Hydrocarbons iron catalysts

The role of carbides in the synthesis of hydrocarbons has been widely considered ever since the carbide theory was first postulated by Fischer and Tropsch in 1926 (20). Although recent experimental studies indicate that the carbide theory is largely incorrect, that is, that bulk-phase carbides are not intermediates in the formation of higher hydrocarbons, iron catalysts converted to Hagg carbide or cementite are usually more active than similar raw or reduced catalysts (21). (For a review of the carbide theory up to 1950, see p. 571 of reference 22.) The selectivity of carbided iron catalysts is essentially the same as that of corresponding reduced catalysts. Nitrides of iron are usually more active than reduced or carbided catalysts, and the catalyst selectivity is significantly different. 

Hydrocarbons from Synthesis Gas and Methanol. Two very important catalytic processes in which hydrocarbons are formed from synthesis gas are the Sasol Eischer-Tropsch process, in which carbon monoxide and hydrogen obtained from coal gasification are converted to gasoline and other products over an iron catalyst, and the Mobil MTG process, which converts methanol to gasoline range hydrocarbons using ZSM-5-type 2eohte catalysts. 

Shortly after World War I, Badische Amlin patented the catalytic conversion of synthesis gas to methanol, and Fischer and Tropsch (F-T) announced a rival process in which an iron catalyst converted synthesis gas into a mixture of oxygenated hydrocarbons. Later,... 

The catalyst losses in either system are moderate and not excessively costly when inexpensive iron catalyst is used (as for production of liquid hydrocarbons). It is questionable, however, whether comparable losses of expensive nickel catalysts (for methanation) could be tolerated. For this reason, it is quite likely that the fluidized catalyst system will be used for methanation only after a cheap methanation catalyst is developed. 

Hydrogen cyanide reactions catalysts, 6,296 Hydrogen ligands, 2, 689-711 Hydrogenolysis platinum hydride complexes synthesis, 5, 359 Hydrogen peroxide catalytic oxidation, 6, 332, 334 hydrocarbon oxidation iron catalysts, 6, 379 reduction... 

The first reported work on the kinetics of hydrogenolysis reactions of simple hydrocarbons appears to be that of Taylor and associates at Princeton (2-4, 14, 15), primarily on the hydrogenolysis of ethane to methane. The studies were conducted on nickel, cobalt, and iron catalysts. More recently, extensive studies on ethane hydrogenolysis kinetics have been conducted on all the group VIII metals and on certain other metals as well (16,28-83). 

Fischer-Tropsch synthesis can be regarded as a surface polymerization reaction since monomer units are produced from the reagents hydrogen and carbon monoxide in situ on the surface of the catalyst. Hence, a variety of hydrocarbons (mainly n-paraffines) are formed from hydrogen and carbon monoxide by successive addition of C, units to hydrocarbon chains on the catalyst surface (Equation 12.1). Additionally, carbon dioxide (Equation 12.3) and steam (Equations 12.1 and 12.2) are produced C02 affects the reaction just a little, whereas H20 shows a strong inhibiting effect on the reaction rate when iron catalysts are used. 

Van der Laan, G.P., Beenackers, A.A.C.M. 1999. Hydrocarbon selectivity model for the gas-solid Fischer-Tropsch synthesis on precipitated iron catalysts. Ind. Eng. Chem. Res. 38 1277. 

Traditionally, iron-based catalysts have been used for FT synthesis when the syngas is coal derived, because of their activity in both FTS and WGS reactions. Complex mixtures of straight-chain paraffins, olefins, and oxygenate (in substantial proportions) compounds are known to be formed during iron-based FTS. Olefin selectivity of iron catalysts is typically greater than 50% of the hydrocarbon products at low carbon numbers, and more than 60% of the produced olefins are a-olefins.13 For iron-based catalysts, the olefin selectivity decreases asymptotically with increasing carbon number. 

The new Brownsville, Tex., plant for the manufacture of synthetic liquid fuels from natural gas makes use of this reaction to increase the octane number of its product by as much as 20 units. Synthetic naphtha produced over iron catalyst is highly olefinic and contains substantial amounts of straight-chain isomers with terminal double bonds (8). The shifting of these double bonds toward the center of the molecule may be accomplished by vapor-phase treatment employing synthetic cracking catalyst in the fluid state, under mild catalytic cracking conditions. Oxygenated compounds also present are converted under the isomerization conditions to hydrocarbons and water. 

Robert B. Anderson. Iron catalysts apparently do not isomerize hydrocarbons however, there is little experimental evidence besides the products of the Fischer-Tropsch synthesis. In hydrocracking of paraffins on nickel and cobalt catalysts the isomerization does not occur. 

Natural Gas Upgrading via Fischer-Tropsch. In the United States, as in other countries, scarcities from Wodd War II revived interest in the synthesis of fuel substances. A study of the economics of Fischer synthesis led to the conclusion that the large-scale production of gasoline from natural gas offered hope for commercial utility. In the Hydrocol process (Hydrocarbon Research, Inc.) natural gas was treated with high purity oxygen to produce the synthesis gas which was converted in fluidized beds of iron catalysts (42). 

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. 

It was found that an iron carbide catalyst produced by laser pyrolysis and a commercially available ultrafine iron oxide catalyst are not as active for FTS as a precipitated iron catalyst. Operating under industrial conditions, it was found that the unpromoted precipitated catalyst had a hydrocarbon productivity 93% of that reported by Kolbel while the novel catalysts were far below Kolbel s benchmark. It was found, however, that at similar CO conversion, the iron carbide catalyst had a higher hydrocarbon production rate and had a better selectivity for C5+ hydrocarbons. 

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