Hydrotalcite-supported gold catalyst for the oxidant-free dehydrogenation of benzyl alcohol: Studies on support and gold size effects

Article abstract of DOI:10.1002/chem.201002469

Gold nanoparticles with uniform mean sizes (a≈3 nm) loaded onto various supports have been prepared and studied for the oxidant-free dehydrogenation of benzyl alcohol to benzaldehyde and hydrogen. The use of hydrotalcite (HT), which possesses both strong acidity and strong basicity, provides the best catalytic performance. Au/HT catalysts with various mean Au particle sizes (2.1-21 nm) have been successfully prepared by a deposition-precipitation method under controlled conditions. Detailed catalytic reaction studies with these catalysts demonstrate that the Au-catalyzed dehydrogenation of benzyl alcohol is a structure-sensitive reaction. The turnover frequency (TOF) increases with decreasing Au mean particle size (from 12 to 2.1 nm). A steep rise in TOF occurs when the mean Au particle size becomes smaller than 4 nm. Our present work suggests that the acid-base properties of the support and the size of Au nanoparticles are two key factors controlling the alcohol dehydrogenation catalysis. A reaction mechanism is proposed to rationalize these results. It is assumed that the activation of the β-Ci￡H bond of alcohol, which requires the coordinatively unsaturated Au atoms, is the rate-determining step.

Full text of DOI:10.1002/chem.201002469

FULL PAPER
DOI: 10.1002/chem.201002469
Hydrotalcite-Supported Gold Catalyst for the Oxidant-Free
Dehydrogenation of Benzyl Alcohol: Studies on Support and Gold Size
Effects
[a]
Wenhao Fang, Jiashu Chen, Qinghong Zhang,* Weiping Deng, and Ye Wang*
Abstract: Gold nanoparticles with uni-
form mean sizes (ꢁ3 nm) loaded onto
various supports have been prepared
and studied for the oxidant-free dehy-
drogenation of benzyl alcohol to ben-
zaldehyde and hydrogen. The use of
hydrotalcite (HT), which possesses
both strong acidity and strong basicity,
provides the best catalytic perfor-
mance. Au/HT catalysts with various
mean Au particle sizes (2.1–21 nm)
have been successfully prepared by a
deposition–precipitation method under
controlled conditions. Detailed catalyt-
ic reaction studies with these catalysts
demonstrate that the Au-catalyzed de-
hydrogenation of benzyl alcohol is a
structure-sensitive reaction. The turn-
over frequency (TOF) increases with
decreasing Au mean particle size (from
12 to 2.1 nm). A steep rise in TOF
occurs when the mean Au particle size
becomes smaller than 4 nm. Our pres-
ent work suggests that the acid–base
properties of the support and the size
of Au nanoparticles are two key factors
controlling the alcohol dehydrogena-
tion catalysis. A reaction mechanism is
proposed to rationalize these results. It
is assumed that the activation of the b-
CꢀH bond of alcohol, which requires
Keywords: alcohols · dehydrogena-
tion · gold · heterogeneous cataly-
sis · nanoparticles
the coordinatively unsaturated Au
atoms, is the rate-determining step.
Introduction
nanoparticles, and the Au–support interaction) may also
[3–12]
affect catalytic performance.
Undoubtedly, understand-
Gold catalysis has become an exciting research area since
ing of the effect of Au particle size and the function of the
support in various Au-catalyzed reactions will be helpful for
the rational design of highly efficient Au catalysts.
the discovery that Au particles <3–5 nm in diameter are cat-
[1]
alytically active. Supported Au nanoparticles have shown
unique catalytic properties in many important reactions,
such as hydrogenations of alkenes, a,b-unsaturated carbonyl
compounds, and nitro compounds, CO oxidation, propylene
epoxidation, hydrogen peroxide synthesis, aerobic oxidation
The dehydrogenation of alcohols to give carbonyl com-
pounds is one of the most essential transformations in or-
ganic synthesis. A number of studies have contributed to the
development of efficient heterogeneous catalysts for the oxi-
dative dehydrogenation of alcohols to aldehydes and ke-
[2,3]
of alcohols, and C–C coupling reactions.
The size of the
[13–24]
Au nanoparticles and the nature of the support are believed
to be pivotal in determining the catalytic behavior of the
supported Au nanoparticles, although many other issues
tones by O2,
which is more environmentally benign and
“greener” than stoichiometric oxidation with a metal-con-
taining inorganic oxidant such dichromate or permanganate.
[13]
(
e.g., the chemical state of Au, the morphology of the Au
Together with Pd- and Ru-based catalysts, some support-
ed Au nanoparticles such as Au/CeO , Au/TiO , Au/
2
2
Cu MgAl O , Au/Ga Al O (x=2, 3, or 4), Au/MnO , Au/
5
2
x
x
6ꢀx
9
2
[
a] W. Fang, J. Chen, Prof. Dr. Q. Zhang, Dr. W. Deng, Prof. Dr. Y. Wang
State Key Laboratory of Physical Chemistry of Solid Surfaces
and National Engineering Laboratory
for Green Chemical Productions of Alcohols, Ethers and Esters
College of Chemistry and Chemical Engineering
Xiamen University, Xiamen 361005 (China)
Fax : (+86)592-2183047
hydrotalcite (HT), Au/PCPs (PCP=porous coordination
polymers), and Au–Pd bimetallic nanoparticles have been
reported to show excellent catalytic performances for the
[14–24]
aerobic oxidation of alcohols.
However, the presence of
O may cause problems of overoxidation and explosion, or
2
[13c]
flammability of organic solvent or alcohol reactants.
The
E-mail: zhangqh@xmu.edu.cn
safety problem becomes particularly serious for practical
large-scale production. In this context, the dehydrogenation
of alcohols under anaerobic conditions is desirable.
wangye@xmu.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201002469.
Chem. Eur. J. 2011, 17, 1247 – 1256
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1247

[
35]
Transfer dehydrogenation of alcohols to aldehydes or ke-
tones in the presence of a hydrogen acceptor, such as al-
kenes (e.g., ethylene, cyclohexene, styrene) or ketones, cata-
lyzed by supported Pd and Cu catalysts has been report-
tion of benzyl alcohol in our previous communication.
