Subnanoscale size effect of dendrimer-encapsulated Pd clusters on catalytic hydrogenation of olefin

Article abstract of DOI:10.1246/cl.2011.180

Dendrimer-encapsulated subnano Pd clusters catalyze selective hydrogenation of 1,3-cyclooctadiene to cyclooctene. The catalytic activity increases with the size of the subnano Pd clusters. The activity of the threefold hollow sites of the subnano Pd clust

Full text of DOI:10.1246/cl.2011.180

doi:10.1246/cl.2011.180
180
Published on the web January 15, 2011
Subnanoscale Size Effect of Dendrimer-encapsulated Pd Clusters
on Catalytic Hydrogenation of Olefin
Zen Maeno,1 Takayuki Kibata,1 Takato Mitsudome,1 Tomoo Mizugaki,1 Koichiro Jitsukawa,1 and Kiyotomi Kaneda*1,2
1Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531
2Research Center for Solar Energy Chemistry, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531
(Received November 15, 2010; CL-100954; E-mail: kaneda@cheng.es.osaka-u.ac.jp)
Dendrimer-encapsulated subnano Pd clusters catalyze se-
N
(A)
N
lective hydrogenation of 1,3-cyclooctadiene to cyclooctene. The
catalytic activity increases with the size of the subnano Pd
clusters. The activity of the threefold hollow sites of the subnano
Pd clusters plays an important role in the hydrogenation
reaction.
N
N
N
N
Tertiary amino groups
N
Metal nanoparticles (NPs) have received much attention in
various fields, including catalysis, optics, and electronics, due
to the difference in their chemical and physical properties
compared to mononuclear metal atoms and bulk metals.1 Among
NPs, small clusters of a range of diameters less than 1 nm,
known as subnano metal clusters, are considered a new material
since they bridge the gap between mononuclear metal atoms and
NPs.2 Subnano metal clusters have significant advantages in
terms of efficiency because they possess extremely large surface-
to-volume ratios, and also large numbers of coordinatively
unsaturated surface atoms, which are expected to show unique
catalytic properties. Several attractive methods for synthesizing
subnano metal clusters have been developed using organic
polymers3a-3c and inorganic materials.3d-3g However, size-selec-
tive synthesis of subnano metal clusters has been challenging
due to their instability®they have a tendency to aggregate into
larger NPs®and consequently, the catalysis of subnano metal
clusters and the size effect are still unclear.4
G5-TEBA dendrimer
(B)
Pd4
Pd8
Pd16
Pd16 × 2
G5-Pd0
G5-Pd0
G5-Pd0
G5-Pd0
32
16
4
8
Figure 1. (A) Structure of G5-TEBA dendrimers. (B) G5-Pd0
(y = 4, 8, 16, and 32) and subnano Pd clusters within G5-TEBA.
y
Pd-Pd
5
(h)
(g)
We recently succeeded in the controlled synthesis of
subnano-ordered Pd clusters consisting of a specific number of
Pd atoms (Pd4, Pd8, and Pd16) within fifth generation poly-
(f)
(e)
(propylene imine) (PPI) dendrimers (G5-Pd0 , y = 4, 8, and 16).5
y
(d)
(c)
Herein, we investigate the application of dendrimer-encapsu-
lated subnano Pd clusters in the hydrogenation of 1,3-cyclo-
octadiene (1,3-COD) in order to reveal size-dependent catalysis.
(b)
(a)
Dendrimer-encapsulated subnano Pd clusters of G5-Pd0 ,
4
0
1
2
3
4
5
-Pd0 , and -Pd016 were synthesized according to a previous paper
8
Interatomic distance/Å
from our laboratory.5 Briefly, an appropriate amount of aqueous
solution of Na2PdCl4 was added to a chloroform solution of fifth
generation triethoxybenzamide-terminated PPI dendrimer, G5-
TEBA (Figure 1A), giving dendrimer-encapsulated Pd2+ ions,
Figure 2. Fourier transforms of k3-weighted Pd K-edge
EXAFS experimental data for (a) G5-Pd0 , (b) used G5-Pd0 ,
4
4
(c) G5-Pd0 , (d) used G5-Pd0 , (e) G5-Pd016, (f) used G5-Pd0
,
16
(g) G5-Pd0832, and (h) used G5-Pd0
.
32
8
G5-PdII (y denotes the number of precursor Pd ions in one
y
dendrimer; y = 4, 8, and 16), and these were reduced with an
aqueous solution of KBH4, with vigorous stirring, to form
The preparation of G5-Pd0 using larger amounts of Pd2+
y
Pd0 clusters, G5-Pd0 . Curve-fitting analysis of inverse Fourier-
ions was also examined. Notably, the CN of the Pd-Pd shell in
y
transform (FT) of k3-weighted Pd K-edge extended X-ray
absorption fine structure (EXAFS) (Figure 2) revealed that the
G5-Pd0 was close to that of G5-Pd016, as confirmed by Pd K-
32
edge EXAFS (Table S212). This indicated that G5-Pd0 might
32
coordination numbers (CNs) of the Pd-Pd shell of G5-Pd0
consist of two Pd16 clusters encapsulated within one dendrimer
(Figure 1B). The internal void spaces of the G5-TEBA dendri-
mer may fit the size of the Pd16 cluster.
y
(y = 4, 8, and 16) were 3.2, 4.6, and 5.9, respectively
(Table S212). The nuclearities of the Pd clusters were estimated
to be 4, 8, and 16, respectively, which match the number of
preorganized Pd2+ ions within each dendrimer (Figure 1B).
The catalytic performances of the dendrimer-encapsulated
subnano Pd clusters G5-Pd0 (y = 4, 8, 16, and 32) were
y
Chem. Lett. 2011, 40, 180-181
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

