Efficient microwave oxidation of alcohols using low-loaded supported metallic iron nanoparticles.

Metallic nanoparticles (MNPs) have attracted a great deal of attention owing to their potential in a wide range of current and future applications in fields as diverse as medicine, sensors, optics and catalysis. We have recently reported various protocols for the preparation of supported metallic nanoparticles on a range of supports such as metal oxides, biopolymers and mesoporous materials. 8] Different approaches were investigated including impregnation, co-precipitation, sonication and microwave-assisted synthesis to achieve an understanding of the formation and the control of the sizes and shapes of the nanoparticles. In particular, the microwave-assisted synthesis was found to overcome inherent drawbacks of the unsupported nanoparticles with regards to stability, agglomeration and reusability as well as the use of reductants, allowing an efficient and greener method to obtain highly active and well-dispersed supported MNPs (Au, Ag, Pd and Ru). Compared to these extensively investigated transition and noble metals, iron-based catalysts are inexpensive, environmentally benign and relatively non-toxic. Iron compounds are also abundant and biocompatible, playing a vital role in many biological systems, and are becoming increasingly important in environmentally friendly catalytic protocols. With regards to catalysis, supported MNPs display a large surface to volume ratio as compared to their bulk metal analogues which makes them attractive candidates as catalysts for a wide range of reactions including hydroisomerisations, hydrogenations, C C coupling and oxidations. The oxidation of alcohols to carbonyl compounds is one of the key transformations in synthetic organic chemistry. Alternative systems have been developed based on greener oxidants, including hydrogen peroxide and molecular oxygen, as well as heterogeneous catalysts such as Fe2O3 nanoparticles, [13] Ag-NPs supported on hydrotalcites, Au-NPs supported on metal oxides and Pd-NPs supported on SBA-15. Following our microwave-assisted approach, we report the preparation of highly active and stable supported Fe-NPs on a range of supports and their catalytic activities in the oxidation of alcohols under microwave irradiation. To our knowledge, this is the first report of directly prepared and catalytically active low-loaded supported Fe-NPs. Si-MCM-41 was synthesized according to our previously reported methodology. Commercial microporous carbon (DARCO) was used as purchased. Starch and cellulose were employed as nonporous supports for comparison. The supported Fe-NPs were prepared under microwave irradiation as follows: The support (0.2 g) was suspended in EtOH (2 mL) containing 100 mg of the iron precursor (iron(II) chloride tetrahydrate, FeCl2·4H2O), and the mixture was heated in a microwave at 200 W for 15 min (average temperature 100 8C, maximum temperature 120 8C). The solids were then filtered off, washed with excess ethanol and acetone and dried overnight at 100 8C to afford red-orange fine powders. The properties of the supported MNPs are summarised in Table 1. The Fe loading on the materials was found to be very low, typically between 0.3 and 0.35 wt% (Table 1), although no significant differences in loading were found between the porous and the nonporous materials.