However, the mean sizes of Au nanoparticles in those cata-
[35]
lysts were quite varied and often greater than 5 nm. It is
clear that a rigorous study of support effects requires the
use of catalysts with uniform Au particle sizes.
[
25–27]
ed,
but the transfer process consumes the hydrogen ac-
ceptor and produces organic byproducts, and thus is not an
atom-economical process. With respect to atom economy,
the dehydrogenation of alcohols under an inert atmosphere
Thus, the first task of this work was to prepare Au nano-
particles on various supports with uniform mean sizes
(ꢁ3 nm) by several different techniques. Haruta has sum-
marized the techniques for preparing Au nanoparticle cata-
to produce aldehydes or ketones and H is the most desira-
2
[3a,f]
ble process [Eq. (1)]:
lysts.
silica), SBA-15 (a mesoporous silica), and carbon nanotubes
CNTs), sonication-aided impregnation was useful for pre-
We clarified that, for SiO2 (Cab-O-Sil, a fumed
(
paring finely dispersed Au nanoparticles. For metal oxide
supports, such as Al O , TiO , ZrO , La O , CeO , and
2
3
2
2
2
3
2
MgO, deposition–precipitation with urea as a homogeneous
precipitant was suitable for preparing supported Au nano-
particles with smaller mean sizes (<4 nm). Deposition–pre-
cipitation with NaOH as a precipitant could give rise to Au/
HT catalysts containing homogeneously distributed Au
nanoparticles with mean sizes less than 4 nm.
By regulating the conditions used in various preparation
techniques, we were able to control the mean size of Au
nanoparticles on various supports at around 3 nm. Typical
TEM micrographs and corresponding Au particle size distri-
butions for the supported Au catalysts are shown in
Figure 1. The mean sizes (d) of Au nanoparticles for all
these samples were in the range of 2.9–3.6 nm. All our cata-
The oxidant- and acceptor-free dehydrogenation of alcohols
may also provide a novel and promising route for H synthe-
2
[28]
sis and storage. By using benzyl alcohol as an example,
we made a thermodynamic analysis for this reaction. Our
analysis reveals that the standard Gibbs free-energy change
A
(
DG ) for the dehydrogenation of benzyl alcohol to benzal-
ꢀ1
dehyde and H is 33.4 kJmol at 298 K, and that this de-
creases to 20.6 kJmol as the temperature rises to 393 K.
The equilibrium conversion is estimated to be around 4% at
2
ꢀ
1
393 K. Although the dehydrogenation is a thermodynamical-
ly limited reaction, the removal of H from the system by an
lysts underwent reduction by H at 523 K to ensure the com-
2
2
inert gas flow could drive the reaction to the product side,
as has been reported several times for the dehydrogenation
of alcohols.
plete reduction of Au to the metallic state. X-ray photoelec-
tron spectroscopy (XPS) showed that the binding energies
for Au 4f7/2 in these catalysts were 83.5–83.8 eV, which con-
[29–35]
0
Early studies showed that Ru-based catalysts, including
firmed the presence of Au in the samples.
the Shvo-type Ru complex grafted on SiO , Ru/AlO(OH),
The performances of these catalysts in the dehydrogena-
tion of benzyl alcohols in Ar gas flow are shown in Table 1.
The Au loading measured by inductively coupled plasma
mass spectrometry (ICPMS), the BET surface area derived
2
and Ru(OH) /Al O , are effective for the dehydrogenation
x
2
3
[29–31]
of alcohols without hydrogen acceptors.
Recently, sup-
ported Ag and Cu nanoparticles were also shown to catalyze
[
32–34]
the dehydrogenation of various alcohols.
In a recent
from N physisorption, and the mean Au particle size ob-
2
[35]
short communication, we demonstrated, for the first time,
that supported Au nanoparticles could catalyze the oxidant-
and acceptor-free dehydrogenation of alcohols to aldehydes
tained from TEM are also listed in Table 1. No correlations
between catalytic performance and the BET surface area
could be deduced. Au nanoparticles loaded on SiO (Cab-O-
2
(
or ketones) and H . In particular, HT-supported Au nano-
Sil) and SBA-15 were almost inactive, and the Au/CNT also
showed a low activity. Medium benzyl alcohol conversions
(17–55%) were obtained with TiO , ZrO , La O , or CeO
2
2
particles exhibited high activity and selectivity for the oxi-
dant-free dehydrogenation of a wide range of alcohols.
[35]
2
2
2
3
There is little information about Au-catalyzed dehydrogena-
tion reactions in the absence of an oxidant. Herein, we
report our recent studies on the elucidation of the effects of
support and Au particle size in the oxidant-free dehydrogen-
ation of benzyl alcohol.
as the support. On the other hand, Al O -, MgO-, hydroxya-
2 3
patite (HAP)-, and HT-supported Au catalysts showed sig-
nificantly higher benzyl alcohol conversions (>80%). Ben-
zaldehyde selectivity also depended on the identity of the
support. Au/SiO and Au/SBA-15 showed significantly lower
2
selectivity towards benzaldehyde, and the main byproduct
was toluene. The formation of toluene was also observed
with high selectivity in the dehydrogenation of benzyl alco-
Results and Discussion
[36]
hol over an Au–Pd/TiO catalyst. Our calculation reveals
2
Support effects: A deep understanding of the role of the
support may lead to significant improvements in supported
Au catalysts for alcohol dehydrogenation. Au nanoparticles
loaded onto various supports prepared by the conventional
impregnation method were examined for the dehydrogena-
that the disproportionation of benzyl alcohol into benzalde-
hyde and toluene [Eq. (2)] is thermodynamically feasible
[DG ACHTGNUTRENNUNG
A
ꢀ1
2
ð2Þ
2
3
2
1248
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Chem. Eur. J. 2011, 17, 1247 – 1256

Hydrotalcite-Supported Gold Catalyst
FULL PAPER
Thus, Equation (2) may proceed simultaneously with the de-
hydrogenation of benzyl alcohol to benzaldehyde and H2.
