Boronate microparticle-supported nano-palladium and nano-gold catalysts for chemoselective hydrogenation of cinnamaldehyde in environmentally preferable solvents

Article abstract of DOI:10.1039/c4gc00383g

Dispersible self-assembled boronate polymers can serve as support materials for metal nanoparticle catalysts. As a proof-of-concept, catalytic systems for the chemoselective hydrogenation of cinnamaldehyde (CA) were prepared by the NaBH₄ reduction of PdCl₄^2- and AuCl ₄^- in methanol, in the presence of polyethyleneimine (PEI)-coated boronate particles (BPs) composed of polymeric 3-benzo-2,4,8,10- tetraoxa-3,9-diboraspiro[5.5]undecane. The Pd-deposited BP (Pd/BP) showed highly selective catalytic activity for the hydrogenation of CA to hydrocinnamaldehyde (HCA) under 0.1 MPa of H₂ at 25 °C in environmentally preferable solvents such as water and methanol. Of particular note is the recyclability of the hydrogenation catalyst in methanol; repeated 4 h reactions afforded HCA in >90% selectivity with a reaction conversion of 100%. On another front, when the corresponding Au catalyst (Au/BP) was used in the hydrogenation in water, a favorable selective reduction of CO to cinnamyl alcohol (CAL) (selectivity ≈ 80%) was obtained with 78% reaction conversion under 0.8 MPa of H₂ at 80 °C for 12 h. The potential use of BP-supported metal nanoparticles as green catalysts is discussed. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4gc00383g

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PAPER
Boronate microparticle-supported nano-palladium
and nano-gold catalysts for chemoselective
hydrogenation of cinnamaldehyde in
Cite this: Green Chem., 2014, 16,
230
3
environmentally preferable solvents†
a
a
a
a,b
Seika Fujiwara, Naoto Takanashi, Ryuhei Nishiyabu and Yuji Kubo*
Dispersible self-assembled boronate polymers can serve as support materials for metal nanoparticle cata-
lysts. As a proof-of-concept, catalytic systems for the chemoselective hydrogenation of cinnamaldehyde
2
−
and AuCl −
(
CA) were prepared by the NaBH
ethyleneimine (PEI)-coated boronate particles (BPs) composed of polymeric 3-benzo-2,4,8,10-tetraoxa-
,9-diboraspiro[5.5]undecane. The Pd-deposited BP (Pd/BP) showed highly selective catalytic activity for
the hydrogenation of CA to hydrocinnamaldehyde (HCA) under 0.1 MPa of H at 25 °C in environmentally
4
reduction of PdCl
4
4
in methanol, in the presence of poly-
3
2
preferable solvents such as water and methanol. Of particular note is the recyclability of the hydro-
genation catalyst in methanol; repeated 4 h reactions afforded HCA in >90% selectivity with a reaction
conversion of 100%. On another front, when the corresponding Au catalyst (Au/BP) was used in the
hydrogenation in water, a favorable selective reduction of CvO to cinnamyl alcohol (CAL) (selectivity ≈
Received 4th March 2014,
Accepted 12th April 2014
DOI: 10.1039/c4gc00383g
8
2
0%) was obtained with 78% reaction conversion under 0.8 MPa of H at 80 °C for 12 h. The potential use
www.rsc.org/greenchem
of BP-supported metal nanoparticles as green catalysts is discussed.
motivated us to apply BP-supported metal nanoparticles
as catalysts for sustainable chemical synthesis. Because of
Introduction
1
2
There has been growing interest in various fields of materials environmental concerns, reactions in non-flammable and
science in developing highly ordered self-assemblies of mole- environmentally friendly solvents such as water at room temp-
cular modules into larger supramolecular entities with well- erature are desirable. In addition, simple alcohols such as
1
13
defined morphologies. Such an approach is advantageous in methanol and ethanol are environmentally preferable solvents
providing support materials for heterogeneous catalysts; to that are water-soluble, biodegradable, and occur naturally in
2
3
date, polymeric nanotubes, microporous polymers, globular- the environment.
