Simple one-pot synthesis of Rh-Fe3O4 heterodimer nanocrystals and their applications to a magnetically recyclable catalyst for efficient and selective reduction of nitroarenes and alkenes

Article abstract of DOI:10.1039/c0cc04816j

A simple synthesis of Rh-Fe₃O₄ heterodimer nanocrystals was achieved by controlled one-pot thermolysis. The nanocrystals exhibited excellent activities for the selective reduction of nitroarenes and alkenes. Furthermore the nanocrystal catalyst could be easily separated by a magnet, and recycled eight times without losing the catalytic activity.

Full text of DOI:10.1039/c0cc04816j

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COMMUNICATION
Simple one-pot synthesis of Rh–Fe3O4 heterodimer nanocrystals
and their applications to a magnetically recyclable catalyst for
efficient and selective reduction of nitroarenes and alkenesw
a
b
Youngjin Jang,z Seyoung Kim,z Samuel Woojoo Jun,a Byung Hyo Kim,a Sunhwan Hwang,c
In Kyu Song,c B. Moon Kim*b and Taeghwan Hyeon*a
Received 5th November 2010, Accepted 7th January 2011
DOI: 10.1039/c0cc04816j
A simple synthesis of Rh–Fe3O4 heterodimer nanocrystals was
achieved by controlled one-pot thermolysis. The nanocrystals
exhibited excellent activities for the selective reduction of
nitroarenes and alkenes. Furthermore the nanocrystal catalyst
could be easily separated by a magnet, and recycled eight times
without losing the catalytic activity.
chemoselective, and effective catalysts for the reduction of
organic compounds are highly desired.
Because it is difficult to separate a homogeneous catalyst
in the mixture solution after reactions, alternative solutions
have been explored. The semi-heterogeneous system using
well-dispersed nanoparticles is one of the most promising
candidates because nanoparticles show a good catalytic activity
along with easy separation capability.5,6 Recently, studies were
focused on the magnetic separation of catalysts using magnetic
nanocomposite materials.7 Even though there have been
several studies on the immobilization of catalytic metal nano-
particles in magnetic materials, there is no report on the
synthesis of Rh–Fe3O4 heterostructured nanoparticles for
catalytic applications. In this study, we report on the simple
and one-pot synthesis of Rh–Fe3O4 heterodimer nanocrystals
by using ‘‘heat-up process,’’8 and demonstrated their applications
to magnetically recyclable catalyst for efficient and selective
reduction of nitroarenes and alkenes.
The reduction of nitro compounds to amines is one of the most
important chemical reactions because organic amines are
essential materials for the production of agrochemicals, dyes,
pharmaceuticals, polymers, and rubbers.1 Numerous catalysts
using a variety of metals including Cu, Au, Fe, Pd, Pt, and Rh
have been investigated for these reduction reactions.2,3 For
example, Corma and Serna reported that gold nanoparticles
supported on TiO2 catalyzed chemoselective hydrogenation of
functionalized nitroarenes with H2.3a The selective reduction
of a nitro group in organic compounds containing other
reducible functional groups is a very challenging issue in
organic synthesis.3 In addition, the reduction of aromatic nitro
compounds is often associated with side products including
hydroxylamines, hydrazines, and azoarenes since the reaction
can stop at an intermediate stage.4 Therefore easily-separable,
We prepared Rh–Fe3O4 heterodimer nanocrystals using a
consecutive heating approach in one-pot synthesis. Rh(acac)3
(27 mg) and Fe(acac)3 (0.70 g) were mixed with a solution
containing 6 mL of oleylamine and 4 mL of oleic acid, and the
resulting mixture was slowly heated to 200 1C, then to 300 1C,
and aged at 300 1C for 30 min (see the detailed procedure
in ESIw). After cooling to room temperature, the mixture
solution was precipitated by adding EtOH, and finally the
nanocrystals were retrieved by centrifugation. The nanocrystals
were dispersible in many organic solvents such as hexane and
chloroform. Transmission electron microscopy (TEM) images
showed that the Rh–Fe3O4 heterodimer nanocrystals are
consisted of Rh nanoparticles with an average diameter of
2–3 nm and Fe3O4 component of B16 nm (Fig. 1a and b).
Inductively coupled plasma atomic emission spectroscopy
(ICP-AES) analysis revealed that the catalyst is composed of
1.09 wt% of Rh and 63.3 wt% of Fe atoms. X-Ray diffraction
(XRD) pattern (Fig. 1c) shows that these heterodimer nano-
crystals are composed of a face-centered cubic (fcc) Rh
(JPCDS 05-0685) and fcc Fe3O4 (JCPDS 19-0629). X-Ray
photoelectron spectroscopy (XPS) analysis (Fig. 1d and Fig.
S1, ESIw) showed that iron oxide component is magnetite
(Fe3O4) and Rh species is Rh0. No peak corresponding to
a National Creative Research Initiative Center for Oxide
Nanocrystalline Materials, World Class University Program of
Chemical Convergence for Energy & Environment, and School of
Chemical and Biological Engineering, Seoul National University,
Seoul 151-744, Korea. E-mail: thyeon@snu.ac.kr;
Fax: +82 2-886-8457; Tel: +82 2-880-7150
b Department of Chemistry, College of Natural Sciences,
Seoul National University, Seoul 151-747, Korea.
E-mail: kimbm@snu.ac.kr; Fax: +82 2-872-7505;
Tel: +82-2-880-6644
c School of Chemical and Biological Engineering, Institute of Chemical
Processes, Seoul National University, Seoul 151-744, Korea
w Electronic supplementary information (ESI) available: Detailed
experimental procedures, XPS and TPR spectrum, SQUID diagram
of Rh–Fe3O4 heterodimer nanocrystals, TEM image of Rh–Fe3O4
heterodimer nanocrystals after 8 cycles of the catalytic reaction and
photographs showing the magnetic separation of the catalyst, and
catalytic result on the reduction of nitrobenzene and trans-stilbene
using various catalysts. See DOI: 10.1039/c0cc04816j
z These authors contributed equally to this work.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 3601–3603 3601

