Effect of alumina phases on hydrocarbon selectivity in Fischer–Tropsch synthesis

Different alumina phases i.e. δ-, θ-, and α-Al2O3 were prepared from two different samples of γ-Al2O3 by heat treatment. The alumina samples were categorised as small and medium pore according to the pore size distribution. Nominally 12 wt% cobalt was loaded onto the supports by incipient wetness impregnation. By varying the amount of deionised water and ethylene glycol, the Co3O4 crystallite size could be controlled. After reduction, a relatively constant cobalt metal particle size of 8–10 nm was obtained. Temperature-programmed reduction profiles showed that the medium pore catalysts were reduced at lower temperatures as compared to the small pore catalysts. The Fischer–Tropsch synthesis was carried out for all samples under the same experimental conditions (P = 20 bar, T = 483 K, H2/CO = 2.1). The medium pore catalysts exhibited higher selectivity to C5+ than the small pore catalysts. Furthermore, higher C5+ selectivity was observed for δ-and α-Al2O3 samples compared to γ-and θ-Al2O3, with α-Al2O3 giving the highest selectivity. Steady-state isotopic transient kinetic analysis (SSITKA) showed that the amount of CHx surface intermediates followed the same trend as the C5+ selectivity with respect to pore size and alumina phase.

Graphical abstractFigure optionsDownload full-size imageDownload high-quality image (41 K)Download as PowerPoint slideResearch highlights▶ Effect of Al2O3 phase, pores and Co particle size on selectivity has been separated. ▶ Co on δ- and α-Al2O3 exhibit high selectivities to C5+ compared to γ- and θ-Al2O3. ▶ Al2O3 with large pores gave enhanced selectivities compared to small pores.