Palladium-platinum bimetallic nanoparticle catalysts using dendron assembly for selective hydrogenation of dienes and their application to thermomorphic system

Article abstract of DOI:10.1246/cl.2005.272

Self-assembly of dendrons allows a new approach for size control of bimetallic Pd-Pt nanoparticles, which affords reusable dendritic nanoreactors for selective hydrogenation of dienes and alkynes to monoenes in thermomorphic systems. Copyright

Full text of DOI:10.1246/cl.2005.272

272
Chemistry Letters Vol.34, No.2 (2005)
Palladium–Platinum Bimetallic Nanoparticle Catalysts Using Dendron Assembly
for Selective Hydrogenation of Dienes and Their Application to Thermomorphic System
Makoto Murata, Yuko Tanaka, Tomoo Mizugaki, Kohki Ebitani, and Kiyotomi Kanedaꢀ
Department of Materials Engineering Science,Graduate School of Engineering Science, Osaka University,
1-3 Machikaneyama, Toyonaka, Osaka 560-853
(Received November 19, 2004; CL-041394)
Self-assembly of dendrons allows a new approach for size
1a)4 and these products were characterized by NMR, IR, MS,
dynamic light scattering (DLS), and TEM. Treatment of the
dendrons with a solution of [PdCl(C3H5)]2 and PtCl2(PhCN)2,
followed by reduction using LiB(C2H5)3H, led to a dark brown
solution containing dendritic mixed metal nanoparticles (Gn Py–
C6 Pd–Pt; n ¼ 1; 2; 3).5 Using a similar procedure, Gn Py–C6
Pd(0)6 and Py–C6 Pt(0) were prepared from [PdCl(C3H5)]2 and
PtCl2(PhCN)2, respectively. Sequential reduction of the Pd
complex after formation of Gn Py–C6 Pt(0) afforded Gn Py–
C6 Pd–(Pt).
control of bimetallic Pd–Pt nanoparticles, which affords reusable
dendritic nanoreactors for selective hydrogenation of dienes and
alkynes to monoenes in thermomorphic systems.
Metal nanoparticles composed of two metal elements are of
interest in materials science, particularly in catalysis, because
they afford the synergistic effect on the catalytic activity and se-
lectivity.1 Simultaneous and sequential reduction of two metal
complexes results in corresponding bimetallic nanoparticles,
such as Pd–Pt, Pd–Ni, and Pt–Au, in the presence of stabilizing
agents.2 Pd–Pt bimetallic nanoparticles were prepared by co-re-
1H NMR spectra of Gn Py–C6 exhibited broad, weak signals
of internal methylene and pyridyl core protons, which implies a
reversed micelle-like assembly of the dendrons.7 DLS and TEM
images provided information about the size of the dendritic Pd–
Pt nanoparticles, as shown in Figure 1 and Table 1. DLS meas-
urements of G2 Py–C6 revealed the formation of globular assem-
blies with average diameters of 10:9 ꢂ 2:0 nm (Entry 1). After
co-reduction of the Pd and Pt complexes, size distributions for
G2 Py–C6 Pd4–Pt1 (Pd/Pt = 4) were similar to those for the pa-
rent dendron assemblies (Entry 2). TEM images revealed an
average metal nanoparticle size of 5:3 ꢂ 1:4 nm for G2 Py–C6
Pd4–Pt1, and X-ray energy dispersive spectroscopy (EDS) anal-
ysis provided atom % of 79% and 21% for Pd and Pt, respective-
ly, similar to those of the parent Pd and Pt mol ratios. In addition,
average particle sizes of G2 Py–C6 Pd(0), Py–C6 Pt(0), and Py–
C6 Pd–(Pt) were 5:3 ꢂ 1:7, 5:3 ꢂ 1:3, and 5:4 ꢂ 1:5 nm, respec-
tively (Entries 3–5). As shown in Figure 1b, the size distribution
of the dendron assemblies agreed well with a distribution de-
rived from the combination of the mixed metal nanoparticle
and dendron sizes.8 These data suggest that the dendrons assem-
ble to form monolayer capsules with polar voids, which act as
useful templates for metal nanoparticles having ca. 5 nm diame-
ter, which are generated within the voids of the assemblies.6
The catalytic ability of dendritic Pd–Pt bimetallic nanopar-
ticles was investigated in the hydrogenation of 1,3-cycloocta-
diene, which yielded cyclooctene exclusively without formation
of an insoluble metal aggregate. G2 Py–C6 Pd–Pt nanoparticles
2ꢁ
2ꢁ
duction of PdCl4 and PtCl4 with poly(N-vinyl-2-pyrroli-
done) (PVP)1 or unimolecular dendrimers.2a Here, we report a
new approach for the preparation of Pd–Pt bimetallic nanoparti-
cles via the self-assembly of dendrons, which was applied to a
