Redox-active catalyst based on poly(anilinesulfonic acid)supported gold nanoparticles for aerobic alcohol oxidation in water

Article abstract of DOI:10.1002/adsc.201000451

The hybrid consisting of gold nanoparticles and poly(2-methoxyaniline-5- sulfonic acid), which works as a redox mediator for transferring protons and electrons, catalyzed the oxidation reaction of various alcohols in water under molecular oxygen.

Full text of DOI:10.1002/adsc.201000451

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DOI: 10.1002/adsc.201000451
Redox-Active Catalyst Based on Poly(Anilinesulfonic Acid)-
Supported Gold Nanoparticles for Aerobic Alcohol Oxidation in
Water
Daisuke Saio,a Toru Amaya,a and Toshikazu Hiraoa,*
a
Affiliation: Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Yamada-oka, Suita,
Osaka 565-0871, Japan
Fax : (+81)-6-6879-7415; phone: (+81)-6-6879-7413; e-mail: hirao@chem.eng.osaka-u.ac.jp
Received: June 9, 2010; Published online: September 8, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000451.
Abstract: The hybrid consisting of gold nanoparti-
cles and poly(2-methoxyaniline-5-sulfonic acid),
which works as a redox mediator for transferring
protons and electrons, catalyzed the oxidation reac-
tion of various alcohols in water under molecular
oxygen.
completely oxidized form PMAS (ox), half oxidized
form PMAS (half ox), and completely reduced form
PMAS (red), and usually exists in the state of PMAS
(half ox) (Figure 1, b). Au NPs were employed for the
catalytic aerobic alcohol oxidation. To the best of our
knowledge, there are no reports of such a catalyst
design for the alcohol oxidation reaction using a
Keywords: aerobic oxidation; gold; green chemis-
try; redox chemistry
Sustainable molecular transformation reactions are
found in biological systems. For example, catalytic ox-
idation reactions are allowed with the help of redox
mediators in vivo using molecular oxygen as a termi-
nal oxidant.[1] On the other hand, oxidation of alco-
hols is one of the most important transformations in
organic chemistry and biochemistry. The discovery of
the catalytic properties of gold nanoparticles (NPs)
for aerobic alcohol oxidation[2] has activated this re-
search field for a decade. Generally, metal NPs need
a stabilizer (polymers are often used) to prevent
metal NPs from aggregation in the catalytic reac-
tion.[3] We envisioned the idea to give the function of
redox actitivity to the stabilizer to mediate the redox
reaction, such as aerobic alcohol oxidation. For this
purpose, we chose polyanilines as a redox-active p-
conjugated polymer. We have constructed artificial
multi-redox systems based on d,p-conjugation of poly-
anilines or polypyrroles with various transition metals
or metal nanoparticles (NPs), and developed some
catalytic oxidation reactions (Figure 1, a).[4] In the
present sudy, we decided to use the water-soluble
polyaniline, poly(2-methoxyaniline-5-sulfonic acid)
(PMAS)[5] from the ecological point of view. PMAS
Figure 1. (a) Concept for the catalyst design based on the
redox-active conductive polymers with various transition
metals or metal NPs. (b) Three representative oxidation
states of PMAS: complete oxidized form PMAS (ox), half
oxidized form PMAS (half ox), and complete reduced form
has three representative oxidation states as follows, PMAS (red).
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Table 1. Oxidation of benzhydrol (1).
redox-active polymer as the catalyst support, in spite
of a variety of investigations on the metal NPs-cata-
lyzed alcohol oxidation reactions.[6] Herein, we report
on the reusable PMAS/Au NPs catalyst for the oxida-
tion reaction of alcohols under molecular oxygen in
water, where PMAS works as a redox mediator for
transferring protons and electrons.
Entry Catalyst
Atmosphere Yield[a]
[%]
PMAS/Au NPs were prepared according to the pro-
cess shown in Figure 2, a. PMAS was treated with
N2H4·H2O, then NaAuCl4 was added to the reaction
mixture to give PMAS (half ox)/Au NPs. The trans-
mission electron microscopy (TEM) image and the di-
ameter histogram of the PMAS/Au NPs are shown in
Figure 2, b. Each particle was dispersed and the aver-
age diameter was 7.9 nm. The thus-obtained aqueous
solution of the PMAS/Au NPs was used for the cata-
lytic reaction, and could be stored without the parti-
cles growing in size for at least 6 months (the TEM
image is shown in the Supporting Information, Fig-
ure S1).
1
2
3
4
5
PMAS/Au NPs
O2
O2
Air
Ar
Ar
quant
0
PMAS[b]
PMAS/Au NPs
PMAS/Au NPs
PMAS/Au NPs
(400 mol%)
quant[c]
3
quant
6
PVP/Au NPs[d]
O2
47
[a]
[b]
1
Determined by H NMR.
