Fischer—Tropsch catalyst

Modification of Surface Reactions AND OF Isolated Site Kinetics 

Olefin hydrogenation reactions control C5+ selectivity by intercepting reactive a-olefins. Readsorption and chain initiation steps reintroduce olefins into chain growth pathways and reverse the most frequent chain termination 

Our readsorption model shows that carbon number distributions can be accurately described using Flory kinetics as long as olefin readsorption does not occur (/3r = 0), because primary chain termination rate constants are independent of chain size (Fig. 24). The resulting constant value of the chain termination probability equals the sum of the intrinsic rates of chain termination to olefins and paraffins (j8o + Ph)- As a result, FT synthesis products become much lighter than those formed on Co catalysts at our reaction conditions (Fig. 24, jSr = 1.2), where chain termination probabilities are much lower than jS -I- Ph for most hydrocarbon chains. The product distribution for /3r = 12 corresponds to the intermediate olefin readsorption rates experimentally observed on Co/Ti02 catalysts, where intrapellet transport restrictions limit the rate of removal of larger olefins, enhance their secondary chain initiation reactions, and increase the average chain size of FT synthesis products. 

Carbon number distribution plots also become linear when olefins readsorb very rapidly (large /3r) or when severe intrapellet transport restrictions (large ) prevent their removal from catalysts pellets before they convert to paraffins during chain termination (Fig. 24, jSr = 100). In this case, chain termination to olefins is totally reversed by fast readsorption, even for light olefins. Chain termination occurs only by hydrogen addition to form paraffins, a step that is not affected by secondary reactions and for which intrinsic kinetics depend only on the nature of the catalytic surface. The product distribution again obeys Flory kinetics, but the constant chain termination probability is given by )8h, instead of po + pH- Clearly, bed and pellet residence times above those required to convert all olefins cannot affect the extent of readsorption or the net chain termination rates and lead to Flory distributions that become independent of bed residence time. 

The presence of hydrogenation sites outside FT synthesis pellets (e.g., as a physical mixture of FT and hydrogenation pellets) is much less effective (Fig. 25, curve C). Such extrapellet sites merely capture (low fugacity) olefins that leave FT synthesis catalyst pellets unreacted and prevent their subsequent readsorption along the catalyst bed. They do not prevent the chain initiation reactions that olefins undergo predominantly within 

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Synthetic Fuels. Hydrocarbon Hquids made from nonpetroleum sources can be used in steam crackers to produce olefins. Fischer-Tropsch Hquids, oil-shale Hquids, and coal-Hquefaction products are examples (61) (see Fuels, synthetic). Work using Fischer-Tropsch catalysts indicates that olefins can be made directly from synthesis gas—carbon monoxide and hydrogen (62,63). Shape-selective molecular sieves (qv) also are being evaluated (64). 

Promoters. Many industrial catalysts contain promoters, commonly chemical promoters. A chemical promoter is used in a small amount and influences the surface chemistry. Alkali metals are often used as chemical promoters, for example, in ammonia synthesis catalysts, ethylene oxide catalysts, and Fischer-Tropsch catalysts (55). They may be used in as Httie as parts per million quantities. The mechanisms of their action are usually not well understood. In contrast, seldom-used textural promoters, also called stmctural promoters, are used in massive amounts and affect the physical properties of the catalyst. These are used in ammonia synthesis catalysts. 

The production of hydrocarbons using traditional Fischer-Tropsch catalysts is governed by chain growth or polymerization kinetics. The equation describing the production of hydrocarbons, commonly referred to as the Anderson-Schulz-Flory equation, is ... 

Dr. Moeller We have done this, and we compared an iron catalyst used for the Fischer-Tropsch plant and a nickel catalyst used in the methanation plant. By the same x-ray techniques, we found no nickel carbide on the used methanation catalyst, but we did find iron carbide on the used Fischer-Tropsch catalyst. 

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... 

Unfortunately, it is difficult to ascertain the identity of the actual catalytic species, and it is not clear whether catalysis by a true intercalation compound has been established. For instance, a frequent method for ammonia and Fischer-Tropsch catalyst generation is the following ... 

Iron Fischer-Tropsch Catalysts Surface Synthesis at High Pressure... 

