Ruthenium at work in Ru-hydroxyapatite during the aerobic oxidation of benzyl alcohol: An in situ ATR-IR spectroscopy study

The structure of Ru in Ru-substituted hydroxyapatite (RuHAp) has been investigated using infrared spectroscopy in the attenuated total reflection mode (ATR-IR). Oxidation–reduction cycles, 12CO and 13CO adsorption have been used to determine the state of Ru in the presence of cyclohexane solvent. The spectroscopic data support the existence of a hydrated RuOxRuOx-like phase, which is identified by overtone signals of Ru–O bonds at ca. 1850 cm−1 and is likely organized as a two-dimensional phase on the apatite. This phase coexists with isolated Ru3+Ru3+ cations likely located in the hardly accessible channels of the apatite structure. The RuOxRuOx-like phase is shown to be highly reactive at the solid–liquid interface and exhibits a number of carbonyl species upon CO adsorption. The relevance of the RuOxRuOx-like phase for the liquid-phase aerobic oxidation of benzyl alcohol has been investigated under close to reaction conditions. Although weak signals of benzaldehyde indicate that conversion is likely low under flow conditions, their presence makes the results relevant to draw structure–activity relationships. Blocking of accessible Ru sites using CO prior to reaction reveals that only selected CO species on the hydrated RuOxRuOx-like phase are depleted when reaction occurs. The likely formation of HCO3− in the absence of the alcohol indicates that benzyl alcohol oxidation and HCO3− formation may occur on similar Ru sites providing evidence that these sites are located on the RuOxRuOx phase.