Genesis of catalytically active gold nanoparticles supported on hydrotalcite for base-free selective oxidation of glycerol in water with molecular oxygen

Article abstract of DOI:10.1246/cl.2011.150

Hydrotalcite-supported gold nanoparticles (Au/HT) were synthesized by depositionprecipitation using NH₃ and successive calcination at various temperatures. Gold nanoparticles of less than 5 nm with narrow size distribution were deposited onto hydrotalcite surfaces, which were determined by TEM measurements. Au L_III-edge XANES spectra clearly indicated that calcination at temperatures higher than 373K leads to the generation of metallic Au which is the catalytically active site for selective glycerol oxidation to glycolic acid in water with molecular oxygen under mild reaction conditions.

Full text of DOI:10.1246/cl.2011.150

doi:10.1246/cl.2011.150
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Published on the web January 8, 2011
Genesis of Catalytically Active Gold Nanoparticles Supported on Hydrotalcite
for Base-free Selective Oxidation of Glycerol in Water with Molecular Oxygen
³
Atsushi Takagaki, Akihiro Tsuji, Shun Nishimura, and Kohki Ebitani*
School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa 923-1292
(
Received October 14, 2010; CL-100883; E-mail: ebitani@jaist.ac.jp)
Hydrotalcite-supported gold nanoparticles (Au/HT) were
temperatures from room temperature to 473 K. Effect of
calcination temperature on both particle size and oxidation state
of gold were investigated. In addition, their oxidation activity
was evaluated by base-free glycerol oxidation in pure water with
molecular oxygen.
synthesized by deposition­precipitation using NH3 and succes-
sive calcination at various temperatures. Gold nanoparticles of
less than 5 nm with narrow size distribution were deposited onto
hydrotalcite surfaces, which were determined by TEM measure-
ments. Au LIII-edge XANES spectra clearly indicated that
calcination at temperatures higher than 373 K leads to the
generation of metallic Au which is the catalytically active site
for selective glycerol oxidation to glycolic acid in water with
molecular oxygen under mild reaction conditions.
Au/HT was synthesized as follows.10 Hydrotalcite (Mg/
Al = 5, 1.0 g) was added to aqueous solution (100 mL)
containing HAuCl ¢4H O (0.25 mmol). After stirring for
4
2
2 min, 1 mL of 25% NH3 (aq) was added followed by 2 h
stirring at room temperature. The resulting solid was collected
by filtration and washed with deionized water. Finally, the solid
catalyst was obtained after drying in air at various temperatures
from room temperature to 473 K for 12 h. The amount of loaded
Au was determined to be 4.56 wt % by ICP.
Gold nanoparticles have been widely studied as highly
active oxidation catalysts including CO oxidation, alcohol
oxidation, epoxidation, and hydrogen peroxide synthesis, which
afford environmentally benign reaction using molecular oxygen
The X-ray diffraction measurement confirmed that crystal
structure of Au/HT is identical to that of parent HT. Figure 1
shows TEM images of Au/HT calcined at various temperatures,
indicating that small Au particles were well dispersed on HT for
1
,2
as an oxidant. Mg­Al hydrotalcite, an anionic clay composed
of Mg Al (OH) CO is a superior basic support for a variety of
6
2
16
3
3
10
metals such as platinum, ruthenium, and palladium.
all samples (see also Supporting Information (SI), Figure S1 ).
Recently, Kaneda et al. demonstrated oxidation of various
monoalcohols and diols to the corresponding carbonyl com-
pounds and lactones using hydrotalcite-supported gold (Au/HT)
The size distributions of Au species are also shown in Figure 1.
The uncalcined sample (room temperature) has the smallest Au
particles of 2.0 nm among samples prepared. The average
particle size increased from 2.2 to 4.8 nm by calcination from
323 to 473 K. The size distributions of Au were also influenced
4
catalyst in the presence of molecular oxygen. They prepared
Au/HT by addition of reducing agents such as KBH to form
4
metal gold nanoparticles of 2.7 nm with narrow size distribution.
The reaction mechanism for alcohol oxidation was proposed to
be initiated by the proton abstraction from alcohol by a basic site
of HT, formation of gold metal­alcolate intermediate and
successive ¢-hydride elimination affording gold­hydride spe-
cies. They also showed that particle size of Au plays key roles in
the reaction, which means small gold particles exhibited high
catalytic activity. A similar study was reported by Wang et al.
(A)
(B)
RT
1
0 %
RT
d = 2.0 nm
σ = 0.5 nm
2
0 nm
5
for oxidant-free dehydrogenation of alcohols using Au/HT.
323 K
d = 2.2 nm
σ = 0.6 nm
They used supported gold nanoparticles of 2.7 nm which were
obtained by deposition­precipitation (DP) and successive
calcination at 393 K. The oxidation state of gold of this active
catalyst, which is expected to be an important factor for gold
catalysis is however, unclear.
3
73 K
3
73 K
d = 3.5 nm
σ = 0.5 nm
Oxidation of glycerol, a simple triol, has received much
attention because of utilization of glycerol as a by-product from
2
0 nm
423 K
6
d = 3.9 nm
biodiesel manufacture. Supported Au metal catalysts, e.g., Au/
σ = 1.2 nm
7
­9
473 K
C are known to exhibit high catalytic activity for the reaction.
However, high oxygen pressure and addition of strong base
NaOH) are necessary to promote the reaction. The latter results
4
73 K
(
d = 4.8 nm
in the formation of products as Na-salts, which requires excess
energy consumption for purification of products in free form. In
these regards, glycerol oxidation in water without addition of
homogeneous base under ambient oxygen pressure would be
desirable.
σ = 2.0 nm
2
0 nm
0
5
10 15 20
Au particle diameter / nm
Figure 1. TEM images (A) and Au particle size distributions
(B) of hydrotalcite-supported gold catalysts (Au/HT) prepared
at various calcination temperature.
Here we synthesized hydrotalcite-supported gold nano-
particles by DP and successive heat treatment at various
Chem. Lett. 2011, 40, 150­152
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