Other catalysts showed higher selectivity towards benzalde-
hyde, which indicated that dehydrogenation was the main
reaction. Among the catalysts examined in this work, Au/
HT and Au/MgO demonstrated outstanding benzaldehyde
selectivity (>99%). We measured the amount of H formed
2
over the Au/HT catalyst; the molar ratio of H to benzyl al-
2
cohol was around 1:1. This further confirms that benzyl alco-
hol dehydrogenation [Eq. (1)] proceeds over our catalyst in
the absence of any oxidant or hydrogen acceptor.
To make a better comparison among Au/Al O , Au/MgO,
2
3
Au/HAP, and Au/HT catalysts, which gave similarly higher
benzyl alcohol conversions (84–89%) after 6 h of reaction
(
Table 1), we performed the dehydrogenation of benzyl al-
cohol for 1 h over these catalysts. Au/Al O exhibited the
2
3
highest benzyl alcohol conversion but the lowest benzalde-
hyde selectivity, whereas Au/MgO showed the highest ben-
zaldehyde selectivity but the lowest benzyl alcohol conver-
sion (Figure 2). Similar to Au/MgO, Au/HT also exhibited
2 3
Figure 2. Catalytic performances of Au/Al O , Au/HAP, Au/MgO, and
Au/HT for the oxidant-free dehydrogenation of benzyl alcohol. Reaction
conditions: catalyst 0.20 g (Au amount 4–5 mmol); benzyl alcohol
ꢀ1
1
1
mmol; solvent (p-xylene) 5 mL; Ar flow rate 3 mLmin ; T=393 K; t=
h.
high benzaldehyde selectivity (99.5%), but its benzyl alco-
hol conversion was higher than Au/MgO. Al O is a typical
2
3
solid acid, whereas MgO is known as a basic oxide; thus,
these results suggest that the acid–base properties of the
support influence the catalytic behavior of the supported Au
catalysts in the dehydrogenation of benzyl alcohol.
We examined the acid–base properties of some typical
supported Au catalysts by temperature-programmed desorp-
tion (TPD) of NH and CO . As shown in Figure 3, desorp-
3
2
tion of NH was observed from Au/HT, Au/Al O , and Au/
3
2
3
HAP, which indicates that acidic sites exist on these cata-
Figure 1. TEM micrographs and corresponding Au particle size distribu-
lysts. From the temperature and intensity of the NH de-
3
tions for Au catalysts loaded on various supports: a) Au/SiO
SBA-15, c) Au/CNT, d) Au/Al , e) Au/TiO , f) Au/ZrO , g) Au/MgO,
h) Au/La , i) Au/CeO , j) Au/HAP, k) Au/HT.
2
, b) Au/
sorption peak, we can see that Au/HT and Au/Al O possess
2
O
3
2
2
2
3
2
O
3
2
relatively strong acidity. On the other hand, no desorption
of NH3 was detectable from Au/SiO , Au/La O , or Au/
2
2
3
MgO, which suggests that these catalysts have no significant
acidity. CO -TPD profiles in Figure 4 suggest that Au/HT
2
Chem. Eur. J. 2011, 17, 1247 – 1256
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1249

Q. Zhang, Y. Wang, et al.
Table 1. Catalytic performances of Au nanoparticles loaded on various supports for the oxidant-free dehydro- whereas neither acidic nor basic
[
a]
genation of benzyl alcohol.
Catalyst Au content
[wt%]
sites exist on Au/SiO . Au/HAP
2
[b]
Surface area
Au size
[nm]
Conversion
[%]
Selectivity [%]
benzaldehyde toluene
also possesses both acidity and
basicity, but the strength of the
acidity and the number of basic
sites are lower. Strongly acidic
2
ꢀ1
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
A
H
U
G
R
N
U
G
]
Au/SiO
Au/SBA-15
Au/CNT
2
0.50
0.50
0.50
0.44
0.48
0.45
0.46
0.47
0.46
0.43
0.40
262
352
97
118
7.0
9.0
38
8.0
6.0
32
39
2.9
3.5
3.0
3.2
3.6
3.5
3.0
3.1
2.9
3.3
3.1
2.1
1.8
7.4
88
27
17
84
55
37
87
49
55
40
27
94
5.1
sites exist on Au/Al O , but its
Au/Al
Au/TiO
Au/ZrO
Au/MgO
Au/La
Au/CeO
2
O
3
96
87
93
>99
97
97
85
>99
3.5
8.8
4.1
0.1
1.7
2.5
2.5
0.1
2
3
basicity is weaker. On the other
2
2
hand, Au/MgO and Au/La O3
2
have relatively strong basic
sites, but they do not possess
acidic sites.
In summary, we have ob-
served significant support ef-
fects in the Au-catalyzed oxi-
dant-free dehydrogenation of
benzyl alcohol, and our studies
2
O
3
2
Au/HAP
Au/HT
89
[
a] Reaction conditions: catalyst 0.20 g (Au amount 4–5 mmol); benzyl alcohol 1 mmol; solvent (p-xylene)
ꢀ
1
5
mL; Ar flow rate 3 mLmin ; T=393 K; t=6 h. [b] The remaining byproduct was benzene.
suggest that the acid–base properties of the support play
pivotal roles in determining both the activity and the selec-
tivity. The catalysts with neither acidity nor basicity (e.g.,
Au/SiO or Au/SBA-15) were almost inactive and less selec-
2
tive, whereas that with both strong acidity and strong basici-
ty (i.e., Au/HT) exhibited both high activity and excellent
selectivity. Further analysis indicates that catalysts with
stronger basicity but lower or no acidity (e.g., Au/MgO)
afford higher selectivity towards benzaldehyde, whereas
those with stronger acidity but lower basicity (e.g., Au/
Al O ) are more active for the conversion of benzyl alcohol.
2
3
Preparation and characterization of Au/HT catalysts with
controlled mean Au particle sizes: The preparation of sup-
ported Au catalysts with controlled Au sizes is still a chal-
lenge. We found that, by regulating the concentration of Au
precursor, the aging temperature and the time used in the
deposition–precipitation process could lead to the prepara-
tion of Au/HT catalysts with controlled Au particle sizes.