aggregates, and polymeric nanofibers, as well as metal–
organic frameworks and covalent organic frameworks, have tive hydrogenation reactions of α,β-unsaturated aldehydes,
been reported. The use of phenylboronic acid complexation which have been widely investigated in catalyst chemistry.
with diols and their congeners is an alternative way to Among them, cinnamaldehyde (CA), being composed of vinyl
construct well-defined supramolecular aggregates. Indeed, (CvC) and carbonyl (CvO) groups, is an important substrate
4
5
With this in mind, our attention has been focused on selec-
6
7
1
4
8
9
sequential boronate esterification of benzene-1,4-diboronic for hydrogenation because the reduction of the vinyl segment
acid with pentaerythritol produced mono-dispersed flower-like gives hydrocinnamaldehyde (HCA), which is a key intermediate
1
5
support materials (BP) where gold nanoparticles (Au NPs) in the preparation of pharmaceuticals. Cinnamyl alcohol
could be developed to provide heterogeneous catalysts for the (CAL) is produced through hydrogenation of the carbonyl
1
0
16
reduction of nitroaromatic compounds. During this work, we groups and is widely used in perfumery and as a deodorant.
1
1
found that BP forms a stable dispersion in water, which Further hydrogenation leads to the production of 3-phenyl-
propanol (PPL). As such, the selective synthesis of both HCA and
CAL is highly desired in fine chemical synthesis. To date, a
a
Department of Applied Chemistry, Graduate School of Urban Environmental
Sciences, Tokyo Metropolitan University, 1-1 Minami-ohsawa, Hachioji, Tokyo
number of transition metal-catalyzed hydrogenation methods
1
7,18
have been applied to this end in chemoselective reactions.
1
92-0397, Japan. E-mail: yujik@tmu.ac.jp; Fax: +81 42-677-3134; Tel: +81 42-677-3134
Pd NP-based catalysts are one of the most promising systems
for selective hydrogenation of the CvC bond; however, there
are few reports on Pd catalysts that are applicable to green
b
Research Center for Gold Chemistry, Tokyo Metropolitan University, Japan
†
Electronic supplementary information (ESI) available: Experimental data
(Fig. S1–S10). See DOI: 10.1039/c4gc00383g
3230 | Green Chem., 2014, 16, 3230–3236
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solvent processes in which comparatively larger amounts of membrane filter (pore size, 0.2 μm (GRADE: K020A047A),
1
9
20
catalysts
or higher pressures of H2
are required. In Advantec Ltd) by filtration were employed. The sample was
addition, regarding the selective hydrogenation to CAL in coated with Au on an EIKO IB3 ion coater. TEM measurement
2
1
water, the use of polymer-supported Au NPs as catalysts was conducted as follows: a suspension of Pd/BP or Au/BP was
remains challenging. put on a micro grid (Cu200mesh, JEOL Ltd) and the excess
Herein, we describe well-defined boronate self-assemblies solution was immediately absorbed using a filtration paper.
