Preparation of silica-supported cobalt catalysts through chemisorption of cobalt (II) and cobalt (III) acetylacetonate

摘要

Silica-supported cobalt catalysts, Co/SiO_2, were prepared by the saturating chemisorption of gaseous cobalt acetylacetonate reactants, Co(acac)_2 and Co(acac)_3. Thermoanalytical investigation indicated that both reactants are suitable for rthe preparation of cobalt catalysts in the temperature range 170-220 deg C. Co (acac)_2 and Co(acac)_3 gave identical cobalt loading on silica. Moreover, diffuse reflectance Fourier transform infrared spectroscopy showed that the chemisorbed species formed from the two reactants were identical. The identical species are explained by the transformation of Co(acac)_3 to Co(acac)_2 before reaction with the silica. Ligand exchange reaction between the reactants and the OH groups of silica led to surface-bound Co-acac structures. In addition, the reactants adsorbed molecularly on the surfaces, at least on surfaces that contained a low number of reactive OH groups. Chemisorption of the reactants reached saturation level with an acac ligand density of about 2.8 acac per square nanometer. Calcination at 450 deg C removed the acac ligands of the surface complex. Through repetition of the cycles of Co(acac)_3 reaction and calcination up to five times, Co/SiO_2 catalysts were prepared with 6 to 19 wt.% Co, that is, 2 to 8 Co atoms per nm~2.