Pt nanoparticles supported on carbon coated magnetic microparticles: An efficient recyclable catalyst for hydrogenation of aromatic nitro-compounds

Article abstract of DOI:10.1039/c3ra41161c

A novel magnetically recoverable Pt nanoparticle catalyst was prepared by in situ NaBH₄ reduction of a Pt⁴⁺ species bound to hydroxyl modified carbon-coated magnetic microparticles. The prepared 'green' catalyst allows for rapid and effective hydrogenation of aromatic nitro-compounds with excellent product yields as well as easy catalyst separation and recovery. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ra41161c

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Pt nanoparticles supported on carbon coated magnetic
microparticles: an efficient recyclable catalyst for
hydrogenation of aromatic nitro-compounds
Cite this: DOI: 10.1039/c3ra41161c
Miao Xie, Fengwei Zhang, Yu Long and Jiantai Ma*
Received 11th March 2013,
Accepted 8th April 2013
A novel magnetically recoverable Pt nanoparticle catalyst was prepared by in situ NaBH4 reduction of a Pt4+
species bound to hydroxyl modified carbon-coated magnetic microparticles. The prepared ‘green’ catalyst
allows for rapid and effective hydrogenation of aromatic nitro-compounds with excellent product yields as
well as easy catalyst separation and recovery.
DOI: 10.1039/c3ra41161c
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heterogeneous catalysts are based on silica or carbon
supports.18–20 This is primarily because these types of support
Introduction
Metal-catalyzed hydrogenation provides a clean route to
heterogeneous aromatic amines, with lower economical and
environmental impacts than alternative non-catalytic pro-
cesses.1,2 Typically, noble metals (ruthenium,3–5 palladium,6–
display some advantageous properties, such as excellent
stability, good accessibility, porosity, and the fact that organic
groups can be robustly anchored to their surfaces to provide
catalytic centers. However, these catalysts are not stable and
the Pt nanoparticles may be leached from the magnetic
support. In addition, these catalysts are prepared by an
expensive method, using costly materials such as ferric
acetylacetonate and acetyl acetone platinum. The conditions
are rigorous (the temperature needed by the catalyst is usually
high which is not easy to control) and use organic solvents
which are harmful to the environment. Therefore, it is highly
desirable to develop a functionalized magnetite support for
immobilizing Pt nanoparticles.
Herein, we have adopted novel carbon-coated magnetic
microparticles as the support.21 The thin carbon layer on the
magnetite plays two important roles. It can protect the
magnetite particles from being corrupted under acid condi-
tions and being oxidized under an oxygen atmosphere. In
addition, these hydroxyl carbon layers possess functional –OH
groups on their surface, which provide an important chemical
environment for coordination with the catalytically active Pt
species by electrostatic interaction in alkaline solution, in
order to prevent their loss. Through this coordinate bonding,
the hydroxyl groups on the Fe3O4@C anchor the Pt nanopar-
ticles instead of simply adsorbing them. We have also
investigated the catalytic activity of the as-synthesized mag-
netic nanocatalyst (Fe3O4@C@Pt). The results showed that the
catalytic hydrogenation of aromatic nitro-compounds can
achieve yields of over 95% and the catalyst may be reused 10
times with no obvious decrease in the conversion efficiency
and selectivity by a simple magnetic separation method.
The process for the preparation of the carbon-modified
magnetite microparticle catalyst is described in Scheme 1.
Briefly, magnetite particles were firstly prepared by a co-
8
rhodium,9–11 and platinum,12–14 etc.) have proven to be
highly active catalysts for reactions under a hydrogen atmo-
sphere. Particularly, platinum nanoparticles (Pt NPs) display
high catalytic activity for hydrogenation reactions under mild
reaction conditions.
The catalytic hydrogenation of aromatic nitro-compounds
over supported metal catalysts is of fundamental interest in
chemical laboratories and industries.14,15 However, Pt is too
rare and expensive for widespread application. To improve the
efficiency and save resources, there has been an increasing
trend toward the use of magnetically retrievable nanoparticles
(MNPs) in efficient green chemical synthesis. Using a catalyst
containing magnetic particles allows it to be recovered and
reused using an external magnet, thus avoiding the need for
catalyst separation by filtration. The recovery of expensive
catalysts after catalytic reactions and reusing them without
loss of activity are essential features in the sustainable process
development.