(
BPs) with Pd or Au NPs on their surface and characterize the The resulting grid was dried in a vacuum and measured
1
3
BPs using spectroscopic and electron microscopic data. It is without staining. Solid-state C cross-polarization-magic angle
interesting to note that Pd NPs with an average diameter of spinning (CP-MAS) NMR was measured by a JEOL ECA 400SS
1
.52 ± 0.44 nm were evenly deposited on the surface, so that spectrometer. The observation frequency was 100.53 MHz for
1
3
the Pd-deposited BP (Pd/BP) exhibited an excellent catalytic
C. The spectrometer is equipped with a 3.2 mm MAS probe-
activity for the hydrogenation of CA to HCA with a high selec- head capable of producing an MAS speed of 18 kHz. Spectra
1
13
tivity under 0.1 MPa of H
2
at 25 °C in environmentally prefer- were obtained using a H– C CP with a contact time of 2 ms,
able solvents such as water and MeOH. Its reusability for the an acquisition time of 25.5 ms, a recycle delay of 5 s between
1
3
hydrogenation in MeOH was also investigated. On the other scans, and an MAS speed of 18 kHz. The C chemical shifts
hand, BP-supported Au NPs (Au/BP) showed a fair activity for were calibrated using adamantine (δ = 29.5 ppm) as an external
selective hydrogenation to CAL under 0.8 MPa of H at 80 °C standard relative to tetramethylsilane (TMS; δ = 0 ppm). The
2
1
1
in water. We highlight that boronate self-assemblies are advan-
tageous as support materials for preparation of green catalysts. measured using boric acid (H
δ = 19.49 ppm) as an external standard relative to boron tri-
fluoride etherate (BF ·O(C ; δ = 0 ppm). Powder X-ray diffr-
B dipolar decoupling (DD)-MAS NMR spectrum was also
3
BO ) saturated aqueous solution
3
(
3
2 5 2
H )
Experimental section
Materials
action (PXRD) data were collected by a Rigaku RINT-TTR III
X-ray diffractometer with Cu Kα radiation. Thermal gravimetric
analysis (TGA) was conducted using a SHIMADZU DTG-60H
Sodium borohydride (NaBH ) and methanol were used as
−1
4
with a scan rate of 5 °C min and a scan range from 24.1 to
received. Tetrahydrofuran, anisole and cinnamaldehyde (CA)
were distilled before use. Palladium(II) chloride (PdCl ), hydro-
gen tetrachloroaurate(III) tetrahydrate (HAuCl ·4H O), and
linear polyethyleneimine (MW: 25000) were purchased from
Wako, Kanto Chemical, and Polymersciences, respectively.
Boronate microparticles (BPs) composed of polymeric 3-benzo-
−1
8
65.7 °C under a N flow (50 mL min ) where an alumina pan
2
2
was used. Zeta potential measurements for BP and polyethylene-
−1
4
2
imine (PEI)-coated BP in methanol (1 g L ) were performed
using an ELSZ-2 (OTSUKA ELECTRONICS) instrument. Atomic
absorption spectroscopy for determination of the amount of
metal (Pd or Au) deposited on the corresponding hybrids was
performed using a SHIMADZU AA-6200. The X-ray photo-
electron spectrum (XPS) was acquired using a JPS-9010MX
instrument where Mg Kα radiation (hν = 1253.6 eV) was
employed. For preparation of sample solutions, BP-supported
metal nanoparticles were added to aqua regia and the mixture
was allowed to stand for 12 h to complete the digestion of pal-
ladium and gold. The specific surface area was calculated by
the BET (Brunauer–Emmett–Teller) method using Micrometer
Trister 3000. Identification and quantitative analysis of the
products for catalytic tests were performed using a SHIMADZU
GC-2010 equipped with an Agilent Technologies 7890A
2
,4,8,10-tetraoxa-3,9-diboraspiro[5.5]-undecane were prepared
from benzene-1,4-diboronic acid and pentaerythritol according
to previously reported procedures (Fig. 1), and their averaged
diameters were 1.6 ± 0.3 μm (Fig. S1 in the ESI†). PEI-coated
1
0
SiO , being used as a support material of a control catalyst,
2
was purchased from Aldrich (246743-10ML).
Characterization
Field-emission scanning electron microscopy (FE-SEM) was
performed using a JEOL JSM-7500F (acceleration voltage of
5
kV). Transmission electron microscopy (TEM) was performed
using a JEOL JEM-3200FS (acceleration voltage, 300 kV). For
FE-SEM measurements, BPs collected on a polycarbonate type
(
GC) with an HP-5 capillary column (0.32 mm i.d., 30 m),
respectively.
