Oxidation of Ethylbenzene in the Presence of an MCM-41-Supported or Ionic Liquid-Standing Bischlorocopper(II) Complex
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Oxidation of Ethylbenzene in the Presence of an MCM-41-Supported or Ionic Liquid-Standing Bischlorocopper(II) Complex Sandra Gago • Sofia M. Bruno • Diogo C. Queiro´s Anabela A. Valente • Isabel S. Gonc¸alves • Martyn Pillinger
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Received: 13 January 2011 / Accepted: 3 April 2011 / Published online: 19 April 2011 Ó Springer Science+Business Media, LLC 2011
Abstract A CuCl2 complex with a pyridyl-ethanimine ligand has been prepared and examined as a catalyst for the oxidation of ethylbenzene with tert-butyl hydroperoxide at 30 °C, using acetonitrile or the ionic liquid 1-butyl-3methyl-imidazolium hexafluorophosphate as solvent. A heterogenised version of the complex based on a chemically modified ordered mesoporous silica is described. Keywords Copper(II) Ethylbenzene Acetophenone Catalytic oxidation MCM-41 Ionic liquids
1 Introduction Acetophenone (PhCOMe) is commercially used in the production of perfumes, drugs, pharmaceuticals and resins, and may be obtained by the partial oxidation of ethylbenzene (PhEt) in the liquid phase, using an appropriate catalyst [1]. The primary industrial method involves the oxidation of PhEt using cobalt salts as homogeneous catalysts (e.g., cobalt acetate or cobalt cycloalkanecarboxylate), manganese and/or bromide species as promoters,
S. Gago S. M. Bruno D. C. Queiro´s A. A. Valente (&) I. S. Gonc¸alves M. Pillinger (&) Department of Chemistry, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal e-mail: [email protected] M. Pillinger e-mail: [email protected] Present Address: S. Gago REQUIMTE, Departamento de Quı´mica, Faculdade de Cieˆncias e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal
acetic acid as solvent, and molecular oxygen or air as the oxidant [2]. This method suffers from several disadvantages, which include the corrosive nature of acetic acid, difficulty in solvent and catalyst recovery, and explosion hazards associated with the solvent and dioxygen. Considerable research has therefore focused on finding not only more efficient catalysts but also alternative solvents and oxidants. Interesting results have been reported for copper complexes combined with tert-butyl hydroperoxide (TBHP) as oxidant and acetonitrile as solvent [3–11], giving PhCOMe yields in the range of 40–60% (for reaction temperatures of 60–70 °C, except 25 °C in ref. [10], and reaction times of 6–24 h) and PhCOMe selectivity above 84%. The replacement of acetonitrile as a reaction solvent would be an important development, given its flammability and toxicity. Surprisingly, only a few papers have dealt with the use of ionic liquids (ILs) as solvents for the oxidation of PhEt [12, 13], despite the intensive research that has been carried out in the last decade in the area of catalysed oxidations in ILs [14]. ILs are potential ‘‘green’’ alternatives to volatile organic solvents, with the added advantage of facile catalyst and solvent recycling. Seddon and Stark [12] reported that the Co(acac)2 catalysed oxidation of PhEt in 1-hexyl-2,3-dimethylimidazolium tetrafl
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