Selective Hydroperoxygenation of Olefins Realized by a Coinage Multimetallic 1-Nanometer Catalyst

Article abstract of DOI:10.1002/anie.202010190

The science of particles on a sub-nanometer (ca. 1 nm) scale has attracted worldwide attention. However, it has remained unexplored because of the technical difficulty in the precise synthesis of sub-nanoparticles (SNPs). We recently developed the “atom-hybridization method (AHM)” for the precise synthesis of SNPs by using a suitably designed macromolecule as a template. We have now investigated the chemical reactivity of alloy SNPs obtained by the AHM. Focusing on the coinage metal elements, we systematically evaluated the oxidation reaction of an olefin catalyzed by these SNPs. The SNPs showed high catalytic performance even under milder conditions than those used with conventional catalysts. Additionally, the hybridization of multiple elements enhanced the turnover frequency and the selectivity for the formation of the hydroperoxide derivative. We discuss the unique quantum-sized catalysts providing generally unstable hydroperoxides from the viewpoint of the miniaturization and hybridization of materials.

Full text of DOI:10.1002/anie.202010190

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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Accepted Article
Title: Selective Hydroperoxygenation of Olefin Realized by Coinage
Multimetallic 1-nanometer Catalyst
Authors: Kimihisa Yamamoto, Tatsuya Moriai, Takamasa Tsukamoto,
Makoto Tanabe, and Tetsuya Kambe
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202010190
Link to VoR: https://doi.org/10.1002/anie.202010190

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Selective Hydroperoxygenation of Olefin Realized by Coinage
Multimetallic 1-nanometer Catalyst
Tatsuya Moriai,+ [a] Takamasa Tsukamoto,+ [a,b] Makoto Tanabe, [b] Tetsuya Kambe, [a,b] and
Kimihisa Yamamoto*[a,b]
[a]
[b]
T. Moriai, [+,++] Dr. T. Tsukamoto, [+,++] Dr. T. Kambe, Prof. Dr. Yamamoto
Institute of Innovative Research
Tokyo Institute of Technology
4259 Nagatsuta, Midori-ku, Yokohama 226-8503 (Japan)
E-mail: yamamoto@res.titech.ac.jp
Dr. T. Tsukamoto, [+,++] Dr. M. Tanabe, Dr. T. Kambe, Prof. Dr. Yamamoto
JST-ERATO, Yamamoto Atom Hybrid Project
Tokyo Institute of Technology
4259 Nagatsuta, Midori-ku, Yokohama 226-8503 (Japan)
These two authors contributed equally to this work.
Co-first authors.
[+]
[
++
]
Supporting information for this article is given via a link at the end of the document.
Abstract: The science of particles on a sub-nanometer (~ 1 nm)
scale has attracted worldwide attention. However, this field of
science has remained unexplored because of the technical difficulty
in the precise synthesis of sub-nanoparticles (SNPs). Thus, we
recently developed the ‘atom-hybridization method (AHM)’ that
Recently, we focused on sub-nanoparticles (SNPs) with an
13]
extremely small diameter of around 1 nm.[12,
On the sub-
nanometer scale, materials exhibit specific electronic states
depending on the number of constitutive atoms due to the lack
of lattice structures in contrast to the bulk and NPs. Additionally,
almost all atoms are exposed on their surface and serve as
reaction sites. Therefore, SNPs are expected to have
extraordinary functions beyond the extension of the bulk and
NPs. However, it has been impossible to finely control the size
and composition of each particle on a sub-nanometer scale by
conventional methods such as chemical vapor deposition,[14]
ligand protection,[15,16] and solid phase pyrolysis.[17] To overcome
this obstacle, we developed the ‘atom-hybridization method
precisely synthesizes the SNPs by using
a suitably-designed
macromolecule as a template. In this study, toward revealing this
unexplored field, we newly investigated the chemical reactivity of
alloy SNPs obtained by the AHM. Focusing on the coinage metal
elements, the oxidation reaction of an olefin catalyzed by these
SNPs was systematically evaluated. The SNPs showed a high
catalytic performance even under the milder condition in contrast to
conventional catalysts. Additionally, the hybridization of multiple
elements enhanced the turnover frequency and the selectivity of the
hydroperoxide derivative. We discuss the unique quantum-sized
catalysts providing generally unstable hydroperoxides from the
viewpoint of miniaturization and hybridization of the materials.
