Aerobic oxidation of alcohols in the liquid phase with nanoporous gold catalysts

Article abstract of DOI:10.1039/c2cc17245c

Aerobic oxidation of alcohols in the liquid phase proceeded smoothly in the presence of nanoporous gold catalyst. The catalyst is reusable multiple times without leaching and loss of the catalytic activity. The reaction was applied successfully to a flow system. Adsorptions of O₂ and 1-phenylethanol into the AuNPore were confirmed by TDS analysis. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc17245c

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COMMUNICATION
Aerobic oxidation of alcohols in the liquid phase with nanoporous gold
catalystsw
Naoki Asao,*a Naoya Hatakeyama,b Menggenbateer,c Taketoshi Minato,de Eisuke Ito,fg
Masahiko Hara,fh Yousoo Kim,e Yoshinori Yamamoto,*a Mingwei Chen,a Wei Zhangi and
Akihisa Inouei
Received 21st November 2011, Accepted 19th January 2012
DOI: 10.1039/c2cc17245c
Aerobic oxidation of alcohols in the liquid phase proceeded
smoothly in the presence of nanoporous gold catalyst. The
catalyst is reusable multiple times without leaching and loss of
the catalytic activity. The reaction was applied successfully to a
flow system. Adsorptions of O2 and 1-phenylethanol into the
AuNPore were confirmed by TDS analysis.
AuNPore were confirmed by thermal desorption spectroscopy
(TDS) analysis.
The requisite AuNPore was prepared by dealloying of
Au30Ag70 alloy thin film (100 mm in thickness) with HNO3
at rt.5 The composition of the resulting material was found to
be Au97Ag3 and a scanning electron microscopy (SEM) study
revealed the formation of nanoporous structure with a ligament
size of around 30 nm (see ESIw). The specific surface area of
the catalyst was electrochemically measured to be 7.38 m2 gÀ1
(see ESIw). The Au 4f 7/2 peak was observed at 84.0 eV from the
Au 4f X-ray photoelectron spectroscopy (XPS) measurements.
This observation indicated that the oxidation state of Au in the
AuNPore was Au(0) (see ESIw).9
We examined the catalytic oxidation of 1-phenylbutanol 1a with
the AuNPore catalyst and the results are summarized in Table 1.
The reaction proceeded in MeOH as a solvent under an O2
atmosphere (balloon) at 60 1C for 10 h and the corresponding
ketone product 2a was obtained in 96% yield (entry 1). The
turnover frequency (TOF) of 52.4 hÀ1 was achieved after the initial
30 minutes (see ESIw). On the other hand, no reaction proceeded
at all with simple gold thin film or undealloyed Au30Ag70 thin film.
These results clearly indicated that the nanoporous structure is
necessary for this transformation. The catalyst we used here was a
thin plate with a size of around 3 Â 3 mm2. Therefore, the catalyst
can be recovered easily by tweezers and any cumbersome
Selective oxidation of alcohols is a fundamental transformation in
organic synthesis.1 From an environmental viewpoint, catalytic
oxidation processes employing molecular oxygen as an oxidant are
attractive.2 Recently, we were successful in using a monolithic
nanoporous gold skeleton material (AuNPore) as an effective
catalyst for oxidation of organosilanes with water, leading to
organic silanols in high yields.3 In comparison with supported
catalysts,4 the nanoporous gold catalyst can be fabricated easily by
selective leaching of Ag from gold–silver alloys.5 Particularly, due
to the robust 3D network structure composed of ligaments, it
shows high durability. These results prompted us to further explore
the feasibility of carrying out a wide range of molecular transfor-
mations with this catalyst.6–8 In this communication, we wish to
report that AuNPore exhibits a remarkable catalytic activity in the
aerobic oxidation of alcohols with O2 at atmospheric pressure
in the liquid phase in batch and flow systems, respectively.
Furthermore, adsorptions of O2 and 1-phenylethanol into the
a WPI Advanced Institute for Materials Research, Tohoku University,
2-1-1 Katahira, Sendai 980-8577, Japan.
