Nitrogen-doped magnetic carbon nanoparticles as catalyst supports for efficient recovery and recycling

Article abstract of DOI:10.1039/b616660a

Palladium nanoparticles were deposited with high dispersion and stability on nitrogen-doped magnetic carbon nanoparticles by a simple impregnation method, and their catalytic performance was investigated for Heck, Suzuki, and Sonogashira coupling reactions. The Royal Society of Chemistry.

Full text of DOI:10.1039/b616660a

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Nitrogen-doped magnetic carbon nanoparticles as catalyst supports for
efficient recovery and recycling{
Hyeonseok Yoon, Sungrok Ko and Jyongsik Jang*
Received (in Cambridge, UK) 15th November 2006, Accepted 15th January 2007
First published as an Advance Article on the web 5th March 2007
DOI: 10.1039/b616660a
matrix would provide strong catalyst–support interactions in
anchoring catalyst particles onto the carbon support without the
aid of any functionalization processes.8
Palladium nanoparticles were deposited with high dispersion
and stability on nitrogen-doped magnetic carbon nanoparticles
by a simple impregnation method, and their catalytic perfor-
mance was investigated for Heck, Suzuki, and Sonogashira
coupling reactions.
We have previously reported on the fabrication of several types
of carbon nanomaterials using nanosized polymer precursors.9
Notably, magnetically functionalized carbon nanotubes and
nanoparticles could be readily obtained via the carbonization of
iron-impregnated polymer nanomaterials.10,11 Herein, we investi-
gate the formation of palladium nanoparticles onto nitrogen-
doped magnetic carbon nanoparticles (N-MCNPs), and their
catalytic activity was also examined for several coupling reactions.
First, magnetic carbon nanoparticles were prepared using iron-
doped polypyrrole nanoparticles as the carbon precursor
(carbonization temperature, 800 uC).11 The resulting carbon
nanoparticles include nitrogen atoms in the carbon matrix because
the pyrrole repeating unit consists of one nitrogen, four carbons,
and five hydrogens: the CHNS elemental analysis indicated that
the as-prepared carbon nanoparticles had a nitrogen content of ca.
4–5 wt%. It is also important to note that the N-MCNPs are
characterized as a microporous nanoarchitecture. To deposit
palladium onto the N-MCNPs, 0.1 g of N-MNCPs was dispersed
in 0.8 mL of water, and subsequently 0.76 mL of 2.5 wt% aqueous
Pd(NO3)2 solution was added in the solution at 25 uC. After
drying, the samples were calcined at 300 uC in air, and then
reduced at 300 uC in a hydrogen atmosphere.
Although homogeneous catalysts have many advantages over their
heterogeneous counterparts, the majority of industrial catalysts
consist of metal or metal compound particles dispersed on an
appropriate support.1 The use of catalyst supports provides
effective opportunities for separation of the reaction product and
the catalyst.2 Of a wide range of catalyst supports, in particular,
carbon materials have been extensively employed in heterogeneous
catalysis due to their features such as chemical inertness, good
mechanical stability, and electrical conductivity.3 Since the
supports can strongly affect catalyst activity, numerous studies
have been performed to modify the carbon supports in an effort to
improve catalyst performance. First, advances in the synthesis of
materials have made it possible to prepare carbon nanostructures
with well-defined morphologies (e.g., surface area and porosity).
Typically, carbon nanotubes and mesoporous carbon materials
have been highlighted as promising catalyst supports.4 Secondly,
chemical modification of the carbon surface has been explored to
stabilize catalyst–support interactions. Functional groups including
hydroxyls, carboxyls and amines were mostly introduced via
ozonolysis, plasma activation, acid or base treatment, and so
forth.5 However, these approaches may deteriorate the mechan-
ical/electronic properties of carbon materials, and the functional
groups generated are often unstable under certain catalysis
conditions.
