Green, Multi-Gram One-Step Synthesis of Core–Shell Nanocomposites in Water and Their Catalytic Application to Chemoselective Hydrogenations

Article abstract of DOI:10.1002/chem.201604763

We devise a new and green route for the multi-gram synthesis of core–shell nanoparticles (NPs) in one step under organic-free and pH-neutral conditions. Simply mixing core and shell metal precursors in the presence of solid metal oxides in water allowed for the facile fabrication of small CeO₂-covered Au and Ag nanoparticles dispersed on metal oxides in one step. The CeO₂-covered Au nanoparticles acted as a highly efficient and reusable catalyst for a series of chemoselective hydrogenations, while retaining C=C bonds in diverse substrates. Consequently, higher environmental compatibility and more efficient energy savings were achieved across the entire process, including catalyst preparation, reaction, separation, and reuse.

Full text of DOI:10.1002/chem.201604763

A Journal of
Accepted Article
Title: Green One-Step Synthesis of Core-Shell Nanocomposites
in Water and Their Catalytic Application to Chemoselective
Hydrogenations
Authors: Teppei Urayama, Takato Mitsudome, Zen Maeno, Tomoo
Mizugaki, Koichiro Jitsukawa, and Kiyotomi Kaneda
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To be cited as: Chem. Eur. J. 10.1002/chem.201604763
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Chemistry - A European Journal
10.1002/chem.201604763
Green One-Step Synthesis of Core-Shell Nanocomposites in Water and
Their Catalytic Application to Chemoselective Hydrogenations
1
1
1
1
Teppei Urayama, Dr. Takato Mitsudome, Dr. Zen Maeno, Dr. Tomoo Mizugaki,
1
1, 2,*
Prof. Dr. Koichiro Jitsukawa, Prof. Dr. Kiyotomi Kaneda
1
Department of Materials Engineering Science, Graduate School of Engineering Science,
Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan
Fax: (+81) 6-6850-6260, E-mail: kaneda@cheng.es.osaka-u.ac.jp
2
Research Center for Solar Energy Chemistry, Osaka University, 1-3 Machikaneyama,
Toyonaka, Osaka 560-8531, Japan
Abstract
We devise a new and green route for the synthesis of core-shell NPs in one-step under
organic-free and neutral conditions. Simply mixing core and shell metal precursors in
the presence of solid metal oxides in water allowed for the facile fabrication of small
CeO -covered Au and Ag nanoparticles dispersed on metal oxides in one-step. The
2
CeO -covered Au nanoparticles acted as a highly efficient and reusable catalyst for a
2
series of chemoselective hydrogenations while retaining C=C bonds. Consequently,
higher environmental compatibility and more efficient energy saving were achieved
across the entire process including catalyst preparation, reaction, separation, and reuse.
Text
Core-shell (CS) metal nanoparticles (NPs) are promising nanomaterials in various
[
1]
[2]
[3]
fields such as sensors,
optical devices,
and quantum dots
due to their
multi-functionality and enhanced properties that cannot be obtained from their
mono-metal counterparts. In addition to the above applications, CS nanocomposites
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Chemistry - A European Journal
10.1002/chem.201604763
[
4]
have great advantages derived from their structural properties in the catalytic field.
For example, shell layers protect catalytically active metal NPs in the core from
[
4b]
aggregation at high temperature, providing long life-times of the catalysts. The close
interaction between the core and shell components allows a strong metal-support
[
4c]
interaction (SMSI), leading to high catalytic activity. Furthermore, acid and base sites
[4f]
are isolated in the shell and core, respectively, enabling one-pot cascade reactions.
To date, the CS NP catalysts have been synthesized by using the seeded-growth
[
5]
[6]
[7]
method,
galvanic replacement method,
and decomposition method.
These
methods are quite useful for the precise synthesis of the CS NP catalysts. However, they
are generally complicated, and often require time-consuming and multiple steps with
harmful and expensive reagents such as surfactants, reductants, and organic solvents,
thus resulting in the production of large amounts of waste, the consumption of the
energy and limiting the wide applicability of the CS NPs. Furthermore, the presence of
un-removed surfactants often decreases the catalytic activity of the CS NPs by blocking
active metal sites. Therefore, the development of a facile and clean synthesis of CS NPs
with high catalytic performance has been highly desired and still a challenging issue.