In our previous studies, we found that increasing the Au
loading by increasing the concentration of Au precursor
3
Figure 3. NH -TPD profiles for some typical supported Au catalysts.
(
HAuCl ) solution, while keeping the volume of solution
4
used for deposition–precipitation constant, increased the
[35]
mean Au particle size over HT. This observation implies
that the concentration of the HAuCl solution might be key
4
to changing the Au particle size. Herein, we attempt to pre-
pare Au/HT samples with uniform Au loadings (ꢁ0.5 wt%)
but varying mean Au particle sizes by changing the concen-
2
Figure 4. CO -TPD profiles for some typical supported Au catalysts.
tration of HAuCl solution and the volume of the solution
4
while keeping the total amount of Au constant.
In our experiment, the concentration of HAuCl solution
4
ꢀ
1
and Au/MgO possess higher concentrations of basic sites,
was varied from 0.08 to 3.17 mmolL , whereas the total
amount of Au in the solution was fixed, to provide a target
Au loading of 0.5 wt%. The actual Au loadings in the ob-
tained Au/HT catalysts, measured by ICPMS, were 0.40–
0.46 wt% (Table 2), which indicated that most (80–90%) of
the Au precursors were deposited on the HT support during
the deposition–precipitation process at a pH of 8.0. Figure 5
shows the TEM micrographs and the Au particle size distri-
butions for these Au/HT samples. The mean size of Au
whereas no basic sites can be observed on Au/SiO . The de-
2
sorption of CO was also observed from Au/Al O and Au/
2
2
3
La O ; the latter catalyst must be much more basic accord-
2
3
ing to the higher desorption temperature. Only very weak
CO desorption peaks could be discerned in the case of Au/
2
HAP, which suggests that this catalyst has fewer basic sites.
In short, our NH -TPD and CO -TPD studies reveal that
3
2
Au/HT possesses both strong acidity and strong basicity,
1250
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Chem. Eur. J. 2011, 17, 1247 – 1256

Hydrotalcite-Supported Gold Catalyst
FULL PAPER
Table 2. Mean sizes and dispersions of Au nanoparticles in Au/HT cata-
lysts prepared by deposition–precipitation with various concentrations of
of Au precursors per unit volume in the solution would de-
crease the probability of collision among the particles
during particle growth, and would thus lead to smaller Au
nanoparticles.
HAuCl
4
.
[
a]
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[HAuCl
mmolL
4
ꢀ
]
Au loading
[wt%]
Mean Au
size [nm]
Au disper-
sion [%]
Au disper-
sion [%]
1
[b]
[c]
[d]
[
]
By assuming a spherical model of Au nanoparticles, the
dispersion of Au (D) can be roughly estimated by using the
0
0
0
0
0
1
3
.08
.16
.21
.32
.63
.06
.17
0.40
0.41
0.43
0.43
0.42
0.46
0.44
2.1
2.5
3.1
4.0
7.4
11
56
47
38
29
16
11
10
51
49
39
28
19
14
12
[3b]
following relationship, D=1.17/d(nm)ꢁ100%. The calcu-
lated Au dispersion is listed in Table 2. We also measured
the dispersion of Au over the Au/HT catalysts by H –O ti-
2
2
[37]
tration. The Au dispersion obtained from the H –O titra-
2
2
12
tion decreased with increasing HAuCl concentration (see
4
[
a] Measured by ICPMS. [b] Obtained from TEM. [c] Estimated from Au
Table 2). Moreover, the values of Au dispersion evaluated
from the H –O titration were quite close to those estimated
particle size. [d] Measured by H –O titration.
2
2
2
2
from the Au size obtained by TEM. This indicates that the
average size of Au nanoparticles evaluated by TEM is ac-
ceptable.
The aging temperature and the time used for deposition–
precipitation may also affect the size of the Au nanoparti-
cles. We have clarified that increasing the aging temperature
from 273 to 353 K slightly decreased the average size of Au
from 3.7 to 2.5 nm (see Figure S1 in the Supporting Informa-
tion,). In other words, the higher aging temperature resulted
in slightly smaller Au nanoparticles. It is known that the size
of a particle formed in solution is determined by the rates of
nucleation and growth. The decrease in size of Au nanopar-
ticles at higher aging temperature is understandable because
the nucleation rate may be higher at increased temperature.
On the other hand, increasing the aging time from 1 to 16 h
significantly increased the mean size of Au nanoparticles
over HT from 2.5 to 21 nm (see Figure S2 in the Supporting
Information). This is probably due to Ostwald ripening,
which is caused by the dissolved smaller particles precipitat-
[38]
ing onto the larger particles.
Catalytic behavior of Au/HT catalysts with various Au sizes
in the oxidant-free dehydrogenation of benzyl alcohol:
Table 3 shows the dependence of catalytic performances on
the mean size of Au nanoparticles in the Au/HT catalysts
prepared by the deposition–precipitation method with vari-
ous HAuCl concentrations, aging temperatures and times.
4
The loadings of Au in these catalysts were consistent (0.40–
0
.46 wt%). Very high selectivity towards benzaldehyde
(
>99%) was obtained for all these catalysts, which indicates
that the Au particle size did not affect product selectivity.
Product selectivity is mainly determined by the nature of
the support. Table 3 shows that the conversion of benzyl al-
cohol depends strongly on the mean size of Au nanoparti-
cles. The smaller Au size gave higher benzyl alcohol conver-
sion.
To elucidate the effect of Au particle size on the intrinsic
catalytic reactivity of Au sites in the dehydrogenation of
benzyl alcohol, we monitored the conversion of benzyl alco-
hol at the initial reaction stages over Au/HT catalysts.
