Application of mobilized gold nanoparticles as sole catalyst for the oxidation of secondary alcohols into ketones

Article abstract of DOI:10.1016/j.apcata.2010.07.050

Oxidation of alcohols into corresponding ketones is efficiently catalyzed by heterogeneous gold nanoparticles. Use of H₂O₂ instead of O₂ (g) is the main feature of our methodology since maintaining the required pressure of O₂ (g) at elevated temperature is not easy. The reaction does not require any co-catalyst. After completion of the reaction, the catalyst could be recovered by centrifugation and reused several times with only a slight decrease in activity. Thus our present protocol, which is comprised of green reagent, solvent and catalyst, is completely clean.

Full text of DOI:10.1016/j.apcata.2010.07.050

Applied Catalysis A: General 387 (2010) 1–4
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Application of mobilized gold nanoparticles as sole catalyst for the oxidation of
secondary alcohols into ketones
Mazaahir Kidwai∗, Saurav Bhardwaj
Green Chemistry Research Laboratory, Department of Chemistry, University of Delhi, Delhi 110007, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 June 2010
Received in revised form 17 July 2010
Accepted 26 July 2010
Available online 3 August 2010
Oxidation of alcohols into corresponding ketones is efficiently catalyzed by heterogeneous gold nanopar-
ticles. Use of H2O2 instead of O2 (g) is the main feature of our methodology since maintaining the required
pressure of O2 (g) at elevated temperature is not easy. The reaction does not require any co-catalyst. After
completion of the reaction, the catalyst could be recovered by centrifugation and reused several times
with only a slight decrease in activity. Thus our present protocol, which is comprised of green reagent,
solvent and catalyst, is completely clean.
Keywords:
© 2010 Elsevier B.V. All rights reserved.
Gold nanoparticles
Heterogeneous catalyst
Oxidation
Green chemistry
1. Introduction
ous examples using toxic metals like vanadium, ruthenium and
sophisticated palladium under rigorous reaction conditions have
Catalysis using gold nanoparticles is a topic of current inter-
est [1–4]. The catalytic activity of gold is directly related to the
particle size on the nanometer length scale. Gold catalysis is a
paradigmatic example of those properties that are only observed
in nanoparticles and can disappear completely as the particle size
is grown into micrometric scale [5]. Considering the current status
of nanoscience, it is understandable that all the aspects related to
the preparation of small nanoparticles of narrow size distribution,
their stabilization and their unique properties compared to larger
particle size are appealing to a large community of researchers
from the fields of material science, computational chemistry and
catalysis. Hence, to understand the reasons of this catalytic activ-
ity, expanding the scope to other reactions, optimizing and finding
the reaction mechanisms for gold catalyzed reactions is a new field
in heterogeneous catalysis [6–10].
The oxidation of sec-alcohols into ketones is one of the most
important functional group transformation reactions in organic
synthesis, because these ketones act as synthons in various drugs,
agro-chemicals, and fragrances [11,12].
Much attention has been paid in the past decades to developing
efficient methods for this transformation. Stoichometric amounts
of hazardous oxidants, such as chromium (VI) and manganese
reagents were used for the oxidation reactions [13–15]. Numer-
been reported [16–18].
Sustainability is our ultimate goal and green chemistry is a way
to achieve it. In continuation to our progressive program towards
the development of green and sustainable synthetic methodologies
[19,20] for organic synthesis and the role of transition metal cata-
lysts in such syntheses [21,22], herein we are reporting mobilized
gold nanoparticles as the sole catalyst for the synthesis of ketones.
2. Experimental
2.1. General
The materials procured from Sigma–Aldrich and Merck were
used without further purification. IR spectra were obtained on
Perkin Elmer FT-IR spectrometer spectrum-2000 using potassium
bromide pellets or as liquid films between two sodium chloride
pellets. Mass spectra were recorded in a TOF-mass spectrometer
model no. KC455. 1H NMR and 13C NMR spectra were recorded
on a Bruker Spectrospin instrument at 400 and 100 MHz, respec-
tively. All NMR samples were run in CDCl3 and chemical shifts are
expressed as ppm relative to internal Me4Si.
2.2. Experimental
Twenty mol% of gold nanoparticles were added through sep-
tum to a mixture of diphenylcarbinol (1 mmol) and H2O2 (1 mmol)
in ethanol (3 ml) under close atmosphere. This reaction mixture
∗
Corresponding author. Tel.: +91 11 27666235; fax: +91 11 27666235.
E-mail address: kidwai.chemistry@gmail.com (M. Kidwai).
0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.07.050

