Advances in the development of novel cobalt Fischer-Tropsch catalysts for synthesis of long-chain hydrocarbons and clean fuels.

Catalyst productivity and selectivity to C5+ hydrocarbons are critical design criteria in the choice of Fischer-Tropsch synthesis (FTS) catalysts and reactors. Cobalt-based catalysts appear to provide the best compromise between performance and cost for the synthesis of hydrocarbons from CO/H2 mixtures. Optimum catalysts with high cobalt concentration and site density can be prepared by controlled reduction of nitrate precursors introduced via melt or aqueous impregnation methods. FTS turnover rates are independent of Co dispersion and support identity over the accessible dispersion range (0.01–0.12) at typical FTS conditions. At low reactant pressures or conversions, water increases FTS reaction rates and the selectivity to olefins and to C5+ hydrocarbons. These water effects depend on the identity of the support and lead to support effects on turnover rates at low CO conversions. Turnover rates increase when small amounts of Ru (Ru/Co<0.008 at.) are added to Co catalysts. C5+ selectivity increases with increasing Co site density because diffusion-enhanced readsorption of α-olefins reverses, β-hydrogen abstraction steps and inhibits chain termination. Severe diffusional restrictions, however, can also deplete CO within catalyst pellets and decrease chain growth probabilities. Therefore, optimum C5+ selectivities are obtained on catalysts with moderate diffusional restrictions. Diffusional constraints depend on pellet size and porosity and on the density and radial location of Co sites within catalyst pellets. Slurry bubble column reactors and the use of eggshell catalyst pellets in packed-bed reactors introduce design flexibility by decoupling the characteristic diffusion distance in catalyst pellets from pressure drop and other reactor constraints.

Design, synthesis, and use of cobalt-based Fischer-Tropsch synthesis catalysts

Due to the large volume of existing literature on Fischer–Tropsch (FT) synthesis, the diversity of the subject and the actually reoriented interest, it seemed indicated to write a historical sketch about the process, putting also emphasis on present trends and future options. The matter is complicated and may be regarded from different positions. Thus history and trends have been divided into several lines which are elaborated individually: • The changing environment of demand and supply of fossil energies and the more stringently upcoming aspects of pollution control, of cleanliness of the automotive fuels and of energy saving. • The development of FT-reactors and processes, an area of highest present industrial activity and progress. • Preparation and performance of catalysts with particular concern about cobalt as the base metal. • FT intermediates and elemental reactions, difficult subjects as the FT multistep conversion features on the catalyst surface in the adsorbed state and kinetic principles as selective inhibition and spatial constraints seem to rule the regime. • Kinetic modelling of FT synthesis which has made great progress recently, particularly on the basis of newly provided detailed experimental data. Of course, presenting history and trends of FT synthesis on a few pages means generalising from many individual investigations and developments and also selection of only a few citations. So I want to apologise for all the contributions to science and technology around FT synthesis which I have missed to include into the article.

Short history and present trends of Fischer–Tropsch synthesis

A critical review of the kinetics and selectivity of the Fischer–Tropsch synthesis (FTS) is given. The focus is on reaction mechanisms and kinetics of the water–gas shift and Fischer–Tropsch (FT) reactions. New developments in the product selectivity as well as the overall kinetics are reviewed. It is concluded that the development of rate equations for the FTS should be based on realistic mechanistic schemes. Qualitatively, there is agreement that the product distribution is affected by the occurrence of secondary reactions (hydrogenation, isomerization, reinsertion, and hydrogenolysis). At high CO and H2O pressures, the most important secondary reaction is readsorption of olefins, resulting in initiation of chain growth processes. Secondary hydrogenation of α-olefins may occur and depends on the catalytic system and the process conditions. The rates of the secondary reactions increase exponentially with chain length. Much controversy exists about whether these chain-length dependencies stem from differe...

Kinetics and Selectivity of the Fischer–Tropsch Synthesis: A Literature Review

National Natural Science Foundation of China [20625310, 20923004]; National Basic Research Program of China [2005CB221408, 2010CB732303]; Research Fund for the Doctoral Program of Higher Education [20090121110007]; Key Scientific Project of Fujian Province [2009HZ0002-1]

H. Oosterbeek

Papers

Non-ASF Product Distributions Due to Secondary Reactions during Fischer–Tropsch Synthesis

Chain Length Dependence of α-Olefin Readsorption in Fischer-Tropsch Synthesis

Investigation of promoter effects of manganese oxide on carbon nanofiber-supported cobalt catalysts for Fischer–Tropsch synthesis