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Table 1 Summary on the reduction of various substituted nitroben-
zenes
Hydrazine
equiv.
Reaction
time/h
Yielda
(%)
Entry Substrate
Product
1
2
2
4
1
2
99
99
3
4
5
4
8
6
1
10
5
99b
99
94
Fig. 1 (a) TEM image, (b) HRTEM image, (c) XRD pattern and
(d) XPS spectra of Rh–Fe3O4 heterodimer nanocrystals, and Fe3O4
and g-Fe2O3 powder references.
6
7
6
6
5
2
99
97
the reduction of RhOx was observed in the temperature-
programmed reduction (TPR) spectrum (Fig. S2, ESIw),
confirming that the oxidation state of Rh species is zero.
The magnetic properties of Rh–Fe3O4 heterodimer nano-
crystals were examined by superconducting quantum interference
device (SQUID) measurement at 5 K and 300 K (Fig. S3,
ESIw). It indicates that the heterodimer nanocrystals exhibit
soft ferrimagnetic behavior. It was observed that the hetero-
dimer nanocrystals have a saturation magnetization value of
105.1 emu gꢀ1 (Fe) and 95.9 emu gꢀ1 (Fe), and a coercivity
value of 212.7 Oe and 8.7 Oe at 5 K and 300 K, respectively. A
high saturation magnetization value at 300 K demonstrates
that the Rh–Fe3O4 heterodimer nanocrystals can be used for
magnetic separation.
8
9
6
6
1
1
99b
99b
a
b
Yields of isolated products. Yield was determined by GC analysis.
Table 2 Recycling result on the reduction of nitrobenzene
Run
1st
99
2nd
99
3rd
99
4th
99
5th
99
6th
99
7th
99
8th
99
Yield (%)a
When we tested the catalytic activity of the Rh–Fe3O4
heterodimer nanocrystals for the reduction of nitrobenzene
using hydrazine, the reaction was finished in 1 h yielding
exclusively aniline (Table 1, entry 1). To examine the scope
of the reduction reaction using the Rh–Fe3O4 heterodimer
nanocrystals, we investigated the reduction of structurally
diverse nitro compounds. Table 1 shows that the heterodimer
nanocrystals have excellent activity for nitro reduction. Arene
with more than one nitro group can be reduced with good
efficiency; the reduction of 2,6-dinitrotoluene gave 2,6-diamino-
toluene as a single product (Table 1, entry 2). The nitro group
of nitroarenes with different functional groups was selectively
reduced to the amino moiety (Table 1, entries 4–9). No
reduction of benzyloxy, N-carbobenzyloxy (Cbz), ester, and
amide groups was observed by Rh–Fe3O4 heterodimer nano-
crystals (Table 1, entries 4–7). It is clear that halogen atoms
remained intact under the reaction conditions (Table 1, entries
8 and 9). In all cases examined in Table 1, excellent yields of
amine products were obtained as exclusive products.
a
All the yields were determined by GC analysis using anisole as an
internal standard.
catalyst estimated before the reaction and after 8 cycles were
1.09 wt% and 1.02 wt%, respectively, and no Rh was observed
in the filtrate obtained after separating the catalysts in the 1st,
2nd, 4th, 6th, and 8th reactions to the detectable level. These
results show no leaching of Rh species during the reaction and
recycling. Fig. S5 (ESIw) indicates that the shape and size of
catalysts separated after 8 cycles remain almost the same.
These results explain the consistently high catalytic efficiency
of the catalyst in 8 cycles.
The olefin reductions were also carried out with the
Rh–Fe3O4 heterodimer nanocrystals, and the results are
summarized in Table 3. The catalyst exhibited good catalytic
efficiency in the reduction of olefins. The catalyst was also
highly chemoselective in the reduction of the double bond.
Only the olefinic bonds were reduced without affecting several
reducible functional groups including benzyloxy, amide, ether,
and ester groups (Table 3, entries 6–9). These results clearly
show that the catalyst has remarkable selectivity for the
reduction of a variety of nitrobenzene compounds and olefins.
As a control experiment, we compared the catalytic activity of
The Rh–Fe3O4 heterodimer nanocrystals were easily separated
by using a magnet after the reduction of nitrobenzene and
reused in the next reaction (Fig. S4, ESIw). It was observed
that the catalysts had excellent catalytic activity at least until 8
times of the catalytic reaction (Table 2). ICP results of the
c
3602 Chem. Commun., 2011, 47, 3601–3603
This journal is The Royal Society of Chemistry 2011

