Fischer-Tropsch synthesis nitrides

Iron has a rich surface coordination chemistry that forms the basis of its important catalytic properties. There are many catalytic applications in which metallic iron or its oxides play a vital part, and the best known are associated with the synthesis of ammonia from hydrogen and nitrogen at high pressure (Haber-Bosch Process), and in hydrocarbon synthesis from CO/C02/hydrogen mixtures (Fischer-Tropsch synthesis). The surface species present in the former includes hydrides and nitrides as well as NH, NH2, and coordinated NH3 itself. Many intermediates have been proposed for hydrogenation of carbon oxides during Fischer-Tropsch synthesis that include growing hydrocarbon chains. 

Studies of the Fischer-Tropsch synthesis on nitrided catalysts at the Bureau of Mines have been described (4,5,23). These experiments were made in laboratory-scale, fixed-bed testing units (24). In reference 5, the catalyst activity was expressed as cubic centimeters of synthesis gas converted per gram of iron per hour at 240°C. and at a constant conversion of 65%. Actually, the experiments were not conducted at 240°C., but the activity was corrected to this temperature by the use of an empirical rate equation (25). Conditions of catalyst pretreatment for one precipitated and two fused catalysts are given in Table IV. 

Fig. 7. Product from Fischer-Tropsch synthesis with nitrided fused iron catalyst. Reprinted by permission of the copyright holder, the American Chemical Society.

Although metals or even promoted metals have very low sulfur tolerances in synthesis reactions, other materials, such as metal oxides, nitrides, borides, and sulfides, may have greater tolerance to sulfur poisoning because of their potential ability to resist sulfidation (18). The extremely low steady-state activities of Co, Ni, and Ru metals in a sulfur-contaminated stream actually correspond to the activities of the sulfided metal surfaces. However, if more active sulfides could be found, their activity/selectivity properties would be presumably quite stable in a reducing, H2S-containing environment. This is, in fact, the basis for the recent development of sulfur active synthesis catalysts (211-215), which are reported to maintain stable activity/ selectivity properties in methanation and Fischer-Tropsch synthesis at H2S levels of 1% or greater. Happel and Hnatow (214), for example, reported in a recent patent that rare-earth and actinide-metal-promoted molybdenum oxide catalysts are reasonably active for methanation in the presence of 1-3% H2S. None of these patents, however, have reported intrinsic activities... 

The Synol and isosynthesis processes, as well as the Oxyl and iron nitride variations of the Fischer-Tropsch synthesis, were developed to produce special products from synthesis gas. All but the isosynthesis are designed to produce high yields of oxygenated materials. The isosynthesis yields highly branched aliphatic hydrocarbons and aromatics. While none of these processes has been used commercially, they could be of interest at some future time. 

Chain Growth and Iron Nitrides in the Fischer-Tropsch Synthesis... 

In research on the Fischer-Tropsch synthesis, FTS, at the Bureau, mechanisms of the growth of the carbon chain and the use of iron nitrides as catalysts were developments that were not anticipated by previous German work. Herington (2) in 1946 was the first to consider chain growth in FTS. He defined a probability 3 that the chain will desorb rather than grow at the surface, where... 

Kam FS, Shultz JP, Anderson RB Kinetics of the Fischer-Tropsch synthesis on iron catalysts. Pressure dependence and selectivity of nitrided catalysts, J Phys Chem 64(4) 446—451, 1960. 

Iron Nitrides as Fischer-Tropsch Catalysts Robert B. Anderson Hydrogenation of Organic Compounds with Synthesis Gas Milton Orchin The Uses of Raney Nickel Eugene Lieber and Fred L. Morritz... 

Developments in the Fischer-Tropsch synthesiis at the Bureau of Mines from 19 5 to I960 include a simple mechanism for chain growth and the use of iron nitrides as catalysts. The chain-growth schene can predict the carbon-number and isomer distributions for products from most catalysts with reasonable accuracy using only 2 adjustable parameters. Iron nitrides are active, durable catalysts that produce high yields of alcohols and no wax. During the synthesis, the nitrides are converted to carbonitrides. 

Simultaneously, the synthesis of ammonia from the elements was also tested over RE-TM intermetallic compounds by the same group (42). Here, 36 intermetallic compounds of rare earth elements and the transition metals Fe, Co, and Ru were evaluated. In the case of the ammonia synthesis, the rare earth component is transformed to the corresponding nitride and the transition metal is finely divided thereon. Some of the compounds showed an even higher specific activity than commercial catalysts used at that time. Later, the catalysts based on the intermetallic compound CeNis-j Cox (x = 0-5) were oxidized in a controlled way prior to the catalytic characterization in the Fischer-Tropsch reaction (43). The reasons to oxidize the compound before use were twofold. On the one hand, the material can be handled in air afterwards, whereas the intermetallic compound itself is not stable in air. Second, the oxidation can be better controlled and reproduced if it is not performed in situ. A review on the early work on these catalysts is available by Wallace (44). 

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. 

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