A redox-coprecipitation method is attractive and promising for the facile synthesis of
CS NPs, where the redox reaction between core and shell metal precursors
spontaneously occurs in the presence of bases, yielding CS NPs in one step without any
[
8]
reductants (Figure 1, a). However, the redox-coprecipitation method generally yields
large CS NPs (> 100 nm) unsuitable for catalytic applications due to their low surface
area to volume ratio and toxic bases such as NaOH are essentially required. Recently, a
new method using the combination of the reverse micelle technique with
redox-coprecipitation was reported to synthesize small CS NP catalysts (Figure 1, b). In
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Chemistry - A European Journal
10.1002/chem.201604763
this strategy, the redox reaction between core and shell metal precursors proceeds in
nanoscale aqueous phase inside the reverse micelles, producing small CS NPs such as
[
9b-d]
[9e]
[9a]
CeO -covered Ag,
Au, and Pt with a mean diameter of less than 10 nm in the
2
core. However, the unavoidable use of surfactants, organic solvents, and additional
bases is still costly and environmentally problematic.
Toward a more effective synthesis of CS NPs, we herein introduce an alternatively
new and green route for the synthesis of small CS NPs without any organic reagents
under neutral conditions. Namely, simply mixing core and shell metal precursors in the
presence of metal oxides with basicity in water enables the one-step fabrication of small
CS NPs without the requirement of the reductants, surfactants, organic solvents or
additional toxic bases that are essential in existing methods (Figure 1, c). In this method,
the basic site on the metal oxides acts as a nanoscale reaction place to promote the redox
reaction between core and shell precursors, efficiently affording CS NPs highly
dispersed on the solid surface with suppressing the agglomeration of the CS NPs. The
sizes of CS NPs are small and suitable for the catalytic application because the in-situ
generated CS NPs are immediately immobilized on the metal oxides without
overgrowth of metal NPs. Furthermore, the metal oxide-supported CS NPs can be
obtained by simple filtration and the conventional calcination process for removing the
residual surfactants is not necessity. The obtained metal oxide-supported CS NPs are
also easily handled as heterogeneous catalysts.
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Figure 1. Comparison of the synthetic methods of CS NPs
First, Au, Ce and hydrotalcite (HT: Mg Al (CO )(OH) ·4(H O)) was chosen as core,
6
2
3
16
2
shell precursors and the inorganic base material, respectively for the synthesis of CS
NPs. To the aqueous suspension of HT, a mixture of an aqueous solution of cerium
nitrate and chloroauric acid was added dropwise. Interestingly, the color of the
suspension became dark gray during the stirring of the resulting mixture at 15 °C for 1 h
in air, indicating that the redox reaction between Au(III) and Ce(III) to Au(0) and
Ce(IV) occurred on HT. The obtained slurry was filtered, washed with deionized water,
and then dried at room temperature in vacuo, yielding the HT containing Au and Ce as
gray powder.
The obtained powder was characterized using transmission electron microscopy
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(TEM). The TEM image showed that size-ordered NPs with diameters of 10 nm were
highly dispersed on HT (Figure 2, a). The NP consisted of an electron-dense core area
and electron-lucent shell area, revealing the formation of CS NPs. A HAADF-STEM
image showed spherical core NPs with a mean diameter of 5 nm and the shell with a
thickness of around 4 nm (Figure 2, b). The lattice fringes of the core and shell NPs had
the d-spacing attributed to Au {111} (2.3 Å) and CeO {111} (3.2 Å), respectively.
2
Energy-dispersive X-ray spectroscopy mapping clearly proved that the CS NPs were
comprised of Au NPs in the core and CeO in the shell (Figure 2, c-e). To the best of our
2
knowledge, this is the first success of a green one-step synthesis of small CS NPs in
water without the use of surfactants, organic solvents, reductants, or toxic bases. This
method is quite different from those previously reported for the CS NPs in the
viewpoints of organic reagents-free and neutral conditions. The CeO -covered Au NPs
2
immobilized on HT is denoted as Au@CeO /HT.
