Design of Core-Pd/Shell-Ag Nanocomposite Catalyst for Selective Semihydrogenation of Alkynes

Article abstract of DOI:10.1021/acscatal.5b02518

We designed core-Pd/shell-Ag nanocomposite catalyst (Pd@Ag) for highly selective semihydrogenation of alkynes. The construction of the core-shell nanocomposite enables a significant improvement in the low activity of Ag NPs for the selective semihydrogenation of alkynes because hydrogen is supplied from the core-Pd NPs to the shell-Ag NPs in a synergistic manner. Simultaneously, coating the core-Pd NPs with shell-Ag NPs results in efficient suppression of overhydrogenation of alkenes by the Pd NPs. This complementary action of core-Pd and shell-Ag provides high chemoselectivity toward a wide range of alkenes with high Z-selectivity under mild reaction conditions (room temperature and 1 atm H₂). Moreover, Pd@Ag can be easily separated from the reaction mixture and is reusable without loss of catalytic activity or selectivity.

Full text of DOI:10.1021/acscatal.5b02518

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Letter
Design of Core-Pd/Shell-Ag Nanocomposite Catalyst
for Selective Semihydrogenation of Alkynes
Takato Mitsudome, Teppei Urayama, Kenji Yamazaki, Yosuke Maehara, Jun Yamasaki,
Kazutoshi Gohara, Zen Maeno, Tomoo Mizugaki, Koichiro Jitsukawa, and Kiyotomi Kaneda
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02518 • Publication Date (Web): 22 Dec 2015
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Design of Core-Pd/Shell-Ag Nanocomposite Catalyst for
Selective Semihydrogenation of Alkynes
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†
†
‡
‡
§
Takato Mitsudome, Teppei Urayama, Kenji Yamazaki, Yosuke Maehara, Jun Yamasaki,
‡
†
†
†
,†,‖
Kazutoshi Gohara, Zen Maeno, Tomoo Mizugaki, Koichiro Jitsukawa, Kiyotomi Kaneda*
†
‖
Department of Materials Engineering Science, Graduate School of Engineering Science and
Research Center for Solar Energy Chemistry, Osaka University, 1-3, Machikaneyama, Toyonaka,
Osaka 560-8531, Japan
‡
Division of Applied Physics, Graduate School of Engineering, Hokkaido University, Kita 13,
Nishi 8, Kita-Ku, Sapporo 060-8628, Japan
§
Research Center for Ultra-High Voltage Electron Microscopy, Osaka University, 7-1,
Mihogaoka, Ibaraki, Osaka 567-0047, Japan
Abstract
We designed core-Pd/shell-Ag nanocomposite catalyst (Pd@Ag) for highly selective
semihydrogenation of alkynes. The construction of the core-shell nanocomposite
enables a significant improvement in the low activity of Ag NPs for the selective
semihydrogenation of alkynes because hydrogen is supplied from the core-Pd NPs to
the shell-Ag NPs in a synergistic manner. Simultaneously, coating the core-Pd NPs with
shell-Ag NPs results in efficient suppression of overhydrogenation of alkenes by the Pd
NPs. This complementary action of core-Pd and shell-Ag provides high
chemoselectivity toward a wide range of alkenes with high Z-selectivity under mild
reaction conditions (room temperature and 1 atm H ). Moreover, Pd@Ag can be easily
2
separated from the reaction mixture and is reusable without loss of catalytic activity or
selectivity.
Keyword
core-shell, silver, palladium, semihydrogenation, alkyne
Text
Nano-structured composites with a core (inner material) and shell (outer layer)
structure, so-called core-shell materials, have attracted enormous attention as highly
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functionalized nanoarchitectures in diverse areas such as sensors , quantum dots ,
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photonic crystals , and catalysts . The extensive study on design of core-shell materials
stems from their own intriguing characteristics, including optical, electronic, and
chemical properties. In the field of the catalyst design, core-shell nanoparticles have
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exhibited unique and promising properties, such as high stability against sintering ,
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high activity through maximization of core-shell interfacial interaction , and
multi-functionalized catalysis for one-pot tandem reaction through accumulation of
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different active species in the core and shell . Fine-tuning the composition and
morphology of such composites will enable rational design of novel core-shell
nanoparticle catalysts with fascinating tailored activities.
The semihydrogenation reaction, which converts alkynes to alkenes, is one of the
most important and fundamental reactions in manufacturing processes of bulk and fine
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chemicals . In this context, Lindlar catalyst (Pd/CaCO treated by Pb salts) has been a
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good first port of call in the synthesis of diverse alkenes . However, the Lindlar catalyst
has serious drawbacks such as high toxicity of Pb species and the low alkene selectivity
when employing terminal alkynes due to the rapid overhydrogenation of terminal
alkenes to alkanes. Therefore, the development of the alternative Pb-free catalysts using
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Pd , Ni , Cu , Ru and Au have been extensively studied.