2
M. Kidwai, S. Bhardwaj / Applied Catalysis A: General 387 (2010) 1–4
Fig. 1. (a) and (b) TEM image of Au-nanoparticles. The scale bar corresponds to 10 nm in the TEM image.
was continuously stirred at 75 ◦C in an oil bath for 8 h. The progress
of the reaction was continuously monitored by TLC. After comple-
tion of the reaction, an excess of water was added in the reaction
mixture. In the case of solid products, the reaction mixture was
filtered to get the solid product. This solid product was dissolved
in ethyl acetate followed by centrifugation to recover the Au-
nanoparticles. But in case of liquid product, the reaction mixture
was extracted with diethyl ether and dried over sodium sulfate. The
etheral solution (organic layer) was concentrated under reduced
pressure. Subsequently the crude products were purified by column
chromatography using hexane and chloroform (4:1) as an eluent.
Benzophenone (Table 4, entry 1) (White solid) IR ꢀmax (KBr)
1660 cm−1 (CO). 1H NMR (400 MHz, TMS, CDCl3): ı 7.48–7.50 (m,
4H), 7.62–7.75 (2H, m), 7.81. (4H, m). 13C NMR (100 MHz, TMS,
CDCl3): ı 129.92, 132.30, 137.58, 197.22. m/z (HR-MS): 182.07 (M+).
Fluorene-9-one(Table 4, entry 4) (White solid) IR ꢀmax (KBr)
1721 cm−1 (CO).1H NMR (400 MHz, TMS, CDCl3): ı 7.17–7.21 (2H,
m), 7.38–7.41 (m, 4H), 7.56 (2H, J = 8 Hz, d). 13C NMR (100 MHz, TMS,
CDCl3): ı 120.26, 124.27, 129.03, 134.11, 134.63, 144.39, 193.85.
m/z (HR-MS): 179.90 (M+).
Fig. 2. UV spectra of gold nanoparticles.
1-Naphthalene-2-yl-ethanone (Table 4, entry 6) (Brownish
solid) IR ꢀmax (KBr)1678 cm−1 (CO). 1H NMR (400 MHz, TMS,
CDCl3): ı 2.63 (3H, s), 7.43–7.53 (2H, 2m), 7.77–7.81 (2H, m),
7.86 (1H, J = 8 Hz, d), 7.92–7.96 (1H, Jab = 8 Hz, Jac = 4 Hz, dd), 8.37
(1H, s). 13C NMR (100 MHz, TMS, CDCl3): ı 26.66, 123.83, 126.73,
127.73, 128.37, 130.16, 132.45, 134.41, 135.53, 198.08. m/z (HR-
MS): 168.96 (M+).
conditions only 20% yield of product was obtained. The same reac-
tion was performed in the presence of 20 mol% of Au-nanoparticles.
It was observed that the yield of the desirable product increases up
to 90% (Scheme 1).
To compare the catalytic efficiency of Au-nanoparticles with
some other metal nanoparticles, we performed some model reac-
tions as above using different nanoparticles. As seen from Table 1,
Au-nanoparticles showed the best result among all the nanoparti-
cles.
During optimization of the reaction conditions, different oxi-
dizing agents were used to oxidize diphenyl carbinol under similar
reaction conditions. Hydrogen peroxide is found to be an excellent
oxidizing agent. This is due to the availability of higher oxygen in
case of hydrogen peroxide than other oxidants [25] (Table 2).
2.3. Synthesis of Au-nanoparticles
The HAuCl4 (0.1 g) was dissolved in 500 ml distilled water
in a beaker with constant stirring on a hot plate. To this boil-
ing solution 2 ml of 1% solution of trisodium citrate dehydrate
(Na3C6H5O7·2H2O) was added. The gold sol gradually forms as the
citrate reduces the gold (III). The solution was removed from heat-
ing as it turned red. Hence, the particles that formed were stabilized
by citrate ions [23,24]. The resulted gold nanoparticles were char-
acterized by using TEM (Fig. 1) and UV (Fig. 2) spectroscopy.
3. Results and discussion
In the beginning, efforts were made towards the catalytic
evaluation of Au-nanoparticles. In an initial endeavor, a blank reac-
tion was carried out using diphenylcarbinol (1 mol) and aqueous
solution of H2O2 (1 mol) in ethanol. This reaction mixture was con-
tinuously stirred at 75 ◦C in an oil bath for 8 h. Under these reaction
Scheme 1. Model reaction for oxidation using Au-nanoparticles.