Figure 6 shows the time course for some typical Au/HT cat-
alysts with mean Au sizes ranging from 2.1 to 12 nm. The
conversion of benzyl alcohol over HT was zero, which con-
firms that the Au nanoparticles were the active surface for
Figure 5. TEM micrographs and corresponding Au particle size distribu-
tions for the Au/HT catalysts prepared by using various concentrations of
HAuCl
g) 3.17 mmolL
4
:
a) 0.08, b) 0.16, c) 0.21, d) 0.32, e) 0.63, f) 1.06, and
ꢀ
1
.
nanoparticles in these samples increased gradually on in-
creasing the concentration of HAuCl . By varying the con-
4
ꢀ
1
centration in the range of 0.08–3.17 mmolL , we obtained
Au/HT samples with mean Au particle sizes ranging from
2.1 to 12 nm (Table 2). We speculate that a smaller number
Chem. Eur. J. 2011, 17, 1247 – 1256
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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1251

Q. Zhang, Y. Wang, et al.
[
a]
Table 3. Catalytic performance of Au/HT catalysts with varying mean Au nanoparticle size.
lyzed oxidant-free dehydrogen-
ation of benzyl alcohol is a
[c]
Preparation
Au loading
[wt%]
Mean Au size
[nm]
Conversion [%]
Selectivity [%]
aldehyde toluene
[
b]
conditions
ACHTUNGTRENNUNG
structure-sensitive
reaction.
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[HAuCl
[HAuCl
[HAuCl
4
4
4
]=0.08 mm
]=0.16 mm
]=0.21 mm
0.40
0.41
0.43
0.44
0.42
0.41
0.43
0.44
0.42
0.45
0.46
0.44
0.41
2.1
2.5
3.1
3.4
3.5
3.7
4.0
5.5
7.4
8.5
11
>99
>99
89
85
85
78
78
73
65
44
38
36
11
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.2
0.3
0.2
0.5
When the mean size of Au de-
creases from 12 to around
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
4
nm, the TOF increases slight-
T
T
T
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(aging)=323 K
(aging)=293 K
(aging)=273 K
ly. However, a further decrease
in the mean Au size increases
the TOF steeply. Decreasing
the mean Au particle size from
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[HAuCl
4
]=0.32 mm
t
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(aging)=4 h
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[HAuCl
4
]=0.63 mm
4
to 2.1 nm increases the TOF
t
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(aging)=8 h
from about 300 to about
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[HAuCl
4
]=1.06 mm
]=3.17 mm
ꢀ
1
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[HAuCl
4
12
21
800 h .
(
The
TOF
value
ꢀ
1
t
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(aging)=16 h
ꢁ800 h ) with the present
[
3
a] Reaction conditions: catalyst 0.20 g; benzyl alcohol 1 mmol; solvent (p-xylene) 5 mL; Ar flow rate catalyst for the oxidant-free de-
ꢀ
1
ꢀ1
mLmin ; T=393 K; t=6 h. [b] If not mentioned, conditions used were: [HAuCl
(aging)=353 K; aging time t(aging)=1 h.
4
]=0.16 mmolL ; aging
hydrogenation of benzyl alco-
hol is better than those mea-
temperature T
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
ACHTUNGTRENNUNG
Table 4. Initial reaction rates with the Au/HT catalysts for varying mean
Au nanoparticle size in the oxidant-free dehydrogenation of benzyl alco-
[
a]
hol.
Au loading
[wt%]
[
b]
Mean Au size
[nm]
Initial reaction rate
[mmolh g (cat.)]
ꢀ
1
ꢀ1
ACHTUNGTRENNUNG
A
H
U
G
R
N
U
G
0
0
0
0
0
0
0
.40
.41
.43
.43
.42
.46
.44
2.1
2.5
3.1
4.0
7.4
11
8.28
5.65
2.74
1.87
0.94
0.58
0.50
12
[
a] Reaction conditions: catalyst 0.10–0.20 g; benzyl alcohol 1–2 mmol;
ꢀ
1
solvent (p-xylene) 5 mL; Ar flow rate 3 mLmin ; T=393 K. [b] Mea-
sured from Figure 6.
Figure 6. Time courses for the conversions of benzyl alcohol during the
initial stage with Au/HT catalysts with varying mean Au nanoparticle
size (*: 2.1, *: 2.5, ~: 3.1, !: &: 7.4,
&
: 11, and ^: 12 nm). Reaction condi-
tions: catalyst 0.10–0.20 g; benzyl alcohol 1–2 mmol; solvent (p-xylene)
ꢀ
1
5
mL; Ar flow rate 3 mLmin ; T=393 K.
the dehydrogenation reaction. In all of the runs over Au/HT
catalysts shown in Figure 6, benzaldehyde selectivity was
always higher than 99%, which further demonstrates that
the Au/HT catalyst was highly selective for the oxidant-free
dehydrogenation of benzyl alcohol. Figure 6 shows that the
conversion of benzyl alcohol increases almost linearly with
reaction time over the initial 40 min. Thus, we can calculate
the initial reaction rate by using the slope of the straight
line for each catalyst. The results are summarized in Table 4.
It is clear that the initial reaction rate increases with de-
creasing mean Au nanoparticle size.
Figure 7. Dependence of TOF on the mean Au nanoparticle size for the
Au/HT-catalyzed oxidant-free dehydrogenation of benzyl alcohol. Reac-
tion conditions: catalyst 0.10–0.20 g; benzyl alcohol 1–2 mmol; solvent
ꢀ
1
(p-xylene) 5 mL; Ar flow rate 3 mLmin ; T=393 K.
Because only the surface Au atoms contribute to the cata-
lytic reactions, we have evaluated the turnover frequency
ꢀ
1
(
TOF), that is, the moles of substrate converted at the initial
sured with many other catalysts such as Ag/HT (590 h ),
ꢀ
1
ꢀ1
stage per mole of surface Au atoms per unit time, for each
Au/HT catalyst. The TOF was calculated by using the initial
reaction rate, and the Au dispersion evaluated by H –O ti-
Ag/Al O (ꢁ10 h ), Cu/HT (<10 h ), and Pd/HT
2
1
3
ꢀ
[32–34]
(36 h ).