(AHM)’ using
a phenylazomethine dendrimer (DPA) as a
macromolecular template, which allows us to precisely control
and diversely design the size and composition of the SNPs.[18]
Actually, metallic SNPs synthesized by this method have shown
a catalytic activity far superior to commercial bulk and NP
catalysts.[19,20] In addition, such functions are expected to be
more enhanced by applying not only miniaturization effect but
also the alloying effect of the materials. However, the studies on
multimetallic SNPs are very few reported[21] and even their
fundamental principle has not been elucidated in detail.
In this study, we focused on the monometallic, bimetallic, and
trimetallic SNPs composed of group 11 metal elements that
have the similar electron configurations (Cu: [Ar]3d104s1, Ag:
[Kr]4d105s1, and Au: [Xe]4f145d106s1). Through the catalytic
oxidation reaction of an alkene as a simple model, we
systematically evaluated the effects induced by the atomic-level
mixing of multiple elements in this quantum size region.
Developing the innovative properties from the finite chemical
elements on the earth is the essential concept toward building a
sustainable society. One of the effective approaches acquired
through the many attempts over the years is the use of nano-
sized materials (5–500 nm). Actually, nanoparticles (NPs) have
been applied in various fields such as chemicals,[1] medicines,[2,3]
photonics,[4,5] and electronics[6] due to their very small size. Such
the miniaturization of materials to a nanometer scale induces the
discretization of energy levels and increase in the band gaps
(quantum size effect), leading to the high reactivity and
semiconducting property not found in the bulk states.[7]
Additionally, NPs with a large surface area tend to show a high
reactivity because the atoms exposed on the material surface
are generally activated due to unsaturated bonds. Even Au that
is a chemically inert element in the bulk state is activated in the
NP state by these effects and exhibits a remarkable catalytic
activity.[8,9] On the other hand, the mixing of multiple elements
often induces the unique properties beyond a unary system and
thus multimetallic NPs have also been investigated with a
central focus on the catalysts.[10] However, such an activation
originating from the interface between xenogeneic metals is
limited because the NPs also undergo non-uniform mixing or
phase-separation as is the case in the bulk state.[11]
First, we precisely synthesized multimetallic SNPs by
adopting
the
4th-generation
tetraphenylmethane-cored
phenylazomethine dendrimer (DPA G4) (Figure 1A) as a
macromolecular template. Because DPA G4 possesses the
unique intramolecular potential gradient providing a stronger
Lewis basicity to imines in the inner layer, Lewis acidic metal
salts are accumulated in the order of 4, 12, 28, and 60 equivs
from the inner layer in a stepwise fashion.[22] By chemical
reduction of such metal salts assembled on the DPA G4,
metallic SNPs with
a
fine-controlled atomicity were
synthesized.[23] Additionally, the synthesis of multimetallic SNPs
with a fine-controlled composition was also accomplished by
1
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arrangement of the various metal species on each layer in the
DPA G4.[18]
Atomic resolution observations at a high magnification confirmed
that the SNPs are composed of multiple atoms (Figure 2A).
Subsequently, the constitutive elements of the SNPs were
investigated by energy dispersive X-ray spectrometry
(STEM/EDS). Spectral peaks attributed to Au, Ag, and Cu were
detected in the region where a particle exists in the STEM image
(Figure 2B), while no peaks for these elements were detected in
the other region without a particle. This fact indicated that the
SNPs were composed of three metals accumulated on DPA G4.
Additionally, elemental mapping images with the uniform
distribution of three elements on a particle suggested that three
metals were mixed in the SNP at an atomic level (Figure 2C). As
results, the synthesis of the Au-Ag-Cu ternary alloy SNPs with a
fine-controlled size and composition was accomplished by the
AHM.
