ZnAl-Hydrotalcite-Supported Au25Nanoclusters as Precatalysts for Chemoselective Hydrogenation of 3-Nitrostyrene

Article abstract of DOI:10.1002/anie.201610736

Chemoselective hydrogenation of 3-nitrostyrene to 3-vinylaniline is quite challenging because of competitive activation of the vinyl group and the nitro group over most supported precious-metal catalysts. A precatalyst comprised of thiolated Au₂₅nanoclusters supported on ZnAl-hydrotalcite yielded gold catalysts of a well-controlled size (ca. 2.0 nm)—even after calcination at 500 °C. The catalyst showed excellent selectivity (>98 %) with respect to 3-vinylaniline, and complete conversion of 3-nitrostyrene over broad reaction duration and temperature windows. This result is unprecedented for gold catalysts. In contrast to traditional catalysts, the gold catalyst is inert with respect to the vinyl group and is only active with regard to the nitro group, as demonstrated by the results of the control experiments and attenuated total reflection infrared spectra. The findings may extend to design of gold catalysts with excellent chemoselectivity for use in the synthesis of fine chemicals.

Full text of DOI:10.1002/anie.201610736

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201610736
German Edition: DOI: 10.1002/ange.201610736
Hydrogenation Reactions
ZnAl-Hydrotalcite-Supported Au Nanoclusters as Precatalysts for
2
5
Chemoselective Hydrogenation of 3-Nitrostyrene
Yuan Tan, Xiao Yan Liu,* Leilei Zhang, Aiqin Wang, Lin Li, Xiaoli Pan, Shu Miao,
Masatake Haruta, Haisheng Wei, Hua Wang, Fangjun Wang, Xiaodong Wang, and Tao Zhang*
[
1,3]
Abstract: Chemoselective hydrogenation of 3-nitrostyrene to
-vinylaniline is quite challenging because of competitive
molecule.
Generally, preferential activation of the nitro
3
group can be promoted by modifying the platinum-group
activation of the vinyl group and the nitro group over most
supported precious-metal catalysts. A precatalyst comprised of
thiolated Au25 nanoclusters supported on ZnAl-hydrotalcite
yielded gold catalysts of a well-controlled size (ca. 2.0 nm)—
even after calcination at 5008C. The catalyst showed excellent
selectivity (> 98%) with respect to 3-vinylaniline, and com-
plete conversion of 3-nitrostyrene over broad reaction duration
and temperature windows. This result is unprecedented for
gold catalysts. In contrast to traditional catalysts, the gold
catalyst is inert with respect to the vinyl group and is only active
with regard to the nitro group, as demonstrated by the results of
the control experiments and attenuated total reflection infrared
spectra. The findings may extend to design of gold catalysts
with excellent chemoselectivity for use in the synthesis of fine
chemicals.
metal with other metals. For example, a RhIn/SiO catalyst
showed good catalytic performance under mild conditions.
2
[4]
However, activation of the C=C bond in styrene was not easy
to avoid over a prolonged reaction period, according to the
results of the control experiment. Moreover, preferential
activation of the nitro group could also be promoted by the
[
5,6]
addition of phosphorous additives.
But the use of harmful
modifiers would be detrimental to sustainable chemistry.
Herein, we try to solve this problem by developing a mono-
metallic catalyst that only activates the nitro group instead of
other unsaturated groups.
Supported Au nanocatalysts are notable for their high
[
7]
selectivity in many catalytic redox reactions. Nitrostyrene,
bearing nitro and vinyl groups in the same molecule, is one of
the most important and typical functionalized nitroarenes for
fundamental investigation. Notably, chemoselective hydro-
genation of the nitro group is quite demanding in nitrostyrene
because of the facile hydrogenation of the C=C bond. In 2006,
F
unctionalized aromatic amines are important industrial
intermediates for pharmaceuticals, agrochemicals, fine chem-
[1]
[8]
icals, dyes, and polymers. They are generally synthesized by
chemoselective hydrogenation of the corresponding function-
alized nitroarenes. Previously, we developed a rationally
Corma et al first reported that Au/TiO2 and Au/Fe O
2
3
catalysts are much more selective for hydrogenation of
3-nitrostyrene to 3-vinylaniline than Pd/C and Pt/C catalysts.
