Thiolate-mediated selectivity control in aerobic alcohol oxidation by porous carbon-supported Au25 clusters

Article abstract of DOI:10.1021/cs501010x

Supported Au₂₅ clusters were prepared through the calcination of Au₂₅(SC₁₂H₂₅)₁₈ on hierarchically porous carbon nanosheets under vacuum at temperatures in the range of 400-500 °C for 2-4 h. TEM and E

Full text of DOI:10.1021/cs501010x

Letter
pubs.acs.org/acscatalysis
Thiolate-Mediated Selectivity Control in Aerobic Alcohol Oxidation
by Porous Carbon-Supported Au Clusters
2
5
†
‡,§
‡
‡
†
Tatchamapan Yoskamtorn, Seiji Yamazoe, Ryo Takahata, Jun-ichi Nishigaki, Anawat Thivasasith,
Jumras Limtrakul,†,∥,⊥ and Tatsuya Tsukuda*,‡,§
†Department of Chemistry and NANOTEC Center for Nanoscale Materials Design for Green Nanotechnology, Faculty of Science,
Kasetsart University, Bangkok 10900, Thailand
‡
Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
§
Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Katsura, Kyoto 615-8520, Japan
∥
Center for Advanced Studies in Nanotechnology and Its Applications in Chemical, Food and Agricultural Industries, Kasetsart
University, Bangkok 10900, Thailand
⊥
PTT Group Frontier Research Center, PTT Public Company Limited, 555 Vibhavadi Rangsit Road, Chatuchak, Bangkok 10900,
Thailand
*
S Supporting Information
ABSTRACT: Supported Au25 clusters were prepared through the
calcination of Au (SC H ) on hierarchically porous carbon nanosheets
2
5
12 25 18
under vacuum at temperatures in the range of 400−500 °C for 2−4 h.
TEM and EXAFS analyses revealed that the thiolate coverage on Au25
gradually decreased with increasing calcination temperature and period
and became negligibly small when the calcination temperature exceeded
500 °C. The catalysis of these Au25 clusters was studied for the aerobic
oxidation of benzyl alcohol. Interestingly, the selectivity for benzaldehyde
formation was remarkably improved with the increase in the amount of
residual thiolates on Au , while the activity was reduced. This observation
2
5
is attributed to the dual roles of the thiolates: the reduction of the
oxidation ability of Au25 by electron withdrawal and the inhibition of the
esterification reaction on the cluster surface by site isolation.
KEYWORDS: Au25 clusters, carbon support, thiol, aerobic alcohol oxidation, selectivity
INTRODUCTION
reduced coverage because the ligands usually act as site-
■
blocking agents or catalytic poisons. Partial removal of the
ligands from Au (SR) and Au (PR ) Cl makes them
Ultrasmall (≈1 nm) metal clusters are promising candidates for
novel catalytic sites owing to their unique geometrical and
electronic structures. Current interest has been focused on gold
clusters because they show size-specific catalysis when the
2
5
18
11
7,18
3 7
3
1
catalytically active for CO oxidation,
although it has been
reported that Au clusters fully protected by thiolates also act as
2
,19−21
1
−4
catalysts.
Improvement in catalytic selectivity by the
2
diameter is smaller than 2 nm.
For an understanding of the
2−25
ligand monolayer has been demonstrated.
These observa-
origin of catalysis and optimization of the catalytic performance,
key structural parameters such as the number of constituent
atoms (cluster size) and chemical compositions must be
defined with atomic precision. We and others have successfully
synthesized a series of well-defined Au clusters such as Au ,
tions suggest that the chemical modification of the cluster
surface is another promising approach for controlling the
catalytic performance.
In this work, we study the effect of dodecanethiolates
referred to as C12S hereafter) on the catalytic activity and
11
(
Au , Au , Au Pd, Au , and Au on solid supports by
1
3
25
24
55
144
selectivity of Au25 clusters for the aerobic oxidation of benzyl
alcohol. The surface coverage of C12S on Au25 was controlled
by changing the calcination temperature and period of
Au (SC12) . As a support, we used hierarchically porous
removing the thiolate (RS) or phosphine (R P) ligands
originally acting as a protecting layer.
supported catalysts, the effects of cluster size and single-atom
doping on catalysis have been demonstrated.
