et al. (1984), Jackson and Catlow (1988) and Tomlinson et al. (1990) showed that with Born model potentials it is possible accurately to reproduce the crystal structures of silicates including zeolites. A typical example is shown in Plate I which illustrates the calculated and experimental structures of a purely siliceous (i.e. pure Si02 polymorph) zeolite, silicalite. (A closely related, isostructural material, ZSM-5, which contains a low concentration of Al is an effective isomerization and hydrocarbon synthesis catalyst.) The agreement between theory and experiment is evidently good more discussion follows in Chapter 9. 

It is well admitted that the first step of the hydrocarbons synthesis from syngas is the CO dissociation on a metallic center (Co°, Fe°, or Co-Fe in the present case). CO dissociation into Cgurf and CO2 has been studied on the partially Lai j,Coo.4Feo,603 5 solids [43]. The results clearly show that the rate of CO dissociation increases almost linearly with the lanthanum deficiency. This is associated with the increased amount of reduced metal (2.1 wt% for y = 0-10.9 wt% for y=0A). The metal particles size has also an effect. For y = 0.4, CO dissociation is lower when catalysts were initially calcined at 900 °C (12% of CO conversion for 14.1 wt% of metal of average particle size of 28 nm) compared to those calcined at 750 °C (19% of CO conversion for 10.9 wt% of metal of average particle size of 10 nm). The larger size of particles led to a lower surface/volume ratio and to a decrease of CO dissociation. 

Czerwosz et al. s findings might be of particular interest to readers familiar with carbon formation on nickel and nickel-coated catalysts that had been exposed to hydrocarbons or carbon monoxide in hydrocarbon synthesis or in so-called re-forming reactions carried out in petroleum refineries. For example, the formation of filamentous carbon on such solids at temperatures in the same range as that used by Czerwosz et al. was reported by McCarthy in 1982 [115]. However, these authors did not analyze the carbon deposits by Raman spectroscopy, nor were they aware of the existence of fullerenes. Their concern was the removal of these carbons by steam or by combustion, because these carbons inactivated the catalyst. It was also unknown to them that these carbons had the lubricating properties that were demonstrated by Lauer and co-workers [60,62]. By using these catalysts under conditions of continuous wear, they could maintain the catalytic effect of the surface. 

Although allylic brommations and chlormations offer a method for attaching a reactive functional group to a hydrocarbon framework we need to be aware of two important limitations For allylic halogenation to be effective m a particular synthesis... 

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