For the precise synthesis of multimetallic SNPs, it is
indispensable that the metal salts-DPA G4 complex as the
precursor be precisely designed. Therefore, as the first step for
the synthesis of the Au-Ag-Cu ternary alloy SNPs, the
complexation behaviors of AuIIICl3, AgIIF2, and CuIIBr2 on DPA
G4 were monitored by UV-vis adsorption spectrophotometry. A
control experiment using the 1st-generation dendrimer (DPA G1)
clarified that the order of the coordination strength for metal salts
was AuIIICl3 > AgIIF2 > CuIIBr2 (Figure S1 in the Supporting
Information). According to this order of Lewis acidity, each metal
salt is stepwise accumulated from the stronger basic imines in
the inner layer to the weaker ones in the outer layer. During the
addition of the metal salts to DPA G4, the absorption of free
imine sites on the DPA G4 at around 300 nm decreased while
the adsorption of the metal-imine complexes increased at
around 400 nm (Figure 1B). At the same time, four changes in
the positions of the isosbestic points were observed while 4, 8,
16, and 32 equivs of each metal salt were added to DPA G4
(Figure 1B). This result indicates that each metal salt complexed
with each imine site at a ratio of 1:1 in each layer maintained the
order of priority for complexation (Figure 1A).
Figure 2. STEM observation and EDS analysis of the Au-Ag-Cu trimetallic
SNPs. A) STEM image of the SNPs supported on KB at low magnification and
(inset) atomic-resolution. The particles are highlighted by the yellow arrows. B)
STEM/EDS spectra of the Au4Ag8Cu16 SNPs on KB and blank. C) STEM/EDS
elemental mapping of the Au4Ag8Cu16 SNPs on KB focusing on Au Lα, Ag Lα,
and Cu Kα (scale bar: 2 nm).
We evaluated the catalytic reactivity of the multimetallic
SNPs by using cyclohexene as a model substrate, which is one
of the important C6 compounds in the chemical industry.[24,25]
Although many studies of the catalytic oxidation reaction of
cyclohexene have been reported especially for the NP catalysts,
they have required severe reaction conditions such as high
temperature, high pressure, use of strong oxidants, and addition
of initiators.[26,27,28,29,30] In this study, using the highly reactive
SNP catalysts, we newly investigated this reaction under the
very mild condition at low temperature, at atmospheric pressure,
with molecular oxygen as a weak oxidant, and without initiators.
First, as monometallic and bimetallic SNPs, Au28/KB, Ag28/KB,
Cu28/KB, Au12Ag16/KB, Au12Cu16/KB, and Ag12Cu16/KB were also
synthesized by the same method for trimetallic Au4Ag8Cu16/KB
as already described. The reactions were carried out in a
heterogeneous system suspending the catalyst in a solution of
cyclohexene/acetonitrile = 1:1 (v/v) at 80 °C for 5 h with stirring
at O2 1 atm. The products in the reaction mixture were analyzed
by 1H-NMR (Figures S2 and S3).
Figure 1. Multimetallic accumulation using DPA G4 as a molecular template.
A) Molecular structure of the DPA G4 and schematic reaction scheme of
stepwise complexation of each metal salt with DPA G4. B) UV-vis absorption
spectral changes during addition of 4 equiv. AuIIICl3, 8 equiv. AgIIF2, and 48
equiv. CuIIBr2 to DPA G4, and (inset) isosbestic points observed in each layer.
Based on these results, the multinuclear trimetallic dendrimer
complex (AuIIICl3)4(AgIIF2)8(CuIIBr2)16@DPA G4 as a precursor of
the SNPs was precisely prepared. The metal salts on the DPA
G4 were reduced by NaBH4 and produced Au4Ag8Cu16 SNPs
supported on carbon (Ketjen Black (KB)). This SNP was
confirmed by high-resolution scanning transmission electron
microscopic (STEM) images (Figure 2). STEM observations at
low magnification demonstrated that the SNPs with a diameter
of around 1 nm were supported on KB in high yield (Figure 2A).