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51
O
O
O
[O]
[O]
HO
OH
HO
OH
HO
OH
Au O
2
3
OH
OH
OH
HAuCl4
Glycerol
Glyceric acid
GA)
Tartronic acid
(TA)
(
Au foil
C-C bond
cleavage
O
O
[O]
HO
OH
OH
RT
OH
O
Glycolic acid
Hydroxyacetic acid, HA)
Oxalic acid
(OA)
3
3
4
4
23 K
73 K
23 K
73 K
(
Scheme 1. A possible reaction pathway of glycerol oxidation.
Table 2. Glycerol oxidation using hydrotalcite-supported gold
a
catalysts in water in the presence of oxygen
Calcination
temp/K
Selectivity/%
GAb TAc HAd OAe
Entry Catalyst
Conv./%
1
1.85
11.90
11.95
12.00
12.05
1
2
3
4
5
6
7
8
Au/HT
Au/HT
Au/HT
Au/HT
Au/HT
Au/HT
RT
323
373
373
423
473
®
0
0
0
0
1
0
0
0
0
0
0
0
4
0
3
3
0
0
0
0
18
0
0
0
Photon energy / keV
70
78
71
76
0
Figure 2. Au LIII-edge XANES spectra for Au/HT catalysts.
f
53 15
22
24
0
0
0
0
0
Table 1. Average particle size and metal concentration of
hydrotalcite-supported gold prepared by calcination at various
temperatures
g
Au/C
g
HT
®
0
0
Au oxidation state/%b
Calcination Au average particle
temp/K
a
size/nma
Au0
Au3+
2
Reaction conditions: Glycerol (0.5 mmol), H O (5 mL), Au/
¹
1
b
HT (0.1 g), under O2 flow (10 mL min ), 333 K, 6 h. Glyceric
RT
2.0
2.2
3.5
3.9
4.8
0
0
80
100
100
100
100
20
0
c
d
acid. Tartronic acid. Hydroxyacetic acid (or Glycolic acid).
3
3
4
4
23
73
23
73
e
f
g
Oxalic acid. 293 K, 72 h. O2 atmosphere. 0.1 g.
The addition of NH to HAuCl aqueous solution during
3
4
0
the catalyst preparation could form amino­hydroxo or
amino­hydroxo­aquo cationic complex [Au(NH3)2(H2O)2¹x-
OH)x]
aEstimated from TEM measurements. bEstimated from the
intensity of white line at Au LIII-edge XANES.
(3¹x)+ 13,14
3+
(
.
This complex with Au species is thought
to be stable on HT until calcination below 323 K, and
decompose above 373 K, resulting in formation of gold metal
Au .
with increase of average particle size on calcination. Above
0
13
423 K, large particles over 10 nm could be observed, resulting in
wide size distribution.
The catalytic activity of Au/HTs was evaluated for glycerol
1
0
The oxidation state of Au on HT was determined by using
XAFS. XANES provides powerful information of the oxidation
state of gold species.11 In particular, for the case of HT, this
analysis is very powerful compared with XPS spectroscopy.
XPS spectra for Au/HT have the peaks of Au3 at 86.0 (Au
oxidation in water with molecular oxygen. The reaction was
performed using 0.5 mmol of glycerol, 5 mL of water, and 0.1 g
of Au/HT at 333 K for 6 h. All experiments were carried out in
a Schlenk tube attached to a reflux condenser under O2 flow
+
¹1
(10 mL min ). After the reaction, the vessel was cooled to room
4
f7/2) and 89.7 eV (Au 4f5/2), which should be overlapped with
temperature, and the catalyst was separated by filtration. The
conversions and yields were estimated using high-performance
liquid chromatography (HPLC). Oxidation of glycerol afforded
a variety of products including glyceric acid (GA), tartronic acid
(TA), glycolic acid (another name; hydroxyacetic acid (HA)),
and oxalic acid (OA) as shown in Scheme 1. Glycolic acid is a
useful chemical for skin care products, intermediates for organic
synthesis, monomer for biocompatible copolymers, and food
the big peak of Mg 2s at ca. 90 eV, resulting in difficulty in
1
2
deconvolution. Figure 2 shows Au LIII-edge XANES spectra
for Au/HTs prepared at various calcination temperatures. Au
metal foil, Au2O3, and HAuCl4 were used as references with
known oxidation state. For samples uncalcined (dried in vacuo
at room temperature) and calcined at 323 K, Au species
remained completely oxidized Au3 . A significant change of
the spectra was clearly observed for samples calcined at
temperatures higher than 373 K. The portion of each Au species
+
6
additives, which is formed by C­C bond cleavage of tartronic
acid.
and particle sizes were described in Table 1. Sample calcined at
Table 2 lists the results of glycerol oxidation using Au/HT
under mild reaction conditions. Au/C showed no activity under
the same conditions due to lack of base. Hydrotalcite itself also
did not transform glycerol (Entry 8). The samples prepared by
calcination below 323 K showed no activity for the reaction
0
3
73 K have high concentration of Au (80%) along with cationic
3+
Au species. Increase of calcination temperature increased the
portion of Au , resulting in up to 100% Au for samples calcined
0
0
at 423 and 473 K.
Chem. Lett. 2011, 40, 150­152
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