We have performed more detailed studies for the
0.40 wt% Au/HT catalyst (with a mean Au size of 2.1 nm),
which exhibits excellent performance in the oxidant-free de-
2
2
tration. The dependence of TOF on the mean size of Au
nanoparticles plotted in Figure 7 suggests that the Au-cata-
1252
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Chem. Eur. J. 2011, 17, 1247 – 1256

Hydrotalcite-Supported Gold Catalyst
FULL PAPER
hydrogenation of benzyl alcohol. TEM measurements for
this catalyst confirm that the mean Au particle size did not
change after the reaction. We measured the Au content in
both the filtrate and the recovered solid catalyst after the re-
action, and confirmed that no leaching of Au occurred
during the reaction.
hols. Tsukuda and co-workers compared the catalytic prop-
erties of colloidal Au nanoclusters of varying size (1.3–
10 nm) stabilized by poly(N-vinyl-2-pyrrolidone) or poly(al-
lylamine) for the dehydrogenation of benzylic alcohols in
the presence of O in water, and found that the smaller Au
2
[6]
nanoclusters exhibited higher catalytic activity. They fur-
ther demonstrated, through various spectroscopic investiga-
tions, that the higher catalytic activity of the smaller Au
nanoclusters was due to the increased negative charge on
the Au core surrounded by the organic stabilizer, which fa-
We also examined the recyclability of this Au/HT catalyst
for the oxidant-free dehydrogenation of benzyl alcohol at
393 K. As shown in Figure 8, the conversion of benzyl alco-
hol and the selectivity of benzaldehyde did not undergo sig-
nificant changes over repeated usage for at least five cycles,
which demonstrates that the present Au/HT catalyst is very
stable.
cilitated the activation of O to superoxo- or peroxo-like
2
[42]
species for alcohol dehydrogenation.
Recently, Tsukuda
and co-workers further disclosed that the catalytic activity
of Au nanoclusters loaded on SBA-15 decreased monotoni-
cally with increasing Au cluster size in the oxidative dehy-
drogenation of benzyl alcohol to benzaldehyde by H O
2
2
[43]
under microwave irradiation. They argued that the activa-
tion of H O might be enhanced by the smaller Au nano-
2
2
clusters. However, with TiO - or CeO -supported Au cata-
2
2
lysts of various Au particle size (from 1.3 to 11.3 nm), the
activity for benzyl alcohol oxidation by O increased with
2
the mean size of Au, and reached a maximum at a mean Au
[44]
size of 6.9 nm.
A comparison among Cu–Mg–Al–O
mixed-oxide-supported Au catalysts with mean Au particle
sizes of 2, 9, and 30 nm also suggested that the catalyst with
a medium mean size (9 nm) was the most active for the
[45]
aerobic oxidation of 1-phenylethanol.
We observed a sharp increase in TOF with the decrease
in the mean Au particle size from 4.0 to 2.1 nm (Figure 7).
In our case, no oxidant was used for the conversion of
benzyl alcohol. Therefore, we believe that Au nanoparticles
activate the substrate (i.e., benzyl alcohol). Undoubtedly,
knowledge about the reaction mechanism would be useful
for understanding the size effect observed in our system. It
can be expected that the dehydrogenation of benzyl alcohol
involves the cleavage of the b-CꢀH bond. This CꢀH bond
Figure 8. Change in catalytic performance of the 0.40 wt% Au/HT (Au
2
.1 nm) catalyst with recycling number, in the oxidant-free dehydrogena-
tion of benzyl alcohol. *: benzyl alcohol conversion,
: benzaldehyde se-
lectivity. Reaction conditions: catalyst 0.20 g; benzyl alcohol 1 mmol; sol-
&
ꢀ
1
vent (p-xylene) 5 mL; Ar flow rate 3 mLmin ; T=393 K; t=4 h.
The size effect and the mechanism of the Au-catalyzed de-
hydrogenation of benzyl alcohol: The most important fea-
ture of Au catalysis is the strong size effect. In CO oxida-
tion, it is well known that the Au particle size exerts a major
effect on catalytic activity; the TOF increases sharply with a
activation, or b-H elimination, has been proposed as the
rate-determining step in the oxidative dehydrogenation of
[1–3]
[46]
decrease in Au diameter from around 5 nm.
The combi-
alcohols by O over several catalysts such as Ru/Al O ,
2
2
3
[47]
[48]
[49]
nation of catalytic results with the aberration-corrected
high-angle annular dark-field-scanning TEM (HAADF-
STEM) studies for Au/FeOx prepared by coprecipitation
suggests that the high catalytic activity for CO oxidation can
be correlated with the presence of Au clusters of around
Pd/hydroxyapatite, Pd/Al O , Pd/SiO –Al O , and the
2 3 2 2 3
hydrogen-transfer dehydrogenation of alcohols catalyzed by
[50]
Cu/Al O
It is reasonable to propose that the b-H elimi-
2
3.
nation also determines the rate in our case. It is well known
that the cleavage of s bonds, including CꢀH bonds, over
[
9]
0
.5 nm in diameter (ꢁ10 Au atoms) on FeO . A recent
metal particles requires coordinative unsaturation of surface
x
[51,52]
study demonstrated that the Au/FeO catalyst containing Au
atoms (edge and corner atoms).
We speculate that this
x
particles with a mean size of (2.1ꢂ0.54) nm prepared by
colloidal deposition is also very active for CO oxidation,
and it is argued that a size of around 0.5 nm for the Au clus-
may result in the size effect observed in Figure 7.
Typically, the fraction of coordinatively unsaturated atoms
[53]
increases with decreasing metal nanoparticle size. By as-
suming a model of Au nanoparticles (e.g., icosahedron, trun-
cated octahedron, or cubooctahedron), the fraction of
higher-coordination-number atoms (face atoms) and lower-
coordination-number atoms (edge and corner atoms) can be
estimated. Icosahedron and truncated octahedron are typi-
cally used as models for unsupported and supported Au
[
39]
ters is not mandatory for achieving the high activity. Gen-
erally, the unique electronic structure and the high fraction
of low-coordinated metal atoms are proposed to account for
the specific catalytic properties of small metal nanopartic-
[
3i,7,40,41]
les,
origin of the structure sensitivity in Au catalysis.
but there is significant debate regarding the
[3,4,7]
[7,54–56]
Several studies have contributed to elucidating the size
effect in Au-catalyzed oxidative dehydrogenation of alco-
nanoclusters.
