Vivek Polshettiwar and Rajender S. Varma*
Sustainable Technology Division, National Risk Management Research Laboratory, U. S. Environmental Protection Agency, MS 443, Cincinnati, Ohio 45268, USA. E-mail: polshettiwar.vivek@epa.gov; varma.rajender@epa.gov
First published on 13th November 2008
A magnetic nanoparticle-supported Pd catalyst was readily prepared from inexpensive starting materials and shown to catalyze various oxidation reactions with high turnover number (TON) and excellent selectivity. The ease of recovery using an external magnetic field, high activity, and the intrinsic stability of the catalyst make this protocol economic and sustainable.
Recently, functionalized magnetic nanoparticles have emerged as viable alternatives to conventional materials, as robust, readily available, high-surface-area heterogeneous catalyst supports.13 They offer an added advantage of being magnetically separable, thereby eliminating the requirement of catalyst filtration after completion of the reaction. Engaged in the development of greener and sustainable pathways for organic transformations,14 nanomaterials,15 and nano-catalysis,16 herein, we report a simple and efficient synthesis of a nano-ferrite-supported, magnetically recyclable, and inexpensive Pd catalyst and its application in oxidation reactions. At the outset of this study, no example of magnetic nanoparticle-supported Pd catalysts had been reported for oxidation reactions, despite the potential inherent stability and activity of such materials.
The first step in the accomplishment of this goal was the synthesis and functionalization of magnetic nanoparticles (Scheme 1). The catalyst was prepared by sonicating nano-ferrites with dopamine (which acts as a robust anchor and avoids Pd-leaching) in water for 2 h, followed by addition of sodium palladium (Pd) chloride at a basic pH. Material with Pd nanoparticles on the amine-functionalized nano-ferrites was obtained in excellent yield.
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Scheme 1 Synthesis of functionalized nano-ferrite-Pd. |
Catalyst characterization by X-ray diffraction (XRD) (Fig. 1b and d) and transmission electron microscopy (TEM) (Fig. 1a and c) confirm formation of single-phase Fe3O4 nanoparticles, with spherical morphology and a size range of 10–16 nm, which is comparable with the crystallite size calculated from the X-ray spectrum, by using the Scherer formula (10.23 nm). FT-IR evidently confirms the anchoring of dopamine on ferrite surfaces (S4, ESI†). The signals of palladium metal were not detected in XRD, showing that the Pd species is highly dispersed on ferrites. The weight percentage of palladium was found to be 10.23% by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) analysis.
This Pd-coated nanomaterial was then explored as a heterogeneous catalyst for the oxidation of alcohols (Scheme 2).
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Scheme 2 Nano-ferrite-Pd catalyzed alcohol oxidation. |
Initially, experiments were performed to optimize reaction conditions for oxidation of benzyl alcohol as a substrate, using one equivalent of hydrogen peroxide, to get a high TON of product (Table 1). First the reaction was conducted using nano-α-Fe2O3 with particle sizes from 2–3 μm (see ESI S7 for TEM images and XRD data†) and conversion with low TON (160) was observed. Then, we tested nano-Fe3O4 with particle sizes from 10–15 nm (Fig. 1a and b) as catalyst, and the TON reached 240 with high selectivity. However, we wanted to develop a high TON protocol and thus prepared Pd-coated nano-Fe3O4 (as discussed in Scheme 1) with particle sizes from 10–16 nm (Fig. 1c) and tested for this oxidation protocol. We observed conversion with very high TON (720), with >99% selectivity.
