Elucidating the nature of the cobalt centres in CoAPO-18 acid catalysts

A combined UV-Vis and FTIR spectroscopic approach, supplemented by the use of probe molecules (N₂, CO, C₂H₄ and H₂O) as selective adsorbates, has clarified the atomic environment of cobalt centres in CoAPO-18 solid acid catalysts. These catalysts have both framework Co(II) ions in tetrahedral coordination and coordinatively-unsaturated Co(II) Lewis acid centres. But, only approximately half of the tetrahedral ions generate Brönsted OH groups (bridged between Co and P) and are oxidizable to framework Co(III). These bridged OHs, which are the active catalytic centres in acid-catalysis, are strongly influenced by the adsorbates in that the Co(II) ions of the [O₃CoO(H)] units undergo significant changes in the coordination sphere: the stronger is the ligand (adsorbate), the larger is the number of catalytic centres that are modified.