Aerobic oxidation of alcohols over carbon nanotube-supported Ru catalysts assembled at the interfaces of emulsion droplets

https://doi.org/10.1016/j.apcata.2010.04.046Get rights and content

Abstract

Carbon nanotube (CNT)-supported ruthenium catalysts, assembled at the interfaces of emulsion droplets, show excellent activity, selectivity, and stability for the selective oxidations of benzyl alcohol to benzaldehyde with oxygen or air as oxidant in the presence of water. The selective oxidation of benzyl alcohol over Ru/CNTs catalysts is greatly enhanced and quickened due to the presence of water. A reaction pathway is proposed, in which the promotion effect of water on the catalytic activity of Ru/CNTs is discussed. The as-made Ru/CNTs catalysts are also active for the aerobic oxidation of a variety of alcohols with a sulfur or nitrogen atom or a carbon–carbon double bond in the multiphase reaction system. Moreover, after the reactions, the catalysts can be easily separated and recycled by sedimentation.

Graphical abstract

Carbon nanotube (CNT)-supported ruthenium catalysts, assembled at the interfaces of emulsion droplets, show excellent activity, selectivity, and stability for the selective oxidation of a variety of alcohols with oxygen or air as oxidant. After the reactions, the catalysts can be easily separated and recycled by sedimentation.

  1. Download : Download high-res image (90KB)
  2. Download : Download full-size image

Introduction

Oxidation of alcohols to carbonyl compounds is one of the most fundamental and important processes for synthesis of organic chemicals. A variety of methods for the oxidation of alcohols have been developed; nevertheless, traditional methods such as non-catalytic methods with stoichiometric amounts of heavy metal reagents [1] or moisture-sensitive expensive oxidants [2] are still widely used. These processes are often conducted in environmentally undesirable media such as chlorinated solvents. From environmental and atom-economical points of view, it would be ideal to oxidize alcohols using air or molecular oxygen under atmospheric pressure conditions. Obviously, this cheaper, safer, and more environmentally benign oxidation process requires novel yet highly efficient heterogeneous catalysts. The research efforts have led to a number of efficient catalysts for the aerobic oxidation of alcohols such as hydroxyapatite (HAP) bound Ru/HAP [3] and Pd/HAP [4], [5], Ru/Al2O3 [6], Au/CeO2 [7], and Au–Pd/TiO2 [8]. Up to now, oxidation reactions at low temperature in appropriate solvents are still highly demanded for alcohols with high melting points or low stability at high temperatures, or in cases where a minimal amount of an alcohol is needed for scientific research [9], [10], [11], [12], [13], [14].

With the discovery of carbon nanotubes (CNTs) and their large scale production, much attention has been paid to their potential applications such as in electronic devices, biosensors, and catalysis due to their unique electron conductivity, thermal stability, and high mechanical strength [15], [16], [17], [18], [19], [20], [21], [22], [23]. Compared with traditional catalyst supports, CNTs with high external surface area and aspect ratio display unusual behaviors, such as being able to significantly increase the contact surface between the reactants and active sites of catalysts, and to greatly minimize the diffusion limitations. These features make CNTs very attractive as catalyst supports in liquid phase reactions. Here we report that, with CNT-supported ruthenium as catalysts, several types of alcohols with a sulfur or nitrogen atom, or a carbon–carbon double bond in the molecule, can be effectively oxidized to their corresponding carbonyl compounds using oxygen or air as oxidant under mild conditions, as illustrated in Eq. (1). It is interesting to note that the CNT-supported ruthenium catalysts assemble and form droplets in the emulsion-like multiphase reaction system. After the reaction, the catalysts can be easily separated from the liquid products by sedimentation and can be recycled.

Section snippets

Materials

CNTs with a diameter of 10–20 nm, a length of 1–2 μm, and N2 surface area of 151 m2/g were used. Coconut shell based active carbon (AC) with N2 surface area of 986 m2/g was purchased from Beijing Broad Activated Carbon Co., Ltd. (Beijing, China). TiO2 (Degussa P25) with a N2 surface area of 59 m2/g was purchased from Shanghai Haiyi Scientific & Trading Co., Ltd. (Shanghai, China). RuO2 was purchased from Alfa Aesar Co., Ltd. Oxygen with a purity of 99.995% was purchased from Dalian Guangming Special

Catalyst characterization

The modification of the CNTs aims to remove amorphous carbon and to increase the number of surface oxygen-containing functional groups that are useful for the metal deposition and dispersion. The FT-IR spectra of the CNTs before and after the modification are shown in Fig. 1, showing that new peaks at 1710 cm−1 and 1190 cm−1 appear after modification, which indicates that the carboxylic groups are formed on the CNTs. The BET surface area of the modified CNTs is 162 m2/g.

The TPR profile of RuCl3

Conclusions

Effective catalytic oxidation of alcohols under mild conditions has been realized in an emulsion system, in which CNT-supported ruthenium composites function both as catalysts and as emulsifying agents. Ru/CNTs catalysts are prepared by the traditional wetness impregnation method, and show excellent activity, selectivity, and stability for the selective oxidation of benzyl alcohol with oxygen or air as oxidant. The catalytic activity of Ru/CNTs for the selective oxidation of benzyl alcohol is

Acknowledgements

This 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.

References (37)

  • H.-B. Ji et al.

    Catal. Commun.

    (2002)
  • S. Martín et al.

    Tetrahedron Lett.

    (2002)
  • W.Z. Li et al.

    Carbon

    (2002)
  • C.H. Liang et al.

    J. Catal.

    (2002)
  • C. Wang et al.

    Catal. Commun.

    (2008)
  • K.C. Taylor

    J. Catal.

    (1975)
  • P. Betancourt et al.

    Appl. Catal. A: Gen.

    (1998)
  • P.G.J. Koopman et al.

    J. Catal.

    (1981)
  • X. Wang et al.

    Chem. Phys. Lett.

    (2001)
  • L. Duclaux

    Carbon

    (2002)
  • H. Nagahara et al.

    Appl. Surf. Sci.

    (1997)
  • X.M. Yang et al.

    Catal. Commun.

    (2008)
  • W.J. Mijs et al.

    Organic Syntheses by Oxidation with Metal Compounds

    (1986)
  • M. Hudlicky

    Oxidations in Organic Chemistry, ACS Monograph 186

    (1990)
  • K. Yamaguchi et al.

    J. Am. Chem. Soc.

    (2000)
  • K. Mori et al.

    J. Am. Chem. Soc.

    (2002)
  • K. Mori et al.

    J. Am. Chem. Soc.

    (2004)
  • K. Yamaguchi et al.

    Angew. Chem. Int. Ed.

    (2002)
  • Cited by (0)

    View full text