181
hydrogenation reaction. Furthermore, the preparation of G5-Pd0
y
using a larger number of Pd2+ ions succeeded in the formation
of two Pd16 clusters within one dendrimer.
70
60
50
40
30
20
10
0
This work was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science and Technology of Japan. The XAFS experiments were
performed at the BL01B1 and BL14B2 in the SPring-8 with
approval of the Japan Synchrotron Radiation Research Institute
(Nos. 2009A1856, 2009A1860, 2009B1854, 2009B1506,
2010A1781, and 2010A1788). One of the authors T. K.
expresses his special thanks for the Global COE (center of
excellence) Program “Global Education and Research Center for
Bio-Environmental Chemistry” of Osaka University.
4
8
16
32
y (Pd2+/G5-TEBA)
Figure 3. TOFs for hydrogenation of 1,3-cyclooctadiene using
G5-Pd0 .
y
References and Notes
examined in the hydrogenation of 1,3-COD.6 All G5-Pd0y tested
promoted hydrogenation selectively, giving cyclooctene in over
99% yield. Interestingly, the initial turnover frequency (TOF/
min¹1), normalized to the total number of surface Pd atoms in
1
2
3
a) D. V. Talapin, J.-S. Lee, M. V. Kovalenko, E. V. Shevchenko,
Chem. Rev. 2010, 110, 389. b) Metal Nanoclusters in Catalysis
and Materials Science: The Issue of Size Control, 1st ed., ed. by
B. Corain, G. Schmid, N. Toshima, Elsevier, Netherlands, 2008.
a) A. Roucoux, J. Schulz, H. Patin, Chem. Rev. 2002, 102, 3757.
b) D. Astruc, F. Lu, J. R. Aranzaes, Angew. Chem., Int. Ed.
2005, 44, 7852. c) A. W. Castleman, Jr., S. N. Khanna, J. Phys.
Chem. C 2009, 113, 2664.
Recent examples of the synthesis and catalysis of subnano metal
clusters: a) K. Okamoto, R. Akiyama, H. Yoshida, T. Yoshida,
S. Kobayashi, J. Am. Chem. Soc. 2005, 127, 2125. b) K.
Yamamoto, T. Imaoka, W.-J. Chun, O. Enoki, H. Katoh, M.
Takenaga, A. Sonoi, Nat. Chem. 2009, 1, 397. c) B. S.
González, M. J. Rodríguez, C. Blanco, J. Rivas, M. A. López-
Quintela, J. M. G. Martinho, Nano Lett. 2010, 10, 4217. d) K.
Okumura, K. Nota, K. Yoshida, M. Niwa, J. Catal. 2005, 231,
245. e) T. Mitsudome, K. Nose, K. Mori, T. Mizugaki, K.
Ebitani, K. Jitsukawa, K. Kaneda, Angew. Chem., Int. Ed. 2007,
46, 3288. f) W. E. Kaden, T. Wu, W. A. Kunkel, S. L. Anderson,
Science 2009, 326, 826. g) Y. Liu, H. Tsunoyama, T. Akita, T.
Tsukuda, J. Phys. Chem. C 2009, 113, 13457.
the Pd clusters within G5-Pd0 , increased with increasing Pd
y
cluster size; the TOFs of G5-Pd0y (y = 4, 8, and 16) were 10, 31,
and 62, respectively (Figure 3).7 Moreover, the TOF of G5-Pd0
32
(65) was similar to that of G5-Pd0 (62), which is consistent
16
with the presence of two Pd16 clusters within one dendrimer
(vide supra).
After the 1,3-COD hydrogenation reaction, FT of the k3-
weighted Pd K-edge EXAFS spectra of G5-Pd0 (y = 4, 8, 16,
y
and 32) gave similar peak intensities and CNs for the Pd-Pd
shell to those of fresh clusters (Figure 2 and Table S212).
Furthermore, the TOFs of G5-Pd0 were maintained in repeated
y
addition of 1,3-COD (Table S312). These phenomena suggest
that the original sizes of the Pd clusters remain unchanged
during the hydrogenation reaction.
