A green and efficient oxidation of alcohols by supported gold catalysts using aqueous H2O2 under organic solvent-free conditions

Article abstract of DOI:10.1039/b820197h

The use of supported gold nanoparticles as an efficient, green and reusable catalyst for the oxidation of various alcohols to the corresponding carbonyl compounds using aqueous hydrogen peroxide as an environmentally benign oxidant is presented. The reaction proceeds with good to excellent yields in particular for nonactivated alcohols under base-free conditions.

Full text of DOI:10.1039/b820197h

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/greenchem | Green Chemistry
A green and efficient oxidation of alcohols by supported gold catalysts using
aqueous H O under organic solvent-free conditions†
2
2
Ji Ni, Wen-Jian Yu, Lin He, Hao Sun, Yong Cao,* He-Yong He and Kang-Nian Fan
Received 12th November 2008, Accepted 6th March 2009
First published as an Advance Article on the web 13th March 2009
DOI: 10.1039/b820197h
The use of supported gold nanoparticles as an efficient,
green and reusable catalyst for the oxidation of various
alcohols to the corresponding carbonyl compounds using
aqueous hydrogen peroxide as an environmentally benign
oxidant is presented. The reaction proceeds with good
to excellent yields in particular for nonactivated alcohols
under base-free conditions.
we report the development of a new efficient H
exhibiting significantly enhanced activity for the oxidation
of alcohols under base-free aqueous conditions. Hydrogen
2
O –Au system
2
peroxide (H
2
2
O ), previously established to be an ideal oxidant for
the green oxidation of alcohols with homogeneous metal peroxo
14
complexes, has shown to be particularly effective for gold
catalyzed selective oxidation of nonactivated alcohols. To the
best of our knowledge, this study also forms the first hectogram-
scale clean synthesis of aliphatic acids using gold catalysts.
First of all, 1-phenylethanol was used as a model substrate
to study the catalytic activity of the different supported gold
catalysts. The results of these studies (Table 1) show that gold
supported on carbon leads to very low conversion (ca. 12%).
Oxidation of alcohols to their corresponding carbonyl com-
pounds is one of the most important processes for production
1
of fine and specialty chemicals. Traditional methods involve
use of toxic and expensive stoichiometric metal oxidants, such
2
3
as chromate and permanganate, or harmful organic solvents,
4
or require vigorous reaction conditions. From both the en-
vironmental and economic points of view, there is a strong
incentive to develop a green, economic and efficient alcohol
The Au/Fe
ca. 34–37%), while Au/TiO
Council and Au/CeO give a higher conversion value (ca. 58–
2%), gold supported on TiO supplied by Mintek afford the
2
O
3
and Au/Al
2
O
3
catalysts show moderate activity
(
2
supplied by the World Gold
2
5
oxidation process. In this context, much recent attention has
6
2
been directed toward the aerobic oxidation of alcohols with
reusable heterogeneous solid catalysts in aqueous media under
highest conversion (ca. 99%), with yields of isolated product of
between 98 and 100%.‡ No conversion was found in the absence
6
mild conditions, which is particularly suitable for industrial
of catalyst or the use of Au-free TiO catalysts under similar
2
practices. However, very few catalysts are sufficiently active for
reaction conditions. Additional examination of the relationship
between the alcohol oxidation activities and a series of Mintek
7
nonactivated alcohols, like aliphatic and alicyclic alcohols. In
particular, bases such as KOH or K
2
CO are frequently needed
3
Au/TiO
2
catalysts calcined at elevated temperatures (Fig. S1–
for oxidation of primary aliphatic alcohols (e.g. 1-octanol)
4
in ESI)† revealed that use of smaller Au nanoparticles gave
8
in water, which is clearly not green and presents notorious
higher activity, as shown in Fig. 1. This result, together with
problems such as corrosion and waste base treatment. In this
respect, the development of a novel catalytic system that exhibits
a wide range of substrate tolerance under mild and base-free
aqueous conditions still remains a major challenge.
the fact that the small Au NPs can substantially facilitate
15
the crucial H
2
O
2
decomposition, strongly suggests that the
high dispersion of Au NPs in combination with a beneficial
synergetic interaction with the TiO
2
support is the key factor
Supported gold catalysts have attracted tremendous recent
attention owing to their unique catalytic properties for a
a
2 2
Table 1 Oxidation of 1-phenylethanol by aqueous H O
9
broad array of organic transformations, especially for aerobic
6
c,d,9a,10–12
oxidation of alcohols under mild conditions.