Organic ligands on the cluster surface not only impose steric
restriction on the accessibility of reactants but also modulate
3
5
−12
By using these
25
18
carbon nanosheets (HPCSs) with a typical BET surface area as
2
large as 2300 m /g. This porous carbon allowed us to calcine a
13
the electronic states of the clusters. As a result, the monolayer
of the ligands plays vital roles in catalysis, such as chiral
modification, molecular recognition, site isolation, and charge
Received: July 16, 2014
Revised: August 30, 2014
Published: September 18, 2014
1
4−16
transfer.
However, these effects will come into play at a
©
2014 American Chemical Society
3696
dx.doi.org/10.1021/cs501010x | ACS Catal. 2014, 4, 3696−3700

ACS Catalysis
Letter
1
0
larger amount of Au (SC12) than on other supports without
sintering.
at 450 °C for 2 h. This difference is probably due to the much
2
5
18
2
larger surface area of the HPCSs (2300 m /g) than CNTs
2
(
200−300 m /g) as well as the higher population of defective
RESULTS AND DISCUSSION
Supported Au25 clusters with various coverages of C12S were
synthesized as summarized briefly below. First, Au (SC12)
sites on which the clusters can be strongly anchored or
immobilized. The detailed structures of the Au clusters
prepared under different calcination conditions were studied
■
26
25
18
using Au L −edge EXAFS (Figure S5) and Fourier transformed
FT) EXAFS (Figure 2). The structural parameters obtained by
and HPCSs were synthesized independently according to the
3
2
7,28
(
reported methods.
The successful synthesis of molecularly
pure Au (SC12) was confirmed by UV−vis spectroscopy,
25
18
matrix-assisted laser desorption ionization (MALDI) mass
spectrometry, and transmission electron microscopy (TEM), as
2
6,29
shown in Figure S1 (Supporting Information (SI)).
Scanning electron microscopy (SEM), TEM, N2 sorption
measurements, and Raman spectroscopy demonstrated that
HPCSs possessed a nanosheet-aggregate structure with not
only a large surface area, but also high micro- and mesoporosity
2
6
(
SI Figure S2). Then, Au (SC12) clusters were adsorbed
25 18
onto HPCSs by mixing them in toluene. The Au (SC12) /
2
5
18
HPCS composite collected by filtration was dried in vacuum for
h. The Au loading with respect to the support was
determined from the optical absorbance to be 0.2 or 1.0 wt
. Finally, the composite was calcined under vacuum in the
4
%
temperature range 400−500 °C for 2−4 h. Hereafter, the
resulting catalysts will be referred to as XAu (SC12) /HPCS-
2
5
n
Y-Z, where X, Y, and Z represent the Au loading (wt %),
calcination temperature (°C), and calcination period (h),
respectively.
Figure 2. FT of Au L -edge EXAFS of (a) 1.0Au (SC12) /HPCS
3
25
18
Figure 1 shows typical TEM images of 0.2Au (SC12) /
and 1.0Au (SC12) /HPCS-Y-Z with (Y, Z) = (400, 2), (450, 2),
25
n
25 n
HPCS-500-4 and 1.0Au (SC12) /HPCS-500-4, which were
(450, 4), (500, 2) and (500, 4) for (b)−(f), respectively.
2
5
n
the curve fitting analysis of Figure 2 are listed in Table 1. Table
1
shows a gradual decrease in the coordination number (CN)
values for the Au−S bonds with increasing calcination
temperature and period. Calcination at 500 °C for >2 h is
required for complete removal of the thiolates on HPCSs, while
the thiolates are completely removed at 450 °C for 2 h on
CNTs. This may be due to the stronger interaction between
Au (SC12) and HPCSs with rough surfaces and porous
10
25
18
frameworks than CNTs with smooth surfaces. In contrast, the
CN value for the AuAu bond before the calcination is much
smaller than that expected for Au (SC12) and increases with
2
5
18
12
the calcination. Underestimation of the CN value for the
AuAu bond in 1.0Au (SC12) /HPCS has been ascribed to
25
n
the fact that the AuAu bonds in Au (SC12) , having
2
5
18
30
multiple lengths due to various coordination environments
and thermal fluctuation, were fitted by assuming a single
3
1
32
length. Thus, the increase in the CN value does not indicate a
growth in cluster size, as demonstrated in Figure 1b, but rather
an equalization of the AuAu bond lengths during the
reconstruction of the Au framework. The CN values of ≈7 after
calcination at 500 °C for 4 h are reasonable for Au . Mass
spectra of the desorbed species (SI Figure S3) showed that
the C12S ligands are desorbed mainly in the form of 1-
dodecene, 1-dodecanethiol, and dodecane in the temperature
range of 240−500 °C. The observation of sulfur-free species is
probably due to thermal decomposition of thiols and disulfides
Figure 1. Typical TEM images and size distributions of Au clusters in
(
a) 0.2Au (SC12) /HPCS-500-4 and (b) 1.0Au (SC12) /HPCS-
25 n 25 n
500-4.
prepared under the harshest calcination conditions employed in
this work. The Au clusters were highly dispersed on the porous
carbon supports. The average diameters of the Au clusters of
both samples were determined to be 1.2 ± 0.3 nm, which are
comparable to that of the initial Au (SC12) (1.1 ± 0.3 nm).