In order to investigate the size effect of the subnano Pd
clusters, preliminary kinetic studies were carried out. The initial
4
Although the chemical vapor method with quadrupole mass is
applicable to size-selective synthesis of subnano metal clus-
ters,3f investigations of the catalytic activity of subnano metal
clusters prepared by this method are limited to gas-phase
reactions.
reaction rate of the 1,3-COD hydrogenation using G5-Pd0
y
(y = 4, 8, and 16) was dependent on the partial pressure of
H2 and independent of the concentration of 1,3-COD
(Figure S2).8,12 The kinetic isotope effect was observed for the
hydrogenation using D2 (kH/kD = 1.5). From the above results,
dissociative adsorption of H2 is considered as the rate-determin-
ing step in the 1,3-COD hydrogenation.9 Zhi et al. and others
reported that the density functional theory (DFT) calculations of
dissociative adsorption of H2 on small Pd clusters revealed that
the configuration with H atoms adsorbed on Pd threefold hollow
sites is the most stable structure.10 In our case, the TOF
normalized to the total number of the threefold hollow sites in
the Pd cluster also increased with increasing the size of subnano
Pd clusters (see the Supporting Information).12 Namely, the size
effect on catalytic activity in the hydrogenation reaction was
derived from the difference of activity of the threefold hollow
sites of the Pd cluster.11
In conclusion, investigation of the catalysis of dendrimer-
encapsulated subnano Pd clusters in the 1,3-COD hydrogenation
showed that the initial rate of the hydrogenation was dependent
on the size of the Pd clusters, and the TOF of the hydrogenation
reaction increased with the Pd cluster size. The activity of the
threefold hollow sites of the subnano Pd clusters in dissociative
adsorption of H2 played an important role in the 1,3-COD
5
6
T. Mizugaki, T. Kibata, K. Ota, T. Mitsudome, K. Ebitani, K.
Jitsukawa, K. Kaneda, Chem. Lett. 2009, 38, 1118.
The total amount of Pd (mol) used for each hydrogenation
reaction was constant. See Supporting Information for the
reaction conditions.12
7
8
See Supporting Information for a definition of TOF.12
In other Pd NP catalyst systems, the reaction rate for 1,3-COD
hydrogenation shows similar dependence on H2 pressure and
1,3-COD concentration. See: H. Arnold, S. Göbel, D. Reinig,
J. Gaube, Chem. Ing. Tech. 1994, 66, 727.
9
S. Naito, M. Tanimoto, J. Chem. Soc., Faraday Trans. 1 1988,
84, 4115.
10 a) M. Ni, Z. Zeng, THEOCHEM 2009, 910, 14. b) E. D.
German, I. Efremenko, M. Sheintuch, J. Phys. Chem. A 2001,
105, 11312. c) J. Roques, C. Lacaze-Dufaure, C. Mijoule,
J. Chem. Theory Comput. 2007, 3, 878.
11 The activation enthalpies (¦H‡/kJ mol¹1) for 1,3-COD hydro-
genation using G5-Pd0 (y = 4, 8, and 16) at 313 K were 41.3,
y
38.1, and 34.3, respectively, while the activation entropies
(¦S‡/kJ mol¹1 K¹1) were ¹141, ¹142, and ¹149, respectively.
12 Supporting Information is available electronically on the CSJ-
Journal Web site, http://www.csj.jp/journals/chem-lett/index.
html.
Chem. Lett. 2011, 40, 180-181
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/