(nBu4N)4W10O32-catalyzed selective oxygenation of cyclohexane by molecular oxygen under visible light irradiation

https://doi.org/10.1016/j.apcatb.2014.08.045Get rights and content

Highlights

  • Using decatungstate as catalyst for photooxidation of cyclohexane and other compounds.

  • Utilization of more accessible visible light.

  • An excellent selectivity for cyclohexanone.

  • The mediated strong or strong acidic aqueous promotes this photocatalyzed oxidation.

Abstract

The development of mild and efficient process for the selective oxygenation of organic compounds by molecular oxygen (O2) can be one of the key technologies for synthesizing oxygenates. Here, the photo-oxygenation of cyclohexane to cyclohexanol and cyclohexanone over tetrabutylammonium decatungstate (W10O324−) was carried out under visible light irradiation and pure O2 atmosphere. The W10O324− was found to be active to this photo-oxygenation in a pure acetonitrile (MeCN), which can achieve ca. 8.1% cyclohexane conversion with ca. 64.3% selectivity for cyclohexanone under sustained visible light irradiation of 12 h. Notably, the above-described photo-catalysis oxygenation was improved to some extent in the presence of some acidic additives such as10 M HCl, H2SO4, or H3PO4 aqueous solution and benzenesulfonic acid, providing ca. 12.4–14.3% cyclohexane conversion with ca. 59.7–69.4% cyclohexanone selectivity. Moreover, this acid-promoted effect was further magnified in the case of adding a suitable amount of water, which can lead to enhancing conversion (16.8–20.0%) and improving cyclohexanone selectivity (68.2–78.3%). This acidic aqueous solution-promoted effect was also observed upon the W10O324−-photocatalyzed oxygenation of other substrates like toluene, ethylbenzene and butanone. This is likely due to the acidic aqueous solutions playing important roles in improving photo-redox cycling of W10O324− and preserving its stability, as supported by the UV–vis spectra and cyclic voltammetry measurements.

Introduction

There is considerable interest in the search for efficient catalysis system that induces oxidation of inactivated Csingle bondH bonds by molecular oxygen under mild conditions. The oxidation of cyclohexane to cyclohexanone and cyclohexanol (as KA-oil), intermediate products in the synthesis of synthetic fibbers and fine chemicals [1], [2], is an example of this transformation. The reaction remains in the center of interest of many research groups [2], [3], [4] as it is one of the least efficient of all major industrial chemical processes. In industrial process, large-scale oxygenation of cyclohexane to KA-oil, employs soluble cobalt or manganese salts as homogeneous catalysts under relatively harsh conditions (443–503 K and 10–20 atm of air) and produces KA-oil with 75–80% selectivity, cyclohexane conversion is usually limited to lower than 10% in order to prevent over-oxidation of the target products [3], [4], [5], [6]. Some efficient catalysis processes have been developed for selective oxygenation of cyclohexane by molecular oxygen [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. However, most of these direct oxidation processes by molecular oxygen under heating generally need to employ harsh operating conditions because the triplet nature of molecular oxygen hampers the reaction with organic compounds in its singlet state, which rarely leads to the chemistry.

During recent years, photo-catalytic oxygenation has received enormous attention because of its potential application in environmental treatment [17], [18], [19] and the synthesis of fine chemicals [20], [21]. On the other hand, the use of solar light and molecular oxygen as reagents in catalysis oxidations contributes to realizing innovative and economically advantageous processes for transformation of hydrocarbons into oxygenated products and, at the same time, to move toward a sustainable chemistry that has a minimal environmental impact. Up to now, some efficient photo-catalysts, such as TiO2 [22], [23], [24], Fe–TiO2 [25] and double-shelled WO3@TiO2 [26], Fenton reagent-assisted WO3 [27], Cr–SiO2 [28], V2O5–Al2O3 [29], NaY zeolite [30], [31], [32], trans-dioxoosmium(VI) complex [33], iron(III) porphyrin complexes [34], [35], iron(III) chloride [36], [37], [38], copper(II) chloride [39], [40] and decatungstate [41], [42], [43], [44], [45], [46], [47], [48], [49] have been successfully applied to the field of UV or visible light-driven aerobic oxygenation or degradation of organic compounds. Among them, the decatungstate has been especially studied for its very important photo-catalytic properties and important advances have been achieved in the transformation of organic compounds to the corresponding oxidative products catalyzed by the decatungstate under UV light [47], [48], [50], [51], [52], [53], [54]. However, based on the facts that most of the solid catalysts commonly show a low photo-catalysis efficiency, the Fe porphyrin and especially Os complexes as efficient photo-catalysts are expensive, the efficient photo-catalysis system originating from a very cheap catalyst iron(III) or copper(II) chloride inevitably goes with considerable amounts of chlorination and the photo-catalysis activity of decatungstate is largely dependent on UV light irradiation. To the best of our knowledge, the decatungstate-catalyzed oxidation reactions under visible light has not been reported so far and how to mediate the photo-catalytic performance of decatungstate is hardly concerned. Herein, we report initial results obtained from using the visible light-driven decatungstate to catalyze aerobic oxidation of cyclohexane to KA oil in the presence of some acidic additives under pure O2 atmosphere.