2
5
26
2
5
18
This result indicates that the aggregation of clusters is negligible
even after the calcination of 1.0 wt % of Au (SC12) on
10
that are desorbed from Au (SC12)
but are captured by
2
5
18
25
18
HPCSs at 500 °C for 4 h. Therefore, it is safe to conclude that
the aggregation did not occur at the lower calciantion
temperature or in the shorter calcination period. This is in
sharp contrast to the results of the calcination of 1.0 wt %
Au (SC12) on carbon nanotubes (CNTs); they were
porous carbon support.
The catalytic performances of XAu (SC12) /HPCS-Y-Z
prepared under various calcination conditions were compared
in the aerobic oxidation of benzyl alcohol. Table 2 summarizes
the results after 6 h of the reaction. Pristine HPCSs and
uncalcined composite Au (SC12) /HPCS did not exhibit any
25
n
2
5
18
converted to larger (>2 nm) Au nanoparticles upon calcination
25
18
3
697
dx.doi.org/10.1021/cs501010x | ACS Catal. 2014, 4, 3696−3700

ACS Catalysis
Letter
Table 1. Results of Curve Fitting Analysis of Au L -edge EXAFS
3
b
c
d
2
e
f
catalyst
atom
CN
r (Å)
Δσ
R factor (%)
1
.0Au (SC12) /HPCS
S
1.6(2)
0.6(3)
0.6(1)
3.1(4)
0.4(1)
6.4(9)
0.2(1)
6.4(9)
7.4(9)
2.309(4)
2.691(5)
2.303(5)
2.800(3)
2.228(7)
2.799(3)
2.256(11)
2.817(3)
2.796(3)
2.805(3)
0.0004(10)
0.0002(47)
0.0001(11)
0.0007(6)
0.0001(9)
0.0028(10)
0.0006(102)
0.0015(9)
0.0022(8)
14.9
25
18
Au
S
a
a
a
(1.0, 400, 2)
(1.0, 450, 2)
(1.0, 450, 4)
4.9
8.6
8.2
Au
S
Au
S
Au
Au
Au
a
a
(
1.0, 500, 2)
1.0, 500, 4)
16.0
13.5
(
7.0(8)
0.0015(6)
a
b
c
d
e
2
(
X, Y, Z) for XAu (SC12) /HPCS-Y-Z. Bonding atom. Coordination number. Bond length. Relative Debye−Waller factor: Δσ
=
25
n
2
f
data
fit
2
data 2 1/2
(σsample−σreference) . R factor = (Σ(χ − χ ) /Σ(χ ) ) .
Table 2. Catalytic Performances of XAu (SC12) /HPCS-Y-
Z for Aerobic Oxidation of Benzyl Alcohol
catalytic activity (Entries 1 and 2), which indicates that HPCSs
are not involved in the catalysis and that a thiolate layer
completely suppresses the catalyst and acts as a site-blocking
reagent. The calcined XAu (SC12) /HPCS-Y-Z catalysts
2
5
n
a
2
5
n
produced benzaldehyde (1), benzoic acid (2), and benzyl
benzoate (3). The conversion increased gradually with
increasing temperature and period of calcination at both
loadings of 0.2 (Entries 3−7) and 1.0 wt % (Entries 9−13).
The conversions reached 84 and 98% for 0.2Au (SC12) /
d
selectivity (%)
d
entry
catalyst
conversion (%)
1
2
3
2
5
n
1
2
3
4
5
6
7
8
9
HPCS
0
0
0
0
0
0
HPCS-500-4 (Entry 7) and 1.0Au (SC12) /HPCS-500-4
25 n
0.2Au (SC12) /HPCS
0
0
(Entry 13), respectively. The calcination-induced enhancement
of the catalytic activity can be ascribed to the exposure of active
sites upon the removal of the thiolates. The Au clusters (1.2 ±
2
5
18
b
b
b
b
b
(0.2, 400, 2)
(0.2, 450, 2)
(0.2, 450, 4)
(0.2, 500, 2)
(0.2, 500, 4)
3
67
97
98
97
74
40
98
97
90
67
15
95
33
0
0
4
3
25
10
3
0
2
0.2 nm) on CNTs showed higher activities than those on
HPCSs under the same calcination condition (Entries 4 and 8).
