Chemistry of Silica-Supported Cobalt Catalysts Prepared by Cation Adsorption. 1. Initial Localized Adsorption of Cobalt Precursors

The speciation of cobalt ethanediamine (en) complexes [Co(en) x (H 2 O) 6 - 2 x ] 2+ ( x = 1, 2, and 3) in aqueous solution changes when a silica support is introduced into the solution ((en/Co = 1), (en/Co = 2), and (en/Co = 3) preparations). The pH-buffering effect of the silica support causes speciation shifts, especially for the precursor complex [Co(en) 2 (H 2 O) 2 ] 2+ in solution, which transform into [Co(en) 1 (H 2 O) 4 ] 2+, as the main species. Once adsorbed on silica and after drying (25 and 100 °C), UV−visible and XAS characterization show that, for the (en/Co = 1) and (en/Co = 2) preparations, the Co(en) 1 complexes form dimers bonded to silica through one silanol group per Co ([(SiO)(en)(H 2 O) 2 Co II ] 2 (μ-O)). For the (en/Co = 3) preparation, the adsorbed Co(II) complexes are monomeric and contain two ethanediamine ligands. Upon drying at 100 °C, residual water molecules in the coordination sphere of adsorbed Co(II) complexes may be lost reversibly, causing the establishment of an octahedral/tetrahedral coordination equilibrium. Upon calcination at 450 °C, the ethanediamine ligands are eliminated. The monomeric complex in (en/Co = 3) probably becomes grafted onto silica through two bonds ((SiO) 2 Co II (OH 2 ) 4 ), while the dimer initially formed in (en/Co = 1) and (en/Co = 2) mostly gives rise to species reminiscent of Co silicate germs.