Fischer Tropsch reactions

The Fischer-Tropsch reaction is essentially that of Eq. XVIII-54 and is of great importance partly by itself and also as part of a coupled set of processes whereby steam or oxygen plus coal or coke is transformed into methane, olefins, alcohols, and gasolines. The first step is to produce a mixture of CO and H2 (called water-gas or synthesis gas ) by the high-temperature treatment of coal or coke with steam. The water-gas shift reaction CO + H2O = CO2 + H2 is then used to adjust the CO/H2 ratio for the feed to the Fischer-Tropsch or synthesis reactor. This last process was disclosed in 1913 and was extensively developed around 1925 by Fischer and Tropsch [268]. 

The classic catalyst consists of Co-Th02-MgO mixtures supported on Kieselguhr (see Ref. 269) group VIII metals, especially Ni, generally are active, 

The mechanism of the Fischer-Tropsch reactions has been the object of much study (note Eqs. XVI11-55-XV111-57) and the subject of much controversy. Fischer and Tropsch proposed one whose essential feature was that of a metal carbide—patents have been issued on this basis. It is currently believed that a particular form of active adsorbed carbon atoms is involved, which is then methanated through a series of steps such as 

This active carbon is considered to be produced through the reaction 

Sequences such as the above allow the formulation of rate laws but do not reveal molecular details such as the nature of the transition states involved. Molecular orbital analyses can help, as in Ref. 270 it is expected, for example, that increased strength of the metal—CO bond means decreased C=0 bond strength, which should facilitate process XVIII-55. The complexity of the situation is indicated in Fig. XVIII-24, however, which shows catalytic activity to go through a maximum with increasing heat of chemisorption of CO. Temperature-programmed reaction studies show the presence of more than one kind of site [99,1(K),283], and ESDIAD data show both the location and the orientation of adsorbed CO (on Pt) to vary with coverage [284]. 

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Fischer-Tropsch reaction The catalytic reaction of hydrogen and carbon monoxide (synthesis gas ) to produce high-molecular weight hydrocarbons. 

The second reaction is called the Fischer-Tropsch synthesis of hydrocarbons. Depending on the conditions and catalysts, a wide range of hydrocarbons from very light materials up to heavy waxes can be produced. Catalysts for the Fischer-Tropsch reaction iaclude iron, cobalt, nickel, and mthenium. Reaction temperatures range from about 150 to 350°C reaction pressures range from 0.1 to tens of MPa (1 to several hundred atm) (77). The Fischer-Tropsch process was developed iadustriaHy under the designation of the Synthol process by the M. W. Kellogg Co. from 1940 to 1960 (83). 

Sasol Fischer-Tropsch Process. 1-Propanol is one of the products from Sasol s Fischer-Tropsch process (7). Coal (qv) is gasified ia Lurgi reactors to produce synthesis gas (H2/CO). After separation from gas Hquids and purification, the synthesis gas is fed iato the Sasol Synthol plant where it is entrained with a powdered iron-based catalyst within the fluid-bed reactors. The exothermic Fischer-Tropsch reaction produces a mixture of hydrocarbons (qv) and oxygenates. The condensation products from the process consist of hydrocarbon Hquids and an aqueous stream that contains a mixture of ketones (qv) and alcohols. The ketones and alcohols are recovered and most of the alcohols are used for the blending of high octane gasoline. Some of the alcohol streams are further purified by distillation to yield pure 1-propanol and ethanol ia a multiunit plant, which has a total capacity of 25,000-30,000 t/yr (see Coal conversion processes, gasification). 

Fischer-Tropsch. Caibon monoxide is catalyticaily hydrogenated to a mixtuie of straight-chain aliphatic, olefinic, and oxygenated hydrocarbon molecules in the Fischer-Tropsch reaction (eq. 11) (see Fuels, synthetic). 

CO + H2 — paraffins -organic compounds) (Fischer-Tropsch reaction)... 

A hst of 74 GLS reacdions with hterature references has been compiled by Shah Gas-Liquid-Solid Reactions, McGraw-HiU, 1979), classified into groups where the solid is a reactant, or a catalyst, or inert. A hst of 75 reactions made by Ramachandran and Chaudhari (Three-Phase Chemical Reactors, Gordon and Breach, 1983) identifies reactor types, catalysts, temperature, and pressure. They classify the processes according to hydrogenation of fatty oils, hydrodesulfurization, Fischer-Tropsch reactions, and miscellaneous hydrogenations and oxidations. 

The Fischer-Tropsch reaction is highly exothermic. Therefore, adequate heat removal is critical. High temperatures residt in high yields of methane, as well as coking and sintering of the catalyst. Three types of reac tors (tubular fixed bed, fluidized bed, and slurry) provide good temperature control, and all three types are being used for synthesis gas conversion. The first plants used tubular or plate-type fixed-bed reactors. Later, SASOL, in South Africa, used fluidized-bed reactors, and most recently, slurry reactors have come into use. 