It is well known that unmodified Pd NPs inherently shows high catalytic activity for
hydrogenation of alkynes, but low selectivity for alkenes due to overhydrogenation. In
contrast, Ag NPs has extremely low catalytic activity for semihydrogenation, requiring
high temperature or high H2 pressure, despite intrinsically showing high
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alkene-selectivity due to the rather weak complexation of Ag with alkenes . In
consideration of beneficial properties of Pd and Ag NPs, we anticipated that the
construction of a bimetallic core-shell nanocomposite catalyst consisting of Pd NPs in
the core and Ag NPs in the shell (Pd@Ag) would address the trade-off between activity
and selectivity for Pd and Ag NPs in semihydrogenation. Our design concept of Pd@Ag
represents the building of a compatible relationship between Pd, with its high activity,
and Ag, with its high alkene-selectivity, by construction of a core-shell structure. Pd
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NPs in the core store hydrogen to form palladium-hydride (PdH) through the
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H -permeable Ag shell nano-layer . The inner PdH serves hydrogen to the outer Ag
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shell, providing the Pb-free and selective semihydrogenation of alkynes occurring on
the Ag surface. Simultaneously, coating the Pd NPs by a Ag layer results in efficient
suppression of the overhydrogenation of alkenes at the Pd surface. This complementary
action of Pd and Ag, in which the advantages of Pd and Ag are integrated while their
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disadvantages are remedied in a synergistic manner, allows selective semihydrogenation
under mild reaction conditions (Figure 1).
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Figure 1 Design concept of complementary bimetalic core-Pd/shell-Ag
catalyst for selective semihydrogenation of alkynes
Pd@Ag was synthesized by the seeding method (for details, see Supporting
Information). Pd NPs with a mean diameter of 24.1 ± 5.7 nm were synthesized by
reduction of Pd(acac) in the presence of polyvinyl alcohol (PVA), with ethylene glycol
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as a solvent and reducing reagent. Next, the resulting Pd NPs were used as seeds for the
formation of Pd@Ag by reduction of AgNO , with different Ag/Pd ratios (Ag/Pd = 0.10,
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.15, 0.20, 0.25, 0.50), using ascorbic acid in water. Hereafter, the resulting Pd@Ag
catalysts are denoted Pd@Ag-X, where X is the Ag/Pd ratio.
Representative images of Pd@Ag-0.20 observed by transmission electron microscopy
(
TEM) and high-angle annular dark-field STEM (HAADF-STEM) are depicted in
Figure 2. It can be seen that the NPs had a mean diameter of 26.2 ± 5.7 nm (Figure 2a).
In the HAADF-STEM image of Pd@Ag, well-defined lattice fringes were observed
(
Figure 2b). The detected lattice fringes had d-spacing attributed to Ag {111} (2.36 Å)
and Ag {200} (2.04 Å). This fact indicates that crystalline Ag grows on the surface of
Pd NP seed. The elemental distribution of Ag on the Pd NPs in Pd@Ag (Figure 2c) was
examined by scanning TEM (STEM) coupled with energy-dispersive X-ray
spectroscopy (EDX) (Figures 2d, e, and f). Elemental mapping of Pd@Ag based on
quantification analysis of EDX spectra clearly revealed that it comprised Pd NPs in the
core and a nano-layer of Ag with a thickness of ca. 1 nm in the shell. The formation of a
thin shell of Ag was also supported by XRD analysis, as no peak attributed to the
formation of Ag nanoparticles or an alloy of Ag and Pd was observed around 35–50°
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(Figure S11).
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Figure 2 Composition and structural analysis of Pd@Ag-0.20. a)
TEM image. b) and c) HAADF-STEM images. Elemental
mapping images of d) Pd and e) Ag. f) Composite overlay image
formed from d) and e).
After loading onto hydroxyapatite (HAP) which was chosen due to easy handling and
to ensure high dispersion, the Pd@Ag-X composites were used as catalysts in the
batchwise semihydrogenation of 1-octyne (1) at room temperature under atmospheric
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pressure of H . In order to compare catalytic activity, both single Pd NPs (Pd seed for
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Pd@Ag) and Ag NPs loaded on HAP were prepared. The results are shown in Table 1.
