The performance optimum of Fischer–Tropsch (FT) catalyst, which catalyzes transformation of synthesis gas into longer hydrocarbons, is a strong function of the CO adsorption equilibrium, the rate of CO conversion to reactive CHx,ads intermediate (kCOCHx), the rate of incorporation of these CHx,ads intermediates into the growing hydrocarbon chain (kCCf), and the rate of chain growth termination (kt). A necessary requirement for high selectivity is that rates of CO dissociation and formation of CHx,ads fragments are fast compared to the rate of methane formation by hydrogenation of CHx,ads intermediates (ktm). The C2+-yield optimum (the rate of production of hydrocarbons other than methane) is determined by fast rate of CHx,ads formation by C–O bond cleavage, fast rate of the chain growth reaction, and slow rate of chain growth termination. With increasing surface reactivity, the rate of CO consumption increases and goes through a maximum, after which it decreases since the rate of hydrocarbon-chain termination becomes rate-limiting. The theoretically optimum catalyst has high rate of chain growth and CHx,ads formation, low rate of CHx,ads transformation to methane, and intermediate rate of chain growth termination. We present microkinetics simulations based on the carbide reaction mechanism, which proposes the hydrocarbon chain growth process to proceed through incorporation of monomeric CHx,ads species into the growing hydrocarbon chain. The reactivity data used as input have been adapted from available quantum chemical studies of the Fischer–Tropsch reaction. They imply reversibility of the chain growth reaction step, which is usually neglected in conventional kinetics simulation models. The effects of chain growth reversibility have been analyzed in detail. An important computational consequence is that, in the simulations, cutoff effects appear when the simulation is limited to formation of a small number of longer hydrocarbons. Secondly, it implies that those reaction channels become favored for which the chain growth reaction is exothermic. The equilibrium of chain growth becomes less exothermic or may even become endothermic for very reactive surfaces. This implies increased methane formation when the catalyst surface becomes too reactive. Details of a lumped molecular kinetics study provide substantial new insights into the structure sensitivity and catalyst composition dependence of this reaction. Essential is the understanding that Fischer–Tropsch catalysis can yield high chain growth Anderson–Schulz–Flory (ASF) a values with very different surface overlayer compositions as well as CO coverage. When the rate of chain growth termination becomes too slow, CO conversion is suppressed because the reactive CO dissociation activation centers become blocked by growing hydrocarbon chains. Having different sites for CO dissociation and hydrocarbon chain growth, that is, the dual reaction center site model, is shown to give a substantially higher rate of CO consumption than the single reaction center site case. As long as kCCf»kCOCHx»kt, Fischer–Tropsch kinetics is related to polymerization kinetics. The overall conversion rate is controlled by the rate of CHx,ads formation. When chain growth becomes limiting (kt«kCCf«kCOCHx), the CO consumption rate is proportional to ktkCCf and the surface is covered with a high fraction of growing chains. The generally observed decreases in rate of CO conversion with decreasing particle size can be due to a suppression of the relative concentration of selective Fischer–Tropsch sites, a decreased rate of hydrocarbon termination, or a decreased rate of CO dissociation. As we will show, transient isotope exchange experiments can be simulated that can discriminate between these options when combined with steady-state CO conversion and selectivity data.
|Titel||Bridging Heterogeneous and Homogeneous Catalysis : Concepts, Strategies and Applications|
|Redacteuren||C. Li, Y. Liu|
|Plaats van productie||New York|
|ISBN van elektronische versie||9783527675906|
|ISBN van geprinte versie||978-3-527-33583-1|
|Status||Gepubliceerd - 2014|
Santen, van, R. A., Ghouri, M. M., Markvoort, A. J., & Hensen, E. J. M. (2014). The molecular kinetics of the Fischer-Tropsch reaction. In C. Li, & Y. Liu (editors), Bridging Heterogeneous and Homogeneous Catalysis : Concepts, Strategies and Applications (blz. 553-606). New York: Wiley. https://doi.org/10.1002/9783527675906.ch16