Preparation of boronate-supported metal nanoparticles
−
3
To a dispersion solution of BPs (1.4 g, 6.09 × 10
mol
−
1 22
unit
)
in MeOH was added polyethyleneimine (PEI)
−2
−1 22
(
525 mg, 1.22 × 10 mol unit
)
at room temperature. The
mixture was sonicated for 5 min using an ultrasonic bath and
was then allowed to stand for 25 min at room temperature. Fil-
tration and drying under vacuum afforded PEI-coated BP as a
white solid (1.33 g). The procedure for the preparation of
Pd/BP is as follows: PEI-coated BP (600 mg) was dispersed in
0
2
.1 N HCl (295 mL), and 0.1 N HCl solution of PdCl (4.68 mL,
−
2
Fig. 1 FE-SEM image of boronate particles (BPs).
2.41 × 10 M) was then added to the solution. The resulting
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mixture was sonicated for 5 min using an ultrasonic bath. 0.3 μm (Fig. S1 in the ESI†). Assignment of the building units
1
3
After being allowed to stand for 25 min, the produced solids of the microparticles was conducted using C CPMAS NMR
1
1
were collected by filtration and were then dispersed in MeOH and B DD-MAS spectra (Fig. S2 in the ESI†). In addition,
300 mL) using an ultrasonic bath. Subsequent addition of powder XRD measurements indicated zigzag stacking of the
(
NaBH (203 mg) into the solution, sonication for 5 min, aging boronate ester polymers (Fig. S3 in the ESI†). The stability of
4
for 25 min, and washing with MeOH afforded Pd/BP (497 mg) the microparticles was assessed by thermogravimetric analysis
as a gray solid, the amount of loaded Pd being 0.66 wt%. As a (TGA). No mass loss was observed up to 483 °C (Fig. S4 in the
comparison, SiO -supported Pd nanoparticles Pd/SiO were ESI†), indicating the suitability of BP as a polymeric support
2
2
prepared in a similar way by stirring PdCl
2
(2.34 mL, 2.41 × for depositing metal nanoparticles. When depositing Pd on
−
2
23
1
0
M) in 0.1 N HCl in the presence of PEI-coated SiO
2
BP,
polyethyleneimine (PEI) was employed as a binder
(
Aldrich, 300 mg), followed by adding NaBH4 (106 mg) to because we reasoned that direct immobilization of the Pd
2
4
produce Pd/SiO
of loaded Pd being 1.40 wt%.
2
as a dark brown solid (264.1 mg), the amount would efficiently occur via ligand exchange. The PEI-coated
BP was characterized by zeta potential measurements, where
For preparing Au/BP, the procedure is as follows: PEI-coated the negative value of BP (−24.16 mV) was shifted to a value of
BP (640 mg) was dispersed in MeOH (317 mL), and +51.86 mV after being treated with PEI. Subsequently, the
2
−
HAuCl
4
·4H
2
O (26.8 mg) dissolved in 3.5 mL MeOH was added reduction of PdCl
4
4
with NaBH in a turbid solution of the as-
to the solution. The resulting mixture was sonicated for 5 min prepared PEI-coated BP afforded Pd-deposited BP (Pd/BP) as a
using an ultrasonic bath. After being allowed to stand for gray solid. Pd/BP was characterized by FE-SEM and STEM
2
5 min at room temperature, the resulting solid was collected measurements, as well as atomic absorption spectroscopy. The
by filtration and was then dispersed in MeOH (320 mL) using shape and morphology of the hybrid seems to be maintained
an ultrasonic bath. Subsequent addition of NaBH (268 mg) during Pd deposition (Fig. 2a). The STEM image shows that
4
into the solution, sonication for 5 min, aging for 25 min, and most of the Pd nanoparticles (Pd NPs) are pseudo-spherical in
washing with MeOH afforded Au/BP as a violet solid (538 mg), shape, and the interface between the Pd NPs and the boronate
the amount of loaded Au being 2.1 wt%.