C. Evangelisti et al.16 reported that c-Fe2O3-supported
platinum nanoparticles could be magnetically recovered from
a reaction mixture and reused in up to 5 catalytic cycles
without showing any appreciable decline of their catalytic
activity and selectivity. C. Lian et al.17 prepared a Pt–Fe3O4
catalyst by adsorbing Pt nanoclusters on a Fe3O4 support. They
proposed that the Pt particles were adsorbed on the outer
surface or in the large pores of the Fe3O4 support, and may
have enhanced resistance to mass transfer. Attempts have
been made to overcome these issues, and the majority of novel
Lanzhou University, College of Chemistry and Chemical Engineering, Lanzhou,
China. E-mail: majiantai@lzu.edu.cn
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to ultrasonic treatment. After 30 min, this material was
washed with water and isolated with the aid of a magnet.
The resultant material was re-dispersed into 225 mL water
containing 15 g glucose by ultrasonic decentralization. The
mixture was transferred into a Teflon-lined stainless-steel
autoclave and the autoclave was placed in an oven and kept at
200 uC for 12 h. After cooling to room temperature, the
precipitated black solid product (MFC) was collected from the
solution by an external magnet and washed with water several
times. Finally, the black products were dried under vacuum for
24 h at 60 uC.
Synthesis of the Fe3O4@C@Pt catalyst. The preparation of Pt
nanoparticles supported on MFC was as follows: 500 mg MFC
and 1.5 mL 0.01 M H2PtCl6 were dispersed in 200 mL
deionized water under ultrasonic treatment for 1 h. Then
200 mL (0.01 M) NaBH4 solution was added dropwise to the
mixture under continuous vigorous stirring at room tempera-
ture. About 2 h later, the solid products were collected by
applying a magnet, and were washed with water and ethanol
several times. The products were dried under vacuum at 60 uC
for 24 h.26
Scheme 1
crystallization method and then Pt(0) was immobilized on the
surface of the magnetite microparticles (the hydroxyl groups
on the surface of the microparticles act as a robust anchor and
avoid Pt leaching).
The Fe3O4@C@Pt catalyst was prepared by means of
dropwise addition of NaBH4 solution to
Fe3O4@C and H2PtCl6 under continuous vigorous stirring at
room temperature.24 The Fe3O4 microparticles were synthe-
sized by a solvothermal method according to our previous
work.25 On the basis of that work, Fe3O4@C composites were
prepared by in situ carbonization of glucose in the presence of
Fe3O4 microparticles under hydrothermal conditions.26 After
the reaction, the solid products were collected by applying a
magnet, and were washed with water and ethanol several
times. The products were dried under vacuum at 60 uC for 24
h. The Pt4+ loaded carbon-modified solid contains 4.89 wt% of
Pt, as determined by atomic absorption analysis (ICP-OES).
a mixture of
3. Catalytic reaction
The Fe3O4@C@Pt catalyst was tested on the hydrogenation of
aromatic nitro-compounds reactions.27 10 mg of Fe3O4@C@Pt
catalyst and 0.025 mmol substrate were added to 5 mL alcohol.
The reactions were carried out at room temperature with
electromagnetic stirring for 1 h. The catalysts were then
collected by external magnets, and the reaction systems were
analyzed by gas chromatography (GC).
Experimental
1. Chemicals
Results and discussion
Iron(III) chloride hydrate, sodium acetate, poly-N-vinylpyrrole-
2-one (PVP, K-30), ethylene glycol, ethanol, glucose, hexachlor-
oplatinic acid and Pt/C were purchased from Sigma-Aldrich.
Various reaction reagents were purchased from Alfa Aesar. All
chemicals were of analytical grade and used as received
without further purification. De-ionized water was used
throughout the experiments.