Instinctively it would seem that Step C would be rate controlling and the slowest. In the case of the Fischer-Tropsch reaction, one would postulate that the surface would offer more methylene groups for chain polymerization. This mechanism differs from that of Vlasenko and Uzefo-vich (4) essentially in the concept that the whole molecule interacts with the surface. Furthermore, the HCOH intermediate is wholly horizontal to the surface rather than perpendicular. 

Calderbank et al. (C6) studied the Fischer-Tropsch reaction in slurry reactors of 2- and 10-in. diameters, at pressures of 11 and 22 atm, and at a temperature of 265°C. It was assumed that the liquid-film diffusion of hydrogen from the gas-liquid interface is a rate-determining step, whereas the mass transfer of hydrogen from the bulk liquid to the catalyst was believed to be rapid because of the high ratio between catalyst exterior surface area and bubble surface area. The experimental data were not in complete agreement with a theoretical model based on these assumptions. 

As was demonstrated in the preceding sections, structure-sensitivity phenomena are mostly confined to particle size regimes smaller than 3-4 nm. A process of industrial relevance was investigated by de Jong et al. [127] in their study on cobalt particle size effects in the Fischer-Tropsch reaction. Earlier works noted distinct drop in activity for Co particles smaller than lOnm and ascribed this phenomenon to either a partial oxide or carbide formation which should be enhanced for particles in this size regime [128-139]. In order to avoid similar effects, de Jong used... 

The carbonylation step that is involved in both hydroformylation and the Fischer-Tropsch reaction can be reversible. Under appropriate conditions, rhodium catalyst can be used for the decarbonylation of aldehydes285 and acyl chlorides.286... 

The polymers were converted to supported catalysts corresponding to homogeneous complexes of cobalt, rhodium and titanium. The cobalt catalyst exhibited no reactivity in a Fischer-Tropsch reaction, but was effective in promoting hydroformylation, as was a rhodium analog. A polymer bound titanocene catalyst maintained as much as a 40-fold activity over homogeneous titanocene in hydrogenations. The enhanced activity indicated better site isolation even without crosslinking. 

Catalysis. Beitel et al. (1997) have employed RAIRS to study in situ the co-adsorption behaviour of CO and hydrogen on single-crystal cobalt (0001) catalysts at pressures up to 300mbar and temperatures between 298 and 490 K. The behaviour of these adsorbates is of considerable importance in relation to their commercial importance as catalysts for the Fischer-Tropsch reaction in the... 

Fischer-Tropsch reaction of CO/H2 to produce ethanol followed by the Sasol Kvaemer process. 

The Fischer-Tropsch reaction has now been known for almost 70 years and is of great importance partly for itself and also as part of a coupled set of processes whereby steam or oxygen plus coal or coke is transformed into methane, alkenes, alcohols, and gasolines. According to Eqs.I-IV in the most... 

A variety of attempts has been made to model the single steps of the Fischer Tropsch reaction on a molecular level. Naturally, the question of the catalytic activity of intermediate carbene and carbyne complexes has been pursued [4],... 

Storch, H. H. Golumbic, N. Anderson, R. B. The Fischer-Tropsch Reaction and Related Syntheses Wiley New York, 1951. 

The induction of steric effects by the pore walls was first demonstrated with heterogeneous catalysts, prepared from metal carbonyl clusters such as Rh6(CO)16, Ru3(CO)12, or Ir4(CO)12, which were synthesized in situ after a cation exchange process under CO in the large pores of zeolites such as HY, NaY, or 13X.25,26 The zeolite-entrapped carbonyl clusters are stable towards oxidation-reduction cycles this is in sharp contrast to the behavior of the same clusters supported on non-porous inorganic oxides. At high temperatures these metal carbonyl clusters aggregate to small metal particles, whose size is restricted by the dimensions of the zeolitic framework. Moreover, for a number of reactions, the size of the pores controls the size of the products formed thus a higher selectivity to the lower hydrocarbons has been reported for the Fischer Tropsch reaction. 

Catalysts were tested for activity in the Fischer-Tropsch reaction using a fixed-bed reactor. The catalyst (0.4 g) was reduced in situ in flowing hydrogen at 425°C for 7 h prior to testing. The test was performed under 2/1 H2/CO at 20 bar total pressure. The initial flow was 64 ml/min, but this was reduced after 24 h to increase the conversion. A final reading of activity and selectivity was taken after 100 h on stream. 

The mannitol-modified catalyst showed significantly increased activity in the Fischer-Tropsch reaction (Table 1.3). After 20 h on stream, the mannitol-modified catalyst is 286% as active as the unmodified catalyst, and 262% as active after 100 h. The selectivity values of the catalysts are similar. Hence, the characteristics of a mannitol-modified catalyst are that it has a higher activity than but the same selectivity as an unmodified catalyst. Increased activity for the FT reaction... 

Bezemer G.L., Bitter J.H., Kuipers H.P.C.E., Oosterbeek H., Holewijn J.E., Xu X., Kapteijn F., van Dillon A.J., and de Jong K.P. 2006. Cobalt particle size effects in the Fischer-Tropsch reaction studied with carbon nanofibre supported catalysts. J. Am. Chem. Soc. 128 3956-64. 

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