Fischer–Tropsch synthesis over un-promoted and Re-promoted γ-Al2O3 supported cobalt catalysts with different pore sizes

The effect of rhenium on the Fischer–Tropsch synthesis activity and selectivity of γ-Al2O3 supported cobalt catalysts was investigated in fixed-bed reactors at T = 483 K, P = 20 bar, and H2/CO = 2.0. Catalysts containing 20 wt.% cobalt and 0 or 0.5 wt.% rhenium were prepared by incipient wetness impregnation of different γ-Al2O3 supports with aqueous solutions of cobalt nitrate hexahydrate and for the Re-promoted catalysts, also perrhenic acid. The γ-Al2O3 supports had very different pore characteristics. The post-calcination Co3O4 crystallite size was predominantly controlled by the γ-Al2O3 support pore diameter. Presence of Re had only a minor effect on the crystallite size. For all catalysts, supported Co3O4 was reduced in two steps to Co0 with CoO as intermediate species. However, while reduction of Co3O4 to CoO took place in the same temperature range for all catalysts, the reduction temperature of CoO to Co0 was dependent on the catalyst properties. Large particles present in wide pores were easier to reduce than small particles located in narrow pores. In addition, Re promoted the reduction of CoO. The effect of rhenium as a reduction promoter was less pronounced at increasing pore size and particle size. Re also had a similar positive impact on the cobalt dispersion of the catalysts. Although Re significantly increased the Fischer–Tropsch synthesis cobalt-time yield, it did not modify the site-time yield. The deactivation rates of all the catalysts were also similar up to 100 h on stream. Positive correlations were found between the catalyst pore diameter and the C5+ selectivity and between the cobalt particle size and the C5+ selectivity. Re had a consistent positive, albeit small, effect on the C5+ selectivity.