Table 1 Aerobic oxidation of 1a with AuNPore catalysta
E-mail: asao@m.tohoku.ac.jp, yoshi@m.tohoku.ac.jp;
Fax: +81 22 217 6165; Tel: +81 22 217 6165
b Department of Chemistry, Tohoku University,
Sendai 980-8578, Japan
c Center for Integrated Nano Technology Support, Tohoku University,
Sendai 980-8577, Japan
d International Advanced Research and Education Organization,
Tohoku University, Sendai 980-8578, Japan
e Surface & Interface Laboratory, RIKEN, Wako 351-0198, Japan
f Flucto-Order Functions Asian Collaboration Team, RIKEN,
Wako 351-0198, Japan
Entry
Catalyst
Yield of 2ab (%)
1
2
3
4
5
Fresh
96
97
97
95
96
g Institute of Nano Science and Technology, Hanyang University,
Republic of Korea
Reuse 1
Reuse 2
Reuse 3
Reuse 4
h Department of Electronic Chemistry, Tokyo Institute of Technology,
Yokohama 226-8502, Japan
i Institute for Materials Research, Tohoku University,
Sendai 980-8577, Japan
a
Reactions were carried out with 1a (0.30 mmol) and O2 balloon in the
presence of 10 mol% of AuNPore catalyst at 60 1C for 10 h. b Determined
by 1H NMR analysis with p-xylene as an internal standard.
w Electronic supplementary information (ESI) available: SEM micro-
graphs, CV, XPS spectra, and experimental details. See DOI: 10.1039/
c2cc17245c
c
4540 Chem. Commun., 2012, 48, 4540–4542
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Table 2 Aerobic oxidation of alcohols with AuNPore catalysta
separation procedures, such as filtration and centrifugation,
were not required. We repeated the reaction five times and the
turnover number (TON) reached up to 1400, but no significant
loss of the catalytic activity was observed (entries 1–5). The SEM
image of the recovered catalyst after five uses has no differences
with that of the fresh one and these results clearly indicate that
the catalyst is reusable due to its robustness (see ESIw).
Generally, most supported gold catalysts require excess
amount of bases for promotion of the reaction.2 On the
contrary, this catalytic system proceeded smoothly without
any additives. This unique feature makes the process much
easier and simpler than those of ordinary catalytic systems.10
The substrate generality was examined and the results are
summarized in Table 2. Phenylethanol 1b was oxidized
smoothly to give acetophenone 2b in 88% yield (entry 1).
The reaction also proceeded even under air instead of an O2
atmosphere although longer reaction time was needed (entry 2).
Besides MeOH, 1,4-dioxane was a suitable solvent in this
reaction (entry 3). Compared to gas-phase reactions, the present
liquid-phase reaction system is suitable for the oxidation of
solid alcohols like 1d (entry 4). Even with alcohols 1h and 1i,
having heteroaromatics, the corresponding products were
obtained in high yields (entries 8 and 9). The reaction of benzyl
alcohol 1j in MeOH gave methyl benzoate in 78% yield.11
However, when the reaction was carried out in 1,4-dioxane
instead of MeOH, benzaldehyde 2j was obtained as a sole product
(entry 10). Not only aromatic but also aliphatic secondary alcohols
1k and 1l were oxidized in high yields by increasing the loading
amount of the catalyst (entries 11 and 12).
Entry
1
Alcohol
1
Time/h
10
2
Yieldb (%)
88
1b
2b
2c
3d
4
1b
1c
1d
24
7
2b
2c
2d
88
85
97
9
5
1e
10
2e
94
6
7
1f
28
24
2f
91
98
1g
2g
Besides the batch system mentioned above, we conducted this
catalytic reaction with a flow system. Recently, several catalytic
reactions using microchannel reactors have been reported,12
however, metal-catalyzed aerobic oxidation of alcohols has not
been studied well.13 The catalyst in powder form was fabricated
by dealloying the commercially available Au35Ag65 thin film
(100 nm in thickness). The resulting catalyst (43 mg, 0.2 mmol)
was put into a stainless steel tube column (15 cm length, 2 mm
inner diameter). Solutions of the substrates in MeOH and oxygen
gas were mixed via a micromixer using syringes under the control
of two syringe pumps to form a clear segmented flow, which were
introduced to the column filled with the catalyst. The column
temperature can be controlled by a column oven.