The fractions of coordinatively unsaturat-
ed atoms and face atoms estimated by using both icosahe-
Chem. Eur. J. 2011, 17, 1247 – 1256
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1253

Q. Zhang, Y. Wang, et al.
Figure 9. Calculated fractions of edge+corner (* and *) Au atoms and
face (& and ) Au atoms on the surfaces of Au nanoparticles with various
sizes. Two Au nanoparticle models (icosahedron (& and *) and truncated
&
[
54,56]
octahedron (
and *)) were used for the calculations.
&
Scheme 1. Possible reaction mechanism for the oxidant-free dehydrogen-
ation of benzyl alcohol over HT-supported Au nanoparticles to give ben-
zaldehyde and H .
2
[54,56]
dral and truncated octahedral models
are plotted in
Figure 9. Both models show that the fraction of coordina-
tively unsaturated Au atoms increases with decreasing Au
particle size, and such an increase becomes particularly sig-
nificant as the Au particle size decreases from around 4 nm.
Comparison of the trends in Figures 7 and 9 suggests that
the variation in TOF for alcohol dehydrogenation with Au
particle size corresponds with the estimated fractions of sur-
face-corner and -edge atoms. In other words, the coordina-
tively unsaturated Au sites on Au nanoparticles play domi-
nant roles in the dehydrogenation of benzyl alcohol.
basic sites. In the second step, the b-H is eliminated by an
Au nanoparticle to produce benzaldehyde. This is expected
to be the rate-determining step, and the coordinatively unsa-
turated Au atoms are more active for the cleavage of this
CꢀH bond. Thus, the catalyst with smaller Au particles ex-
hibits higher activity. The final step is the conversion of Au
hydride to H . For the oxidative dehydrogenation of alco-
2
hols, the hydride species is believed to be oxidized by
[
46–48]
In addition to the coordinatively unsaturated sites on Au
nanoparticles, we have demonstrated that the acidic and
basic sites on the support also play pivotal roles in the dehy-
drogenation of benzyl alcohol. A base is often added as a
promoter into the catalytic system for aerobic oxidation of
O2.
In our case, because no oxidant is consumable, we
d+
speculate that the Brønsted acid sites (i.e., AlO–H ) on
the HT support might participate in the reaction with the
Au hydride to produce molecular H . We will carry out fur-
2
ther studies to elucidate this mechanism in our future work.
[13]
alcohols. For the Au-catalyzed oxidative dehydrogenation
of alcohols, a base additive such as K CO also worked as
2
3
an efficient promoter, especially when the support was not a
Conclusion
[
6,16,21,22,42,43]
basic material, or without a support.
The base
might act to cleave the OꢀH bond of the alcohol to form an
We prepared two series of supported Au catalysts; namely,
Au nanoparticles with uniform mean sizes (2.9–3.6 nm)
loaded onto various supports, and the HT-supported Au
nanoparticles with controlled mean sizes ranging from 2.1 to
[21]
alkoxide intermediate. It has been proposed that coadsor-
bed oxygen atoms on Au surfaces could also act as a Brønst-
[
57]
ed base, thereby facilitating OꢀH bond activation.
The
OꢀH bond was not activated, and no alkoxide was formed,
21 nm. These catalysts were characterized by TEM, H –O2
2
in the absence of such coadsorbed oxygen species on Au sur-
titration, NH -TPD, and CO -TPD. It was shown that soni-
3
2
[57]
faces. Thus, in the case of oxidant-free dehydrogenation
of alcohols (no coadsorbed oxygen species), a base additive
or basic support becomes particularly important. In other
words, the basic sites on HT support in our case should play
a key role in the activation of the OꢀH bond of the alcohol,
cation-aided impregnation was suitable for the preparation
of small Au nanoparticles on SiO , SBA-15, and CNTs,
2
whereas deposition–precipitation with urea or NaOH was
applicable to the preparation of small Au nanoparticles on
many metal oxides, such as Al O , TiO , ZrO , La O , CeO ,
2
3
2
2
2
3
2
to facilitate formation of an adsorbed alkoxide intermediate.
A reaction mechanism that could explain our observations
is shown in Scheme 1. The activation of the OꢀH bond of
MgO, HAP, and HT. We demonstrated that the concentra-
tion of the Au precursor (HAuCl ) solution, the aging tem-
4
perature, and the aging time used in the deposition–precipi-
tation process were the main parameters controlling Au par-
ticle size in the Au/HT catalysts.
The two series of supported Au catalysts were studied in
detail for the oxidant- and acceptor-free dehydrogenation of
benzyl alcohol to elucidate the support and Au particle size
dꢀ
benzyl alcohol by the basic Mg–OH site on the HT sup-
port proceeds in the first step to form the Mg alkoxide inter-
mediate. We speculate that this step may determine the se-
lectivity, although we currently have no information about
the conversion path of benzyl alcohol in the absence of
1254
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 1247 – 1256

Hydrotalcite-Supported Gold Catalyst
FULL PAPER
effects. Among the supports investigated, HT, which has
both strong acidity and strong basicity, provided the best
catalytic performance. Excellent activity and very high ben-
zaldehyde selectivity (>99.5%) were achieved with the Au/
HT catalyst. The acid–base properties of the support were
proposed to play key roles in the dehydrogenation of benzyl
alcohol. Product selectivity, in particular, was strongly af-
fected by the nature of the support.
ature. Then, the calcined HT (carbonate free) was added, with stirring.
Aging for
a certain time resulted exclusively in the deposition of
[
3a]
Au(OH) precipitates on the HT surfaces. After cooling to room tem-
3
perature, the solid was recovered by filtration, followed by washing with
ꢀ
deionized water until no Cl remained. The resulting compound was
dried in air at 393 K overnight, and finally reduced by H
h. The concentration of HAuCl , as well as the aging temperature and
time, affected the mean Au nanoparticle size. The standard conditions
2
at 523 K for
2
4
ꢀ
1
4
were as follows: [HAuCl ]=0.16 mmolL ; aging temperature=353 K;
and aging time=1 h.
