Efficient aerobic oxidation of alcohols using magnetically recoverable catalysts

Article abstract of DOI:10.1016/j.catcom.2014.05.023

A simple, highly efficient and mild catalytic aerobic oxidation of alcohol, in particularly benzoin was studied using iron oxide nanoparticles as a reusable catalyst. Moreover, benzyl was synthesized in large scale using inexpensive, readily available and

Full text of DOI:10.1016/j.catcom.2014.05.023

Efficient Aerobic Oxidation of Alcohols using Magnetically Recoverable
Catalysts
Melad Shaikh, Mansingh Satanami, Kalluri V.S. Ranganath
PII:
DOI:
Reference:
S1566-7367(14)00204-0
doi: 10.1016/j.catcom.2014.05.023
CATCOM 3917
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
28 April 2014
21 May 2014
23 May 2014
Please cite this article as: Melad Shaikh, Mansingh Satanami, Kalluri V.S. Ranganath,
Efficient Aerobic Oxidation of Alcohols using Magnetically Recoverable Catalysts, Catal-
ysis Communications (2014), doi: 10.1016/j.catcom.2014.05.023
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ACCEPTED MANUSCRIPT
Efficient Aerobic Oxidation of Alcohols using Magnetically
Recoverable Catalysts
Melad Shaikh, Mansingh Satanami and Kalluri V.S. Ranganath*
Guru Ghasidas University, Bilaspur, India-495009
E-mail: kallurivsr@gmail.com
Tel: +91-7752-260488; fax: +91-7752-260488
Abstract: A simple, highly efficient and mild catalytic aerobic oxidation of alcohol, in
particularly benzoin was studied using iron oxide nanoparticles as a reusable
catalyst. Moreover, benzyl was synthesized in large scale using inexpensive, readily
available and environmentally friendly protocol in the presence of air. This novel and
green methodology will allow further development not only in the geochemistry, but
also in the oxidation of alcohols to replace expensive metals and corrosive reagents.
Key Words: Benzoin., Heterogeneous., Magnetite., Nitro alcohols., Oxidation.

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1
. Introduction
Magnetite nanoparticles (MNPs) are emerging as promising materials for
applications in information recording, magnetic fluids, drug delivery, in-vivo magnetic
imaging, and catalysis [1-4]. 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 catalysis over
homogeneous 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 metal nanoparticles (NPs) on various supports
[
8-10]. Apart from these catalysts, iron, vanadium and ruthenium catalysts have also
been successfully used in oxidation reactions [11-13]. The oxidation of benzoins to
benzyl has also been reported by variety of homogeneous and heterogeneous
catalysts [14-21]. In catalysis, it is highly desirable to find cheap replacements for the
expensive metal and waste generated catalysts. Recently, Beller and coworkers
have reported Fe O NPs catalyzed oxidation of alcohols in the presence of
2
3
hydrogen peroxide successfully [22]. Similarly, Oliver and coworkers have also
reported the selective oxidation of primary alcohols in the presence of hydrogen
peroxide [23].

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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.
Scheme 1. Aerobic oxidation of benzoin using MNPs.
2
. Experiment
.1. Catalyst preparation
2
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 (NH ) Fe(SO ) and ferric chloride (FeCl )
4
2
4 2
3
mixture, respectively, in alkaline medium. The Fe (II) and Fe (III) salts were mixed in
2+
3+
their respective stoichiometry (i.e., ratio Fe : Fe = 1:2). The resultant mixture was
o
kept at 80 C in 80.0 mL of water. To this boiling solution, NH OH is added drop by
4
drop till the precipitation occurs under constant stirring. Magnetite formed by
conversion of metals into hydroxides, which takes place immediately, and
transformation of hydroxides into ferrites. The resultant solution was maintained at
8
0C for 2 hr. The brown color precipitate, thus formed is then washed several times
by distilled water. The precipitate is then separated by the help of external magnet
o
and is then kept for drying at 80 C.

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2
.2. Catalytic Reaction:
An oven dried flask was charged with Fe O (0.030 g), benzoin (0.127 g, 0.6 mmol).
3
4
To the flask toluene (3.0 mL) was added and heated to 80 C with magnetic stirring.
After completion of the reaction (monitored by TLC and UV-Visible spectroscopy),
the reaction mixture was cooled to rt, and catalyst was magnetically removed and
washed several times with toluene.
3
. 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
9
0 % yield in 15 h. There was no reaction in the absence of MNPs. Very interestingly
MNPs not only acts as a heterogeneous catalyst, but also as an oxidant in the
benzoin oxidation reaction.
We continued our study in the oxidation of benzoin using MNPs in the presence of
air using various solvents and temperatures. Quite interestingly, oxidation reaction of
benzoin to benzyl gave upto 24% yield at room temperature using MNPs in
acetonitrile. The reaction rate appeared to be dependent on the amount of MNP and
also on temperature (Fig. 1).