When the conversion of 3-nitrostyrene attained 98.5% and
95.2%, the selectivities to 3-aminostyrene were 95.9% and
95.1% over Au/TiO and Au/Fe O , respectively. However,
designed FeO -supported Pt single-atom and pseudo single-
x
atom catalyst and found that it showed outstanding reactivity
for hydrogenation of nitro groups, while the hydrogenation of
other unsaturated functional groups was unavoidable when
the reaction time was prolonged and/or the temperature was
2
2
3
the selectivity at complete conversion of 3-nitrostyrene was
[9]
not provided. Subsequently, Matsushima et al. and Shimizu
[2]
[10]
raised. This is a common problem for most catalysts because
they are active for the reduction of the nitro group as well as
for the hydrogenation of other unsaturated functional groups
et al. found that the performance of the Au catalysts were
highly dependent on the particle sizes of Au. When the Au
[9]
particle sizes were decreased to 2.7 nm for Au-BP(DR) and
[
10]
(
for example, C=C, C=O, and so forth) existing in the same
2.5 nm for Au/Al O , the catalysts showed better activity
2 3
than that with larger sizes. Nevertheless, at complete con-
version of 4-nitrostyrene, the selectivity to 4-aminostyrene
obviously decreased with prolonged reaction time (for
[
*] Y. Tan, Prof. X. Y. Liu, Dr. L. Zhang, Prof. A. Wang, Dr. L. Li, X. Pan,
Prof. S. Miao, Prof. M. Haruta, Dr. H. Wei, H. Wang, Prof. F. Wang,
Prof. X. Wang, Prof. T. Zhang
[10]
example, from ca. 90% to ca. 60% within about 4 h.
Although Au catalysts possess great potential when it comes
to promoting high chemoselectivity, to date, no breakthrough
has been made that attains excellent chemoselectivity in
conjunction with 100% conversion of nitrostyrene.
State Key Laboratory of Catalysis, iChEM, Gold Catalysis Research
Center, Dalian Institute of Chemical Physics
Chinese Academy of Sciences
Dalian 116023 (China)
E-mail: xyliu2003@dicp.ac.cn
Recently, atomically precise gold nanoclusters (Au NCs)
taozhang@dicp.ac.cn
have attracted intensive interest owing to their small and
Y. Tan
[11]
uniform size.
Thiolated Au25 NCs are one of the most
University of Chinese Academy of Sciences
Beijing 100049 (China)
typical Au NCs that have been applied to synthesize
supported Au catalysts with well-controlled size. It has been
reported that the thermostability of Au NCs depends highly
Prof. M. Haruta
Department of Applied Chemistry, Tokyo Metropolitan University
Hachioji, Tokyo, 192-0397 (Japan)
[
12,7f–g]
on the nature of the support.
However, the role of the
residual S and the origin of the stability of Au NCs has not yet
been clarified. To date there have been no reports on
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201610736.
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supported thiolated Au NCs as precatalysts for chemoselec-
tive hydrogenation of functionalized nitroarenes to aromatic
amines.
Herein, we chose the thiolated Au25 NCs as the precursor
for synthesizing Au NCs with well-tuned size and ZnAl-
hydrotalcite (ZnAl-HT) as a precursor for the support used to
stabilize the Au NCs against calcination. The catalyst calcined
at the optimized temperatures showed excellent chemo-
selectivity to the 3-vinylaniline at 100% conversion of the
3
-nitrostyrene in wide reaction period and temperature
windows. This result is unprecedented for gold catalysts.
Unlike the catalytic systems previously reported in the
[
1–3,13]
literature,
where the nitro group and the vinyl group
were competitively adsorbed and activated, our results
showed that C=C is hardly activated on our Au catalyst,
Figure 2. Evolution of the product distribution with reaction time over
leading to high chemoselectivity for hydrogenation of the
nitro group. This work could provide useful methods for
synthesis of Au NCs with well-controlled size, and catalyst
fabrication with high chemoselectivity.
the Au /ZnAl-HT-300 catalyst.