Section snippets

Materials and apparatus

Materials and reagents used in this study were cyclohexane, benzene, toluene, ethylbenzene, butanone n-hexanol, acetonitrile (CH3CN), sodium tungstate (Na2WO4), tetrabutylammonium bromide ([(C4H9)4N]4Br), concentrated HCl, H2SO4, and H3PO4, acetic acid (HAc), benzenesulfonic acid, all of which were of analytical grade. Distilled water was used throughout this experiment.

Photo-catalytic performance of W10O324−

Table 1 lists data for the W10O324−-catalyzed oxygenation of cyclohexane with O2 at 36–38 °C in MeCN containing various acidic additives under visible light irradiation. Entry 1 illustrates that the W10O324− was active to this photo-oxygenation, which could achieve ca. 8.1% cyclohexane conversion (turnover frequency (TOF), 0.48) in a pure CH3CN medium under 12 h of sustained visible light irradiation, and afford cyclohexanol (selectivity (Sel.) 35.7%) and cyclohexanone (Sel. 64.3%) as its

Conclusion

In summary, for the first time we have developed a visible light-triggered decatungstate to catalyze the selective oxidation of cyclohexane to cyclohexanol and cyclohexanone by molecular oxygen, and found that the photo-catalysis activity and cyclohexanone selectivity can be improved significantly in the presence of acidic aqueous solutions. Furthermore, this promoted effect commonly exists in the decatungstate-photocatalyzed oxygenation of other some organic compounds by molecular oxygen. It

Acknowledgments

We acknowledge the financial support for this work by the Specialized Research Fund for the Doctoral Program of Higher Education (20124306110005), the National Natural Science Fund of China (20873040), the Natural Science Fund of Hunan Province (10JJ2007, 14JJ2148), the Innovation Platform Open Fund of Hunan College (11K044), the Program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province, the 100 Talents Program of Hunan Province and the

References (66)

  • U. Schuchardt et al.

    J. Mol. Catal. A: Chem.

    (1998)
  • M.H.N. Olsen et al.

    J. Supercrit. Fluids

    (2005)
  • L.N. Ji et al.

    J. Mol. Catal.

    (1991)
  • J.W. Huang et al.

    J. Mol. Catal. A: Chem.

    (2000)
  • C.C. Guo et al.

    Appl. Catal. A: Gen.

    (2003)
  • G. Huang et al.

    J. Mol. Catal. A: Chem.

    (2007)
  • B.C. Hu et al.

    Catal. Commun.

    (2008)
  • D.S. Ma et al.

    Catal. Commun.

    (2009)
  • S.X. Li et al.

    Dyes Pigments

    (2012)
  • M.A. Gonzalez et al.

    J. Catal.

    (1999)
  • C.B. Almquist et al.

    Appl. Catal. A: Gen.

    (2001)
  • S.X. Li et al.

    Appl. Catal. B: Environ.

    (2014)
  • H. Lee et al.

    Appl. Catal. B: Environ.

    (2013)
  • I. Sökmen et al.

    J. Colloid Interface Sci.

    (2003)
  • A. Maldotti et al.

    J. Mol. Catal. A: Chem.

    (1996)
  • G.B. Shul’pin et al.

    Mendeleev Commun.

    (1992)
  • W.F. Wu et al.

    J. Catal.

    (2012)
  • I.N. Lykakis et al.

    Tetrahedron Lett.

    (2004)
  • E. Fornal et al.

    J. Photochem. Photobiol. A: Chem.

    (2007)
  • C. Tanielian

    Coord. Chem. Rev.

    (1998)
  • F. Bigi et al.

    J. Catal.

    (2007)
  • K. Nomiya et al.

    Polyhedron

    (1987)
  • I. Texier et al.

    Chem. Phys. Lett.

    (1999)
  • G.W. Parshall et al.

    Homogenous Catalysis

    (1992)
  • H.H. Szmant

    Organic Building Blocks of the Chemical Industry

    (1989)
  • M.T. Musser

    Cyclohexanol and Cyclohexanone, Ullmann's Encyclopedia of Industrial Chemistry

    (2007)
  • R.A. Sheldon et al.

    Metal-Catalyzed Oxidation of Organic Compounds

    (1981)
  • D.L. Vanoppen et al.

    Angew. Chem. Int. Ed. Engl.

    (1995)
  • E. Amin et al.

    Appl. Catal. A: Gen.

    (2007)
  • Y. Li et al.

    Catal. Lett.

    (2008)
  • F. Cavani et al.
  • X.Y. Li et al.

    J. Chem. Technol. Biotechnol.

    (2003)
  • B. Khennaoui et al.

    J. Environ. Sci. Eng. A

    (2012)
  • Cited by (0)

    View full text