8
9
1h
1i
22
22
2h
2i
83
81
10d,ef
11d,e,f
1j
24
22
2j
85
82
1k
2k
12f
1l
19
2l
96
With the present gas–liquid–solid phase flow reaction system,
the oxidation of 1a was carried out and the results are summarized
in Table 3. The complete conversion of 1a and quantitative
production of 2a were achieved when a 0.1 M solution of 1a in
MeOH and oxygen gas were introduced to the column with flow
rates 3 mL hÀ1 and 30 mL hÀ1, respectively (entry 1). To raise the
synthetic potential of the present system, we increased the concen-
tration of 1a. Even with 0.5 M solution, the conversion and the
chemical yields were not changed at all (entries 2–5). The present
flow system accelerated the reaction significantly in comparison
with the batch system. For example, it needed 1.3 hours for
producing 1.9 mmol of 2a (entry 6). On the other hand, the batch
system required 7 hours for obtaining the same amount of 2a with
the same amount of the AuNPore powder catalyst.14
a
Reactions were carried out with 1 (0.30 mmol) and O2 balloon in the
presence of 10 mol% of AuNPore catalyst at 60 1C unless otherwise noted.
b Determined by 1H NMR analysis with p-xylene as an internal standard.
c Air balloon was used instead of O2 balloon. d 1,4-Dioxane was used as a
solvent. e Reaction temperature was 80 1C. f 20 mol% of catalyst was used.
appeared at 118 and 198 1C from AuNPore (Fig. 1a). These results
obviously indicate that AuNPore has an effective O2 adsorption
property than Au(111). Previously, Behm and his co-workers have
reported that the desorption peak of O2 from fresh AuNPore
appeared at 274 1C together with a shoulder peak at 248 1C in the
TDS spectrum.6e These peaks were assigned to ‘‘initial surface
oxygen’’, which were adsorbed during the preparation process of
AuNPore by electrochemical etching with HClO4. In contrast,
such peaks were not detected with our material, which was
prepared by chemical etching with HNO3. Since the typical
We next conducted the TDS analysis of O2 and phenylethanol
1b with AuNPore and Au(111) as shown in Fig. 1. While no
desorption peak of O2 was observed from Au(111),15,16 clear peaks
c
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Chem. Commun., 2012, 48, 4540–4542 4541

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Table 3 Aerobic oxidation of 1-phenylbutanol 1a with flow systema
2 (a) T. Mallat and A. Baiker, Chem. Rev., 2004, 104, 3037;
(b) B.-Z. Zhan and A. Thompson, Tetrahedron, 2004, 60, 2917.
3 N. Asao, Y. Ishikawa, N. Hatakeyama, M. Bateer, Y. Yamamoto, M.
Chen, W. Zhang and A. Inoue, Angew. Chem., Int. Ed., 2010, 49, 10093.
4 (a) A. S. K. Hashmi and G. J. Hutchings, Angew. Chem., Int. Ed.,
2006, 45, 7896; (b) C. D. Pina, E. Falletta, L. Prati and M. Rossi,
Chem. Soc. Rev., 2008, 37, 2077; (c) A. Corma and H. Garcia, Chem.
Soc. Rev., 2008, 37, 2096; (d) T. Matsumoto, M. Ueno, N. Wang
and S. Kobayashi, Chem.–Asian J., 2008, 3, 196; (e) A. Corma,
Entry
1a in MeOH
Conversionb (%)
Yield of 2ab (%)
1
2
3
4
0.1 M
0.2 M
0.3 M
0.4 M
0.5 M
0.5 M
100
100
100
100
100
100
99
99
99
99
99
99
5
6c
A. Leyva-Perez and M. J. Sabater, Chem. Rev., 2011, 111, 1657;
´
a
(f) S. Kobayashi and R. Matsubara, JP 2007237116 A, 2007.
