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Abstract: The excellent selectivity and activity of ruthenium-hydroxyapatite {RuHAp, HAp=Ca10(PO4)6(OH)2} in the aerobic oxidation of alcohols has been attributed to isolated, monomeric Ru3+ species as the active sites. Here we reinvestigated these catalysts by a DRIFT study of CO adsorption and by electron microscopy (STEM) combined with EDX analysis. The study revealed the presence of two major species: isolated Ru3+ ions and hydrated Ru3+-oxide particles. The latter structure has high redox activity; it is easily oxidized with oxygen to RuO2 and reduced by hydrogen or CO at ambient conditions. The amount of this phase in the catalysts correlates well with the alcohol oxidation activity in the liquid phase at close to ambient conditions. In contrast, the redox activity of isolated Ru3+ ions is significantly lower (or they might be completely inactive). These species may correspond to adsorbed Ru3+ ions, ion-exchanged Ru3+ at the outer surface of HAp, or Ru3+ ions in Ru-phosphate formed by a dissolution–redeposition process from HAp. Our final conclusion is that in RuHAp, the real active sites are small hydrated Ru3+-oxide nanoparticles (1–2 nm) located on the surface of HAp. Application of a low RuCl3 concentration and a short contact time between HAp and the aqueous RuCl3 solution during catalyst preparation help maximize the fraction of the Ru3+-oxide phase and thus the catalytic activity. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]