2
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Figure 3. Catalytic activity of mono-, bi-, and trimetallic SNPs composed of Au, Ag, and Cu. A) TONs of the main reaction products converted from the
cyclohexene oxidation in this study; A: 2-cyclohexen-1-hydroperoxide (green), B: 2-cyclohexen-1-one (red), C: 2-cyclohexen-1-ol (blue), and D: 1,2-
epoxycyclohexane (orange). B) TONs of products for these SNP catalysts. C) Summary of TONs for the catalytic oxidation reaction of cyclohexene.
Interestingly, the SNPs catalyzed the reaction even under
such mild conditions, and 2-cyclohexen-1-hydroperoxide (A), 2-
cyclohexen-1-one (B), 2-cyclohexen-1-ol (C), and 1,2-
epoxycyclohexane (D) were obtained as the main reaction
products (Figure 3A). The catalytic activity was evaluated by the
total of the TON estimated from the amounts of A, B, C, and D
because those of other minor products were negligible (Figures
3B and C). Cu28 showed a remarkably high TON (2150) among
the monometallic SNPs (Figure S3). Although TON value of Ag
was slightly improved by the miniaturization of structure, it was
much lower than one of Cu (Figure S4). However, for the
bimetallic SNPs, the hybridization of Au and Ag with Cu led to
further enhancement of the catalytic activity. The TONs and
selectivity of product A of Au12Cu16 and Ag12Cu16 increased
compared to the single metal Cu28. Finally, by the combination of
all metals, the trimetallic Au4Ag8Cu16 showed the highest TON
(2526) with the highest selectivity of the hydroperoxide
derivative A, which is as unstable compound in contrast to the
other common products (Figures 3B, C and Table S1). On the
other hand, in the case of the control experiment without SNPs,
no products were confirmed (Figure S3). As a result, by utilizing
the ultrasmall metal particles of the quantum size range, we
succeeded in catalyzing the reaction under the first-ever mild
conditions (at low temperature, at atmospheric pressure, with a
weak oxidant, without any initiators). Moreover, their reactivity
was significantly improved in the alloyed systems especially for
the Au-Ag-Cu.
We discuss in details of the factors that enhanced the
catalytic activity of the multimetallic SNPs. It has been reported
that Cu has a high catalytic activity for the oxidation reactions of
organic compounds because various oxidation states, such as
Cu(0), Cu(I), and Cu(II), are available.[31,32] Therefore, we
assumed that the Cu atoms in the SNPs also serve as the main
reactive sites. We first investigated the effect induced by the
miniaturization of the structure. The Cu28 SNPs showed a
notably higher TOF per Cu atom for a 1 h reaction than the
commercially available Cu NPs (60–80 nm) and bulk Cu (<425
μm) (Figures 4A, S5 and Table S2). This result indicated that the
1 nm SNPs contain numerous reactive Cu sites by increasing
the surface area.
We next focused on the effect of the hybridization of the
metals. The Cu atoms alloyed with Au and Ag in the SNPs
showed higher TOFs than the single-elemental Cu28 SNPs,
especially leading to the increase in product A. Among them, the
Au4Ag8Cu16 showed the highest TOF (106 h−1 Cu atom−1), i.e.,
almost 10 times greater than the commercial Cu NPs and
provided the highest amount of the product A. The enhancement
of catalytic activity in the Au-Cu system would be due to charge
transfer between these metals (ligand effect).[33,34] The electrons
of the Cu atoms are withdrawn by the Au atoms with the highest
electronegativity among the metal elements. This speculation is
supported by the fact that the signal of the Cu atoms in the
Au12Cu16 SNPs was shifted to the higher binding energy than
that of Cu28 SNPs in the XPS measurement (Figure S6). This
distinct ligand effect was also supported by the density
functional theory (DFT) calculations (Figure S7).[35] The
computational results also demonstrated that the Cu atom
binding to Au atom had the most stable electronic state for
stabilization for the O2 adsorption by the ligand effect (Figures
S8 and S9). It was reported that the radical species (A′) as a
precursor of product A is generated by the reaction on the Cu(II)
sites (Figures 4B and S10).[29] The control experiment using
2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) as
a
radical
scavenger suggested that all the reaction products in this study
were also produced via radical intermediates (Figure S11).
