Promoted Ru-hydroxyapatite: designed structure for the fast and highly selective oxidation of alcohols with oxygen

Ru-containing hydroxyapatite (HAp) catalysts have been developed for the oxidation of alcohols with molecular oxygen in an organic solvent. The working hypothesis has been that fine-tuning of the location and surroundings of the active Ru species would provide outstanding performance. The highest oxidation activity was achieved when (i) the contact time between the metal salt solution and HAp was limited to a few minutes to minimize restructuring via dissolution-redeposition, (ii) low Ru content was applied, (iii) Co2+ and Pb2+ promoters were incorporated to occupy the poorly accessible “hidden” sites in HAp before the introduction of Ru3+, thus facilitating egg-shell-type Ru distribution in the particles, and (iv) after Ru incorporation the catalysts were dried at moderate temperature to avoid restructuring induced by dehydration of the active sites and their neighborhood. It is assumed that the active sites are Ru(OH)2+ species stabilized by the phosphate O atoms and adsorbed (H-bonded) water. Various methods have been used for characterization of these catalysts, including XPS, DRIFT spectroscopy, inductively coupled plasma optical emission spectroscopy (ICP-OES), XRD, thermogravimetry combined with mass spectroscopy (pulse thermal analysis), and nitrogen adsorption. One of the two best catalysts (RuCoHAp-10min) was used in the partial oxidation of benzylic, allylic, aliphatic and cycloaliphatic alcohols. Most carbonyl compounds were produced in excellent yields without the formation of any detectable by-product. The catalytic activity seems to be higher than that of other Ru-based solid catalysts developed for the aerobic oxidation of alcohols.