2
5
a linear increase with reaction time, accompanied by a linearly
decreasing concentration of 3-nitrostyrene. Until the sub-
strate was completely converted, around the 200 min mark,
other byproducts could hardly be detected. Even the reaction
time was prolonged to 900 min with selectivity maintained at
ꢀ 99%. Moreover, when the reaction temperature was raised
from 90 to 1358C, a 98.1% selectivity to 3-vinylaniline was
still attained with prolonged reaction time at complete
conversion of 3-nitrostyrene (Table 1, entry 2). These results
obviously show that the Au /ZnAl-HT-300 catalyst favored
hydrogenation of the nitro group instead of the C=C bond. In
contrast, severe over-hydrogenation of 3-nitrostyrene to
3-ethylphenylamine over the Au/TiO catalyst (provided by
the World Gold Council (WGC)) happened under the same
reaction conditions (Table 1, entry 3,4). The chemoselectivity
of our catalyst far exceeded those of supported Pt or Pd
catalysts (Table 1, entry 5–7) when the conversion of the
substrates was close to 100%. Although the rationally
Cysteine-capped Au25 (denoted as Au (Cys) ) was syn-
25
18
[14]
thesized by a NaOH-mediated NaBH reduction method.
4
The UV/Vis spectrum (Supporting Information, Figure S1)
showed four peaks positioned at 440, 545, 670, and 780 nm,
which suggested that the Au25 NCs were successfully
obtained. The as-prepared Au (Cys) was deposited onto
2
5
18
ZnAl-HT by an impregnation method with a Au loading of
.1 wt%; the resulting catalysts were denoted as Au /ZnAl-
1
2
5
HT-x (x signifies the calcination temperature). When the
x values were between 300–5008C, the catalysts showed
excellent chemoselectivity (ꢀ 99%) to 3-vinylaniline at 100%
conversion of 3-nitrostyrene (Figure 1). At the same time, the
turnover frequency (TOF) also attained a plateau at the
highest level. When the calcination temperature decreased to
2
5
2
2
008C or increased to 600–7008C, the catalytic activity
decreased correspondingly.
The Au /ZnAl-HT-300 catalyst was taken as an example
2
5
to study the evolution of the product distribution during
chemoselective hydrogenation of 3-nitrostyrene, as shown in
Figure 2. The desired product (3-vinylaniline) presented
designed Ag@CeO system could achieve high selectivity at
complete conversion of the nitro compounds bearing C=C
2
Table 1: Hydrogenation of 3-nitrostyrene using different catalysts.
[
e]
Entry
Catalyst
T
8C]
Conv.
[%]
Yield [%]
[
e]
[
B
C
D
[
a]
a]
1
2
3
4
5
6
7
Au /ZnAl-HT-300
90
135
90
135
80
120
120
100
100
100
100
100
99.0
96.7
99.0
98.1
86.3
20.7
0
1.0
1.9
12.6
52.7
~32.0
23.8
0
0
1.1
2
5
[
Au /ZnAl-HT-300
2
5
[
a,b]
1.5 wt%Au-TiO2
1.5 wt%Au-TiO
1 wt% Pt/ TiO
Pd/ C
[
a,b]
2.1
2
[
c]
~62.0
75.1
43.3
2
[
d]
0
2.8
[
d]
Pt/ C
50.6
Figure 1. The conversion, selectivity, and TOF values (line) over the
Au /ZnAl-HT catalysts calcined at different temperatures. Reaction
[a] Reaction conditions: 3-nitrostyrene (0.4 mmol), Au catalyst
2
5
conditions: 3-nitrostyrene (0.4 mmol); Au catalyst (0.7 mol%);
(0.7 mol%), reaction time 9 h, H pressure (10 atm), [b] catalysts were
2
temperature 908C; H pressure (10 atm); reaction time 4 h. The TOF
value was measured below 20% conversion.
provided by the WGC; [c] Reference [1b]; [d] Reference [8]; [e] deter-
mined by GC.
