Efficient chemoselective reduction of nitro compounds and olefins using Pd-Pt bimetallic nanoparticles on functionalized multi-wall-carbon nanotubes

Article abstract of DOI:10.1016/j.catcom.2013.09.016

We report the synthesis of novel Pd-Pt bimetallic nanoparticle catalysts using functionalized multi-wall carbon-nanotubes and utilization of them to reductions. The carbon nanotube-supported bimetallic nanoparticle catalysts showed improved activity in reduction reactions, compared with that of mono metal-supported catalysts. Under the optimized reaction conditions, various nitro compounds and alkenes were cleanly reduced at ambient temperature. Furthermore, this catalytic system exhibits excellent activity and high chemoselectivity for nitro compounds in the presence of other functional groups labile to hydrogenation. After the reaction, the catalysts could be collected through filtration, and reused for 10 times without any loss of catalytic activity.

Full text of DOI:10.1016/j.catcom.2013.09.016

Catalysis Communications 45 (2014) 25–29
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Efficient chemoselective reduction of nitro compounds and olefins using
Pd–Pt bimetallic nanoparticles on functionalized
multi-wall-carbon nanotubes
Eunsuk Kim, Han Saem Jeong, B. Moon Kim ⁎
Department of Chemistry, Seoul National University, Seoul 151-747, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 July 2013
Received in revised form 12 September 2013
Accepted 17 September 2013
Available online 13 October 2013
We report the synthesis of novel Pd–Pt bimetallic nanoparticle catalysts using functionalized multi-wall
carbon-nanotubes and utilization of them to reductions. The carbon nanotube-supported bimetallic
nanoparticle catalysts showed improved activity in reduction reactions, compared with that of mono
metal-supported catalysts. Under the optimized reaction conditions, various nitro compounds and
alkenes were cleanly reduced at ambient temperature. Furthermore, this catalytic system exhibits
excellent activity and high chemoselectivity for nitro compounds in the presence of other functional
groups labile to hydrogenation. After the reaction, the catalysts could be collected through filtration,
and reused for 10 times without any loss of catalytic activity.
Keywords:
Multiwall carbon nanotube
Heterogeneous catalyst
Bimetallic catalyst
Nitro reduction
© 2013 Elsevier B.V. All rights reserved.
Olefin reduction
Recycling
1
. Introduction
optoelectronic properties, as well as physical and chemical stabilities
[
12]. The activities of CNTs-supported metal nanoparticles could
Metal nanoparticles are characterized by their high surface area and
be altered through metal–support interactions [13]. Various metal
nanoparticle CNTs have been synthesized through the use of metal
nanoparticles of Ni, Co, Fe, Pt, Pd, Ag, and Au [14–19]. A recent report
has demonstrated that well-dispersed metallic nanoparticles on
CNT supports show high catalytic activities and recyclability for
hydrogenation [20]. Therefore, carbon nanotubes are suitable supports
for metal nanoparticle systems [21–25] involved in redox reactions.
Reduction of the nitro group is one of the most important organic
reactions for the preparation of amines, which are used for the
synthesis of many agrochemicals, pharmaceuticals, dyes, rubbers,
polymer, and pigments [26,27].
therefore have attracted much attention. Since their properties are
different from their bulk materials, they often show unexpected
catalytic activity in organic reactions [1]. Over the last few years, many
studies have been reported on bimetallic nanoparticle catalysts [4].
Compared with monometallic nanoparticles, bimetallic nanoparticles
show interesting electronic, optical, chemical or biological properties
due to their improved properties [5,6]. Bimetallic nanoparticles are
commonly synthesized using polymers and surfactant stabilizers
such as polyvinylpyrrolidone (PVP) and quaternary ammonium salts
[
7–10]. However, since such polymers and stabilizers can diminish the
catalytic activity, there is a growing need for the development of new
methods for the synthesis of well-dispersed bimetallic nanoparticles
on solid supports.
The discovery of carbon nanotubes (CNTs) in the early 1990s has
contributed to the development of heterogeneous solid supports
over the past decade [11] because of their special electronic and
Reduction of nitro compounds using stoichiometric reagents often
produces unwanted side products such as hydroxylamine [28], therefore
researchers have searched for efficient and selective methods including
metal-catalyzed reactions. Since the introduction of Béchamp process
[29], Corma and Serna reported that gold nanoparticles supported on
2 2
TiO catalyzed reduction of functionalized nitroarenes with H in a
chemoselective manner [30]. Many metal catalysts such as Cu, Au, Fe,
Pd, Pt, and Rh immobilized on solid supports have been utilized for
reduction of nitro compounds in combination with various hydrogen
2
sources, including H , silanes, and hydrazine derivatives [31–34]. From
our continuing efforts on the study of the bimetallic nanoparticles,
we investigated chemoselective reduction of nitro compounds using
bimetallic nanoparticles on multi-wall carbon nanotubes [35].
⁎
Corresponding author at: Department of Chemistry, College of Natural Sciences, Seoul
National University, Seoul 151-747, Republic of Korea. Fax: +82 2 875 7505.
E-mail address: kimbm@snu.ac.kr (B.M. Kim).
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.09.016

