Selforganization in Fischer–Tropsch synthesis with iron- and cobalt catalysts

•Selforganization in Fischer–Tropsch synthesis on iron and cobalt catalysts.•Mechanism of alkali promoting of iron catalysts.•Time resolved selectivity, chain growth and branching.•Primary and secondary olefin selectivity.•Spatial constraints on growth sites.

Unsteady initial regimes of Fischer–Tropsch synthesis with iron and cobalt catalysts have been investigated for changes in rate and selectivity, applying own-developed methods for temporal resolution of product composition. Using the kinetic model of non-trivial surface-polymerization, probabilities of reactions of chemisorbed intermediates are calculated as function of time and carbon number of species, at several sets of reaction parameters and different catalyst properties. Results are used to elucidate principles of selforganization.The rules of iron catalyst selforganization are dominated by alkali promoting, as controlling iron phase composition and the relative rate of reaction of on-site-carbon for FT-monomer formation, Fe-carbide formation and carbon-phase formation—these as in relation to primary and secondary formation of olefins and paraffins.With cobalt as catalyst, selforganization for creating different kinds of active sites for primary and secondary reactions appears essential, and is explained via the observed selectivity changes. A dynamic structure of active sites is proposed. Probability of linear chain prolongation is merely carbon number dependent, but probability of growth with chain branching declines exponentially with carbon number, indicating increasing spatial constraints on the reaction. Selectivity changes are mechanistically understood from the ordered complexity of product composition.

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