2
2
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[
15]
bonds, the TOF value was much lower than that of our Au
(HAADF-STEM) images clearly showed the distribution of
Au particles on the Au /ZnAl-HT catalyst calcined at
À1
À1
catalyst (5.2 h vs. 69.2 h ).
2
5
To confirm this result, control experiments were con-
ducted using an artificial mixture of nitrobenzene and styrene
as the substrates (Supporting Information, Scheme S1). Over
the Au /ZnAl-HT-300 catalyst, and even though the reaction
different temperatures (Figure 3; Supporting Information,
Figure S4). More than 500 Au particles in arbitrarily selected
areas were measured for each sample. The Au NCs of the
Au /ZnAl-HT catalyst before calcination were uniformly
2
5
25
temperature was raised to 1358C, styrene was hardly con-
verted over a prolonged reaction period, while the nitro-
benzene was completely converted to aminobenzene. On the
contrary, styrene can be readily hydrogenated (64.1%) over
dispersed on the support with a mean diameter of 1.4 nm
(Figure 3a). When the samples were calcined in a temperature
range from 300 to 5008C, the average particle diameter grew
slightly from 1.7 to 2.0 nm (Figure 3b; Supporting Informa-
tion, Figure S4b,c). Even when the calcination temperature
was raised to 6008C, the particle diameter was increased only
to 2.6 nm (Figure 3c). The space lattice of Au (111) is 2.39 ꢀ,
which is very close to that of the support (101) with a space
lattice of 2.44 ꢀ (Figure 3d). This meant there was epitaxial
a Au/TiO (WGC) catalyst under the same reaction con-
2
ditions. Both the nitro and vinyl groups can adsorb on the
[13a]
Au/TiO catalyst, as reported by Corma et al. previously,
2
while the nitro group competed more favorably with adsorp-
tion of the vinyl group. Hence the C=C bond would be
[
16]
hydrogenated when the -NO group was approaching com-
interaction between Au and the support, which helped to
enhance the thermostability of the Au NCs. This result was
also in good agreement with the X-ray diffraction (XRD;
Supporting Information, Figure S5) and UV/Vis (Supporting
Information, Figure S6) measurements, indicating that Au -
2
plete conversion. However, the nitro group and the vinyl
group were not competitively adsorbed and activated on the
surface of the Au /ZnAl-HT-300 catalyst; instead, only the
2
5
nitro group could be activated while the vinyl group could
not. This led to high chemoselectivity for hydrogenation of
the nitro group over our Au catalyst.
Further evidence for inertness to the vinyl group over our
catalyst was provided by attenuated total reflection infrared
spectra (ATR-IR) spectroscopy results, as shown in Figure S2
2
5
(Cys)18 is a good precursor for controlling the particle sizes of
the Au catalyst. When the calcination temperature was
further raised to 7008C, the average particle diameter of Au
grew to 4.5 nm (Supporting Information, Figure S4d). Corre-
spondingly, the catalytic activity dropped sharply (Figure 1).
Thus, we speculated that the diameter of Au NCs was one of
the most important factors that influence the catalytic activity
of the Au /ZnAl-HT-x catalysts.