support material can be clearly observed (Fig. 2b). The histo-
gram of the size distribution, being collected from 300 par-
ticles in the STEM images, suggested that highly dispersed Pd
NPs (0.66 wt%) with a mean diameter of 1.52 nm and a stan-
General procedure for the hydrogenation of CA over Pd/BP or
Pd/SiO
2
Dispersed solution (3 mL) involving CA (39.6 mg, 0.3 mmol) dard deviation of 0.44 nm were loaded on the surface (Fig. 2c).
and Pd/BP (24.4 mg, Pd: 0.5 mol%) was placed in a stainless Note that cluster-sized palladium was dispersed on the boro-
steel autoclave. After adding a magnetic stirring bar into the nate surface. The HR-TEM image allowed us to confirm the Pd
solution, hydrogen was then introduced into the autoclave. deposition on the surface; the space fringes of the Pd were
The mixture was stirred under 0.1 MPa of H at 25 °C. After
2
the reduction, the reaction mixture was extracted with an
organic solvent, filtered, and analyzed by GC using anisole as
an internal standard. A control experiment using Pd/SiO
2
(11.9 mg, Pd: 0.5 mol%) was performed under similar
conditions.
General procedure for the hydrogenation of CA over Au/BP
Au/BP (32.8 mg; Au 1.1 mol%), CA (39.6 mg, 0.3 mmol), H
2
O
(
3 mL), and a magnetic stirring bar were placed in a stainless
steel autoclave. Hydrogen was then introduced into the auto-
clave, and the mixture was stirred under 0.8 MPa of H at
0 °C. After the reaction, hydrogen was removed and the reac-
2
8
tion mixture was extracted with tetrahydrofuran, filtered, and
analyzed by GC using anisole as an internal standard.
Results and discussion
Preparation of boronate-supported metal nanoparticles
The boronate support material BP was easily prepared accord-
1
0
ing to a previously reported procedure. Briefly, a solution of
benzene-1,4-diboronic acid and pentaerythritol was aged in
THF at room temperature for 48 h. Monodispersed flower-like
Fig. 2 (a) FE-SEM and (b) STEM images of Pd/BP. (c) Size distribution of
Pd NPs on the BP support material in the STEM images. (d) HR-TEM of
microparticles were obtained with an average diameter of 1.6 ± Pd/BP.
3232 | Green Chem., 2014, 16, 3230–3236
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Fig. 3
2
N adsorption–desorption isotherms of BP and Pd/BP.
determined from the image to be d = 0.261 nm (Fig. 2d),
approximately consistent with the {110} facet of Pd. The EDX
spectrum, furthermore, showed signals at 2.83 and 21.18 keV,
which were assigned to PdLα and PdKα, respectively (Fig. S5 in
the ESI†). The confirmation of the presence of metallic palla-
dium as a major component came from the XPS measurement
which showed signals at 335.16 and 340.46 eV corresponding
to Pd(0) 3d5/2 and Pd(0) 3d3/2, respectively (Fig. S6 in the ESI†).
The textural properties of BP and Pd/BP were estimated by N
2
adsorption–desorption measurements. According to the IUPAC
classification, typical type IV isotherms with H3 hysteresis
loops were observed for both BP and Pd/BP (Fig. 3), which
suggests the formation of multi-layers of component polymers
Fig. 4 Hydrogenation of CA (0.3 mmol) in H
2
O (a) and MeOH (b) at
25 °C and 0.1 MPa in the presence of Pd/BP (Pd; 0.5 mol%).
with a slit-type pore structure. The Brunauer–Emmett–Teller
2
(
g
BET) surface area (SBET) of Pd/BP was calculated to be 107 m
−
1
2
−1
, which is lower than that (184 m g ) of the support
6% with no detectable amount of CAL (product of the CvO
material (BP).
