Relation between hydrocarbon selectivity and cobalt particle size for alumina supported cobalt Fischer–Tropsch catalysts

The relation between hydrocarbon selectivity and cobalt particle size was studied using different alumina supports. Alumina phases, i.e. δ-, θ- and α-Al2O3 were produced from a small and a medium pore γ-Al2O3 by heat treatment. All catalysts were prepared by incipient wetness impregnation resulting in a cobalt loading of 12, 20 and 30 wt.% cobalt. The catalysts were tested in a fixed bed reactor at 20 bar, 483 K and H2/CO = 2.1. The relation between methane, olefin and C5+ selectivities was studied for 2–14 nm cobalt particles supported on δ- and θ-Al2O3 supports prepared from the two different γ-Al2O3. The C5+ selectivity increases with increasing Co particle size for particles less than 8–9 nm where the selectivity goes through a maximum. The C5+ selectivity decreases for larger Co particles and then seems to approach a constant value independent of the alumina phase and pore size. The lowest CH4 selectivity and the highest olefin to paraffin ratio were obtained at maximum C5+ selectivity.

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► With increasing particle size, the C5+ selectivity passes through a maximum at 8–9 nm.
► δ-Al2O3 as support gives higher C5+ selectivity than θ-Al2O3 for particles ∼<10 nm.
► The olefin/paraffin ratio varies in the same way as the C5+ selectivity for all Al2O3 phases.
► With increasing Co size, the influence from the support becomes less important.