nanoreactor for highly selective hydrogenation of conjugated di-
enes and alkynes to monoenes. The recycling of the dendritic
catalyst was also investigated using the thermoresponsive phase
separation of solvents, called as thermomorphic systems.3
Alkylated amidoamine dendrons (Gn Py–C6; n ¼ 1; 2; 3)
were synthesized from 4-picolylamine with methyl acrylate
and ethylenediamine using the divergent method. Terminal ester
moieties of the branches were alkylated by n-hexylamine (Figure
(a)
NH
O
1. methyl acrylate
2. ethylene diamine
Repeat (1)
3. n-hexyl amine
NH
NH
O
O
N
N
O
O
NH2
H
H
N
N
N
N
N
O
G2 Py-C6
NH
1. [PdCl(C H )]
3
5
2
2
PtCl (PhCN)
2
CH Cl
2
2
2. LiB(C H ) H
5 3
2
Table 1. Average diameter of G2 dendron assemblies and metal
nanoparticles by DLS and TEM measurements
G2 Py-C6 Pd-Pt
(b)
Avarage Diameter/nm
Entry
Nanoparticle distributions (TEM)
+ G2 Py-C6 (2.5 nm) x 2
40
DLSa
Vold spaceb
5.9
TEM
30
20
10
0
1
2
3
4
5
Py–C6
10:9 ꢂ 2:0
G2 Py-C6 Pd4-Pt1 (DLS)
20
Py–C6 Pd4–Pt1(0) 11:2 ꢂ 1:6
5:3 ꢂ 1:4
5:3 ꢂ 1:7
5:3 ꢂ 1:3
5:4 ꢂ 1:5
Py–C6 Pd(0)
Py–C6 Pt(0)
Py–C6 Pd–(Pt)
11:0 ꢂ 1:7
11:1 ꢂ 1:7
11:1 ꢂ 1:7
0
10
Diameter /nm
Figure 1. Structure of dendritic Pd-Pt nanoparticles (G2 Py–C6
Pd–Pt) (a) and the size distributions by DLS and TEM measure-
ments (b). aIntensity was based on the number of particles.
aMeasured in CH2Cl2 at 25 ꢃC. bCalculated by [DLS – G2
dendron size ð2:5Þ ꢄ 2].
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.2 (2005)
273
Homogeneous
Substrate
Catalyst
10
Isolation
Yield /%
catalyst
heptane
DMF
Product
Catalyst
Substrate
Catalyst
65 °C
r.t.
fresh 61
2nd 99
8
6
4
2
0
H2 1atm
solvent, r.t.
3rd
99
Py-C6 Pd-Pt
Recyclable
4rth 99
Figure 3. Thermomorphic system using dendritic catalyst.
Physical mixture
(Py-C6 Pd(0) + Py-C6 Pt(0))
performance of dendritic Pd–Pt bimetallic nanoparticles using
dendron self-assembly. The dendron assemblies act as a tem-
plate of bimetallic nanoparticles. The dendritic nanoparticles
functioned as nanoreactors for the hydrogenation of olefins
and acetylenes, and could be reused under thermosensitive
biphasic conditions. These self-assembled dendrons can be
applied widely for the development of a variety of mono and
mixed metal nanoparticles for materials science.
0
10 20 30 40 50 60 70 80 90 100
Mole percentage of Pd
Figure 2. Hydrogenation of 1,3-cyclooctadiene using G2 Py–
C6 Pd–Pt. Reaction conditions: 1,3-cyclooctadiene 1.0 mmol,
dendron 0.082 mmol, Pd (mmol) + Pt (mmol) 0.005 mmol,
CH2Cl2 3 mL, rt., H2 1 atm.
This work was supported by a Grant-in-Aid for Scientific
Research from JSPS. We thank for the center of excellence
(21COE) program ‘Creation of Integrated Ecochemistry’ of
Osaka University. A portion of the experiments involving
TEM measurements was conducted using a facility in the
Research Center for Ultrahigh Voltage Electron Microscopy at
Osaka University.
possessed higher catalytic activity than that found for a physical
mixture of G2 Py–C6 Pd(0) and Py–C6 Pt(0) (Figure 2). A sig-
nificant bimetallic effect was observed with Pd–Pt dendrimers
containing a Pd content of 80–90 mol %; a similar phenomenon
was reported using polymer-stabilized mixed Pd–Pt bimetallic
nanoparticles.1 No synergistic effect was observed with Py–C6
Pd–(Pt) nanoparticles prepared by sequential reduction. Notably,
the catalytic activity of G3 Py–C6 Pd4–Pt1 was 1.9 times greater
than that of G2 Py–C6 Pd4–Pt1. This effect contrasts with the re-
sults reported for an unimolecular dendrimer-encapsulated Pd(0)
nanoparticle catalyst.9 An increase in dendron generation pro-
vides a less congested Pd surface, which facilitates substrate ac-
cess to active sites.6,10
An extremely high selectivity for the monoene was also
achieved in the hydrogenation of alkynes; e.g., 1-phenyl-1-
propyne afforded 1-phenyl-1-propene in 94% yield (Z:E=
94:6) after complete conversion of the alkyne using G2
Py–C6 Pd4–Pt1 catalyst (Scheme 1). This high monoene selec-
tivity can be attributed to the coordination of the pyridyl core
to the metal nanoparticles.11
References and Notes
1
2
N. Toshima and T. Yonezawa, New J. Chem., 1998, 1179.
Recent examples of bimetallic nanoparticles, see: Pd–Pt: a) R. W.