The resulting palladium-loaded N-MNCPs (Pd/N-MCNPs)
were observed by transmission electron microscope (TEM). As
shown in Fig. 1(a), the diameter of N-MCNPs used as the catalyst
Recently, doping of heteroatoms into carbon materials has
offered new opportunities for tailoring their chemical/physical
properties. The most notable example is nitrogen-doped carbon
materials. Rao and co-workers achieved good yields of nitrogen-
doped carbon nanotubes by pyrolysis of pyridine and nickel
phthalocyanine-thiophene (nitrogen content of 3–10 wt%).6 In
addition, Mokaya and co-workers fabricated nitrogen-doped
carbons in the form of nanotubes and mesoporous structures
by using acetonitrile as a carbon precursor (nitrogen content of
6–9 wt%).7 Importantly, the nitrogenated sites in the carbon
Hyperstructured Organic Materials Research Center and School of
Chemical and Biological Engineering, Seoul National University,
Shinlimdong 56-1, Seoul, 151-742, Korea.
E-mail: jsjang@plaza.snu.ac.kr; Fax: 82 2 888 1604; Tel: 82 2 880 7069
{ Electronic supplementary information (ESI) available: 1. Electron
diffraction pattern of Pd/N-MCNPs; 2. Control experiments (TEM
images, XRD patterns, and textural parameters of Pd-decorated carbon
blacks). See DOI: 10.1039/b616660a
Fig. 1 TEM images of Pd/N-MCNPs: (a) low-magnification bright-field
image (inset: size distribution histogram of the Pd nanoparticles formed),
(b) high-magnification bright-field image for a single nanoparticle, and the
corresponding dark-field images obtained from (c) Pd(111) and (d)
Pd(200) reflections (for electron diffraction image see ESI, Fig. S1{).
1468 | Chem. Commun., 2007, 1468–1470
This journal is ß The Royal Society of Chemistry 2007

View Article Online
support was approximately 50 nm. The palladium nanoparticles
were well-dispersed onto the N-MCNPs and were not agglomer-
ated. The size distribution of palladium nanoparticles was
reasonably narrow, and the average diameter was determined as
3.8 nm by counting 100 nanoparticles in the TEM image (Fig. 1(a)
inset). The dark-field TEM images revealed the (111) and (200)
crystallographic planes of the deposited palladium nanoparticles
(Fig. 1(c), (d)). The powder X-ray diffraction (XRD) pattern of the
Pd/N-MCNPs displayed the characteristic (111), (200), (220) and
(311) peaks of the fcc palladium crystal structure (Fig. 2). The
average size of the palladium crystallites calculated from the line
broadening of the Pd(111) peak using the Scherrer formula was
4.6 nm, which was commensurable to the result obtained from the
TEM observation. Thermogravimetric analysis showed that the
Pd/N-MCNPs contained an extremely high palladium content
(approximately 40 wt%). In general, employing a substantial
amount of metal precursor for such a high metal loading
commonly gives rise to agglomeration of metal nanoclusters
during the reduction process. However, in our case, the excellent
dispersion of palladium nanoparticles with narrow size distribution
could be achieved over the sample through a simple impregnation
method without using any specific ligands or stabilizers. Moreover,
the palladium nanoparticles deposited were retained without
deformation even after sonication for 5–10 min, which indicated
strong binding of the palladium nanoclusters to the carbon
support. These results can be explained in the following three
ways: (i) the well-defined spherical nanostructures may allow a
rapid diffusion rate and wetting of the metal precursor on their
surface compared with irregularly shaped spheres, mesoporous
channel structures, and tubular nanostructures. (ii) The porous
surface of the carbon support can offer steric restriction to confine
the metal precursor and to prevent growth of metal clusters due to
the surface tension of the metal precursor solution. (iii) The
nitrogenated sites are capable of providing a strong metal–support
interaction to anchor efficiently metal precursor on the carbon
support and to stabilize the final metal nanoclusters.
aggregates of spherical particles with the diameter of individual
particles ranging from 10 to 200 nm. Under the same deposition
condition, these carbon blacks could be only decorated with larger
agglomerates of palladium nanoparticles that were poorly
dispersed on the surface: the palladium size distributions were
2–28 and 2–20 nm, respectively (Fig. S2 and S3, ESI{). This
experimental result may be a good evidence that the well-defined
spherical porous nanostructures and the nitrogen doping con-
tribute to the uniform anchoring of palladium nanoparticles with
similar sizes.