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(Entries 1 and 2). In contrast, the samples prepared by
References and Notes
calcination at temperatures higher than 373 K exhibited high
glycerol conversion over 70% (Entries 3­6). Increase of
calcination temperature improved gold metal concentration and
glycerol conversion. The main product was glycolic acid
³
1
2
Present address: Department of Chemical System Engineering,
School of Engineering, The University of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-8656
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presence of NaOH and revealed that hydrogen peroxide was
produced during the reaction under highly basic conditions,
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5
cleavage. Furthermore, Hutchings et al. demonstrated glycerol
oxidation using Au/C in the presence of NaOH with hydrogen
peroxide as an oxidant. They also obtained glycolic acid
selectively and proposed a reaction mechanism for glycolic acid
formation by the decarboxylation of tartronic acid.16 Actually,
we have detected CO2 formation by trapping in Ba(OH)2
aqueous solution. They also showed high reaction temperature
leads to decomposition of hydrogen peroxide, resulting in low
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at low temperature, affording high selectivity toward glycolic
acid (53%) at high glycerol conversion (78%) at 293 K
3
4
a) T. Mitsudome, A. Noujima, T. Mizugaki, K. Jitsukawa, K.
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2
010, 49, 5545.
1
7
alcohol but also as in situ generator of hydrogen peroxide. We
should consider other products for the glycerol oxidation.
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5
6
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1
5
not occur. One possible explanation is adsorption of tartronic
1
0
7
acid on HT. Tartronic acid well adsorbed on HT whereas
glyceric acid and glycolid acid partly adsorbed because of these
acidities (pK = 2.37 (pK ), 3.52, and 3.83 for tartronic acid,
a
a1
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8
9
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In summary, hydrotalcite-supported gold nanoparticle cata-
lysts were synthesized by simple deposition­precipitation and
successive calcination. Without calcination, 2.0 nm average gold
particles were deposited on hydrotalcite. It was found that
average particle size and size distribution of gold on hydrotalcite
gradually increased with increase of calcination temperature.
XANES revealed that uncalcined Au/HT has totally cationic
Au species and calcination at temperatures higher than 373 K
afforded reduction of Au cation to metal, which is catalytically
active for oxidation of glycerol. Au/HT was found to transform
glycerol to glycolic acid in water with molecular oxygen through
C­C bond cleavage. Low-temperature reaction afforded high
selectivity of glycolic acid at high glycerol conversion. Hydro-
talcite as basic support for nanosized gold catalyst may play
important roles in promotion of alcohol oxidation and formation
of Au­alcolate intermediate.
1
0 Supporting Information is available electronically on the CSJ-
Journal Web site, http://www.csj.jp/journals/chem-lett/index.
html.
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7 Glycerol oxidation using Au/HT calcined at 473 K with
hydrogen peroxide was performed (Reaction conditions: glyc-
erol (0.5 mmol), H2O2 (0.5 mmol), H2O (5 mL), Au/HT (0.1 g),
1
The synchrotron radiation experiments were performed at
the BL01B1 in the SPring-8 with the approval of the Japan
Synchrotron Radiation Research Institute (JASRI) (Proposal
Nos. 2008B1328 and 2009B1497). This work was supported
by a Grant-in-Aid for Scientific Research (C) (No. 10005910)
of the Ministry of Education, Culture, Sports, Science and
Technology (MEXT), Japan.
3
33 K, 6 h)). Low glycerol conversion (14%) with 19%
selectivity of glyceric acid, 6% of tartronic acid and trace
amount of glycolic acid were obtained due to rapid decom-
position of H2O2. In situ generation of H2O2 with aldehyde
formation by alcohol oxidation would be beneficial to high
activity.
Chem. Lett. 2011, 40, 150­152
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/