This is because of the easier removal of thiolates on CNTs with
a smooth surface than on HPCSs with an entirely rough surface
and 3D porous structure (SI Figure S2). In a recycling test
11
84
20
19
33
69
84
98
63
2
1
21
7
5
0.2Au /CNT-450−2
53
1
2
5
b
b
b
b
b
(1.0, 400, 2)
(1.0, 450, 2)
(1.0, 450, 4)
(1.0, 500, 2)
(1.0, 500, 4)
(1.0, 450, 4)
1
1
1
1
1
1
0
1
2
3
4
1
2
(
Entries 11 and 14), the conversion was reduced only slightly,
7
3
but the selectivity for 1 was still high (>90%). This indicates
that the catalysts are stable and can be reused.
22
70
2
11
15
3
It appears from Table 2 that there is a trade-off between
selectivity and conversion. At low conversion, 1 is the major
product for both loadings of 0.2 (Entries 3−6) and 1.0 wt %
b,c
a
Reaction conditions: amount of catalyst: 5 mg; amount of
PhCH OH: 1.2 μL; amount of K CO : 1 mg; volume of water: 1
mL; temperature: 30 °C; O pressure: 1 atm; reaction time 6 h. (X, Y,
Z) for XAu (SC12) /HPCS-Y-Z. Recovered from Entry 11.
Determined by gas chromatography.
2
2
3
(
Entries 9−11). In contrast, the selectivity for 1 was reduced,
b
2
c
and those for 2 and 3 were increased at higher conversions for
2
5
n
d
both loadings of 0.2 (Entry 7) and 1.0 wt % (Entries 12, 13),
33
suggesting that the reaction proceeds sequentially. In order to
evaluate more rigorously the catalytic performance for the
sequential reaction, we have to compare the selectivities at
a
Table 3. Comparison of Selectivity at Similar Conversions
d
selectivity (%)
d
entry
catalyst
reaction time (h)
conversion (%)
1
2
3
b
1
2
3
4
5
6
7
8
(0.2, 400, 2)
24
15
20
37
33
32
68
69
77
97
40
95
97
41
71
90
40
2
7
1
53
4
0.2Au /CNT-450−2
6
2
5
b
b
(1.0, 400, 2)
(1.0, 450, 2)
(1.0, 500, 4)
(1.0, 450, 2)
(1.0, 450, 4)
(1.0, 500, 4)
30
1
6
1
2
b
,
c
6
30
18
11
7
41
18
3
b
b
b
6
10 min
47
13
a
Reaction conditions: amount of catalyst: 5 mg; amount of PhCH OH: 1.2 μL; amount of K CO : 1 mg; volume of water: 1 mL; temperature: 30
2
2
3
b
c
d
°C; O2 pressure: 1 atm. (X, Y, Z) for XAu (SC12) /HPCS-Y-Z. The amount of catalyst was reduced to 1 mg. Determined by gas
25 n
chromatography.
3
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ACS Catalysis
Letter
comparable conversions. Table 3 compares the selectivities of
vigorous stirring. After 1 h, the Au (SC12) /HPCS
2
5
18
products 1−3 at three conversions of 15−20% (Entries 1, 2),
composites were filtrated and dried in vacuum for 4 h at
room temperature. The dried composite in a quartz boat was
subsequently placed in a furnace and calcined under vacuum
with the following steps: (1) 50 °C for 30 min and (2) 400,
450, or 500 °C for 2 or 4 h. The final products were collected
and kept in vials.
3
2−37% (Entries 3−5), and 68−77% (Entries 6−8). As
expected, similar conversions were achieved in a shorter
reaction time with further calcination of 1.0Au (SC12) /
25
n
HPCS owing to the suppression of site-blocking effects by the
thiolates. The selectivity for 1 was higher with less calcination,
whereas those for 2 and 3 increased significantly upon
calcination. This result suggests that the residual thiolates
enhance the selectivity for 1 by suppressing the oxidation
reaction of 1 to 2 and the esterification reaction between 1 and
Catalytic Test. The aerobic oxidation of benzyl alcohol was
performed under the conditions modified from refs 7 and 10.