Pd NPs promoted rapid overhydrogenation, quantitatively giving the undesired product
n-octane (3) (Table 1, Entry 1). Ag NPs did not show any catalytic activity under the
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conditions used (Table 1, Entry 2). As previously reported , highly alkene-selective
catalysis of Ag NPs was confirmed under high pressure of H (50 atm), although with
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-1
extremely low activity (turnover frequency = 0.035 h ) (Table 1, Entry 3). In contrast,
the various Pd@Ag-X composites showed differing activity and selectivity toward 2
depending on the value of X. When the Ag/Pd ratio X was increased from 0.10 to 0.20,
alkene-selectivity steadily increased (Table 1, Entries 4–6), with Pd@Ag-0.20
exhibiting the highest catalytic activity, affording 2 in quantitative yield (Table 1, Entry
6
). Upon a further increase in the Ag/Pd ratio to 0.50, complete alkene-selectivity was
retained but the activity of the catalyst gradually decreased (Table 1, Entries 8 and 9).
Interestingly, the excellent alkene-selectivity of Pd@Ag-0.20 was maintained even
when the reaction time was prolonged after full conversion of 1 (Figure S16). Notably,
the C=C bond of 2 remained almost intact up to a H pressure of 50 atm (Table 1, Entry
2
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7). This high performance of Pd@Ag-0.20 arising from the core-shell structure is in
sharp contrast with many previous catalysts, including the Lindlar catalyst and Pd-Ag
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alloys, which easily causes overhydrogenation of terminal alkenes to alkanes .
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Table 1 Semihydrogenation of 1 using various Pd-Ag catalysts
Conv.
2
[%]
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Entry
1
Catalyst
Time [h]
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[%]
[%]
Pd NPs
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2.5
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>99
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>99
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Ag NPs
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Ag NPs
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Pd@Ag-0.15
Pd@Ag-0.20
Pd@Ag-0.20
Pd@Ag-0.25
Pd@Ag-0.50
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>99
>99
>99
>99
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>99
>99
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d
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Reaction conditions: Catalyst (0.10 g, Pd: 0.33 mol%, Ag: 0.033
a
mol%-0.17 mol%), 1 (0.3 mmol), EtOH (2 ml). Determined by
b
GC using internal standard technique. Ag/HAP (Ag: 0.33
c
d
mol%). Ag/HAP (Ag: 50 mol%), H (50 atm). H (50 atm).
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The catalytic potential of Pd@Ag-0.20, which exhibited the best performance, was
investigated using various alkynes in a batch reactor. A wide range of terminal and
internal alkynes containing functional groups such as hydroxyl, carboxylic acid, ester,
amine, cyano, and halogen groups were smoothly converted to the corresponding
alkenes with >99% selectivity (Table 2). After the reaction, the Pd@Ag catalyst loaded
onto HAP was easily recovered by filtration, and proved to be reusable, retaining its
high activity and selectivity during five recycling experiments, which demonstrates its
high durability (Table 2, Entry 2). We also examined the practical utility of Pd@Ag by
using a plug flow reactor for gram-scale synthesis. When 1 (10 mmol, 1.10 g) in ethanol
(0.10 M) was passed through a Pd@Ag-0.20-packed column reactor at a flow rate of 4.2
−1
mL h along with H at atmospheric pressure, 2 was successively obtained in over 99%
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yield (96% isolated yield; 1.07 g).
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Table 2 Semihydrogenation of various alkynes
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Time Conv.
Yield
E/Z
Entry
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Product
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[%]
[%]
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>99
>99
>99 (95)
>99
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>99
>99
>99 (93)
>99 (91)
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>99
>99
>99 (92)
>99 (93)
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c
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>99
>99 (92)
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3.0
3.0
4.0
4.0
5.0
5.5
5.0
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>99
>99
>99
>99
>99
>99
>99
>99
>99
>99 (96)
>99 (95)
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-
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0
>99 (94) 0/100
>99 (98) 0/100
>99 (94) 0/100
>99 (94) 0/100
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1
2
13
1
4
>99 (91)
4/96
d
1
5
>99 (92) 16/84
>99 (93) 0/100
1
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Reaction conditions: Pd@Ag-0.20 (0.10 g, Pd: 0.33 mol%, Ag:
a
0
.066 mol%), alkyne (0.3 mmol), EtOH (2 mL). Determined by
GC using internal standard technique. Values in parentheses are
b
c
d
o
isolated yields. 5th reuse. Hexane (2 mL). 50 C.
