Aerobic oxidation of alcohols over carbon nanotube-supported Ru ...

136 X. Yang et al. / Applied Catalysis A: General 382 (2010) 131–137Scheme 2. Possible reaction mechanism of benzyl alcohol oxidation over the supportedruthenium catalysts.amount of water increases, more interfaces would be available,which results in an increase in the conversion of substrate. Thesolubility of benzaldehyde in toluene is higher than that of benzylalcohol, but its solubility is much lower in water than thatof benzyl alcohol. Toluene extracts benzaldehyde to the organicphase, avoiding further oxidation. 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 of alcohol oxidation, the possible reaction steps of theoxidation of benzyl alcohol are shown in Scheme 2. The O–H bondof 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 of the oxygen atom, resultingin the preferential breaking of 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 carbonyl 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 oxidation of other activated and non-activated alcoholswas also tested in the presence of water (Table 4). Four of thesealcohols contain a sulfur or nitrogen atom, or a carbon–carbondouble bond (entries 4–6). The reaction selectivity is 100% in allcases, and all primary and secondary benzylic alcohols are convertedinto the corresponding benzaldehydes and ketones. Primaryallylic alcohol (typical cinnamyl alcohol) affords the correspondingenal (cinnamaldehyde) without intramolecular hydrogen transferor geometrical isomerization of the double bond. The Ru/CNTs systemalso catalyzes the oxidation of alcohols containing nitrogen orsulfur atoms to the corresponding aldehydes in high yields (entries5 and 6), while monomeric Ru complexes cannot effect catalyticoxidation of these alcohols because of strong coordination to themetal center [37]. Alicyclic alcohols such as cyclopentanol andcyclohexanol are selectively oxidized to the corresponding cyclicketones (entries 8 and 9). Less reactive aliphatic alcohols such as 1-octanol and 2-octanol are also oxidized (entries 10 and 11). Whenair is used as oxidant instead of pure O 2 , the conversion of benzylalcohol remains at the same level, demonstrating that the efficientselective oxidation of benzyl alcohol over the as-made Ru/CNTs catalystscan be realized in air, which is of great potential for industrialapplications. When the oxidation of benzyl alcohol is repeated 4times with the same catalyst of Ru/CNTs, the conversion of benzylalcohol remains 92% without any obvious loss in activity, indicatingthat the Ru/CNTs catalyst has excellent stability.4. ConclusionsEffective catalytic oxidation of alcohols under mild conditionshas been realized in an emulsion system, in which CNT-supportedruthenium composites function both as catalysts and as emulsifyingagents. Ru/CNTs catalysts are prepared by the traditionalwetness impregnation method, and show excellent activity, selectivity,and stability for the selective oxidation of benzyl alcohol withoxygen or air as oxidant. The catalytic activity of Ru/CNTs for theselective oxidation of benzyl alcohol is greatly enhanced by water,which is due to the formation of emulsion droplets where the CNTsupportedcatalysts assemble at the interfaces. Ru/CNTs catalystsare of potential for the selective oxidation of a large variety ofalcohols in the presence of water. Furthermore, the catalyst canbe easily