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

Cobalt-aluminum catalysts were prepared using either Co2+ precipitation ont o freshly prepared Mg-Al or Zn-Al hydrotalcite (promoted samples) or co-pre cipitation of Co2+ and Al3+ (non-promoted samples). The evolution of initia l 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 defect ive inverted spinel-like structure. Cations Co2+ enter the support structur e, and occupy both tetrahedral and octahedral positions. Octahedron coordin ated Co species are reduced at 580-620 degreesC. After the reduction at 470 -480 degreesC catalyst phase composition shows Co-0 supported on inverted s pinel-like structure, which contains Co2+ in the octahedral coordination. F urther reduction at 600 degreesC transforms the support to 'ideal' spinel, which contains no octahedron coordinated Co2+. Chemical properties of the C o-Al catalysts, including their performance in the Fischer-Tropsch synthesi s (FTS), were found to depend on the catalyst reduction temperature, and th us on the support structure. Metal-support interaction is supposed to expla in the observed properties of metallic cobalt. (C) 2001 Elsevier Science B. V. All rights reserved.