Amount of PMAS was calculated based on the aniline
monomer unit.
[c]
[d]
6 h.
Prepared by treatment with N2H4·H2O in the presence of
PVP and NaAuCl4.
action was enhanced by the presence of PMAS. The
redox-mediating effect by PMAS may be discussed to
account for the results (entry 6).
In order to investigate the redox behaviour of
PMAS in the oxidation reaction, the selected entries
of Table 1 were followed by using UV-vis-NIR spec-
troscopy. In the catalytic reaction under an argon at-
mosphere (Table 1, entry 4), a significant spectral
change was observed after the reaction (Figure 3, a),
whereas the yield was poor. The characteristic polar-
on band (747 nm)[7] disappeared after the reaction, in
the meantime a sharp peak appeared at 402 nm,
which is assigned to the reduced form of PMAS[8]
Figure 2. Preparation of PMAS/Au NPs. (b) TEM image
and diameter histogram of PMAS/Au NPs.
To examine the catalytic function of PMAS/Au NPs (Figure 3, a). The surface plasmon band of Au NPs
prepared according to the process in Figure 2, a, the was observed after the reaction. These results suggest
oxidation of benzhydrol (1) was conducted in pH 9.0 the hydrogen transfer from 1 to PMAS. In other
aqueous buffer solution at 808C, where no organic words, the reoxidation of PMAS (red) is essential for
solvent was used (Table 1). Under an oxygen atmos- the catalytic cycle. In the stoichiometric reaction
phere, 1 was quantitatively transformed into benzo- under an argon atmosphere (Table 1, entry 5), PMAS
phenone (2) in the presence of 5 mol% PMAS/Au was reduced, while the alcohol 1 was quantitatively
NPs (based on Au atom) for 1 h (entry 1). On the oxidized to 2 via stoichiometric hydrogen transfer.[9]
other hand, PMAS showed no catalytic activity in the On the other hand, under an oxygen atmosphere, the
absence of Au NPs (entry 2). Even under air, PMAS/ UV-vis-NIR spectra after the reaction did not show
Au NPs transformed 1 into 2 quantitatively (entry 3). any significant changes (Figure 3, b). The catalytic re-
In contrast, the reaction under an argon atmosphere action proceeded quantitatively under these condi-
showed a sharply decreased yield of 2 of merely 3% tions (Table 1, entry 1), suggesting the involvement of
with PMAS/Au NPs (entry 4), suggesting that molecu- molecular oxygen in the reoxidation of PMAS (red).
lar oxygen is essential for the oxidation reaction. With these results in hand, the proposed multi-cata-
Poly(N-vinyl-2-pyrrolidone) (PVP)/Au NPs were tried lytic cycles were proposed as shown in Figure 4. The
as a representative catalyst in which the polymer does latest mechanistic research of alcohol oxidation with
not have redox-active function (average diameter: Au NPs has revealed that the catalyst restoration de-
7.7 nm, see Supporting Information, Figure S2). The pends on the abstraction process of hydride species
[10]
À
yield was lower than with PMAS/Au NPs whereas the from Au H by molecular oxygen.
However, the
average particle size is similar, indicating that the re- availability of molecular oxygen on the Au NPs sur-
2178
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Redox-Active Catalyst Based on Poly(Anilinesulfonic Acid)-Supported Gold Nanoparticles
of the cyclopropyl ring (entry 1). When the catalyst
was reused, there was no loss of the activity, at least
for four times reuse (Supporting Information,
Table S1). Furthermore, its oxidation could be carried
out under organic-solvent-free conditions in all pro-
cess with 81% isolated yield (Supporting Information,
Scheme S1). It is possible to synthesize dicarbonyl
compounds, which are important building blocks for
the synthesis of heterocyclic compounds, as exempli-
fied by the oxidation of benzoin (entry 2). In the oxi-
dation of an a,b-unsaturated alcohol, the C=C bond
remained intact (entry 5). Several benzyl alcohols
were transformed into the corresponding aldehydes,
carboxylic acids, and esters with high conversion
yields (entries 6–8). 1-Adamantaneethanol as a pri-
mary alcohol was found to be oxidized to the corre-
sponding aldehyde in 70% yield in spite of the need
for a longer reaction time (entry 9). Furthermore, the
aerobic oxidation of a non-activated aliphatic primary
alcohol proceeded although 10 mol% PMAS/Au NPs
and 72 h reaction time were required (entries 10 and
11). 1,10-Decanedicarboxylic acid was obtained in
54% yield (entry 10). The oxidation of 1-decanol to
the corresponding carboxylic acid was perfomed in
the presence of 300 mol% K2CO3 (74% yield,
entry 11).