5 (a) A. J. Forty, Nature, 1979, 282, 597; (b) J. Erlebacher, M. Aziz
and A. Karma, Nature, 2001, 410, 450.
6 For CO oxidations, see: (a) V. Zielasek, B. Jurgens, C. Schulz,
¨
J. Biener, M. M. Biener, A. V. Hamza and M. Baumer, Angew.
¨
Reactions were carried out as follows unless otherwise noted: 1a in
MeOH (3 mL hÀ1) and O2 (30 mL hÀ1) at 60 1C for 20 min.
b
Determined by 1H NMR analysis with p-xylene as an internal
c
standard. Reaction was carried out for 1.3 h.
Chem., Int. Ed., 2006, 45, 8241; (b) C. Xu, J. Su, X. Xu, P. Liu,
H. Zhao, F. Tian and Y. Ding, J. Am. Chem. Soc., 2007, 129, 42;
(c) C. Xu, X. Xu, J. Su and Y. Ding, J. Catal., 2007, 252, 243;
(d) A. Wittstock, B. Neumann, A. Schaefer, K. Dumbuya,
C. Kubel, M. M. Biener, V. Zielasek, H.-P. Steinruck,
¨
¨
J. M. Gottfried, J. Biener, A. Hamza and M. Baumer, J. Phys.
¨
Chem. C, 2009, 113, 5593; (e) L. C. Wang, H. J. Jin, D. Widmann,
J. Weissmuller and R. J. Behm, J. Catal., 2011, 278, 219.
7 (a) H. Yin, C. Zhou, C. Xu, P. Liu, X. Xu and Y. Ding, J. Phys.
Chem. C, 2008, 112, 9673; (b) A. Wittstock, V. Zielasek, J. Biener,
C. M. Friend and M. Baumer, Science, 2010, 327, 319; (c) D. Han,
¨
T. Xu, J. Su, X. Xu and Y. Ding, ChemCatChem, 2010, 2, 383;
(d) B. Xu, J. Haubrich, T. A. Baker, E. Kaxiras and C. M. Friend,
J. Phys. Chem. C, 2011, 115, 3703.
8 For electrochemical reactions, see: (a) J. Zhang, P. Liu, H. Ma and
Y. Ding, J. Phys. Chem. C, 2007, 111, 10382; (b) C. Yu, F. Jia, Z. Ai
and L. Zhang, Chem. Mater., 2007, 19, 6065; (c) R. Zeis, T. Lei,
K. Sieradzki, J. Snyder and J. Erlebacher, J. Catal., 2008, 253, 132.
9 J. F. Moulder, W. F. Stickle, P. E. Sobol and K. D. Bomben,
Handbook of X Ray Photoelectron Spectroscopy: A Reference Book
of Standard Spectra for Identification and Interpretation of Xps
Data, Physical Electronics, 1995.
10 For oxidation reactions with metastable oxygen on Au(111), see:
B. K. Min and C. M. Friend, Chem. Rev., 2007, 107, 2709.
11 (a) D. I. Enache, J. K. Edwards, P. Landon, B. Solsona-Espriu,
A. F. Carley, A. A. Herzing, M. Watanabe, C. J. Kiely,
D. W. Knight and G. J. Hutchings, Science, 2006, 311, 362;
Fig. 1 TDS spectra for desorption of (a) O2 and (b) 1-phenylethanol
(1b) from Au(111) (red lines) and AuNPore (blue lines).
desorption temperature of atomic oxygen from Au single crystals
occurs at temperatures of about 257 1C (530 K) and above,17 the
adsorbed oxygen species with our material would be not atomic
oxygen but probably molecular oxygen due to the enrichment of
unsaturated gold atoms and/or residual silver. In particular, the
residual silver on the surface of the nanoporous gold has been
suggested to play a significant role in activating molecular oxygen
although the details of the reaction mechanism are unclear.6d,7b No
detection of O2 desorption over 300 1C indicates that the amount
of O2 adsorbed on the residual Ag is negligible because the O2
desorption from the Ag surface occurs at 300 1C.18 Fig. 1b is the
TDS spectra of 1b. While the desorption peak was observed at
123 1C from Au(111), two peaks were observed at 109 1C and
230 1C from AuNPore. Although the adsorption states of 1b are
not clear at this stage, these results indicated that the adsorption of
phenylethanol on AuNPore is more stable than that on Au(111).