M. Kidwai, S. Bhardwaj / Applied Catalysis A: General 387 (2010) 1–4
3
Table 1
Oxidation of alcohols using different metal nanoparticles.
Table 4
Au-nanoparticles catalyzed oxidations of alcohols into ketones.
Entry
Nanoparticles
Time (h)
Yield (%)b
Entry Alcohols
Products
Time (h) Yield (%)a
1
2
3
4
Cu
Zn
Ni
8
8
8
8
38
40
50
90
1
2
Diphenylcarbinol
1,2,3,4-Tetrahydro-
1-naphthol
Benzophenone
3,4-Dihydro-2H-
naphthalen-1-one
2-
8
8
90
88
Au
3
2-Methylbenzhydrol
10
87
Methylbenzophenone
Fluoren-9-one
4,4ꢀ-
Reaction conditions: diphenylcarbinol (1 mmol), hydrogen peroxide (1 mmol) and
4
5
9-Hydroxyfluorene
4,4ꢀ-
10
8
92
92
20 mol% nanoparticles (20 2) nm; ethanol; temperature 75 ◦C.
b
Isolated yields.
Dimethoxybenzhydrol dimethoxybenzophenone
6
␣-Methyl-2-
naphthalene-
methanol
1-Naphthalen-2-yl-
ethanone
6
90
Table 2
Oxygen availability in various oxidants.
7
8
4-Chlorobenzhydrol
4-
11
12
88
85
Oxidants
% of O2
By product
Chlorobenzophenone
Acetophenone
2-Phenylethanaol
H2O2
47
18
22
22
8
H2O
t-BuOOH
CH3COOOH
NaOCl
t-BuOH
CH3COOH
NaCl
Reaction conditions: diphenylcarbinol (1 mmol), hydrogen peroxide (1 mmol) and
20 mol% Au-np (20 2) nm; ethanol; temperature 75 ◦C.
a
Isolated yields.
KIO4
KIO3
Table 3
Optimization of concentration of H2O2.
Entry
Conc. of H2O2 (mmol)
Yield (%)a
1.
2
3
0.5
1
1.5
2.0
50
92
85
78
4
Reaction conditions: diphenylcarbinol (1 mmol), hydrogen peroxide (x mmol) and
20 mol% Au-np (20 2) nm; ethanol; temperature 75 ◦C.
a
Isolated yields.
It was important to optimize the concentration of H2O2 in order
to get the maximum yield of product. For this, the same amount of
alcohol was treated with different concentrations of hydrogen per-
oxide as depicted in Table 3. We found that an equivalent amount of
hydrogen peroxide gives maximum yield of product without giving
other side products. Notably the reaction could not be performed
without aqueous H2O2 as an oxidant.
Scheme 2. Purposed mechanism for oxidation of alcohol using gold nanoparticles
as catalyst.
Besides this, we observed that the concentration of catalyst
played a crucial role in catalyzing the oxidation reaction. Using
the model reaction, and varying just the concentration of Au-
nanoparticles, we observed that 20 mol% of the Au-nanoparticles
give optimum reaction rate and yield (Fig. 3).
In addition to this, model reactions were carried out in different
solvents. Among the solvents investigated, ethanol was found to
be the best solvent. Hence all the reactions were carried out tak-
ing ethanol as a solvent. After all the standardization, we chose a
variety of structurally different alcohols possessing a wide range
of functional groups to understand the scope and efficiency of the
Au-nanoparticles promoted oxidation reaction. We also observed
that other functional groups such as –CH3, –Cl, –OCH3 and phenolic
OH do not undergoes any reaction and remain as they are (Table 4).
Here the main feature of H2O2 is to facilitate the abstraction of
the ␣-hydrogen as a hydrogen scavenger, resulting in the formation
of a metal alcoholate species during the initial step of the reac-
tion. This Au-alcoholate complex may be further attacked by the
Au-hydridoperoxide complex generated from the reaction of Au0
with H2O2, to form the final carbonyl compound via a ␤-elimination
pathway as shown in Scheme 2 [26].
Fig. 3. Optimization of concentration of Au-nanoparticles.
Fig. 4. Recycling of Au-nanoparticles.

4
M. Kidwai, S. Bhardwaj / Applied Catalysis A: General 387 (2010) 1–4
It is important to stress that the catalyst was recycled up to four
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We observed that a gold nanoparticle is a highly active and recy-
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an oxidizing agent. Thus Au-np having higher specific surface area
shows higher catalytic activity than other catalysts. Our method
avoids the use of hazardous chemicals, or toxic solvents, and pro-
duced water as the only side product. Thus present methodology
will make an impact in the field of sustainable green chemistry.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.apcata.2010.07.050.
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