The magnetic properties of Pd/N-MCNPs were investigated
using a superconducting quantum interference device magnet-
ometer (Fig. 3(a)). The saturation of the magnetization could be
observed with applied magnetic fields above 15 kOe (1 kOe =
0.1 T), and the saturation magnetization from the hysteresis loop
was found to be 6.9 emu g21. The magnetic property of the carbon
support is useful for efficient recovery and recycling of nanoca-
talysts in liquid-phase reactions.12 As an example, when the Pd/N-
MCNPs were dispersed in water giving a black suspension
(Fig. 3(b)), upon applying an external magnetic field, the black
powder was readily harvested and the solution became transparent
(Fig. 3(c)).
To investigate the catalytic activity of the Pd/N-MCNPs, three
different types of coupling reactions were performed, as summar-
ized in Table 1. As a typical example for Heck reaction,
iodobenzene and styrene were selected as a non-activated aryl
halide and an aromatic olefin, respectively (entry 1). In general,
non-activated aryl halides have relatively low conversion yields
compared with activated aryl halides (e.g., 4-iodoanisole and
2-iodotoluene). Nevertheless, the Heck olefination of iodobenzene
with styrene showed high yields of more than 97% when
employing the Pd/N-MCNPs. The yield of the reaction with a
commercial palladium/carbon catalyst was only 53% after 24 h
under the same experimental conditions.13 The Suzuki coupling of
phenylboronic acid and iodothiophene was also catalyzed using
To confirm these explanations, two different types of carbon
blacks containing no nitrogen were employed for control
experiments. One was a Vulcan XC72R (a commercially available
carbon black, Cabot Corporation), and the other was a carbon
black prepared by thermal decomposition of methane using dc
thermal plasma processing (Plasnix Co.). These materials consist of
Fig. 2 Powder XRD pattern of Pd/N-MCNPs (G: graphite, M: c-Fe2O3,
P: palladium). The XRD peaks were collected in the range 2h = 10–85u
Fig. 3 Magnetic property of Pd/N-MCNPs: (a) hysteresis loop measured
at 300 K, and photographs of an aqueous solution containing 0.01 wt%
Pd/N-MCNPs (b) before and (c) after magnetic separation.
with a scan speed of 3u min21 (Cu-Ka radiation source: l = 1.5406 A).
˚
This journal is ß The Royal Society of Chemistry 2007
Chem. Commun., 2007, 1468–1470 | 1469

View Article Online
Table 1 Catalytic activities of Pd/N-MCNPs for Heck, Suzuki and Sonogashira coupling reactions
Yielde (%)
1st cycle 2nd cycle 3rd cycle
Entry Reaction
Reagent 1
Reagent 2
T/uC t/h Product
1b
Heck
120
3
97.6
97.4
97.3
2c
Suzuki
80
3
3
94.2
91.2
94.0
91.6
93.7
91.2
3d
Sonogashira
100
a
b
All reactions were catalyzed using 0.01 g of Pd/N-MNCPs. Reaction carried out in 20 mL of dimethylacrylonitrile with triethylamine
c
(4.46 mmol), iodobenzene (1.95 mmol), and styrene (2.11 mmol). Reaction performed in 50 mL of ethanol with 4-bromoacetophenone
d
(3.00 mmol), phenylboronic acid (6.00 mol), 2-iodothiophene (3.00 mmol), and K3PO4 (12.00 mmol). Reaction carried out in 20 mL of
dimethyl sulfoxide with Na2CO3 (14.47 mmol), 4-bromoacetophenone (10.00 mmol), phenylacetylene (13.33 mmol), and CuI (0.15 6
e
1021 mmol). The yield was calculated by gas chromatography analysis.