1. Characterization
These magnetic micro-materials were characterized by induc-
tively coupled plasma (ICP) mass spectrometry, powder X-ray
diffraction (PXRD), transmission electron microscopy (TEM),
X-ray photoelectron spectroscopy (XPS), Fourier transform-
infrared spectroscopy (FT-IR) and vibrating sample magneto-
metry (VSM). XRD measurements were performed on a Rigaku
D/max-2400 diffractometer using Cu-Ka radiation as the X-ray
source in the 2h range 10–90u. The size and morphology of the
magnetic microparticles were observed by a Tecnai G2 F30
transmission electron microscope and samples were obtained
by placing a drop of a colloidal solution onto a copper grid and
evaporating the solvent in air at room temperature. The Pt
content of the catalyst was measured by inductively coupled
plasma mass spectrometry (ICP) on an IRIS Advantage
analyzer. Magnetic measurements of Fe3O4@C@Pt were
investigated with a Quantum Design vibrating sample mag-
netometer (VSM) at room temperature in an applied magnetic
field sweeping from 28 to 8 kOe. X-ray photoelectron
spectroscopy (XPS) was recorded on a PHI-5702 instrument
and the C 1s line at 284.8 eV was used as the binding energy
reference.
2. Preparation of the Fe3O4@C@Pt (Pt–MFC) catalyst
Synthesis of the Fe3O4 microparticles. Fe3O4 microparticles
were synthesized by a solvothermal method according to our
previous work.24 Firstly, 9 g FeCl3?6H2O, 6 g PVP and 12 g NaAc
were added to 200 mL ethylene glycol. The mixture was stirred
vigorously for 2 h to ensure that all the materials completely
dissolved. Then the mixture was transferred to a Teflon-lined
stainless-steel autoclave, sealed and heated at 200 uC for 8 h.
The precipitated black products were collected from the
solution by an external magnet and washed with ethanol
several times. Finally, the black products were dried under
vacuum for 24 h at 60 uC.
Synthesis of the magnetic Fe3O4@C (MFC) composites.
Fe3O4@C composites were prepared by in situ carbonization of
glucose in the presence of Fe3O4 microparticles under
hydrothermal conditions.25 Typically, 1 g Fe3O4 microparticles
was dispersed in 200 mL of 0.01 mol L21 HCl and was subject
Fig. 1a shows a typical TEM image of the Fe3O4 micro-
spheres prepared by the versatile solvothermal reaction. As can
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Fig. 3 XPS spectrum of the elemental survey scan of Fe3O4@C@Pt catalyst (inset
Fig. 1 TEM image of (a) the Fe3O4 microspheres, (b) the Fe3O4@C composite, (c)
the Fe3O4@C@Pt catalyst, (d) the Fe3O4@C@Pt catalyst at a different
magnification.
is the XPS spectrum of the spent Pt(0) catalyst).
successfully immobilized on the surface of the magnetic
microparticles.
be seen from the image, the average diameter of the as-
synthesized spherical particles is about 500 nm. The TEM
image in Fig. 1b shows that the thickness of the shell carbon
layer of the Fe3O4@C composite is about 30 nm. Meanwhile,
from Fig. 1d, which is magnified from Fig. 1c, it can also be
concluded that the platinum particle size distribution was
centred around 5 nm.
The XRD patterns of the Fe3O4 microspheres (Fig. 2a) and
the Fe3O4@C composite (Fig. 2b) show characteristic peaks of
magnetite nanoparticles and the sharp, strong peaks confirm
the products are well crystallized.22 Fig. 2c shows that apart
from the original peaks, two new peaks appear at 2h = 39.9572
and 46.6102, which are attributed to the Pt species. The results
from XRD imply that the Pt nanoparticles have been
Fig. 3 presents XPS elemental survey scans of the surface of
the Fe3O4@C@Pt catalyst. Peaks corresponding to oxygen,
carbon, iron and platinum can clearly be observed. To
ascertain the oxidation state of Pt, X-ray photoelectron
spectroscopy (XPS) studies were carried out. The XPS analysis
of the spent Pt(0) catalyst is shown in the upper right corner of
Fig. 3, and, as expected, the spectrum of the Pt 4f region
confirms the presence of Pt(0) with a peak binding energy of
71.3 eV (Pt 4f7/2).
The magnetic properties were investigated as shown in
Fig. 4. The curves show no remaining magnetization or
coercivity, indicating that three samples are paramagnetic at
room temperature. The carbon-coated magnetic microparti-
cles (Fig. 4b) show a small reduction in the saturation
magnetization. This slight reduction in the saturation magne-
tization for the material-loaded solid was expected.23 The
decrease of the saturation magnetization (Fig. 4c) suggests the
presence of some platinum particles on the surface of the
Fig. 4 VSM of (a) the Fe3O4 microspheres, (b) the Fe3O4@C composite, (c) the
Fig. 2 XRD patterns of (a) the Fe3O4 microspheres, (b) the Fe3O4@C composite,
(c) the Fe3O4@C@Pt catalyst.