Studies of the Au/HT catalysts with varying mean Au
nanoparticle size demonstrated that the Au-catalyzed dehy-
drogenation of benzyl alcohol was a structure-sensitive reac-
tion. The TOF for benzyl alcohol conversion increased with
decreasing mean Au particle size (from 12 to 2.1 nm), and a
particularly steep rise in TOF was observed when the mean
Au nanoparticle size was <4 nm. We propose that the dehy-
drogenation of benzyl alcohol proceeds through the cleav-
age of the OꢀH bond to form an adsorbed alkoxide inter-
Catalyst characterization: Au loadings in all of the catalysts were mea-
sured by ICPMS with an Agilent ICP-MS 4500 instrument after dissolv-
ing the sample in aqua regia. The Mg/Al ratio of our HT was measured
2
to be 1.80. N physisorption at 77 K was carried out with a Micromeritics
Tristar 3000 surface-area and porosimetry analyzer to examine the sur-
face area of each sample. XPS was carried out on a Quantum 2000 Scan-
ning ESCA Microprobe instrument (Physical Electronics) with AlKa radi-
ation. The binding energy was calibrated by using the C1s photoelectron
peak at 284.6 eV as a reference. TEM measurements were performed on
a JEM-2100 electron microscope operated at an acceleration voltage of
2
00 kV. The mean Au particle size in the Au/HT samples was estimated
mediate that undergoes b-H elimination to produce benzal-
dehyde. The basic sites on the support facilitate the forma-
tion of the alkoxide intermediate. The coordinatively unsa-
turated Au atoms are much more active for b-CꢀH bond
from TEM micrographs by counting around 150–200 particles. Au disper-
sions were measured by H –O titration with an ASAP2010C Micromer-
2
2
[
37]
3 2
itics apparatus, according to published procedures. NH -TPD and CO -
TPD were performed on a Micromeritics AutoChem 2920 II instrument.
Typically, the sample loaded in a quartz reactor was pretreated with high-
activation, and this is the basis of the Au size effect. More-
over, the Brønsted acid sites may participate in the transfor-
mation of Au hydride, formed in the b-H elimination step,
to molecular H2.
purity He at 623 K for 1 h. After cooling the sample to 373 K, NH
sorption was performed by switching the He flow to a NH –He (10 vol%
NH ) gas mixture and then maintaining the sample at 373 K for 1 h. The
gas-phase (and/or weakly adsorbed) NH , was purged by high-purity He
at the same temperature. NH -TPD was then performed in the He flow
by raising the temperature to 1073 K at a rate of 10 Kmin . The desor-
bed NH molecules were detected by using a ThermoStar GSD 301 T2
3
ad-
3
3
3
3
ꢀ
1
3
2
mass spectrometer at the signal of m/z 16. CO -TPD was performed by
using a similar procedure.
Experimental Section
Catalytic reaction: The dehydrogenation of benzyl alcohol was carried
out by using a batch-type reaction vessel with a reflux condenser. Typi-
cally, the powdered catalyst (typically, 0.20 g) was added to the alcohol
with the solvent (p-xylene), and then high-purity Ar gas (99.999%) was
Catalyst preparation: The target Au loading for each catalyst used in this
work was 0.5 wt%, unless otherwise stated. Au/SiO
Au/CNT were prepared by sonication-aided impregnation. The calculated
amount of support was suspended in an aqueous solution of HAuCl , and
the suspension was placed into the sonication bath with irradiation at
0 kHz and 200 W output power. After sonication, the water was re-
moved by evaporation at 353 K. The solid powder was finally reduced in
at 523 K for 2 h. Au/Al , Au/TiO , Au/ZrO , Au/MgO, Au/La
Au/CeO , and Au/HAP were prepared by deposition–precipitation with
urea as the precipitant. In brief, urea with a molar ratio of urea/Au of
00:1 was added to an aqueous solution of HAuCl . After addition of the
2
, Au/SBA-15, and
4
bubbled into the mixture. After purging with Ar gas (flow rate
ꢀ1
8
mLmin ) for 1 h to remove the remaining air in the reaction system,
4
the mixture was heated to the reaction temperature with stirring. The
ꢀ1
flow rate of Ar was typically kept at 3 mLmin during the reaction.
H
2
2
O
3
2
2
2 3
O ,
After the reaction, the catalyst was separated by centrifugation, and the
liquid products were analyzed by GC (Shimazu GC-14B) after the addi-
tion of an internal standard.
2
1
4
support, the suspension was placed in a water bath at 353 K and was vig-
orously stirred. After aging for a certain time (6–10 h), the solid product
was recovered by filtration followed by thorough washing with deionized
ꢀ
water until no Cl could be detected. The resulting compound was dried
Acknowledgements
2
in air at 393 K for 1 h, and finally reduced in H at 523 K for 2 h.
HTs are layered materials composed of positively charged two-dimen-
sional sheets of mixed hydroxides with water and exchangeable charge-
compensating anions, and their general formula can be expressed as
This work was supported by the National Natural Science Foundation of
China (nos. 20625310, 20773099, 20873110, and 20923004), the National
Basic Research Program of China (no. 2010CB732303), the Research
Fund for the Doctoral Program of Higher Education (no.
20090121110007), and the Key Scientific Project of Fujian Province
(2009HZ0002-1). We thank Professors H. B. Zhang and G. D. Lin for
providing the CNTs.
2
+
3+
x+
nꢀ
[58]
[
M
1ꢀx
M
x
(OH)
(OH)16(CO
precipitation method with a mixed solution of Mg and Al chlorides as
2
]
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(A
)
x/n·mH
2
O.
The most popular HT is
Mg
6
Al
2
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
3
)·mH
2
O. We prepared our Mg–Al HT by a simple co-
[
58]
the precursors.
Precipitation occurred after adding a solution of
Na CO and NaOH. The suspension was stirred and aged at 338 K for
2
3
1
8 h. After cooling to room temperature, the resulting material was fil-
ꢀ
tered and washed with deionized water until no Cl could be detected.
The solid was dried in air at 393 K overnight, and then calcined in air at
[
[
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[
58]
precipitation process owing to the “memory effect” of HT.
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lows: The pH of the aqueous solution of HAuCl was first adjusted to 8.0
by NaOH, and the solution was placed in a water bath at a fixed temper-
4
Chem. Eur. J. 2011, 17, 1247 – 1256
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