Entry | Catalyst | Reaction time/h | TONb | Selectivity (%)c |
---|---|---|---|---|
a Reactions were carried out with 10 mmol of benzyl alcohol, 0.025 mole% of catalyst, and 1 equiv of H2O2, at 75–80 °C.b Number of moles of aldehyde formed per mole of catalyst.c Selectivity based on alcohol conversion. | ||||
1 | nano-α-Fe2O3 | 6 | 160 | >99 |
2 | nano-α-Fe2O3 | 12 | 200 | >99 |
3 | nano-Fe3O4 | 6 | 240 | >99 |
4 | nano-Fe3O4 | 12 | 280 | >99 |
5 | nano-Fe3O4-Pd | 6 | 720 | >99 |
6 | nano-Fe3O4-Pd | 12 | 760 | >99 |
Using the above optimized condition, this catalyst was then explored for oxidation of a variety of alcohols (Table 2). Excellent turnover numbers were observed for various aromatic alcohols (entries 1 to 7) as well as aliphatic alcohols (entries 8, 9) within 6 h. However, in the case of hexanol, conversion with low TON is due to volatility of the ensuing aldehyde (entry 9). Heterocyclic alcohols (entries 10, 11), extensively used building-blocks in drug discovery, also underwent oxidation reaction with high TON, proving the suitability of this protocol for assembly of biomolecules. In all the reactions, we observed more than 95% selectivity because of the very low catalyst amount (0.025 mole%). This will be very useful in total synthesis of drug molecules, where it is required that an alcoholic group be selectively oxidized to its aldehyde without any over-oxidation. The catalytic activity of our system is superior to that of Beller's,8i.e., TON of benzyl alcohol oxidation is 720 in 6 h, as compared to TON of 32 in 12 h, with comparable selectivity.
The oxidation of olefinic compounds to the corresponding aldehydes (Scheme 3) is a synthetically important transformation and notably, this catalytic system also oxidized aromatic olefins with high TON and the results are summarized in Table 3.
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Scheme 3 Nano-ferrite-Pd catalyzed olefin oxidation. |
The scope of the catalyst was established for a range of aromatic olefins and the reactions were completed within 6 h. In all the reactions, TON ranging from 600 to 680 were observed, which is superior as compared to the TON of known protocols, which range from 19 to 36.8
Separation and recovery of the catalyst is also an important aspect for the synthesis of fine chemicals; which is generally performed by filtration with reduced efficiency. In our catalytic system, because of the super-paramagnetic nature of the nano-Fe3O4-Pd catalyst, it can be recovered by simply using external magnets, with high efficiency and up to 99% recovery of catalyst (Fig. 2). After completion of the reaction, within 30–45 sec, the reaction mixture turned clear and catalyst was deposited on the magnetic bar (Fig. 2b), which was easily removed from reaction mixture using an external magnet (Fig. 2c).
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Fig. 2 Oxidation of alcohols using nano-ferrite-Pd catalyst, (a) during reaction under stirring, (b) after completion of reaction without stirring, (c) catalyst removal by external magnet. |
For practical applications of heterogeneous systems, the lifetime of the catalyst and its level of reusability are very important factors. To clarify this issue, we established a set of experiments for the oxidation of benzyl alcohol using the recycled nano-Fe3O4-Pd catalyst. After the completion of the first reaction to afford the corresponding aldehyde, the catalyst was recovered magnetically, washed with methanol, and finally dried at 60 °C for 10 min. A new reaction was then performed with fresh reactants and H2O2, under the same conditions. The nano-ferrite-supported Pd catalyst could be used at least 5 times without any change in activity.
Metal leaching was studied by ICP-AES analysis of the catalyst before and after the five reaction cycles. The Pd concentration was found to be 7.91% before reaction and 7.86% after the reaction, and also no Pd metal was detected in the final oxidation product (see ESI S5 for details†), which confirmed negligible Pd leaching. The negligible Pd leaching is due to a well defined amine-binding site located on the surface of the nano-ferrite (Scheme 1), which acts as a pseudo-ligand by non-covalent binding with Pd through a metal–ligand interaction. This non-covalent anchoring minimizes deterioration and metal leaching and allows efficient catalyst recycling. Also, the most important criteria in choosing a catalyst is metal recovery. It would be preferable to use a more accessible, economic Pd catalyst, provided that the process works at high TON and that the catalyst leaves no remnants of metal in the end product, since metal contamination is highly regulated by the pharmaceutical industry. All above conditions were well satisfied by our recyclable nano-ferrite-supported Pd catalyst.
In summary, we have developed a convenient synthesis of a nano-ferrite-supported Pd catalyst which was readily prepared from inexpensive starting materials in water. This catalyst then catalyzed the oxidation of alcohols and olefins with high TON and excellent selectivity. Also, being magnetically separable eliminated the requirement of catalyst filtration after completion of the reaction, which is an additional sustainable attribute of this oxidation protocol.
Footnote |
† Electronic supplementary information (ESI) available: Experimental procedures; recyclability and Pd-leaching of catalyst, TEM, XRD of nano-ferrites; NMR and MS data of compounds. See DOI: 10.1039/b817669h |
This journal is © The Royal Society of Chemistry 2009 |