Fe3O4@C@Pt catalyst.
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Table 2 Hydrogenation of various substrates using the Fe3O4@C@Pt catalysta
Entry Substrate
1
Product
t (min) Yield (%)
60
.99
96b
89c
99
2
3
4
5
60
60
60
60
.99
.99
95
Fig. 5 FT-IR spectra of (a) the Fe3O4 microspheres, (b) the Fe3O4@C composite,
(c) the Fe3O4@C@Pt catalyst.
magnetic supports. Even with this reduction in saturation
magnetization, the catalyst can still be efficiently separated
from the solution with a small permanent magnet.
As shown in Fig. 5, in the low-frequency region, the peaks at
580.6 cm21 (Fig. 5a), 582.5 cm21 (Fig. 5b) and 582.5 cm21
(Fig. 5c) correspond to the stretching vibrations of Fe–O. The
FT-IR spectrum of Fe3O4@C@Pt shows broad peaks at 1621.9
cm21, 1700.2 cm21 and 3433.8 cm21 (Fig. 5b), which are
assigned to the stretching vibrations of COOH, CLO and O–
H.21 The characteristic stretches of COOH, CLO and O–H in
the incompletely carbonized glucose at 1622.0 cm21, 1700.8
cm21 and 3417.4 cm21 (Fig. 5c) are weaker than in Fe3O4@C,
indicating that coordination reactions have occurred between
Pt nanoparticles and ketonic or hydroxylic oxygens on the
surface of Fe3O4@C. Functionalization with hydroxylic oxygens
improved the uptake of Pt4+ ions when the solid was
introduced into an aqueous solution of hexachloroplatinic
acid.
6
60
.99
7
8
60
60
97
.99
9
60
60
.99
2. Optimization of reaction conditions
10
98
In order to optimise the experiment, the reaction conditions
were explored in the presence of the Fe3O4@C@Pt catalyst for
the hydrogenation of nitrobenzene with various reaction times
and different amounts of catalyst. As can be seen from Table 1,
entry 4 resulted in a higher yield than the other entries. In the
11
12
60
60
.99
98
Table 1 Optimization of reaction conditionsa
Entry
Catalyst (mg)
Time (min)
Yield (%)
13
14
60
60
96
98
1
2
3
4
5
6
5
10
15
20
20
20
60
60
60
60
45
30
5
38
75
98
43
12
a
Reaction conditions: nitrobenzene (0.5 mmol) and solvent (10 mL)
at room temperature under a H2 atmosphere.
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groups makes little difference to the yield. As for comparing
the effect of substituent position, the percent conversion for a
meta-compound is a little higher than for the corresponding
ortho-nitroarene and para-nitroarene. The microparticle-based
material shows superior catalytic activity with over 95% yields
in 60 min and can be recycled multiple times without any
significant loss in catalytic activity under mild conditions.
Table 2 (Continued)
Entry Substrate
Product
t (min) Yield (%)
a
Reaction conditions: catalyst = 10 mg; H2 = 1 atm; substrate = 0.25
b
c
mmol; solvent = 5 mL. Yield after 10 runs. Hydrogenation with
10% Pt/C.
following studies, all the reactions were explored using these
optimum conditions.
Acknowledgements
The authors are grateful to the Key Laboratory of Nonferrous
Metals Chemistry and Resources Utilization, Gansu Province
for financial support.
3. Catalyst testing for the hydrogenation reaction
Initially, the catalytic activity was tested on the hydrogenation
of a variety of aromatic nitro-compounds to their correspond-
ing products. The reactions were carried out in ethanol at
room temperature and under 1 atm of H2. Detailed observa-
tions of all the reactions are given in Table 2. We observed that
the catalyst was very active for the hydrogenation reaction
under such mild conditions. Catalytic hydrogenation of
aromatic nitro-compounds can produce yields of over 95%,
and the catalyst can be removed and reused 10 times with no
obvious decrease in the conversion efficiency and selectivity by
a simple magnetic separation method.
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1
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.99
.99
.99
99
6
7
8
9
98
98
96
98
96
.99
10
a
Reaction conditions: catalyst = 50 mg; H2 = 1 atm; substrate = 1.25
mmol; solvent = 25 mL.
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