Aerobic oxidation of alcohols by organically modified ruthenium hydroxyapatite

The activity of ruthenium-hydroxyapatite (RuHAp) in the oxidation of benzyl alcohol with molecular oxygen was more than tripled by the incorporation of Ru into organically modified HAp and no detectable product beside benzaldehyde was formed. The effective modifiers prolinol, proline, benzoic acid, and hexanoic acid have functional groups that can form strong hydrogen bonds with the surface OH and phosphate groups of HAp. On the basis of catalytic, DRIFT, SEM, STEM-EDX, ICP-OES, and BET measurements, we assume that the major reason for the activity enhancement is the higher intrinsic activity of Ru species due to their different location and coordination in organically modified HAp. It is very probable that incorporation of Ru is not an ion-exchange process, but rather an adsorption process controlled by the polar organic compounds. Interestingly, during this process the modifiers leach out and thus do not disturb the catalytic oxidation of alcohols. The slope of the Hammett plot of p-substituted benzyl alcohols indicates that β-hydride elimination from the alcoholate intermediate is the rate-determining step under kinetically controlled reaction conditions. The catalysts are reusable with no sign of Ru leaching.