Therefore, the electron-poor Cu atoms by alloying Au atoms are
expected to lead to the promotion of the A production (Figure
4C). On the other hand, the degree of activation by the ligand
effect in the Ag-Cu system is estimated to be lower than the Au-
Cu one because the Ag and Cu atoms possess similar
electronegativities as demonstrated by DFT calculations (Figure
S7). In this case, the final process for A′ generation would be
promoted by the specific adsorption state of Ag (ensemble
effect).[36,37] The computational results demonstrated that the
energy gap in the process for A′ desorption on Ag is lower than
on the Cu and Au atoms and the bond length of metal-oxygen is
the largest for O2 adsorbed on the Ag atom (Figures S12 and
S13). Therefore, the role of the Ag atom in this catalyst is
estimated to be making the desorption reaction easier (Figure
4D).
3
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Figure 4. Enhancement of reactivity of Cu atoms by the hybridization with Au and Ag on the quantum sized scale. A) TOFs per Cu atom for 1 h reaction obtained
from the cyclohexene oxidation catalyzed by SNPs, NPs, and bulk containing Cu; product A (green), B (red), C (blue), and D (orange). B) Proposed reaction
schemes on Cu atoms containing the adsorption process of O2, the additional reaction process of cyclohexene radical, and the desorption process of cyclohexene
peroxy radical. C, D) Proposed reaction mechanisms promoted by the ligand effect in the Au-Cu alloy system and the ensemble effect in the Ag-Cu alloy system.
Additionally, it is known that products
B and C are
Acknowledgements
secondarily obtained by the decomposition of product A on the
Cu(I) sites (Figure S10).[29] Actually, this reaction scheme was
also observed in this system using the SNP catalysts, judging
from the temporal changes in the amounts of the products
(Figures S14 and S15). However, the extraordinarily high
selectivity to A in this study indicated that this subsequent
reaction is drastically suppressed compared to the other
catalysts without the SNPs. Not only the mild reaction condition,
but also the Cu atoms with lower electron density due to the
ligand effect (Figures S6 and S7) would contribute to the
preservation of the unstable product A in the reaction mixture.
Moreover, the reused Au4Ag8Cu16 also showed high reactivity
and maintained its own product selectivity (Figure S16). This
result indicates that the catalysts stay on supports without
aggregation and leaching behaviors during the catalytic
processes. This certain level of durability and reusability would
be due to the mild reaction condition, which is one of the
advantages of sub-nanocatalysts.
As a conclusion, the mono-, bi-, and trimetallic SNPs
composed of Au, Ag, and Cu were precisely synthesized by the
AHM. This was mainly due to the effect of the extreme
miniaturization of the structures, thus the coinage metals
acquired a high enough reactivity to catalyze the oxidation even
under the mild condition. Additionally, the hybridization of these
metals on the quantum sized scale induced the improvement in
the catalytic performance by the electronic and adsorbing effects.
The trimetallic Au4Ag8Cu16 showed the highest TOF (1700 h−1)
and provided the highest amount of product A compared to the
other SNPs. These results suggest that the proper design of the
constitutive elements of the SNP catalysts allow us to diversely
customize the chemical reactions and to easily supply the high-
energy compounds. Such findings should be the pioneering key
to discover innovative sub-nanomaterials from a wide variety of
elements, leading to the solution of energy and environmental
problems all over the world.
This study was supported by a grant from JST ERATO Grant
Number JPMJER1503 (K.Y.), JSPS KAKENHI Grant Number
JP15H05757 (K.Y.) and JP19K15583 (T.T.), a research grant
from the Advanced Technology Institute (T.T.), and a research
grant from the Yoshinori Ohsumi Fund for Fundamental
Research (T.T.).
Keywords: Nanoparticles • Metal-metal interactions • Supported
Catalysts • Peroxides • Dendrimers
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Entry for the Table of Contents
First-ever efficient and selective hydroperoxygenation of cyclohexene was accomplished under a mild condition by a ultrasmall
catalyst where Au, Ag, and Cu with the individual roles were hybridized each other into 1 nm particle.
6
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