Over the
11
6c
12
last few years, our group, Kobayashi et al. and Corma et al.
reported that Au nanoclusters deposited on metal oxides or
polymers are highly effective for aerobic oxidation of various
alcohols. In general, organic solvents and/or bases are necessary
to achieve high yield and selectivity. The oxidation of alcohols
under solvent- and base-free conditions was reported,
Entry
Catalyst
Conversion (%)
11
b
12,13
1
0.8% Au-C (WGC)
12
34
62
37
99
58
the
2
3
4
5
6
4.5% Au-Fe
1.5% Au-TiO
0.9% Au-Al
1% Au-TiO
1% Au-CeO
2
O
3
(WGC)
(WGC)
(Mintek)
(Mintek)
yield of target product was, however, much lower than that
obtained with organic solvent and/or base conditions. Herein,
2
2
O
3
2
2
Department of Chemistry & Shanghai Key Laboratory of Molecular
Catalysis and Innovative Material, Fudan University, Shanghai, 200433,
P. R. China. E-mail: yongcao@fudan.edu.cn; Fax: (+86-21) 65642978;
Tel: (+86-21) 55665287
a
◦
Reaction conditions: 10 mmol 1-phenylethanol, 10 mL H
2
b
O, 90 C, 5%
H
2
O
2
added dropwise, substrate:H
2
O
2
:Au = 100:150:1. This sample
catalyst was used as-received. According to the characterizations of the
Au/C sample provided by the World Gold Council, the water content
of the Au/C reference catalyst was equal to 40%.
†
Electronic supplementary information (ESI) available: Experimental
section, TEM and XPS data. See DOI: 10.1039/b820197h
7
56 | Green Chem., 2009, 11, 756–759
This journal is © The Royal Society of Chemistry 2009

View Article Online
a
2 2
Table 2 Oxidation of non-activated alcohols by aqueous H O
Conv.
(%) Sel. (%)
Entry Substrate
1
Product
87
100
2
3
98
>99
>99
100
100
100
b
4
b
5
>99 >99
2 2
Fig. 1 1-Phenylethanol conversion and H O decomposition versus
gold particle dimension; oxidation of 1-phenylethanol: 10 mmol 1-
◦
phenylethanol, Au/TiO
added dropwise, substrate:H
.5 wt% H aqueous solution 10 mL, Au/TiO
for 80 min in open air. The rate of decomposition was calculated based
on the amount of H remaining, which was measured by the titration
of 0.1 M Ce(SO with 1.5 w/v% ferroin solution as an indicator.
2
(Mintek), 10 mL H
:Au = 100:110:1; H
(Mintek) 80 mg, 25 C
2
O, 90 C, 5% H
2
O
2
b
c
6
>99
90
2
O
2
2 2
O decomposition:
◦
1
2
O
2
2
◦
2
O
2
a
Reaction conditions: 10 mmol Substrate, 10 mL H
added dropwise, substrate:H
00:250:1. Octyl octanate was found as by-product.
O, 90 C, 5% H
O
2
2
2
b
4
)
2
2
O
2
:Au = 100:150:1. Substrate:H
2
O
2
:Au =
c
1
for achieving high activity in the H
alcohols.
2
2
O -mediated oxidation of
The Mintek Au/TiO
2
catalyst was stable and can be easily
To examine the scope of the alcohol reaction with the
–Au/TiO system, we extended our studies to various
structurally different alcohols. The results are summarized in
Table 2. It was found that primary and secondary alcohols were
oxidized to the corresponding carboxylic acids and ketones,
reused in the oxidation of 1-phenylethanol. After the first alcohol
oxidation, the catalyst was separated from the reaction mixture
by filtration, thoroughly washed with water, and then reused
as catalyst for the next run under the same conditions. As
shown in Fig. 2, the acetophenone yields remained essentially
constant for the five successive cycles, reflecting the high stability
and reusability of the catalyst. This was consistent with the
characterization results for this catalyst. TEM and XPS results
H
2
O
2
2
respectively. Notably, H
2
O
2
–Au/TiO was capable of catalyzing
2
the oxidation of various nonactivated alcohols. For alicyclic
alcohols and secondary aliphatic alcohols, i.e. 2-octanol, they
gave ketones in excellent yields of 87–98% (Table 2, entries 1–
2). Moreover, it was found that the chemoselective oxidation of
allylic alcohols having a terminal double bond, such as 1-hexene-
3-ol, also proceeded efficiently (Table 2, entry 3). Contrastingly, a
(
Fig. S5 and S7 in ESI)† showed essentially no changes in the
mean diameters of the Au nanoparticles and in the metallic state
of Au, respectively, after the five successive runs. Moreover, it
was confirmed by induced coupled plasma (ICP) techniques that
previously reported H
2
O
2
–Pt black system is found to be totally
the Au content of the used Au/TiO
that of the fresh catalyst and that no Au was in the filtrate.