In conclusion, we have demonstrated that the
redox-active PMAS can work in a multi-catalytic pro-
cess as both a stabilizer of Au NPs and redox media-
tor for aerobic alcohol oxidation in water. This design
concept provides a new type of redox catalyst system
for transferring protons and electrons.
Figure 3. UV-vis-NIR spectra of (a) PMAS/Au NPs solid
line) and after the reaction of PMAS/Au NPs with 1 for 1 h
under Ar (dotted line), and (b) PMAS/Au NPs (solid line)
and after the reaction of PMAS/Au NPs with 1 for 1 h
under O2 (dotted line) (1.0ꢁ10À4 M based on the aniline mo-
nomer unit, pH 9.0 buffer solution, 258C).
Experimental Section
Preparation of PMAS/Au NPs
Poly(2-methoxyaniline-5-sulfonic acid) (PMAS), which was
kindly provided by Mitsubishi Rayon Co., was deionized
through cation-exchange resins before use. Other reagents
were used as recieved. The water used in the present study
is of a MilliQ grade.
Figure 4. Proposed multi-catalytic cycles for aerobic alcohol
oxidation with PMAS as a redox mediator in the presence
of Au NPs.
PMAS/Au NPs: An aqueous solution (1 mL) of PMAS
(0.06 mmol based on the aniline monomer unit, 12 mg) was
mixed with 3 mL of 0.5M B(OH)3-NaOH buffer (pH 9.0)
solution. An aqueous solution (1 mL) of N2H4·H2O
(0.06 mmol, 3 mL) was added to the PMAS solution, which
was stirred at room temperature under air for 24 h. Then, an
aqueous solution (1 mL) of NaAuCl4·H2O (0.06 mmol,
24 mg) was added to the stirring solution at 08C. The mix-
ture was stirred at room temperature under air for 24 h. The
thus-obtained aqueous solution of PMAS/Au NPs was ana-
lyzed by UV-vis-NIR spectroscopy (lmax =537, 747 nm) (Hi-
tachi U-3500). The aqueous solution of PMAS/Au NPs was
dropped on copper TEM grids coated with a carbon mesh
(Nissin EM Co., Ltd. Tokyo) for TEM analysis (Hitachi H-
face is known to be low,[11] which often reduces the
catalytic activity. Therefore, the redox-mediating
effect by PMAS is likely to accelerate the reaction.
The utility of the PMAS/Au NPs prepared accord-
ing to the process in Figure 2, a was examined for the
oxidation of a wide variety of alcohols as summarized
in Table 2. Secondary alcohols were oxidized to give
the corresponding ketones in high yields (entries 1–5).
In the case of a-cyclopropylbenzyl alcohol, the oxida-
tion of the hydroxy group occurred without cleavage 8100).
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Daisuke Saio et al.
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Table 2. Oxidation of various alcohols.[a]
Entry
1
Substrate
Time [h]
3
Product
Yield [%][b]
quant (81)[c]
2
3
3
4
quant
quant
quant
4
5
8
8
80
15
43
6
4
38
60
17
7
4
17
64
23
8
4
13
70
9
72
72
trace
54
10[d]
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Redox-Active Catalyst Based on Poly(Anilinesulfonic Acid)-Supported Gold Nanoparticles
Table 2. (Continued)
Entry
Substrate
Time [h]
72
Product
Yield [%][b]
trace
11[d,e]
74
[a]
Reaction conditions: substrate (0.2 mmol), 5 mol% PMAS/Au NPs (Au: 0.01 mmol), 0.25M B(OH)3-NaOH buffer solu-
tion pH 9.0 (1 mL), 808C.
Determined by H NMR.
Isolated yield through organic solvent-free conditions.
10 mol% PMAS/Au NPs, 0.25M B(OH)3-NaOH buffer solution pH 9.0 (2 mL).
300 mol% K2CO3.
[b]
[c]
[d]
[e]
1
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The general procedure for the catalytic aerobic oxidation of
alcohols in water is as follows: a 5-mL two-necked flask was
evacuated and backfilled with atmospheric oxygen. Then, al-
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0.01 mmol, 5 mol%) were added at room temperature. The
mixture was stirred at 808C under molecular oxygen. The
reaction mixture was extracted with ethyl acetate. The or-
1
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Acknowledgements
The authors thank Professor T. Sekino at Tohoku University
and Dr. S. Seino at Osaka University for TEM analysis. This
work was partially supported by a Grant-in-Aid for Scientific
Research on Priority Areas “Chemistry of Concerto Cataly-
sis” from the Ministry of Education, Culture, Sports, Science
and Technology, Japan. This work was also financially sup-
ported in part by Mitsubishi Rayon Co. D.S. expresses his
special thanks for The Global COE (center of excellence)
program “Global Education and Research Center for Bio-
Environmental Chemistry” of Osaka University.
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