To elucidate the reasons for the significant differences in the performance of
Pd@Ag-X depending on X, the shell effect of Ag was investigated by CO adsorption
FTIR spectroscopy (Figure S13). It is well known that CO is not adsorbed onto the Ag
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NP surface but is strongly adsorbed on Pd NPs . In fact, when single Ag NPs and Pd
NPs were treated with CO, no CO adsorption peaks were observed for Ag NPs, while
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two CO adsorption peaks derived from CO stretching at 1930 cm and 2060 cm were
observed for Pd NPs. IR spectra of Pd@Ag-0.10 and Pd@Ag-0.15 treated with CO
showed two peaks in the same position as for single Pd NPs. On the other hand, it is
notable that Pd@Ag-0.20, Pd@Ag-0.25, and Pd@Ag-0.50 showed no CO adsorption
peaks, demonstrating that the Pd NPs were entirely covered by Ag in a core-shell
structure. These results showed that the low alkene-selectivity of Pd@Ag-0.10 and
Pd@Ag-0.15 was strongly associated with the presence of the exposed Pd NPs, causing
overhydrogenation. The steadily decreasing activity of Pd@Ag-X as X increased from
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.20 to 0.50 was explained by the increase in thickness of the Ag shell, which hinders
the supply of hydrogen from the core-Pd NPs to the surface Ag sites. TEM images of
Pd@Ag supported the contention that the thickness of the Ag shell could be controlled
by changing the amount of Ag used. The mean diameters of Pd@Ag-X, where X was
0
.10, 0.15, 0.20, 0.25, and 0.50, were 25.2 ± 4.0, 25.9 ± 4.9, 26.2 ± 5.7, 26.6 ± 5.9, and
27.0 ± 4.5 (Figure S1–S4, S6, and S7).
Among noble metals, Ag nanocrystals have uniquely bifunctional properties with not
only the alkene-selective catalysis but also the high activity for surface-enhanced
Raman spectroscopy (SERS), allowing in-situ monitoring of Ag-catalyzed surface
reaction. Therefore, we investigated the Pd@Ag-catalyzed semihydrogenation by SERS.
Figure 3(A) shows SERS spectra from ethanol suspension of Pd NPs and Pd@Ag in the
presence of phenylacetylene (PA) as a Raman probe. No peaks of PA (0.12 mM) without
nanoparticles or with Pd NPs were confirmed (Figure 3A (a) and (b)). Interestingly,
when the colloidal Pd@Ag was dropped into the ethanol solution of PA, new peaks at
-
1
-1
1980 cm (C≡C stretching mode) and 1590 cm (benzene ring mode) appeared,
demonstrating the SERS activity of Pd@Ag (Figure 3A (c)). The shift in the peak at
-1
-1
2111 cm for the C≡C stretching of free PA to 1980 cm is assigned to surface
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adsorption of PA on Ag (Figure 3A (c) vs. (d)), in agreement with previous finding .
Moreover, the SERS activity of Pd@Ag enabled the in-situ monitoring of the
Pd@Ag-catalyzed semihydrogenation of PA. After adding Pd@Ag to the PA solution
under atmospheric H conditions, the SERS spectrum was recorded (Figure 3B). The
2
peaks attributed to the PA adsorbed on the surface of Ag gradually decreased during the
semihydrogenation of PA, supporting that the semihydrogenation undergoes on the
surface of Ag shell of Pd@Ag where the Pd core serves as a hydrogen source due to the
formation of a PdH species. The formation of the PdH was confirmed by XRD analysis
(Figure S12) where the XRD peaks attributed to Pd core of Pd@Ag shifted to low-angle
side under H atmosphere. Furthermore, the H -D exchange experiment using Ag NPs
2
2
2
catalyst (Scheme S2) revealed the formation of HD, indicating the possibility for
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Figure 3 (A) Raman spectra recorded from ethanol suspension of
nanoparticles in the presence of PA: a) 0.12 mM PA, b) 0.12 mM PA
with Pd NPs, c) 0.12 mM PA with Pd@Ag-0.20 and d) reference PA
(intensity: 1/500). (B) SERS spectra recorded during the
semihydrogenation of PA catalyzed by Pd@Ag-0.20 at r. t. under 1
atm of H2.
In conclusion, we designed core-Pd/shell-Ag nanocomposite catalysts (Pd@Ag) whose
components acted in complementary fashion to achieve Pb-free and highly selective
semihydrogenation of alkynes under mild reaction conditions. The Pd@Ag catalyst
(loaded on HAP) was reusable and applicable to use in a plug flow reactor. It was found
that the core-Pd enhanced the activity of the shell-Ag by serving as a hydrogen source,
while, at the same time, the shell Ag reduced the intrinsically low alkene-selectivity of
Pd by inhibiting contact between Pd and alkenes. As a whole, the complementary
relationship between Pd and Ag derived from the core-shell arrangement resulted in
highly active and selective catalysis in the semihydrogenation of alkynes.
Supporting Information
Experimental details and characterization of catalysts. This material is available free
of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
E-mail:Kaneda@cheng.es.osaka-u.ac.jp
Acknowledgement
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This work was supported by JSPS KAKENHI Grant Nos. 26289303, 26630410,
2
6105003, and 24246129. This work was also supported by the Program for Creating
Future Wisdom, Osaka University, selected in 2014. A part of this work was conducted
in Hokkaido University, supported by Nanotechnology Platform Program of the
Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.
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[
Graphic Abstract
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