reduction). Taking into account the low solubility of H
2
in
2
5
water, Pd/BP showed a significant catalytic activity for hydro-
genation to HCA. When MeOH was employed as the reaction
Hydrogenation of CA over boronate-supported metal
nanoparticles
2
6
solvent (Fig. 4b), a more efficient catalytic activity could be
obtained; the reaction was complete within 4 h to afford HCA
The prepared boronate-supported metal nanoparticles were
tested in the hydrogenation reaction of CA (Scheme 1), which
was carried out in a stainless steel autoclave. Fig. 4a shows the
time-dependence of the hydrogenation over the BP-supported
Pd catalyst (Pd/BP) in water, as monitored by gas chromato-
graphy (GC). The reaction proceeded significantly under
−1
in 92% yield with a TOF value of 50 h . It is noteworthy that
further reduction to PPL from HCA was not observed under
these conditions, meaning that Pd/BP is a selective catalyst for
HCA via hydrogenation of CA using a green process.
Our interest is to understand what role the BP support plays
2
ambient conditions (0.1 MPa of H , 25 °C). For 6 h, the hydro-
in the catalytic activity. We thus prepared Pd/SiO
similar manner, where a MeOH solution of PdCl4 in the
presence of PEI-coated SiO was treated with NaBH . The
resulting Pd/SiO hybrids were characterized by HAADF-STEM,
2
catalysts in a
genation over the hybrid catalyst afforded HCA in 94% yield
with a reaction conversion of 100%. The by-product, PPL (from
the reduction of CvC and CvO bonds) was formed in only
2−
2
4
2
EDX, and atomic adsorption microscopy (see Fig. S7 in the
ESI†); EDX Pd-mapping indicated the presence of well-dis-
persed Pd NPs (1.40 wt%) on the surface, with the average dia-
meter being 1.55 ± 0.31 nm (Fig. S6b in the ESI†). To our
delight, the size of the Pd NPs is consistent with the size
of the Pd NPs in Pd/BP, allowing us to conduct a control
hydrogenation using Pd/SiO as a catalyst. The hydrogenation
2
2
in water for 4 h under similar conditions (0.1 MPa H , 25 °C)
showed a low reactivity (TOF = 12 h− ) with a reaction conversion
1
Scheme 1 Hydrogenation of cinnamaldehyde (CA).
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Fig. 5 (a) TEM image of Au/BP. (b) HR-TEM image of a lattice structure
observed for the {111} facet of the Au NPs.
of 23%, despite a significant HCA selectivity (87%). This
Fig. 6 Amount of CA, HCA, CAL, and PPL as a function of time in the
means that the use of BP as a support material is favorable for hydrogenation of CA over Au/BP (Au; 1.1 mol%) in H O at 0.8 MPa H2
2
and 80 °C.
obtaining a Pd catalyst with enhanced catalytic activity. The
2
2
surface area of the Pd/SiO catalyst was estimated to be 571 m
−
1
g
(Fig. S8 in the ESI†), which is larger than that of Pd/BP.
Therefore, the enhanced reactivity of Pd/BP cannot be inter-
preted on the basis of its surface area. It is known that catalytic
activity is strongly dependent on the atomic structure of
2
7
nano-palladium. Further research in this regard is currently
underway.
In the next stage, we examined the selective hydrogenation
of CA to CAL over Au/BP, where Au NPs (Fig. 5) were deposited
on the surface of BP with an average diameter of 2.7 ± 0.7 nm
(Fig. S9 in the ESI†). In terms of its catalytic activity, the hydro-
genation of CA (0.3 mmol) under 0.1 MPa H at 25 °C over
2
Au/BP (Au: 1.1 mol%) in water (3 mL) hardly proceeded; the reac-
tion conversion was 5.1% after 36 h. This can be interpreted
on the basis of a poor activity for dihydrogen dissociation com-
2
8
pared to the Pd catalysts. After some optimization of the
Fig. 7 Catalytic reusability of Pd/BP for the hydrogenation in MeOH.