In conclusion, we disclosed that the AuNPore exhibited a
remarkable catalytic activity in the aerobic oxidation reaction of
alcohols in both batch and flow systems as a non-supported nano-
structured catalyst. The reaction does not need any additives,
such as bases, stabilizers, and ligands as well as any cumbersome
work-up procedure like filtration or centrifugation. Further appli-
cations of the AuNPore catalyst to other transformations are
currently under investigation in our laboratory.
(b) A. Abad, P. Concepcion, A. Corma and H. Garcıa, Angew.
´ ´
Chem., Int. Ed., 2005, 44, 4066; (c) A. S. K. Hashmi,
C. Lothschutz, M. Ackermann, R. Doepp, S. Anantharaman,
¨
B. Marchetti, H. Bertagnolli and F. Rominger, Chem.–Eur. J.,
2010, 16, 8012; (d) I. S. Nielsen, E. Taarning, K. Egeblad,
R. Madsen and C. H. Christensen, Catal. Lett., 2007, 116, 35.
12 (a) J. Kobayashi, Y. Mori and S. Kobayashi, Chem.–Asian J.,
2006, 1–2, 22; (b) A. Kirschning, W. Solodenko and K. Mennecke,
Chem.–Eur. J., 2006, 12, 5972; (c) T. Razzaq and C. O. Kappe,
Chem.–Asian J., 2010, 5, 1274; (d) S. Suga, D. Yamada and
J. Yoshida, Chem. Lett., 2010, 39, 404; (e) J. Yoshida, K. Saito,
T. Nokami and A. Nagaki, Synlett, 2011, 1189.
13 (a) E. Cao, A. Gavriilidis and W. B. Motherwell, Chem. Eng. Sci.,
2004, 59, 4803; (b) E. Cao, W. B. Motherwell and A. Gavriilidis,
Chem. Eng. Technol., 2006, 29, 1372; (c) N. Wang, T. Matsumoto,
M. Ueno, H. Miyamura and S. Kobayashi, Angew. Chem., Int. Ed.,
2009, 48, 4744; (d) N. Zotova, K. Hellgardt, G. H. Kelsall,
A. S. Jessiman and K. K. Hii, Green Chem., 2010, 12, 2157.
14 The batch reaction was conducted with a 0.1 M solution of 1a.
With a higher concentration of 1a (0.5 M), the chemical yield of 2a
was decreased to 21% yield. This result markedly contrasts with
those obtained by the flow system.
This work was partially supported by the Grant-in-Aid for
Scientific Research on Innovative Areas ‘‘Reaction Integration’’
(No. 2105) and the Grant-in-Aid for Scientific Research on
Priority Areas (No. 474) from the MEXT, Japan.
15 J. J. Pireaux, M. Chtaib, J. P. Delrue, P. A. Thiry, M. Liehr and
R. Caudano, Surf. Sci., 1984, 141, 211.
16 J. M. Gottfried, K. J. Schmidt, S. L. M. Schroeder and
K. Christmann, Surf. Sci., 2002, 511, 65.
17 (a) J. M. Gottfried, K. J. Schmidt, S. L. M. Schroeder and
K. Christmann, Surf. Sci., 2003, 525, 184; (b) V. Zielasek,
B. J. Xu, X. Y. Liu, M. Baumer and C. M. Friend, J. Phys. Chem.
C, 2009, 113, 8924.
18 V. I. Bukhtiyarov, A. I. Boronin and V. I. Savchenko, J. Catal.,
1994, 150, 262.
Notes and references
1 (a) C. L. Hill, Advances in Oxygenated Processes, JAI Press,
London, 1988, vol. 1; (b) M. Hundlucky, Oxidations in Organic
Chemistry, American Chemical Society, Washington, DC, 1990.
c
4542 Chem. Commun., 2012, 48, 4540–4542
This journal is The Royal Society of Chemistry 2012