Short CommunicationEfficient aerobic oxidation of alcohols using magnetically recoverable catalysts
Graphical abstract
Introduction
Magnetite nanoparticles (MNPs) are emerging as promising materials for applications in information recording, magnetic fluids, drug delivery, in-vivo magnetic imaging, and catalysis [1], [2], [3], [4]. Heterogeneous catalysis has been around for a long time, but has still much to groom. Most heterogeneous catalysts are comprised of small nanometer sized, often a metal, and finely dispersed on a cheaper, high surface area support [5]. Industry favors a heterogeneous over homogeneous catalysis due to the advantage of recovery and recycling. The oxidation of alcohols to ketones is an important organic transformation since the resultant compounds are used in a variety of drugs, agro-chemicals and fragrances [6], [7]. Alcohols have been conventionally oxidized by noncatalytic methods using stiochiometric amounts of hazardous oxidizing agents such as chromium and manganese compounds in the presence of mineral acids, which generate large amounts of waste materials. In addition to homogeneous catalysts, heterogeneous protocols have also been developed on various supports [8], [9], [10]. Apart from these catalysts, iron, vanadium and ruthenium catalysts have also been successfully used in oxidation reactions [11], [12], [13]. The oxidation of benzoins to benzyl has already been reported by variety of homogeneous and heterogeneous catalysts [14], [15], [16], [17], [18], [19], [20], [21]. In catalysis, it is highly desirable to find cheap replacements for the expensive metal and waste generated catalysts. Recently, Beller et al. have reported Fe2O3 NP catalyzed oxidation of alcohols in the presence of hydrogen peroxide successfully [22]. Similarly, Oliver et al. have reported the selective oxidation of primary alcohols in the presence of hydrogen peroxide [23].
Herewith, we report MNPs as a catalyst for the oxidation of benzoin to benzyl in high yields in the presence of air (up to 90%) as described in Scheme 1. This simple protocol overcomes the problems of homogeneous conditions and also in conventional heterogeneous conditions. The easy removal of the catalyst makes the procedure very simple and eco-friendly.
Section snippets
Catalyst preparation
The MNPs were prepared by the so called wet-impregnation method as reported in the literature [24]. The ultra fine MNPs are prepared by co-precipitating aqueous solutions of ferrous ammonium sulfate (NH4)2Fe(SO4)2 and ferric chloride (FeCl3) mixture, respectively, in alkaline medium. The Fe (II) and Fe (III) salts were mixed in their respective stoichiometry (i.e., ratio Fe2 +:Fe3 + = 1:2). The resultant mixture was kept at 80 °C in 80.0 mL of water. To this boiling solution, NH4OH is added drop by
Results and discussion
The catalytic properties of the freshly prepared MNPs were initially evaluated in the oxidation of benzoin at 80 °C. We found that MNPs performed as a versatile nano catalyst in the oxidation reaction of benzoin, furnishing the corresponding product in 90% yield in 15 h. There was no reaction in the absence of MNPs. Very interestingly MNPs not only act as a heterogeneous catalyst, but also as an oxidant in the benzoin oxidation reaction.
We continued our study in the oxidation of benzoin using
Acknowledgment
KVSR thanks to DST-SERB for financial support and also dept. of chemistry.
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