An XPS Investigation of iron Fischer-Tropsch catalysts before and after exposure to realistic reaction conditions is reported. The iron catalyst used in the study was a moderate surface area (15M /g) iron powder with and without 0.6 wt.% K2CO3. Upon reduction, surface oxide on the fresh catalyst is converted to metallic iron and the K2CO3 promoter decomposes into a potassium-oxygen surface complex. Under reaction conditions, the iron catalyst is converted to iron carbide and surface carbon deposition occurs. The nature of this carbon deposit is highly dependent on reaction conditions and the presence of surface alkali. 

This XPS investigation of small iron Fischer-Tropsch catalysts before and after the pretreatment and exposure to synthesis gas has yielded the following information. Relatively mild reduction conditions (350 C, 2 atm, Hg) are sufficient to totally reduce surface oxide on iron to metallic iron. Upon exposure to synthesis gas, the metallic iron surface is converted to iron carbide. During this transformation, the catalytic response of the material increases and finally reaches steady state after the surface is fully carbided. The addition of a potassium promoter appears to accelerate the carbidation of the material and steady state reactivity is achieved somewhat earlier. In addition, the potassium promoter causes a build up on carbonaceous material on the surface of the catalysts which is best characterized as polymethylene. 

Pirola C, Bianchi CL, Michele AD, Diodati P, Boffito D, Ragaini V (2010) Ultrasound and microwave assisted synthesis of high loading Fe-supported Fischer-Tropsch catalysts. Ultrason Sonochem 17 610-616... 

Iron Nitrides as Fischer-Tropsch Catalysts Robert B. Anderson... 

Cobalt has been used as a Fischer-Tropsch catalyst in a variety of forms(80). Thus it was not surprising to see that both active forms of cobalt powders were moderate Fischer-Tropsch catalysts. Reacting synthesis... 

Hydrocarbons over a Cobalt-Thoric Fischer-Tropsch Catalyst. J. Amer. chem. Soc. 73, 5213 (1951)-... 

Khodakov A.Y., Chu W., and Fongarland P. 2007. Advances in the development of novel cobalt Fischer-Tropsch catalysts for synthesis of long-chain hydrocarbons and clean fuels. Chem. Rev. 107 1692-744. 

Storsaeter S., Totdal B., Walmsley J.C., Tanem B.S., and Holmen A. 2005. Characterisation of alumina-, silica- and titania-supported cobalt Fischer-Tropsch catalysts. 7. Catal. 236 139-52. 

Morales F., de Smit E., de Groot F.M.F., Visser T., and Weckhuysen B.M. 2007. Effects of manganese oxide promoter on the CO and H2 adsorption properties of titania-supported cobalt Fischer-Tropsch catalysts. J. Catal. 246 91-99. 

Girardon J.-S., Quinet E., Constant-Griboval A., Chemavskii P.A., Gengembre L., and Khodakov A.Y. 2007. Cobalt dispersion, reducibility and surface sites in promoted silica-supported Fischer-Tropsch catalysts. J. Catal. 248 143-57. 

Carbon Nanomaterials as Supports for Fischer-Tropsch Catalysts... 

ICP) measurements. The catalytic performance of the nanocatalysts was finally tested in the Fischer-Tropsch synthesis carried out in a fixed bed reactor. The obtained results were compared with literature data of commercially used Fischer-Tropsch catalysts. 

FIGURE 2.2 TPR profiles of carbon nanomaterial Fischer-Tropsch catalysts (gas mixture 10% H2 in Ar heating rate 10 K/min). 

FIGURE 2.4 Arrhenius plots of the tested carbon nanomaterial catalysts and commercially used Fischer-Tropsch catalysts (reaction conditions p = 3 MPa, CO/H2 = A, V. = 18.5 1/h (NTP)). "... 

Activation Energy, and Collision Factor, Arlt of Carbon Nanomaterial-Supported Co Catalysts and Commercially Used Fischer-Tropsch Catalysts... 

Bezemer, G. L., van Laak, A., van Dillen, A. J., and de Jong, K. P. 2004. Cobalt supported on carbon nanofibers—A promising novel Fischer-Tropsch catalyst. Natural Gas Conversion 147 259-64. 

Van Steen, E., andPrinsloo, F. F. 2002. Comparison of preparation methods for carbon nanotubes supported iron Fischer-Tropsch catalysts. Catalysis Today 71 327-34. 

Hilmen, A. M., Schanke, D., Hanssen, K. F., and Holmen, A. 1999. Study of the effect of water on alumina supported cobalt Fischer-Tropsch catalysts. Appl. Catal. A 186 169-88. 

Puskas, I. 1993. Unusual reactions on a cobalt-based Fischer-Tropsch catalyst. Catal. Lett. 22 283-88. 

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