2
catalyst was the same as
inactive for this substrate, owing to a strong coordination of the
0
16
–C=CH
that H
group to the Pt complex. It is especially noteworthy
–Au/TiO was highly active for the oxidation of
2
2
O
2
2
primary aliphatic alcohols, the most inactive alcohols, in the
absence of any bases (Table 2, entries 4–6). For example, the
yield of 1-hexanoic acid in 1-hexanol oxidation was as high as
9
9% at a 100% conversion.
Furthermore, it was found that the H
2
2
O -mediated oxidations
also proceeded smoothly in a hectogram-scale synthesis to give a
high yield of formation of the corresponding carboxylic acids. As
shown in Scheme 1, by stirring a mixture of 1-hexanol (100 g) in
the presence of a 0.01 molar amount of Au catalyst and 3.0 molar
amount of aqueous H
2
2
O (5%) for 2.5 h, 1-hexanoic acid could
be obtained in a 93% yield (105.6 g). Notably, this is the first
example of a hectogram-scale clean oxidation of alcohols under
base-free conditions in neat water using a heterogeneous gold
catalysts.
Activated alcohols, such as benzylic alcohols, were also
Fig. 2 Recycling of the Mintek Au/TiO
of 1-phenylethanol: 10 mmol substrate, 10 mL H
added dropwise. Substrate:H
:Au = 100:150:1. For comparison, the
corresponding H decomposition rates are also included.
2
catalyst for the oxidation
◦
◦
oxidized at 90 C with H O in the presence of Au/TiO2
O, 90 C, 5% H
2
O
2
2
2
2
2
O
2
to give the corresponding carbonyl compounds in good to
excellent yields (Table 3). Like most of the catalysts reported
2
O
2
This journal is © The Royal Society of Chemistry 2009
Green Chem., 2009, 11, 756–759 | 757

View Article Online
a
Table 3 Oxidation of activated alcohols by aqueous H
2
O
2
conducted, it is known that the a-hydrogen abstraction of an
alcohol may be involved in the rate determining step based
Entry Substrate Product Conv. (%) Sel. (%)
17
on the isotope kinetic study (k
H
/k
D
= 1.7–2.0). In addition,
b
H O was found to be indispensable for the reaction progress as
1
2
3
>99
>99
>99
100
85
2
2
inferred by the data in Fig. 1, which prompted us to propose a
possible mechanism of the present H –Au-mediated alcohol
2
O
2
oxidation, as illustrated in Fig. S8 (ESI).† The key aspect
c
c
of H O is to facilitate the abstraction of the a-hydrogen as
2
2
a hydrogen scavenger, leading to the formation of a meta-
alcoholate species during the initial step of the reaction. The
Au-alcoholate complex may be further attacked by H
2
O
2
or
44
the Au-hydridoperoxide complex generated from the reaction
0
of Au with H
2
2
O , to form the final carbonyl compound via a
b-elimination pathway. The overall effect is that a dramatically
boosted activity for alcohol oxidation is achieved in contrast to
the normal aerobic process in which dioxygen or air was used as
the oxidant.
c
4
5
>99
>99
61
In summary, we have demonstrated that the Au/TiO catalyst
2
c
has great potential for the environmentally benign oxidation
of alcohols with aqueous hydrogen peroxide under mild and
37.5
organic solvent-free conditions. The H
2
O
2
–Au/TiO protocol
2
has been proven to be particularly efficient in the oxidative
transformation of nonactivated alcohols in aqueous media. The
stability of the catalyst has also been demonstrated convincingly
by conducting five successive runs without a significant drop
b
6
>99
100
in the reaction rate. Further extension of the present H
catalytic system to many other key oxidative transformations is
currently being explored.
2
O –Au
2
a
◦
Reaction conditions: 10 mmol substrate, 10 ml H
2
O, 90 C, 5% H
:Au = 100:250:1. Substrate:H :Au =
00:150:1. Aldehyde was found as by-product.
2 2
O
b
added dropwise, substrate:H
1
2
O
2
2 2
O
c
Acknowledgements
Financial support by the National Natural Science Foundation
of China (20633030, 20721063 and 20873026), the National
Basic Research Program of China (2003CB 615807), Shang-
hai Education Committee (06SG03) and Science & Technol-
ogy Commission of Shanghai Municipality (08DZ2270500,
0
7QH14003) is kindly acknowledged.