(
Supporting Information); the asymmetric n (NO ) and
as 2
symmetric n (NO ) IR vibration frequencies at 1526 and
s
2
À1
1
348 cm were observed when nitrobenzene was used as the
2
5
substrate. When nitrostyrene was applied
as the only probe molecule, the IR bands
À1
at 1531 and 1350 cm , attributed to n -
as
(
NO ) and
2
[
10,13a]
n (NO ),
clearly appeared while none
s
2
of the vibrations of the vinyl group were
observed. Further styrene adsorption
experiments implied no n(C=C) was pres-
À1
ent (1630 cm ), and d(=CÀH) vibrations
À1
at 1416 cm appeared, which suggested
that the surface of our catalyst could
adsorb the nitro group rather than the
vinyl group. Consistent with this result,
after five reaction cycles over the Au /
2
5
ZnAl-HT-300 catalyst at 908C the selec-
tivity to 3-vinylaniline remained at ꢀ 99%
when the conversion of 3-nitrostyrene was
1
00% (Supporting Information, Fig-
ure S3). Furthermore, we also applied the
Au /ZnAl-HT-300 catalyst to other sub-
2
5
strates (Supporting Information, Table S1)
and found that it worked well for the
hydrogenation of -NO with a wide scope
2
of substrates. The above results indicated
that the Au /ZnAl-HT-300 catalyst was
2
5
capable of catalyzing the chemoselective
hydrogenation of the functionalized nitro-
arenes to aromatic amines with high
chemoselectivity.
The high-angle annular dark-field
scanning transmission electron microscope
Figure 3. HAADF-STEM images of the catalysts: a) Au /ZnAl-HT; b) Au /ZnAl-HT-300;
2
5
25
c–d) Au /ZnAl-HT-600.
2
5
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Because of the strong coordination between Au and S, it is
difficult to remove S, which is thought to be poisonous to
the mapping results (Supporting Information, Figure S11).
+
2
Because the Zn ion also prefers to interact with S rather
[17]
[19]
catalytically active sites.
However, Jin et al. found that
than O, Au-S-Zn moieties may form, which enhance the
supported/unsupported Au NCs with specific amounts of Au
atoms protected by S-containing ligands can perform catalysis
interaction between Au and the support and promote the
thermal stability of the Au NCs.
[
11c]
directly.
The role of the protecting agents of the thiolated
It is reported that the nitro group can be easily adsorbed
[20]
Au NCs is still controversial in the relevant literature. Herein,
we studied the influence of organic ligands in the Au /ZnAl-
on a basic surface. But the activity of the hydrogenation of
the nitro group would be suppressed if the basicity of the
surface was too strong (for example, MgO, MgAl-HT, and so
forth). In contrast, the basic and acidic sites over the surface
2
5
HT catalyst on the catalytic performance. When we applied
the Au /ZnAl-HT catalyst without calcination to the chemo-
2
5
selective hydrogenation of 3-nitrostyrene, no reactant was
converted, which indicated that the organic ligands of
Au (Cys) might be detrimental to the reaction. At this
time, the X-ray photoelectron spectroscopy (XPS) results
showed that the binding energy (BE) of the Au 4f7/2 was
of Al O3 could potentially promote the activation of the
2
[
10]
reactants.
Herein, relatively weak basicity and acidity
provided by ZnAl-HT-300–500 supports might play an
important role in the selective hydrogenation of the nitro
group. As was demonstrated by the ATR-IR result in
Figure S12 (Supporting Information), ZnAl-HT calcined at
300 and 5008C without deposition of Au adsorbed the nitro
group instead of C=C. But the blank experiment showed that
2
5
18
8
4.4 eV, indicating that Au was positively charged because of
the interaction between Au and the thiol ligand (Supporting
[12]
Information, Figure S7).
Subsequently, we calcined the
Au /ZnAl-HT catalyst at different temperatures from 200 to
the support was not active for the hydrogenation of
3-nitrostyrene. This result indicated that the support itself
could not catalyze the reaction. The active sites might be
positioned at the perimeter between Au NCs and the support.
Au atoms with low coordination numbers could activate
2
5
7
008C. The BE of Au 4f7/2 decreased gradually with increas-
ing temperature. After calcination at 3008C, the BE of
Au 4f7/2 decreased to 83.6 eV, close to that of the final state
8
3.4 eV, which suggested that the thiol ligands on the surface
[13a]
of Au NCs were removed and the Au atoms on the surface
were reduced to a metallic state.
hydrogen according to DFT calculations.
Herein, the
activation of hydrogen might be highly dependent on Au
NCs with a well-controlled size. Thus, the nitro group of
3-nitrostyrene adsorbed on the support could be activated and
hydrogenated at the interfaces.
