Preparation and characterization of Fischer-Tropsch active Co/SiO2 catalysts

Silica supported cobalt catalysts have been prepared by a new method combin ing the precipitation of the metal precursor (cobalt nitrate) by oxalic aci d with the hydrolysis and condensation of the silicium precursor (tetraetho xysilane). Depending on the pH during preparation, the textural properties (BET specific surface area, porosity) of the Co/SiO2 catalysts can be modif ied. In an acid medium (pathway A), the resulting silica is constituted by a polymeric net with few branchings, the catalysts are microporous. In basi c medium (pathway B) silica is composed of more branched polymers leading t o mesoporous catalytic systems. After calcination, the only crystallized ph ase detected by XRD is the Co3O4 spinel. At 773 K, the surface degrees of r eduction of the catalysts at cobalt isocontent (25 wt.% Co) as determined b y XPS is of 81% and 69% for systems prepared by pathways A and B, respectiv ely. The presence of small unreduced Co-II suggests the existence of non-cr ystallized cobalt silicate formed during the reduction by reaction of CoO w ith silica. The activity for the CO + H-2 reaction for the 25 wt.% catalyst s prepared by pathway A increased with the specific surface area which can be controlled by the preparation parameters. The reduction degree has a dir ect influence on the selectivity for the Co/SiO2 catalysts. The presence of a part of unreduced cobalt tin interaction with the support) results in a better selectivity to the C-5-C-13 fraction (gasoline), whereas a higher re duction degree of cobalt favors the production of higher molecular weight h ydrocarbons (waxes) (C-22+ selectivity > 40%). (C) 1999 Elsevier Science B. V. All rights reserved.