reaction conditions, 0.8 MPa of H at 80 °C in water allowed
2
the hydrogenation to proceed significantly, with the conver-
sion ratio being 78% over 12 h. Although the catalytic activity
for the hydrogenation was lower than that of Pd/BP, 80%
chemoselectivity for CAL was obtained, which is higher than
heterogeneous catalyst. With this in mind, we investigated the
stability and reusability of Pd/BP in the hydrogenation using
31
MeOH. As a first run, a MeOH solution of CA over Pd/BP was
1
8d
that of ionic polymer-supported Au NP catalysts
and almost
2
stirred under 0.1 MPa of H at 25 °C for 4 h. After evaluating
1
8a
similar to Au/Al O at 4 MPa.
On the other hand, when
2
3
the reaction results using GC, the catalyst was collected,
washed with MeOH, and then used again in the next reaction.
As shown in Fig. 7, the catalytic activity was maintained with
high selectivity to HCA (>90%) over all the reaction runs,
which indicates an excellent recyclability. In order to examine
the shape and morphology of the Pd/BP catalyst after five reac-
tion runs, FE-SEM and TEM measurements were carried out,
and it seemed that the catalyst was not subject to serious
damage during the reactions (Fig. S10 in the ESI†). The
average diameter of the Pd NPs was estimated to be 1.94 ±
MeOH was used as a solvent Au/BP gave a reaction conversion
of 37% with 91% chemoselectivity for CAL over 12 h under
similar conditions. Accordingly, favorable reactivity over Au/BP
in water (Fig. 6) may be attributed to a hydrophobic active site
of Au/BP. It is known that the existence of Au NPs on the
surface of support materials is an important factor to produce
2
9
Au–H as a reducing agent, leading to favorable participation
in the CvO hydrogenation. Thus, the deposition of gold clus-
ters, the diameter being less than or equal to 1.7 nm, is highly
desired to improve the catalytic activity of Au-deposited BP
0.62 nm, somewhat larger than the Pd NPs of the as-prepared
3
0
towards hydrogenation.
Pd/BP (Fig. 8). This result indicates that some aggregation of
the embedded Pd occurred during the recycled reactions.
However, irrespective of the cycle, the catalyst was invariably
Reusability
As described above, Pd/BP shows good catalytic activity for the active. The amount of loaded Pd on the surface is 0.65 wt%
hydrogenation under mild conditions in MeOH. Accordingly, after five successive runs, which indicates no leaching during
it is valuable to gain insight into its properties as a practical the recycle reactions. In this way, Pd/BP could be reused at
3234 | Green Chem., 2014, 16, 3230–3236
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3
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Fig. 8 (a) TEM images of as-prepared Pd/BP and (b) Pd/BP after suc-
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least four times without appreciable loss of its catalytic activity
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1
(
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(
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Our effort to investigate BP as a support material led to the
fabrication of nano-palladium and nano-gold catalysts for the
selective hydrogenation of CA, a common target reaction in
fine chemical synthesis. It is worth emphasizing that the Pd-
deposited BP (Pd/BP) shows efficient and reusable catalytic
activity with a high selectivity for HCA under ambient
conditions in environmentally preferable solvents. Therefore,
the practical use of Pd/BP as a heterogeneous catalyst in
the sustainable production of fine chemicals merits further
investigation.
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Acknowledgements
This research was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Science, Sports and
Culture of Japan (no. 24350075). We thank Dr A. Taketoshi
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the hydrogenation under 0.8 MPa of H at 80 °C in water
for 12 h. After one cycle, the hydrogenation of CA over
Au/BP under the same conditions afforded CAL in 52%
selectivity with a 29% reaction conversion. The low stability
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8, 8128.
3236 | Green Chem., 2014, 16, 3230–3236
This journal is © The Royal Society of Chemistry 2014