Cobalt–aluminum co-precipitated catalysts and their performance in the Fischer–Tropsch synthesis

Abstract

Cobalt–aluminum catalysts were prepared using either Co 2+ precipitation onto freshly prepared Mg–Al or Zn–Al hydrotalcite (promoted samples) or co-precipitation of Co 2+ and Al 3+ (non-promoted samples). The evolution of initial hydrotalcite structure was monitored during its calcination and reductive treatment. It has been shown that, at moderate temperatures, hydrotalcites results decomposition yields a Co oxide phase supported by a highly defective inverted spinel-like structure. Cations Co 2+ enter the support structure, and occupy both tetrahedral and octahedral positions. Octahedron coordinated Co species are reduced at 580–620°C. After the reduction at 470–480°C catalyst phase composition shows Co 0 supported on inverted spinel-like structure, which contains Co 2+ in the octahedral coordination. Further reduction at 600°C transforms the support to ‘ideal’ spinel, which contains no octahedron coordinated Co 2+. Chemical properties of the Co–Al catalysts, including their performance in the Fischer–Tropsch synthesis (FTS), were found to depend on the catalyst reduction temperature, and thus on the support structure. Metal-support interaction is supposed to explain the observed properties of metallic cobalt.