Notes and references
Scheme 1 Hectogram-scale oxidation of 1-hexanol.
‡
Without the use of H
2
O
2
(under an air atmosphere), acetophenone was
catalyst.
obtained in only <10% yield over the Mintek Au/TiO
2
6
–12
previously,
Au/TiO efficiently catalyzed the oxidation of
2
1
R. A. Sheldon and J. K. Kochi, Metal-Catalyzed Oxidation of Organic
Compounds, Academic Press, New York, 1981.
2 W. J. Mijs and C. R. H. de Jonge, Organic Synthesis by Oxidation
with Metal Compounds, Plenum Press, New York, 1986.
(a) Z. Hou, N. Theyssen, A. Brinkmann and W. Leitner, Angew.
Chem., Int. Ed., 2005, 44, 1346; (b) B. Z. Zhan, M. A. White, T. K.
Sham, J. A. Pincock, R. J. Doucet, K. V. R. Rao, K. N. Robertson
and T. S. Cameron, J. Am. Chem. Soc., 2003, 125, 2195; (c) B. T.
Guan, D. Xing, G. X. Cai, X. B. Wan, N. Yu, Z. Fang and Z. J. Shi,
J. Am. Chem. Soc., 2005, 127, 18004.
secondary aromatic alcohol, such as 1-phenylpropanol (Table 3,
entry 1), almost quantitatively to their target products (~99%
yields). Benzyl alcohol was oxidized to benzoic acid at a 100%
conversion, although with a selectivity of only 85% (Table 3,
entry 2; the other product was mainly benzaldehyde). Benzyl
alcohols with electron-donating groups were also successfully
oxidized, in which a shift in selectivity from acids to benzalde-
hydes was observed (Table 3, entries 3–5). It is worth noting
that chemoselective oxidation of cinnamyl alcohol with the sole
formation of cinnamaldehyde could be accomplished (Table 3,
entry 6). Interestingly, an epoxidized compound of cinnamyl
alcohol was not observed. All these results clearly showed the
3
4
(a) C. X. Zhang, P. Chen, J. Liu, Y. H. Zhang, W. Shen, H. L. Xu and
Y. Ta n g , Chem. Commun., 2008, 3290; (b) J. Shen, W. Shan, Y. H.
Zhang, J. M. Du, H. L. Xu, K. N. Fan, W. Shen and Y. Tang, Chem.
Commun., 2004, 2880.
5
6
T. Mallat and A. Baiker, Chem. Rev., 2004, 104, 3037.
(a) Y. Uozumi and R. Nakao, Angew. Chem., Int. Ed., 2003, 42, 194;
(b) Y. M. A. Yamada, T. Arakawa, H. Hocke and Y. Uozumi, Angew.
Chem, Int. Ed., 2007, 46, 704; (c) H. Tsunoyama, H. Sakurai, Y.
Negishi and T. Tsukuda, J. Am. Chem. Soc., 2005, 127, 9374; (d) H.
Miyamura, R. Matsubara, Y. Miyazaki and S. Kobayashi, Angew.
Chem., Int. Ed., 2007, 46, 4151; (e) K. Mori, T. Hara, T. Mizugaki,
K. Ebitani and K. Kaneda, J. Am. Chem. Soc., 2004, 126, 10657.
efficiency of the H
activated and nonactivated alcohols in water.
Attempting to gain insight into the origin of the enhanced
activity achieved by using aqueous H
2
O
2
–Au/TiO system in the oxidation of both
2
2
2
O as oxidant is important.
Although an extensive mechanistic study has not yet been
7
58 | Green Chem., 2009, 11, 756–759
This journal is © The Royal Society of Chemistry 2009

View Article Online
7
8
T. Wang, C. X. Xiao, L. Yan, L. Xu, J. Luo, H. Shou, Y. Kou and
H. C. Liu, Chem. Commun., 2007, 4375.
(a) X. Wang, H. Kawanami, N. M. Islam, M. Chattergee, T.
Yokoyama and Y. Ikushima, Chem. Commun., 2008, 4442; (b) H.
Tsunoyama, T. Tsukuda and H. Sakurai, Chem. Lett., 2007, 36, 212;
11 (a) F. Z. Su, Y. M. Liu, L. C. Wang, Y. Cao, H. Y. He and K. N. Fan,
Angew. Chem., Int. Ed., 2008, 47, 334; (b) L. C. Wang, L. He, Q. Liu,
Y. M. Liu, M. Chen, Y. Cao, H. Y. He and K. N. Fan, Appl. Catal.,
A, 2008, 344, 150; (c) L. C. Wang, Y. M. Liu, M. Chen, Y. Cao, H. Y.
