136 X. Yang et al. / Applied Catalysis A: General 382 (2010) 131–137Scheme 2. Possible reaction mechanism <strong>of</strong> benzyl alcohol <strong>oxidation</strong> <strong>over</strong> the <strong>supported</strong>ruthenium catalysts.amount <strong>of</strong> water increases, more interfaces would be available,which results in an increase in the conversion <strong>of</strong> substrate. Thesolubility <strong>of</strong> benzaldehyde in toluene is higher than that <strong>of</strong> benzylalcohol, but its solubility is much lower in water than that<strong>of</strong> benzyl alcohol. Toluene extracts benzaldehyde to the organicphase, avoiding further <strong>oxidation</strong>. As such, a maximum yield canbe obtained at a proper water/toluene ratio. A model is proposedand shown in Scheme 1. Following the classical dehydrogenationmechanism <strong>of</strong> alcohol <strong>oxidation</strong>, the possible reaction steps <strong>of</strong> the<strong>oxidation</strong> <strong>of</strong> benzyl alcohol are shown in Scheme 2. The O–H bond<strong>of</strong> benzyl alcohol breaks upon adsorption at the surface sites, yieldingan adsorbed alkoxide and hydrogen [32]. For the adsorbedalkoxide, the -C–H bond is weaker than other C–H bonds dueto the electron-withdrawing effect <strong>of</strong> the oxygen atom, resultingin the preferential breaking <strong>of</strong> the -C–H bond that is the ratedeterminingstep [33], as shown in Scheme 2. Meanwhile, adsorbedactivated oxygen is necessary to oxidize the hydrogen co-product,which helps to shift the equilibrium toward the <strong>carbon</strong>yl compoundand to accelerate the whole reaction by liberating surfacemetallic sites [34], as schematically shown in Scheme 2. A similarmultiphase reaction system was proposed and discussed before[35,36].The <strong>oxidation</strong> <strong>of</strong> other activated and non-activated <strong>alcohols</strong>was also tested in the presence <strong>of</strong> water (Table 4). Four <strong>of</strong> these<strong>alcohols</strong> contain a sulfur or nitrogen atom, or a <strong>carbon</strong>–<strong>carbon</strong>double bond (entries 4–6). The reaction selectivity is 100% in allcases, and all primary and secondary benzylic <strong>alcohols</strong> are convertedinto the corresponding benzaldehydes and ketones. Primaryallylic alcohol (typical cinnamyl alcohol) affords the correspondingenal (cinnamaldehyde) without intramolecular hydrogen transferor geometrical isomerization <strong>of</strong> the double bond. The <strong>Ru</strong>/CNTs systemalso catalyzes the <strong>oxidation</strong> <strong>of</strong> <strong>alcohols</strong> containing nitrogen orsulfur atoms to the corresponding aldehydes in high yields (entries5 and 6), while monomeric <strong>Ru</strong> complexes cannot effect catalytic<strong>oxidation</strong> <strong>of</strong> these <strong>alcohols</strong> because <strong>of</strong> strong coordination to themetal center [37]. Alicyclic <strong>alcohols</strong> such as cyclopentanol andcyclohexanol are selectively oxidized to the corresponding cyclicketones (entries 8 and 9). Less reactive aliphatic <strong>alcohols</strong> such as 1-octanol and 2-octanol are also oxidized (entries 10 and 11). Whenair is used as oxidant instead <strong>of</strong> pure O 2 , the conversion <strong>of</strong> benzylalcohol remains at the same level, demonstrating that the efficientselective <strong>oxidation</strong> <strong>of</strong> benzyl alcohol <strong>over</strong> the as-made <strong>Ru</strong>/CNTs catalystscan be realized in air, which is <strong>of</strong> great potential for industrialapplications. When the <strong>oxidation</strong> <strong>of</strong> benzyl alcohol is repeated 4times with the same catalyst <strong>of</strong> <strong>Ru</strong>/CNTs, the conversion <strong>of</strong> benzylalcohol remains 92% without any obvious loss in activity, indicatingthat the <strong>Ru</strong>/CNTs catalyst has excellent stability.4. ConclusionsEffective catalytic <strong>oxidation</strong> <strong>of</strong> <strong>alcohols</strong> under mild conditionshas been realized in an emulsion system, in which CNT-<strong>supported</strong>ruthenium composites function both as catalysts and as emulsifyingagents. <strong>Ru</strong>/CNTs catalysts are prepared by the traditionalwetness impregnation method, and show excellent activity, selectivity,and stability for the selective <strong>oxidation</strong> <strong>of</strong> benzyl alcohol withoxygen or air as oxidant. The catalytic activity <strong>of</strong> <strong>Ru</strong>/CNTs for theselective <strong>oxidation</strong> <strong>of</strong> benzyl alcohol is greatly enhanced by water,which is due to the formation <strong>of</strong> emulsion droplets where the CNT<strong>supported</strong>catalysts assemble at the interfaces. <strong>Ru</strong>/CNTs catalystsare <strong>of</strong> potential for the selective <strong>oxidation</strong> <strong>of</strong> a large variety <strong>of</strong><strong>alcohols</strong> in the presence <strong>of</strong> water. Furthermore, the catalyst canbe easily separated and recycled by sedimentation.The results reported here also highlight the unique propertyand functions <strong>of</strong> solid catalysts assembled at the interfaces <strong>of</strong> twoliquid phases to form special emulsion catalysis systems, whichleads one to anticipate that such emulsion-stabilizing solid catalystsafter being further tailored will be <strong>of</strong> wide use in a broad range<strong>of</strong> reactions.AcknowledgementsThis work is partly <strong>supported</strong> by NSFC (Nos. 20725619,20836002). We thank Pr<strong>of</strong>. Roel Prins at ETH, Switzerland, and Dr.Dangsheng Su at Fritz Haber Institute <strong>of</strong> the Max Planck Society,Germany, for helpful discussions and suggestions.Appendix A. Supplementary dataSupplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.apcata.2010.04.046.References[1] W.J. 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