Angewandte
Chemie
DOI: 10.1002/anie.201207845
Heterogeneous Catalysis
Metal–Ligand Core–Shell Nanocomposite Catalysts for the Selective
Semihydrogenation of Alkynes**
Takato Mitsudome, Yusuke Takahashi, Satoshi Ichikawa, Tomoo Mizugaki, Koichiro Jitsukawa,
and Kiyotomi Kaneda*
In recent years, hybrid nanocomposites with core–shell
structures have increasingly attracted enormous attention in
many important research areas such as quantum dots,[1]
optical,[2] magnetic,[3] and electronic[4] devices, and cata-
lysts.[5–9] In the catalytic applications of core–shell materials,
core-metals having magnetic properties enable easy separa-
tion of the catalysts from the reaction mixtures by a magnet.[5]
The core-metals can also affect the active shell-metals,
delivering significant improvements in their activities and
selectivities. However, it is difficult for core-metals to act
directly as the catalytic active species because they are
entirely covered by the shell. Thus, few successful designs of
core–shell nanocomposite catalysts having active metal
species in the core have appeared to date.[10] Recently, we
have demonstrated the design of a core–shell catalyst
consisting of active metal nanoparticles (NPs) in the core
and closely assembled oxides with nano-gaps in the shell,
allowing the access of substrates to the core-metal. The shell
acted as a macro ligand (shell ligand) for the core-metal and
the core–shell structure maximized the metal–ligand inter-
action (ligand effect), promoting highly selective reactions.[11]
The design concept of core–shell catalysts having core-metal
NPs with a shell ligand is highly useful for selective organic
transformations owing to the ideal structure of these catalysts
for maximizing the ligand effect, leading to superior catalytic
performances compared to those of conventional supported
metal NPs.
natural products.[12] In this context, the Lindlar catalyst (Pd/
CaCO3 treated with Pb(OAc)2) has been widely used.[13]
Unfortunately, the Lindlar catalyst has serious drawbacks
including the requirement of a toxic lead salt and the addition
of large amounts of quinoline to suppress the over-hydro-
genation of the product alkenes. Furthermore, the Lindlar
catalyst has a limited substrate scope; terminal alkynes cannot
be converted selectively into terminal alkenes because of the
rapid over-hydrogenation of the resulting alkenes to alka-
nes.[13c] Aiming at the development of environmentally benign
catalyst systems, a number of alternative lead-free catalysts
have been reported.[14,15] Recently, we also developed a lead-
free catalytic system for the selective semihydrogenation
consisting of SiO2-supported Pd nanoparticles (PdNPs) and
dimethylsulfoxide (DMSO), in which the addition of DMSO
drastically suppressed the over-hydrogenation and isomer-
ization of the alkene products even after complete consump-
tion of the alkynes.[16] This effect is due to the coordination of
DMSO to the PdNPs. DMSO adsorbed on the surface of
PdNPs inhibits the coordination of alkenes to the PdNPs,
while alkynes can adsorb onto the PdNPs surface because
they have a higher coordination ability than DMSO. This
phenomenon inspired us to design PdNPs coordinated with
a DMSO-like species in a solid matrix. If a core–shell
structured nanocomposite involving PdNPs encapsulated by
a shell having a DMSO-like species could be constructed, it
would act as an efficient and functional solid catalyst for the
selective semihydrogenation of alkynes.
Semihydrogenation of alkynes is a powerful tool to
synthesize (Z)-alkenes which are important building blocks
for fine chemicals, such as bioactive molecules, flavors, and
Herein, we successfully synthesized core–shell nanocom-
posites of PdNPs covered with a DMSO-like matrix on the
surface of SiO2 (Pd@MPSO/SiO2). The shell, consisting of an
alkyl sulfoxide network, acted as a macroligand and allowed
the selective access of alkynes to the active center of the
PdNPs, promoting the selective semihydrogenation of not
only internal but also terminal alkynes without any additives.
Moreover, these catalysts were reusable while maintaining
high activity and selectivity.
[*] Dr. T. Mitsudome, Y. Takahashi, Dr. T. Mizugaki,
Prof. Dr. K. Jitsukawa, Prof. Dr. K. Kaneda
Department of Materials Engineering Science
Graduate School of Engineering Science, Osaka University
1–3, Machikaneyama, Toyonaka, Osaka 560-8531 (Japan)
E-mail: kaneda@cheng.es.osaka-u.ac.jp
Dr. S. Ichikawa
Institute for NanoScience Design Center, Osaka University
Toyonaka, Osaka 560-8531 (Japan)
Pd@MPSO/SiO2 catalysts were synthesized as follows. Pd/
SiO2 prepared according to our procedure[16] was stirred in
n-heptane with small amounts of 3,5-di-tert-butyl-4-hydroxy-
toluene (BHT) and water at room temperature. Next, methyl-
3-trimethoxysilylpropylsulfoxide (MPSO) was added to the
mixture and the mixture was heated. The slurry obtained was
collected by filtration, washed, and dried in vacuo, affording
Pd@MPSO/SiO2 as a gray powder. Altering the molar ratios
of MPSO to Pd gave two kinds of catalysts: Pd@MPSO/SiO2-
1 (MPSO:Pd = 7:1), and Pd@MPSO/SiO2-2 (MPSO:Pd =
100:1).
Prof. Dr. K. Kaneda
Research Center for Solar Energy Chemistry Osaka University
1-3, Machikaneyama, Toyonaka, Osaka 560-8531 (Japan)
[**] This work was supported by the Japan Society for the Promotion of
Science (JSPS) through a Grant-in-Aid for Young Scientists (A)
(23686116). We thank Dr. Uruga, Dr. Tanida, Dr. Nitta, Dr.
Taniguchi, and Dr. Hirayama (SPring-8) for XAFS measurements.
T.M. thanks the JGC-S Scholarship Foundation.
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2013, 52, 1481 –1485
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1481