Effect of promoters on the performance of Co/Al₂O₃ catalysts for Fischer-Tropsch synthesis in slurry reactors

The effect of ZrO₂, Ru and/or Pt promoters on the structure and performance of Co/Al₂O₃ catalysts for Fischer-Tropsch synthesis in a slurry reactor was investigated. The results indicated that ZrO₂ modification may prevent or reduce the formation of CoAl₂O₄ phase on the catalysts and improve the reduction of cobalt species, which is beneficial to the increase of the catalytic activity, the decrease of methane selectivity, as well as the improvement of C₅₊ selectivity. Small amounts of ruthenium and platinum as promoter are able to lower the reduction temperatures of cobalt species (Co₃O₄→CoO and CoO→Co⁰) and to get a high reduction extent; as a result, the catalysts promoted with noble metals exhibit much higher activity in CO hydrogenation. Moreover, the impregnation sequence of various components also has an important influence on the performance of resultant catalysts in Fischer-Tropsch synthesis. Impregnation of γ-Al₂O₃ with Zr at first, the formation of irreducible cobalt aluminate can be prevented; co-impregnation of γ-Al₂O₃ with Co and Ru can enhance the interaction between cobalt and ruthenium, which benefits to the reduction of cobalt oxides; sequential impregnation of γ-Al₂O₃ with Co and Pt is beneficial to the dispersion of cobalt species. As a result, the sequentially impregnated catalyst Co-Pt-ZrO₂/Al₂O₃ performs excellently in the Fischer-Tropsch synthesis.