26
E. Kim et al. / Catalysis Communications 45 (2014) 25–29
O
O
O
N
N
OH
OH
N3
OH
1
,2-dichlorobenzene, 160 o
C
CNT-COOH
O
H
N
N
N
H
OH
1
) SOCl2
Metal
Metal salt, Hydrogen source
Water
O
nanoparticle
H
N
N
CNT
N
H
H
N
OH
2
)
H N
OH
2
CNT-ED-OH
Scheme 1. Synthesis of Pd–Pt CNT.
2
. Experimental
catalyst in the reduction of the nitro group suggests a synergistic
effect of the two metals. Other solvents such as THF, DME,
dichloromethane, and water (entries 4–7) were found to retard
the Pd–Pt-CNT catalyzed reduction of the nitrobenzene (53%, 51%,
83%, and 15% yields, respectively) when compared to ethanol (99%
yield), though there were no apparent differences in dispersibility of
the catalyst.
With the optimized reaction conditions in hand, we carried out
recycling reactions using the Pd–Pt-CNT catalyst (Table 2). After the
reaction was complete, the catalyst was collected by filtration; washed
with ethanol, water, and DCM; and reused for the next reaction. The
results are summarized in Table 2. The catalyst showed consistently
high activity through the 10th reaction. The 11th reaction was slightly
slower (30 min). To examine if metal nanoparticles from the CNT
catalyst leached during the recycling process, the catalyst systems
were analyzed for metals through ICP analysis. The Pd and Pt contents
in the catalyst after 11 reaction cycles were 3.13 and 4.88 wt.%,
respectively, indicating slight decreases from the original 3.44 and
5.46 wt.%. Fig. 3 shows the TEM image of the catalyst after the 11th
2
.1. Catalyst preparation
Multi-wall carbon nanotubes (MWNTs) used in this research
were prepared according to the chemical vapor deposition method
(
(
see Supporting Information). Functionalization of the CNT-ED-OH
Scheme 1) was confirmed by Fourier-transform infrared (FT-IR),
elemental analysis (EA), and thermal gravimetric analysis (TGA).
Detailed experimental procedure and data are included in the
Supporting Information (Fig. S1).
2
.2. Characterization
The Energy Dispersive Spectrometer (EDS) data indicate that
metal and C are the major elements in the composite. Inductively
coupled plasma (ICP) mass data of Pd and Pt concentrations in
the Pd–Pt-CNT catalyst (Fig. 1) were found to be 3.44 wt.% (Pd)
and 5.46 wt.% (Pt). From the EDS results (Fig. 2), we found that
the atom ratio of Pd and Pt in Pd–Pt CNT is 1.08:1.00. Fig. 1
shows the TEM images of the Pd–Pt-CNT. We confirmed that the
nanoparticles of a uniform size (9–10 nm) are evenly distributed.
More TEM images and EDS data are given in the Supporting
Information (Fig. S2).
2
.3. Catalyst activity tests
Pd–Pt-CNT catalyst in degassed EtOH (4mL) was placed in a 25-mL
two-necked round-bottom flask. Nitrobenzene (52 μL, 0.5 mmol) was
added to the mixture under H (1 atm) and the mixture was stirred at
2
ambient temperature. After completion of the reaction, the mixture
was filtered. Pd–Pt CNT residue was washed with dichloromethane
and ethanol, and then dried under vacuum for 2 h to be used in the
next reaction. In addition to nitrobenzene, an array of nitro-substrates
was also subjected to the Pd–Pt-CNT reduction conditions.
3
. Results and discussion
First, the catalytic activity of the Pd–Pt-CNT was tested for the
reduction of nitrobenzene to aniline under various conditions as
seen in Table 1. We evaluated the catalytic activity of metals (Pd,
Pt) separately using 1 mol% (based on Pd) metal-supported CNT.
When the reaction was conducted using Pd-CNT, full conversion
was observed after 1 h; with Pt-CNT catalyst in 2 h (Table 1, entries
1
and 2, respectively). However, when bimetallic Pd–Pt-CNT was
used as a catalyst, the reaction was complete in 20 min (entry 3).
The relatively short reaction time achieved by the bimetallic
Fig. 1. TEM images of Pd–Pt CNT.