Scott, J. A. K. Datye, and R. M. Crooks, J. Am. Chem. Soc., 125,
3708 (2003). Pd–Ni: b) S. U. Son, Y. Jang, J. Park, H. B. Na,
H. M. Park, H. J. Yun, J. Lee, and T. Hyeon, J. Am. Chem.
Soc., 126, 5026 (2004). Pd–Au: c) R. Harpeness and A. Gedanken,
Langmuir, 20, 3431 (2004). Pt–Au: d) H. Lang, S. Maldonada,
K. J. Stevenson, and B. D. Chandler, J. Am. Chem. Soc., 126,
12949 (2004).
3
4
a) D. E. Bergbreiter, Chem. Rev., 102, 3345 (2002). b) M. Ooe, M.
Murata, T. Mizugaki, K. Ebitani, and K. Kaneda, J. Am. Chem.
Soc., 126, 1604 (2004).
a) M. Murata, T. Hara, K. Mori, M. Ooe, T. Mizugaki, K. Ebitani,
and K. Kaneda, Tetrahedron Lett., 44, 4981 (2003). b) K. Aoi,
A. Motoda, M. Ohno, K. Tsutsumiuchi, M. Okada, and T. Imae,
Polym. J., 30, 1071 (1999).
H2 (1 atm)
5
Typical procedures for the preparation of the dendritic Pd4–Pt1
bimetallic nanoparticles (Gn Py–C6 Pd4–Pt1; n ¼ 1; 2; 3) are
as follows. To a CH2Cl2 solution (5 mL) of Gn Py–C6 (0.164
mmol) was added a CH2Cl2 solution (1 mL) of [PdCl(C3H5)]2
(0.004 mmol) and PtCl2(PhCN)2 (0.002 mmol), and stirred vigo-
rously at room temperature for 1 h. The mixture was treated with
LiB(C2H5)3H in THF (1M, 0.02 mL) and stirred at room temper-
ature for 1 h.
G2 Py-C6 Pd4-Pt1, r.t.
94 % yield (Z/E =94/6 )
Scheme 1. 1-phenyl-1-propyne 1.0 mmol, G2 Py–C6 0.164
mmol, Pd (mmol) + Pt (mmol) 0.010 mmol CH2Cl2 3 mL,
42 min.
Recycling of dendritic catalysts has been attempted by sol-
vent precipitation and membrane filtration,12 which often results
in loss of the catalytic activity during the reuse processes. G2
Py–C6 dendron possessed high solubility in DMF, which al-
lowed the transfer of the dendritic catalyst to DMF under the
thermomorphic systems consisting of DMF and n-heptane
(Figure 3).3 In the hydrogenation of 1,3-cyclooctadiene, the
two thermosensitive phases of DMF and heptane became homo-
geneous at 65 ꢃC, but could be readily separated by cooling the
reaction mixture to room temperature. The DMF phase contain-
ing the dendritic catalyst could be recycled after decanting the
n-heptane phase. High catalytic activity was retained during re-
use experiments; yields of cyclooctene in heptane phase were
61% (fresh), 99% (second), 99% (third), and 99% (fourth).13
In conclusion, we report the preparation and high catalytic
6
M. Murata, Y. Tanaka, T. Mizugaki, K. Ebitani, and K. Kaneda,
submitted.
´
7
8
I. Gitsov and J. M. J. Frechet, Macromolecules, 26, 6536 (1993).
The molecular size of G2 Py–C6 dendron calculated by Chem 3D
are 2.5 nm.
M. Ooe, M. Murata, T. Mizugaki, K. Ebitani, and K. Kaneda,
Nano Lett., 2, 999 (2002).
9
10 A similar generation effect has been reported for nanoparticle-
cored dendrimers, see: K. R. Gopidas, J. K. Whitesell, and
M. A. Fox, J. Am. Chem. Soc., 125, 6491 (2003).
11 R. L. Augustine, ‘‘Heterogeneous Catalysis for the Synthetic
Chemist,’’ Marcel Dekker, Inc., New York (1996).
12 B. Yi, Q.-H. Fan, G.-J. Deng, Y.-M. Li, L.-Q. Qiu, and A. S. C.
Chan, Org. Lett., 6, 1361 (2004).
13 The relatively low yield for the fresh run was due to the distribu-
tion of cyclooctene in the DMF phase.
Published on the web (Advance View) January 26, 2005; DOI 10.1246/cl.2005.272