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

Cobalt-aluminum catalysts were prepared using either Co2+ precipitation onto freshly prepared Mg-Al or Zn-Al hydrotalcite (promoted samples) or co-precipitation of Co2+ and Al3+ (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 Co2+ enter the support structure, and occupy both tetrahedral and octahedral positions. Octahedron coordinated Co species are reduced at 580-620 degreesC. After the reduction at 470-480 degreesC catalyst phase composition shows Co-0 supported on inverted spinel-like structure, which contains Co2+ in the octahedral coordination. Further reduction at 600 degreesC transforms the support to 'ideal' spinel, which contains no octahedron coordinated Co2+. 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. (C) 2001 Elsevier Science B.V. All rights reserved.