separated and recycled by sedimentation.The results reported here also highlight the unique propertyand functions of solid catalysts assembled at the interfaces of twoliquid phases to form special emulsion catalysis systems, whichleads one to anticipate that such emulsion-stabilizing solid catalystsafter being further tailored will be of wide use in a broad rangeof reactions.AcknowledgementsThis work is partly supported by NSFC (Nos. 20725619,20836002). We thank Prof. Roel Prins at ETH, Switzerland, and Dr.Dangsheng Su at Fritz Haber Institute of 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. Mijs, C.R.H. de Jonge, Organic Syntheses by Oxidation with Metal Compounds,Plenum, New York, 1986.[2] M. Hudlicky, Oxidations in Organic Chemistry, ACS Monograph 186, ACS, Washington,DC, 1990.[3] K. Yamaguchi, K. Mori, T. Mizugaki, K. Ebitani, K. Kaneda, J. Am. Chem. Soc. 122(2000) 7144–7145.[4] K. Mori, K. Yamaguchi, T. Hara, T. Mizugaki, K. Ebitani, K. Kaneda, J. Am. Chem.Soc. 124 (2002) 11572–11573.[5] K. Mori, T. Hara, T. Mizugaki, K. Ebitani, K. Kaneda, J. Am. Chem. Soc. 126 (2004)10657–10666.[6] K. Yamaguchi, N. Mizuno, Angew. Chem. Int. Ed. 41 (2002) 4538–4542.[7] A. Abad, C. Concepción, A. Corma, H. García, Angew. Chem. Int. Ed. 44 (2005)4066–4069.[8] D.I. Enache, J.K. Edwards, P. Landon, B. Solsona-Espriu, A.F. Carley, A.A. Herzing,M. Watanabe, C.J. Kiely, D.W. Knight, G.J. Hutchings, Science 311 (2006)362–365.[9] H.-B. Ji, K. Ebitani, T. Mizugaki, K. Kaneda, Catal. Commun. 3 (2002) 511–517.[10] S. Martín, D.F. Suárez, Tetrahedron Lett. 43 (2002) 4475–4479.[11] K.S. Colemane, M. Coppe, C. Thomas, J.A. Osborn, Tetrahedron Lett. 40 (1999)3723–3726.[12] J. Muldoon, S.N. Brown, Org. Lett. 4 (2002) 1043–1045.[13] F. Minisci, C. Punta, F. Recupero, F. Fontana, G.F. Peduli, Chem. Commun. (2002)688–689.[14] F. Minisci, F. Recupero, A. Cecchetto, C. Gambarotti, C. Punta, R. Faletti, R.Paganelli, G.F. Pedulli, Eur. J. Org. Chem. (2004) 109–119.[15] C. Dekker, Phys. Today 52 (1999) 22–28.[16] J. Kong, N.R. Franklin, C.W. Zhou, M.G. Chapline, S. Peng, K.J. Cho, H.J. Dai, Science287 (2000) 622–625.[17] J.M. Planeix, N. Coustel, B. Coq, V. Brotons, P.S. Kumbhar, R. Dutartre, P. Geneste,P. Bernier, P.M. Ajayan, J. Am. Chem. Soc. 116 (1994) 7935–7936.[18] A. Chambers, T. Nemes, N.M. Rodriguez, R.T.K. Baker, J. Phys. Chem. B 102 (1998)2251–2258.[19] W.Z. Li, C.H. Liang, J.S. Qiu, W.J. Zhou, H.M. Han, Z.B. Wei, G.Q. Sun, Q. Xin, Carbon40 (2002) 791–794.[20] C.H. Liang, Z.L. Li, J.S. Qiu, C. Li, J. Catal. 211 (2002) 278–282.[21] J.S. Qiu, H.Z. Zhang, C.H. Liang, J.W. Li, Z.B. Zhao, Chem. Eur. J. 12 (2006)2147–2151.[22] C. Wang, J.S. Qiu, C.H. Liang, L. Xing, X.M. Yang, Catal. Commun. 9 (2008)1749–1753.[23] J. Zhang, X. Liu, R. Blume, A. Zhang, R. Schlögl, D.S. Su, Science 322 (2008) 73–77.[24] R.A. DallaBetta, J. Catal. 34 (1974) 57–60.[25] K.C. Taylor, J. Catal. 38 (1975) 299–306.[26] P. Betancourt, A. Rives, R. Hubaut, C.E. Scott, J. Goldwasser, Appl. Catal. A: Gen.170 (1998) 307–314.[27] P.G.J. Koopman, A.P.G. Kieboom, H. van Bekkum, J. Catal. 69 (1981) 172–179.[28] S. Liu, X. Tang, L. Yin, Y. Koltypin, A. Gedanken, J. Mater. Chem. 10 (2000)1271–1272.[29] X. Wang, Y. Liu, D. Zhu, Chem. Phys. Lett. 340 (2001) 419–424.[30] L. Duclaux, Carbon 40 (2002) 1751–1764.[31] S. Carrettin, P. McMorn, P. Johnston, K. Griffin, G.J. Hutchings, Chem. Commun.(2002) 696–697.