2
Figure 2. Composition and structural analysis of Au@CeO /HT. a) TEM image. b)
2
HAADF-STEM image. Elemental mapping images of c) Au and d) Ce. e) Composite
overlay image formed from c) and d).
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It should be noted that this simple preparation of Au@CeO was easily scalable; 100 g
2
of Au@CeO /HT was successfully synthesized in one-step where the morphology of
2
Au@CeO on HT did not significantly change (Figure S1), demonstrating the utility of
2
this method. The present method also allowed the use of various metal oxides with
basicity in the synthesis of Au@CeO (Figure 3, a-c). For example, MgO (Figure 3, a),
2
synthetic layered silicate (Figure 3, b), and amine-functionalized silica (Figure 3, c)
were applicable to form Au@CeO on the surface. In contrast, Au@CeO was not
2
2
formed using non-basic materials such as silica, carbon, and alumina, confirming the
necessity of surface basic sites to promote the redox reaction between Au and Ce.
Moreover, this method was adapted to the fabrication of CS Ag NPs of Ag@CeO /HT,
2
in which core Ag NPs with a mean diameter of 15 nm were coated with the shell CeO2
to a thickness of 10 nm (Figure S3).
The dependence of the morphology of Au@CeO /HT on the preparation temperature
2
was also investigated (Figure 3, d-f). The redox reaction between Au(III) and Ce(III) did
not proceed at 0 °C. Upon increasing the temperature to 15, 30, and 45 °C, the CS
structures of Au@CeO were observed and the mean diameters of Au NPs in the core
2
increased to 5, 10, and 20 nm, respectively. The CS structure was not formed at 60 °C
and bare Au NPs were observed. These results revealed that the present method allowed
the size-controllable synthesis of CS NPs by changing the preparation temperature.
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Figure 3. TEM images of Au@CeO supported on a) MgO, b) synthetic layered silicate
2
(
laponite), and c) amine-functionalized silica. Au@CeO /HT prepared at d) 15 °C, e)
2
3
0 °C, and f) 45 °C.
The catalytic potential of the Au@CeO /HT was investigated in the chemoselective
2
reductions of various target functional groups in the presence of C=C bonds using
molecular hydrogen (H ). These transformations are important in the synthesis of fine
2
[
10]
chemicals such as natural products and pharmaceuticals.
In this context, Au NP
catalysts have attracted attention due to their high chemoselectivity for the
[
10c,11]
hydrogenation of polar functional groups.
However, Au catalyst systems still
[
11a-c]
[11i,j,l,p]
suffer from limited substrate scope,
the requirement of harmful additives,
and
[
11f,m,o]
low atom-efficient hydride reagents,
producing large amounts of waste. Therefore,
the chemoselective reductions of various functional groups catalyzed by Au catalysts
using H as a green reductant under additive-free conditions are still challenging issues.
2
Interestingly, Au@CeO /HT prepared at 15 °C showed high chemoselectivity for the
2
unsaturated aldehyde, citral (1), giving nerol and geraniol (2) in quantitative yield with
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the C=C bonds remaining completely intact during the reduction (Table 1, entry 1).
Au@CeO /HT with larger Au NPs prepared at 30 °C and 45 °C also promoted the
2
selective hydrogenation of 1, but they were considerably less active than that prepared
at 15 °C (Entry 1 vs. 3 and 4), revealing that small Au NPs that are sub-10 nm in
diameter are more effective as a catalyst. In control experiments, bare Au NPs (without
CS structure) supported on CeO and HT were prepared and tested in the hydrogenation
2
of 1. However, these catalysts hydrogenated the C=C bond of 1, resulting in low
selectivity for 2 (Scheme S1). This sharp contrast in the catalytic performance of Au
NPs with or without CS structure discloses that the CS structure is the key for
promoting highly selective hydrogenation. Note that Au@CeO /HT prepared at 15 °C
2
showed the highest catalytic performance (all catalytic activity, product yield and
selectivity) among those of previously reported Au-based catalysts (Table S1).
Furthermore, Au@CeO /HT was able to catalyze versatile chemoselective
2
hydrogenations while retaining C=C bonds under additive-free conditions, such as the
hydrogenation of other unsaturated aldehydes (Entries 5-9), deoxygenation of epoxides
(Entries 10-13), and semihydrogenation of alkynes (Entries 14-18). The selectivity for
the desired products was significantly high (>99%) at high conversion levels in all these
hydrogenations and the perfect regioselectivity for the Z-alkene was obtained in
semihydrogenation of the internal alkyne (Entry 14).