He and K. N. Fan, J. Phys. Chem. C, 2008, 112, 6981.
(
c) G. ten Brink, I. W. C. E. Arends and R. A. Sheldon, Science, 2000,
12 (a) A. Abad, P. Concepcion, A. Corma and H. Garc ´ı a, Angew. Chem.,
Int. Ed., 2005, 44, 4066; (b) A. Abad, C. Almela, A. Corma and H.
Garc ´ı a, Tetrahedron, 2006, 62, 6666.
13 D. I. Enache, D. W. Knight and G. J. Hutchings, Catal. Lett., 2005,
103, 43.
2
87, 1636; (d) B. P. Buffin, J. P. Clarkson, N. L. Belitz and A. Kundu,
J. Mol. Catal. A: Chem., 2005, 225, 111.
9
(a) L. Prati and M. Rossi, J. Catal., 1998, 176, 552; (b) A. Corma and
P. Serna, Science, 2006, 313, 332; (c) C. H. Christensen, B. Jorgensen,
J. RassHansen, K. Egeblad, R. Madsen, S. K. Klitgaard, S. M.
Hansen, M. R. Hansen, H. C. Andersen and A. Riisager, Angew.
Chem., Int. Ed., 2006, 45, 4648; (d) F. Z. Su, L. He, J. Ni, Y. Cao,
H. Y. He and K. N. Fan, Chem. Commun., 2008, 3531; (e) J. E. Bailie
and G. J. Hutchings, Chem. Commun., 1999, 2151; (f) T. Hayashi, K.
Tanaka and M Haruta, J. Catal., 1998, 178, 566; (g) A. K. Sinha, S.
Seelan, S. Tsubota and M. Haruta, Angew. Chem., Int. Ed., 2004, 43,
14 (a) B. M. Trost and Y. Masuyama, Tetrahedron Lett., 1984, 25, 173;
(b) O. Bortolini, V. Conte, F. Di Furia and G. Modena, J. Org. Chem.,
1986, 51, 2661; (c) G. Barak, J. Dakka and Y. Sasson, J. Org. Chem.,
1988, 53, 3553; (d) Y. Ishii, K. Yamawaki, T. Ura, H. Yamada,
T. Yoshida and M. Ogawa, J. Org. Chem., 1988, 35, 35873; (e) R.
Zennaro, F. Pinna, G. Strukul and H. Arzoumanian, J. Mol. Catal.,
1991, 70, 269; (f) C. Venturello and M. Gambaro, J. Org. Chem.,
1991, 56, 5924; (g) A. C. Dengel, W. P. Griffith and B. C. Parkon,
J. Chem. Soc., Dalton Trans., 1993, 2683; (h) R. Neumann and M.
Gara, J. Am. Chem. Soc., 1995, 117, 5066.
1
546.
1
0 (a) S. Carrettin, P. McMorn, P. Johnston, K. Griffin and G. J.
Hutchings, Chem. Commun., 2002, 696; (b) N. F. Zheng and G. D.
Stucky, Chem. Commun., 2007, 3862; (c) P. Haider and A. Baiker,
J. Catal., 2007, 248, 175; (d) C. Burato, P. Centomo, G. Pace,
M. Favaro, L. Prati and B. Corain, J. Mol. Catal. A: Chem,
2 2
15 The higher H O decomposition rate achieved for catalysts with
smaller Au particle size can be attributed to the higher abundance
of low-coordinated Au atoms, which are also believed to be the key
sites for low temperature CO oxidation.
2
005, 238, 26; (e) D. I. Enache, J. K. Edwards, P. Landon, B.
Solsona-Espriu, A. F. Carley, A. A. Herzing, M. Watanabe, C. J.
16 (a) Y. Kon, Y. Usui and K. Sato, Chem. Commun., 2007, 4399; (b) Y.
Kon, H. Yazawa, Y. Usui and K. Sato, Chem. Asian J., 2008, 3, 1642.
17 A. Abad, A. Coma and H. Garc ´ı a, Chem.–Eur. J., 2008, 14, 2.
Kiely, D. W. Knight and G. J. Hutchings, Science, 2006, 311,
3
62.
This journal is © The Royal Society of Chemistry 2009
Green Chem., 2009, 11, 756–759 | 759