Fischer–Tropsch synthesis over un-promoted and Re-promoted γ-Al 2O 3 supported cobalt catalysts with different pore sizes

Abstract

The effect of rhenium on the Fischer–Tropsch synthesis activity and selectivity of γ-Al 2O 3 supported cobalt catalysts was investigated in fixed-bed reactors at T = 483 K, P = 20 bar, and H 2/CO = 2.0. Catalysts containing 20 wt.% cobalt and 0 or 0.5 wt.% rhenium were prepared by incipient wetness impregnation of different γ-Al 2O 3 supports with aqueous solutions of cobalt nitrate hexahydrate and for the Re-promoted catalysts, also perrhenic acid. The γ-Al 2O 3 supports had very different pore characteristics. The post-calcination Co 3O 4 crystallite size was predominantly controlled by the γ-Al 2O 3 support pore diameter. Presence of Re had only a minor effect on the crystallite size. For all catalysts, supported Co 3O 4 was reduced in two steps to Co 0 with CoO as intermediate species. However, while reduction of Co 3O 4 to CoO took place in the same temperature range for all catalysts, the reduction temperature of CoO to Co 0 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 C 5+ selectivity and between the cobalt particle size and the C 5+ selectivity. Re had a consistent positive, albeit small, effect on the C 5+ selectivity.