Journal of Molecular Catalysis A: Chemical 372 (2013) 72–78 Contents lists available at SciVerse ScienceDirect Journal of Molecular Catalysis A: Chemical j o ur nal ho me p age: www.elsevier.com/locate/molcata Dioxygen oxidation of 1-phenylethanol with gold nanoparticles and N-hydroxyphthalimide in ionic liquid Hassan Hosseini-Monfared ∗,1 , Hajo Meyer, Christoph Janiak ∗ Institut für Anorganische Chemie und Strukturchemie, Universität Düsseldorf, Universitätsstr. 1, D-40225 Düsseldorf, Germany a r t i c l e i n f o Article history: Received 30 November 2012 Received in revised form 10 February 2013 Accepted 11 February 2013 Available online xxx Keywords: Gold nanoparticles Oxidation catalysis Dioxygen Ionic liquids Alcohol a b s t r a c t Gold nanoparticles (Au-NPs) of 8 nm average diameter were obtained by thermal reduction under nitrogen from KAuCl 4 in the presence of n-butylimidazol dispersed in the ionic liquid (IL) 1-n-butyl- 3-methylimidazolium tetrafluoroborate (BMIm + BF 4 - ). Characterization of the Au-NP was done by transmission electron microscopy (TEM) and dynamic light scattering (DLS). Catalytic activities of the Au-NP/IL dispersion were evaluated in the oxidation of 1-phenylethanol at 100 and 160 ◦ C under 4 bar pressure of dioxygen in a base-free system. Au-NP in combination with the radical initiator N- hydroxyphthalimide (NHPI) showed good conversion and selectivity for the oxidation of 1-phenylethanol to acetophenone through formation of an -hydroxy carbon radical. The concomitant side products di(1-phenylethyl)ether and di(1-phenylethyl)peroxide were rationalized by an equilibrium due to the IL matrix of the -hydroxy carbon radical with the 1-phenylethoxy radical. Maximum turnover number was ∼5200 based on the total number of moles of gold but a factor of about six larger, TON ≈ 31 300, when only considering the Au-NP surface atoms. The fraction (N S /N T ) of exposed surface atoms (N S ≈ 2560) for an average 8 nm Au-NP (having N T ≈ 15 800 atoms in a ∼17-shell icosahedral or cuboctahedral particle) was estimated here as 0.16. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Selective oxidation of alcohols to carbonyl compounds is a fun- damental transformation in organic synthesis because the target molecules can be obtained directly in one-pot sequences. Alcohols are important precursors for various chemicals and in particular, the oxidation of alcohols and polyols is of interest owing to the large array of biological hydroxy-derivatives [1–3]. Choice of the oxidants determines the practicability and efficiency of the oxida- tion reactions. Oxidants, like MnO 4 - and Cr 2 O 7 2- produce toxic, environmentally and economically unacceptable by-products or like NaOCl have a low (≤30 wt.%) active oxygen content. In this con- text, O 2 (or air) is the most attractive oxidants because of its high contents of active oxygen species (100% for dioxygenase-type, 50% for monooxygenase-type) and co-production of only water. Nanoparticles (NPs) are of high interest in catalysis [4–6]. The small size of nanoparticles results in a large fraction of surface atoms [7]. Interaction of unprotected small particles will, however lead to agglomeration or aggregation from the cohesive surface ∗ Corresponding authors. Tel.: +49 2118112286. E-mail addresses: monfared@znu.ac.ir (H. Hosseini-Monfared), janiak@uni-duesseldorf.de (C. Janiak). 1 Permanent address: Department of Chemistry, University of Zanjan, 45195 313, Zanjan, Iran. energy [8]. As a result of their colloidal instability, many nanopar- ticles need to be stabilized via additional (capping) agents such as surfactants or polymers, which provide a steric, electrostatic or electrosteric particle stabilization [9]. For catalytic applications of NPs strong surface protection is, however, undesirable as it hinders substrate access and interaction with the surface catalytic sites. Instead ligand-free NPs should be advantageous for a high activ- ity. Ionic liquids (ILs) can stabilize M-NPs on the basis of their ionic nature [10], high polarity, high dielectric constant and supramolec- ular network without the need of additional protective ligands [11]. ILs can therefore function both as stabilizer and solvent for the preparation of small (<5 nm) and (generally) kinetically stabilized M-NPs [4,12,13]. The past decade has seen an explosive growth in catalytic reac- tions studied in ILs. Often the IL enables more efficient reactions compared with standard organic solvents and catalysts show good or even enhanced activities when applied in ionic liquids [14–16]. ILs are interesting in the context of green catalysis [17] which requires that catalysts be designed for easy product separation from the reaction products and multi-time efficient reuse/recycling [18]. The Pd-metal-catalyzed oxidation of benzyl alcohol with 1 atm O 2 in the IL BMIm + X - gave good conversion to benzaldehyde, albeit only for X = BF 4 and not for X = Cl or Br and with Pd amounts of equal or larger than 2.8 mol% [19]. Oxidation by gold is of timely interest for green processes requir- ing stable, selective and non-toxic heterogeneous catalysts, as well 1381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molcata.2013.02.007