E. Kim et al. / Catalysis Communications 45 (2014) 25–29
27
cps / eV
3
2
2
1
1
0
0
.0
.5
.0
.5
.0
.5
.0
Pt
C
O
Pd
Pt
Pd
Pt
0
2
4
6
8
10
keV
Fig. 2. EDS spectrum of Pd–Pt CNT.
run, where the nanoparticles appear to be aggregated to a considerable
degree.
benzyl, ester, ketone and halogen groups (entries 4–5, and 10–12)
were not reduced at all. Reduction of 2,6-dinitrotoluene was
complete in 1 h (entry 6). In addition, the substrates containing
electron-withdrawing amide (entry 8), acetyl (entry 10), and
halogen groups (entries 11–12) also showed good reactivity.
Though the reduction of an aliphatic nitro compound took longer
than that for nitroarene compounds, excellent yield of the desired
amine was obtained (entry 13).
To examine the substrate scope of the Pd–Pt-CNT-catalyzed
reduction, we tested reactions of various substrates under the
optimized conditions shown in Table 1. Table 3 shows that the
Pd–Pt-CNT catalyst showed excellent activity on a variety of
substrates. In particular, other reducible functional groups such as
Olefin reductions were also carried out with the optimized conditions
obtained in the nitro reduction. The Pd–Pt-CNT catalyst showed
exceedingly good catalytic efficiency as shown in Table 4. The reduction
of styrene proceeded extremely well and was complete in 30 min
Table 1
a
Optimization of nitrobenzene reduction conditions.
(entry 1). The reduction of di- or tri-substituted cyclic olefins produced
excellent yields (entries 2–3). Sterically hindered and 1,1-disubstituted
olefins were also reduced smoothly (entries 2, 5 and 4, respectively).
Entry Catalyst
Solvent Time (min) GC conversion (%)b TON TOF (h−1
)
1
2
3
4
5
6
7
8
9
Pd-CNT
Pt-CNT
EtOH
EtOH
60
120
20
30
30
30
30
60
60
99
99
99
53
51
83
15
12
22
100 100
100 50
100 300
53 106
51 102
83 166
Pd–Pt-CNT EtOH
Pd–Pt-CNT THF
Pd–Pt-CNT DME
Pd–Pt-CNT DCM
Pd–Pt-CNT Water
15
12
22
30
12
22
Pt/C
Pd/C
EtOH
EtOH
a
Reaction conditions: 0.5 mmol of nitro compound, hydrogen gas (1 atm), 4.0 mL of
ethanol with 1 mol% of metal-CNT catalyst at ambient temperature.
b
All the conversions were determined by GC analysis.
Table 2
a
Recycling test of Pd–Pt CNT catalyst in nitrobenzene reductions.
Run
Time (min)
GC yield (%)b
1
3
5
7
9
20
20
20
20
20
20
30
99
99
99
99
99
99
99
10
1
1
a
Reaction conditions: 0.5 mmol of nitro compound, H
2
1 atm, 4.0 mL of ethanol with
1
mol% of metal-CNT catalyst at ambient temperature.
Anisole was used as an internal standard.
b
Fig. 3. TEM images of Pd–Pt CNT after 11 times of use.

28
E. Kim et al. / Catalysis Communications 45 (2014) 25–29
Table 3
Substrate scope of Pd–Pt CNT catalyzed nitro group reductions.
Table 4
a
a
Substrate scope of Pd–Pt CNT catalyzed olefin group reductions.
Entry
1
Substrate
Product
Time (h)
0.5
Yield (%)b
99c
Yield (%)b
Entry
1
Substrate
Product
Time (h)
0.5
95
99c
2
0.5
2
1
99
3
4
4
99
98
3
4
0.5
0.5
98
97
1.5
5
2
94
5
6
1
1
99
99
6
7
2
99
96
20
8
1
99c
7
8
1
93
94
a
2
Reaction conditions: 0.5 mmol of olefin compound, H 1 atm, 4.0 mL of ethanol with
1
mol% of metal-CNT catalyst at ambient temperature.
0.5
b
Yields of isolated products unless otherwise noted.
c
Determined by GC analysis using anisole as an internal standard.
9
0.5
0.5
87
97
the Pd–Pt-CNT forms an efficient catalyst for the reduction of the nitro
group and alkenes. In addition, the catalyst is air- and moisture-stable.
10
Acknowledgment
1
1
2
1
1
99c
This work was supported by the Nano Material Development
Program through the National Research Foundation of Korea
99c
1
(
NRF) funded by the Ministry of Education, Science and Technology
(
2012M3A7B4049637) and the Mid-career Researcher Program through
1
3
3
98c
National Research Foundation (NRF) grant funded by the Ministry of
Education, Science, and Technology (MEST) (No. 2011–0016187).
a
Reaction conditions: 0.5 mmol of nitro compound, hydrogen gas(1 atm), 4.0 mL of
ethanol with 1 mol% of metal-CNT catalyst at ambient temperature.
b
Yields of isolated products.
Yields were determined by GC analysis using anisole as an internal standard.
c
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2013.09.016.
Allylic alcohol was also reduced cleanly to the corresponding alkanol
(
entry 6). The slow reaction of trans-stilbene (entry 7) may be due to
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