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Published on the web June 26, 2010
Fine Tuning of Pd0 Nanoparticle Formation on Hydroxyapatite and Its Application
for Regioselective Quinoline Hydrogenation
Norifumi Hashimoto,1 Yusuke Takahashi,1 Takayoshi Hara,2 Shogo Shimazu,2 Takato Mitsudome,1
Tomoo Mizugaki,1 Koichiro Jitsukawa,1 and Kiyotomi Kaneda*1,3
1Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531
2Graduate School of Engineering, Chiba University, 1-33 Yayoi, Inage, Chiba 263-8522
3Research Center for Solar Energy Chemistry, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531
(Received May 7, 2010; CL-100444; E-mail: kaneda@cheng.es.osaka-u.ac.jp)
Fine control of the formation of Pd0 nanoparticles with
catalytic hydrogenation of quinolines is the most straight
forward and promising approach due to its high atom efficiency
and the availability of a wide variety of quinolines. There are
many reports on homogeneous and heterogeneous catalyst
systems for the hydrogenation of quinolines. These systems,
however, have often suffered from low catalytic activity or poor
selectivity and the need for harsh conditions such as high
hydrogen pressures and acidic solvents.9 It is desirable, there-
fore, to develop highly active heterogeneous catalysts for the
regioselective hydrogenation of quinolines to 1,2,3,4-tetrahy-
droquinolines under greener process conditions.10
diameters between 1 and 1.5 nm on hydroxyapatite (HAP) was
achieved by adjusting the temperature at which the PdII species
on the HAP surface (PdIIHAP) was reduced in the presence of
1 atm of molecular hydrogen. The HAP-supported Pd0 nano-
particles (Pd0HAP) having an average diameter of 1.5 nm
exhibited significantly high catalytic activity for the regiose-
lective hydrogenation of quinolines to the corresponding
1,2,3,4-tetrahydroquinolines under mild reaction conditions.
Moreover, the Pd0HAP catalyst was reusable without appreci-
able loss of its high catalytic activity or selectivity.
HAP-bound PdCl2 (PdIIHAP) was synthesized according to
a procedure described in our previous report.4a To prepare the
Pd0 NPs, PdIIHAP (0.2 g, Pd: 4 ¯mol) was treated under 1 atm of
H2 for 30 min to reduce the Pd(II) species giving grayish brown
powders (Pd0HAP(T) (T: reduction temperature of 30, 60, or
90 °C)). Palladium K-edge XAFS analysis of the Pd0HAP(T)
was conducted on the samples taken after reduction with H2.
XANES spectra showed that the reduction of the PdII species
occurred within 30 min to form Pd0 species on HAP during the
pretreatment. In the Fourier transforms of the Pd K-edge EXAFS
spectra (Figure 1), a peak around 0.25 nm corresponding to the
Pd-Pd bond was observed in all Pd0HAP(T) catalysts.
A second coordination peak based on the Pd-(Pd)-Pd bond,
however, did not appear in the range between 0.4 and 0.5 nm.
These results confirm the formation of Pd0 NPs on the HAP
surface. The intensity of the peak around 0.25 nm decreased with
increasing reduction temperature, indicating that the particle size
becomes smaller at higher reduction temperatures (Figures 1a-
1c). A curve fitting analysis of the Pd K-edge EXAFS spectra
Size-selective synthesis of metal nanoparticles (NPs) is of
great interest in the areas of materials chemistry and physics
because of the high potential for application of NPs in catalysis,
optics, and electronics.1 In the catalysis area in particular, much
effort has been devoted to improving methods for precisely
controlling metal NP size using various stabilizers such as solid
supports, polymers, and surfactants in order to design high
performance catalysts.2 Systematic control of the size of metal
NPs, however, remains a valuable challenge. We have developed
unique heterogeneous catalysts using hydroxyapatite (HAP),
Ca10(PO4)6(OH)2, as a macroligand for catalytically active
species that are effective in various environmentally benign
organic transformations.3 By using HAP as a catalyst support, a
structurally uniform (P=O)2PdCl2 precursor species is easily
created, which is further reduced to size-controlled Pd0 NPs with
narrow size distribution. It has been demonstrated that Pd0HAPs
with specific particle sizes act as an effective heterogeneous
catalysts for aerobic alcohol oxidations (effective Pd0 NP size:
3.8 or 7.8 nm),4a dehydrogenation of indolines (9 nm),4b hydro-
genolysis of Z-groups (3 nm),4c and hydrodechlorination of
organic halides (3 and 1.2 nm) and DDT derivatives (4 nm).4d-4f
Herein, we found that the fine tuning of the formation of Pd0
NPs on HAP surfaces in the size range 1-1.5 nm was achieved
by changing the reduction temperatures and that Pd0HAP with a
diameter of 1.5 nm exhibited significantly high catalytic activity
for the regioselective hydrogenation of quinolines to the
corresponding 1,2,3,4-tetrahydroquinolines under mild reaction
conditions.
1,2,3,4-Tetrahydroquinoline derivatives have attracted con-
siderable attention due to their importance as synthetically
valuable intermediates for pharmaceuticals,5 agrochemicals,6
and dyes.7 Among several possible methods for the synthesis
of the 1,2,3,4-tetrahydroquinolines such as cyclization of 3-
aminopropyl-o-haloarenes,8a,8b Beckmann rearrangement of ox-
ime sulfonates,8c borohydride reduction of quinolones,8d the
Figure 1. Fourier transforms of k3-weighted Pd K-edge
EXAFS experimental data for (a) Pd0HAP(90), (b) Pd0HAP(60),
(c) Pd0HAP(30), and (d) Pd foil.
Chem. Lett. 2010, 39, 832-834
© 2010 The Chemical Society of Japan