However, according to thermogravimetric analysis
(
TGA) of Au (Cys) (Supporting Information, Figure S8),
25 18
the organic ligands were not removed completely between
00 to 5008C. It seems that the residual ligands did not
3
In summary, using a ZnAl-HT supported Au (Cys) as
2
5
18
suppress the catalytic activity after calcination in this temper-
ature range. When the temperature increased to 6008C most
of the protecting agents were removed, while the activity of
Au /ZnAl-HT-600 began to decrease (Figure 1). At 7008C,
a precatalyst we have obtained Au nanocatalysts blessed by
useful performance: they were active only for the hydro-
genation of the -NO group but inert with respect to the C=C
2
bond over wide reaction duration and temperature windows.
2
5
the weight loss of Au (Cys) reached 20.3%. At this time,
The inertness of the catalyst to the C=C bond was verified by
2
5
18
both the activity and selectivity of Au /ZnAl-HT-700
control experiments and ATR-IR results. The residual S and
the epitaxial interaction between Au and the support
contributed to high thermostability of Au NCs. The results
of this study will inspire the design of efficient Au catalysts for
the synthesis of fine chemicals.
2
5
decreased dramatically. The above results indicate that the
protecting agents, which are detrimental to the reaction on
the surface of the Au NCs, might be removed preferentially
with increased calcination temperature, as also suggested by
the XPS results. Ligands that are relatively difficult to
eliminate might position underneath the Au NCs interacting
with the support and thus play a key role in stabilizing the Au Acknowledgements
NCs and promoting catalytic reactivity.
To further test the interaction of the residual organic
ligands with Au, we studied the local coordination environ-
ment of Au by X-ray absorption spectroscopy (XAS). To
obtain good signal-to-noise ratios, we increased the loading of
Au to 9.2 wt%. The extended X-ray absorption fine structure
The authors are grateful to the National Science Foundation
of China (21303194, 21373206, 21476227, 21522608, 21503219,
21573232, and 21690084). The foundation from the Youth
Innovation Promotion Association CAS (2014163), the
Strategic Priority Research Program of the Chinese Academy
of Sciences (XDB17020100), the National Key Projects for
Fundamental Research and Development of China
(2016YFA0202801), and the department of science and
technology of Liaoning province under contract of
2015020086-101 are also acknowledged. We also thank the
BL 14W at the Shanghai Synchrotron Radiation Facility
(SSRF) for the XAFS experiments.
(
EXAFS) data fitting results are shown in Table S2 and
Figures S9 and S10 (Supporting Information). Before calci-
nation, neighboring Au atoms were observed at 2.77 and
2
.97 ꢀ with the coordination numbers (CN) 1.9 and 1.8,
respectively. The Au-S interaction was the nearest neighbor of
Au at 2.30 ꢀ with a CN equal to 1.4. The data reflects a typical
[
12,18]
structure for thiolated Au25 NC.
When the calcination
temperature was raised from 200 to 5008C, the CN of the
AuÀS bond decreased from 0.9 to 0.1, which suggested some
residual protecting ligands were still present even after
calcination at 5008C. This characteristic also manifested in
4
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Final Article published: && &&, &&&&
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Hydrogenation Reactions
Size-controlled gold nanocatalysts were
prepared from a ZnAl-hydrotalcite-sup-
ported thiolated Au25 precatalyst. Resid-
ual sulfur and epitaxial interactions
between gold and the support stabilized
the catalyst against high-temperature
calcination, promoting complete conver-
sion of 3-nitrostyrene into 3-amino-
styrene with high selectivity for the nitro
group.
Y. Tan, X. Y. Liu,* L. Zhang, A. Wang, L. Li,
X. Pan, S. Miao, M. Haruta, H. Wei,
H. Wang, F. Wang, X. Wang,
T. Zhang*
&&& — &&&
ZnAl-Hydrotalcite-Supported Au25
Nanoclusters as Precatalysts for
Chemoselective Hydrogenation of 3-
Nitrostyrene
6
www.angewandte.org
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
These are not the final page numbers!