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Au@CeO /HT also worked well for gram-scale reactions (Scheme 1). Thus, 20 mmol
2
of substrates were chemoselectively hydrogenated, giving the desired products in
excellent yields. After the hydrogenation of 1, the solid Au@CeO /HT catalyst was
2
easily recoverable by simple filtration and reusable without any loss of its catalytic
activity or selectivity (Entry 2). The absence of Au species in the liquid phase was
confirmed by inductively coupled plasma atomic emission spectroscopy (ICP-AES),
which indicated no leaching of Au species from Au@CeO /HT. The TEM image of the
2
reused Au@CeO /HT showed that the CS structure of Au@CeO was retained without
2
2
the aggregation of Au NPs (Figure S2).
Scheme 1. Gram-scale reactions using Au@CeO /HT
2
Recently, our group reported that CeO -covered Au and Ag CS NP catalysts were
2
effective in the chemoselective reductions of various functional groups while retaining
[9b-e]
C=C bonds.
The high chemoselectivity originated from the cooperative action at the
interface between core-Au or Ag and shell-CeO , where polar hydrogen species of
2
δ-
δ+
[9b-e]
M-H and CeO -H effectively formed through the heterolytic dissociation of H .
2
2
These polar hydrogen species would be active for the targeted functional groups but
inactive for the C=C bond, resulting in high chemoselectivity. To confirm the above idea,
IR study was performed. When Au@CeO /HT was treated with H , new bands appeared
2
2
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-1
-1
around 2120 cm and 3294 cm , respectively (Figure S4, a). These peak positions were
δ-
δ+
[12]
consistent with those attributed to Au-H and CeO -H in previous reports.
The
2
formation of these polar hydrogen species was well supported by the treatment of
Au@CeO /HT with D instead of H (Figure S4, b). Furthermore, the band attributed to
2
2
2
δ-
o
Au-H gradually disappeared during the treatment with 1 at 120 C. These results
clearly revealed that active polar hydrogen species were formed in the selective
hydrogenation reactions.
In conclusion, we devised for the first time a green synthetic route for CS NPs under
organic reagent-free and neutral conditions by using a redox-coprecipitation technique
combined with metal oxides. Simply mixing CS precursors with various metal oxides in
water allowed for the fabrication of small CeO -covered Au and Ag NPs in one step
2
without reductants, surfactants, organic solvents or toxic bases. This facile method was
also applicable to the large-scale synthesis of CS NPs. Furthermore, the synthesized
Au@CeO /HT acted as a highly efficient and reusable heterogeneous catalyst for a
2
series of highly chemoselective hydrogenations of unsaturated aldehydes, alkynes and
epoxides where the desired products were obtained with over 99% selectivities while
retaining C=C bonds under additive-free conditions. Consequently, higher
environmental compatibility and more efficient energy saving were achieved across the
entire process, including catalyst preparation, reaction, separation, and reuse. We
believe that the present simple and powerful method will facilitate the clean synthesis of
other CS NPs.
Acknowledgements
This work was supported by JSPS KAKENHI Grant Nos. 26289303, 26630410,
2
6105003, and 24246129. This work was also supported by the Toyota Physical &
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Chemical Research Institute Scholars. The TEM experiments were carried out at a
facility of the Institute for NanoScience Design Center, Osaka University.
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This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.201604763
COMMUNICATION
Teppei Urayama,Takato Mitsudome,
Zen Maeno, Tomoo Mizugaki, Koichiro
Jitsukawa, Kiyotomi Kaneda*
Page No. – Page No.
Core-shell nanocomposites were successfully synthesized in one-step under
organic-free and neutral conditions. The synthesized core-shell nanoparticles
showed high chemoselectivity in a series of hydrogenations while retaining C=C
bonds without any additives.
Green One-Step Synthesis of
Core-Shell Nanocomposites in Water
and Their Catalytic Application to
Chemoselective Hydrogenations
Key words: core-shell, gold, heterogeneous catalyst, hydrogenations, silver
This article is protected by copyright. All rights reserved.