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Table 3 Summary on the reduction of various substituted olefins
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10
20
99
2
3
4
12
12
8
20
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14
99
97
80
5
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23
99
99
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7
8
14
99b
8
9
a
6
6
6
90
78
20
Yields were determined based on GC analysis and 1H NMR
b
integration. Yield of isolated product.
the current catalyst with other related catalysts including
Fe3O4, Rh2O3, and a mixture of Rh2O3 and Fe3O4. The results
(Tables S1 and S2, ESIw) and the initial rate data of the tested
catalysts (Fig. S6 and S7, ESIw) show that the Rh–Fe3O4
heterodimer nanocrystal catalyst is the most active among the
tested catalysts in both nitro and olefin reductions.
In summary, we developed the facile synthesis of Rh–Fe3O4
heterodimer nanocrystals using one-pot thermolysis of
M(acac)3 (M = Fe, Rh). These heterodimer nanocrystals
had excellent catalytic activity and good selectivity for the
reduction of nitroarenes and alkenes. The catalyst can be
easily separated using a magnet and had good activity until
8 time’s recycling.
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T.H. thanks the Korean Ministry of Education, Science
and Technology (MEST) for financial support through the
National Creative Research Initiative (R16-2002-003-01001-0),
the Strategic Research (2010-0029138), and the World Class
University (R31-10013) Programs of the National Research
Foundation of Korea (NRF). B.M.K. thanks NRF grant
funded by MEST (No. R01-2008-000-20332-0).
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c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 3601–3603 3603