Typically, benzyl alcohol (1.2 μL, 11.6 μmol) and K CO (1
2
3
mg) were mixed well in H O (1 mL) in a test tube. The
2
23
2
on the cluster surface. There are two possible effects of the
mixture was then transferred to the synthesizer under vigorous
residual thiolates on the catalysis: steric and electronic effects.
The gradual decrease of absorption threshold energies at the Au
L3-edge with the calcination temperature and period (SI Figure
S4) suggests that Au25 clusters with the residual thiolates are
more positively charged than the bare Au owing to electron
stirring at 30 °C. The catalyst (5 mg) was added into the
solution before purging with O gas (1 atm). After the desired
2
reaction time, the mixture was quenched with two drops of 2 M
aqueous HCl solution and extracted with toluene. The obtained
organic layer was analyzed with a gas chromatograph with a
frame ionization detector by using an external standard method.
The entire mass balance was greater than or equal to 95%. The
conversion of benzyl alcohol is defined as the percentage of the
total amount of benzyl alcohol consumed in the oxidation
reaction to the total amount of benzyl alcohol at the initial time.
The selectivity of the reaction is denoted as the ratio of benzyl
alcohol converted to the corresponding products.
2
5
13
withdrawal by the thiolates. Thus, the suppression of
oxidation of 1 to 2 is ascribed to the fact that the Au clusters
25
are more positively charged. This trend is in parallel with the
previous observation that the oxidation ability of Au clusters
(
1.4 nm) is significantly reduced by using less-electron-
34
donating polymer as a stabilizer. The suppression of the
formation of 3 by thiolates can be explained by the site-
isolation effect. This effect of thiolates has been also proposed
in the oxidation of 1 over silica-supported Au nanoparticles (5.5
nm) but at a low conversion (<10%). The present
observation clearly demonstrates that catalytic performance of
small metal clusters can be controlled through the modulation
of their electronic structures and steric environments by
chemical modification.
ASSOCIATED CONTENT
Supporting Information
■
2
3
*
S
Details of experimental procedures and characterization results.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
E-mail: tsukuda@chem.s.u-tokyo.ac.jp.
■
CONCLUSION
■
*
We have demonstrated herein that the coverage of dodeca-
nethiolates (C12S) on Au25 plays a vital role not only in the
catalytic activity but also in the selectivity for the aerobic
oxidation of benzyl alcohol (BA). The Au25 clusters fully
covered by the thiolates were inert owing to the site-blocking
effect. On the contrary, thiolate-free Au25 clusters oxidized BA
efficiently, while showing poor chemoselectivity. Interestingly,
the selectivity for benzaldehyde formation was significantly
improved by the presence of residual thiolates on Au ,
although the activity was reduced. It is proposed that the roles
of thiolates are to reduce the oxidation ability of Au25 by
electron withdrawal and to prohibit the esterification reaction
on the cluster surface by site isolation. These findings
demonstrate that chemical modification of the cluster surface
is another important approach for controlling the catalysis of
metal clusters.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Prof. Hiroshi Nishihara (The University of Tokyo)
for allowing us to access the TEM apparatus. This research was
supported financially by the Funding Program for Next
Generation World-leading Researchers (NEXT Program, GR-
003) and “Elements Strategy Initiative for Catalysts & Batteries
(ESICB)”. T.Y. thanks the Development and Promotion of
Science and Technology Talents Project (DPST) from the
Ministry of Science and Technology, the Ministry of Education,
and the Institute for the Promotion of Teaching Science and
Technology (IPST) of Thailand. This work was partially
supported by grants from the National Science and Technology
Development Agency (NANOTEC Center for Nanoscale
Materials Design for Green Nanotechnology funded by the
National Nanotechnology Center), the Commission on Higher
Education, Ministry of Education (the “National Research
University Project of Thailand (NRU)”, and the “National
Center of Excellence for Petroleum, Petrochemical and
Advanced Materials (NCE-PPAM)”).
25
EXPERIMENTAL METHODS
■
Preparation of Catalysts. Typical syntheses of
26−28
Au (SC12) and HPCSs followed previous reports.
A
2
5
18
mixture of 100 mg HPCSs in 200 mL of toluene was sonicated
for 1 h to remove impurities. The suspension was filtrated and
transferred to another flask. After adding the same amount of
toluene, the mixture was cooled to 0 °C and stirred for 20 min.
Then, the toluene suspension of HPCSs was stirred and cooled
to 0 °C for 20 min. Then, the calculated amount of
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dx.doi.org/10.1021/cs501010x | ACS Catal. 2014, 4, 3696−3700