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Fig. 1. Effect of various parameters (temperature, time and amount of MNPs) on the
aerobic oxidation of benzoin.
Very pleasingly, in toluene at 80 C gave the best yields in the presence of MNPs in
the oxidation reaction of benzoin to benzyl selectively (Table 1). Since magnetite has
2+
3+
an inverse cubic spinel structure, it possesses both Fe and Fe ions and it is
3+
2+
2+
expected that Fe take hydrogen from benzoin and reduced to Fe ions. Fe ion is
3+
oxidised to Fe ions in the presence of air. The activity and selectivity of the oxide
catalysts depends upon the oxidation state of the metal ions and their co-ordination
in the lattice, the surface cation-oxygen bond strength, the extent of total oxygen
(both surface and total) and morphology of the material. Surprisingly, no reaction
was observed either in the presence of FeCl or Ammonium iron (II) Sulphate, which
3
is a precursor for the synthesis of MNPs. Interestingly, when the reaction was
carried out in toluene in the presence of MNPs and 30% H O , benzyl product was
2
2
formed in (up to 80% yield) in 4 h. However, catalyst lost its paramagnetic property
after the reaction. No product formation was observed when the catalyst was only
2+
3+
3+
treated either with 30% H O (convert all Fe to Fe ) or with NaBH (convert Fe to
2
2
4
2+
Fe ) for one hr before addition of benzoin to the flask [25]. These results clearly
2+
3+
indicate that both Fe and Fe are required for the oxidation.

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Recycling of the experiment was carried out in toluene and the catalyst could be
recycled for five times without loss of activity. Moreover, atomic absorption
spectroscopy (AAS) analysis of the filtrate indicates, no iron was leaching from the
MNPs. Moreover, the oxidation of benzoin was carried out in large scale (10 m.mol)
and observed that benzyl was formed in 87% yield using MNPs (0.1 g)
The catalyst was characterized by X-ray photo electron spectroscopy (XPS),
in order to understand the change of cations. The ratio of Fe (II) and Fe (III) remain
constant after the oxidation reaction (Table 2). The XPS of the freshly prepared MNP
catalyst exhibits the predominance of Fe (III) species. However, the heterogeneous
catalyst, obtained after partial conversion of benzoin to benzyl shows the increasing
of Fe (II) species (Figure 2). These results provide strong evidence that, upon
treatment with benzoin, Fe (III) in MNP is reduced to Fe (II), which is in turn, oxidized
to Fe (III) during the oxidation of benzoin

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Table 1
Aerobic oxidation of benzoin in the presence of various catalysts
a
Entry Catalyst
Time
h)
Solvent
Toluene
Oxidant Temp(C)
Yield (%)
(
1
-
36
air
80
No
reaction
2
3
4
5
6
7
8
9
1
FeCl3
36
36
36
15
6
Toluene
Toluene
Toluene
acetonitrile
acetonitrile
Toluene
Toluene
Toluene
Toluene
air
air
air
air
air
80
-
(NH ) Fe(SO )
4 2
80
-
4
2
FeCl + FAS
80
-
3
Fe O
R.T
60
24
71
18
-
3
4
4
4
Fe O
3
Fe O
24
24
15
6
H O
2
R.T
R.T
80
3
2
2
-
H O
2
Fe O
air
90
80
3
4
4
0
Fe O
H O
2
RT
3
2
All the reactions were carried out in toluene or acetonitrile (3.0 mL), MNP (30.00 mg)
and benzoin (1.0 m.mol)
To test the substrate scope of MNP catalyzed oxidation in the presence of air, variety
of compounds were examined. Notably, simple primary and secondary alcohols also
underwent using MNPs in the presence of air (Table 3, entries 2 & 3). These
reactions were realized using expensive catalysts like Pd, Ru and Rh under
homogeneous conditions [6]. Interestingly, nitro alcohols were converted into nitro

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ketones successfully in the presence of air using MNPs (Table 3, entry 4 & 5). Short
reaction times and isolation of the product are the advantages of present work
compared to earlier described methods [26-28]. Interestingly, there was no reaction
with amino alcohols in the presence of MNP at 80 C. (Table 3, entry 6)
a
b
c
Intensity /cps
B.E /e.V
Fig. 2. XPS spectrum of Fe 2p (2p3/2 and 2p peaks) a) Freshly prepared MPs b) Used
½
Catalyst c) Partial conversion of benzoin to benzyl.
Table 2
XPS data result of Fe O before and after the reaction (after five cycles)
3
4
3
+
2+
S.No
Catalyst
Fe O
Fe (%) Fe (%)
1
2
3
55.5
57.2
49.8
44.5
42.8
50.2
3
4
Used Fe O
3
4
Fe O (partial
3
4
conversion)
Table 3
Oxidation of various alcoholic groups in the presence of Fe O4
a
3
Entry
1
Substrate
Time (h)
15
Yield (%)
90
2
12
80

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3
18
76
4
5
18
20
72
82
6
15
No
Reaction
a
All the reactions were carried out in toluene (3.0 mL), metal oxide (30.00 mg) at 80

C
In conclusion, we have reported the oxidation of benzoin to benzyl selectively using
MNPs as a reusable catalyst in the presence of air. We also proved that both Fe (II)
and Fe (III) are necessary to complete the transformation. In addition, simple
magnetic removal and recycling of the catalyst was shown to proceed without loss of
activity.
Acknowledgement:
KVSR thanks to DST for financial support.
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Highlights




A simple, highly efficient and green protocol for the oxidation of benzoin.
Fe(II) and Fe(III) are necessary for oxidation.
Magnetite not only acts as a heterogeneous catalyst, but also as an oxidant.
Nitro alcohols were converted into nitro ketones successfully in the presence
of air.