the Pd/N-MCNPs (entry 2). A previous report showed that
0.3 mol% palladium nanoparticles (3.6 nm in diameter) stabilized
by poly(N-vinyl-2-pyrrolidone) gave a yield of 78% after 12 h for
the same reaction.14 Although only 1.2 mol% palladium content
was used under our experimental conditions, high yields of
approximately 94% could be achieved after only 3 h. Finally, the
Pd/N-MCNPs were employed to promote a Sonogashira reaction
of 4-bromoacetophenone and phenylacetylene as aryl halide and
alkyne, respectively (entry 3). The desired products were obtained
in high yields of more than 91% with 0.4 mol% palladium content
for 3 h at 100 uC. This is comparable to the result (92% yield)
obtained using 2 mol% palladium/nickel nanoparticles (3.9 nm in
diameter) for 2 h at 80 uC.15 Thus, the Pd/N-MCNPs gave high
conversion yields for all the three different reactions.
Monodisperse spherical carbon nanoparticles allow the easy
diffusion of reactants and highly dispersed palladium nanoparticles
offer more catalytically active sites for reactants, which may be
responsible for the remarkable catalytic activities. It was also
noteworthy that the Pd/N-MNCPs could be readily reused several
times without losing their catalytic activity. If the bonds between
metal and support were weak, the metal nanoparticles would be
detached from the support. Consequently, it is believed that the
strong interaction between palladium clusters and N-MNCPs
plays a significant role in preventing metal leaching during the
reactions.
Materials Research Center is supported by the Korea Science and
Engineering Foundation.
Notes and references
1 J. Grunes, J. Zhu and G. A. Somorjai, Chem. Commun., 2003, 2257;
D. J. Cole-Hamilton, Science, 2003, 299, 1702.
2 D. Astruc, F. Lu and J. R. Aranzaes, Angew. Chem., Int. Ed., 2005, 44,
7852; M. Gilhespy, M. Lok and X. Baucherel, Chem. Commun., 2005,
1085; C. M. Y. Yeung, K. M. K. Yu, Q. J. Fu, D. Thompsett,
M. I. Petch and S. C. Tsang, J. Am. Chem. Soc., 2005, 127, 18010.
3 C. Moreno-Castilla and F. J. Maldonado-Ho´dar, Carbon, 2005, 43, 455;
F. Rodr´ıguez-Reinoso, Carbon, 1998, 36, 159.
4 G. G. Wildgoose, C. E. Banks and R. G. Compton, Small, 2006, 2, 182;
A.-H. Lu and F. Schu¨th, Adv. Mater., 2006, 18, 1793.
5 D. Tasis, N. Tagmatarchis, A. Bianco and M. Prato, Chem. Rev., 2006,
106, 1105.
6 M. Nath, B. C. Satishkumar, A. Govindaraj, C. P. Vinod and
C. N. R. Rao, Chem. Phys. Lett., 2000, 322, 333; F. L. Deepak,
N. S. John, A. Govindaraj, G. U. Kulkarni and C. N. R. Rao, Chem.
Phys. Lett., 2005, 411, 468.
7 Z. Yang, Y. Xia and R. Mokaya, Chem. Mater., 2005, 17, 4502; Y. Xia
and R. Mokaya, Chem. Mater., 2005, 17, 1553.
8 A. Zamudio, A. L. El´ıas, J. A. Rodr´ıguez-Manzo, F. Lo´pez-Ur´ıas,
G. Rodr´ıguez-Gattorno, F. Lupo, M. Ru¨hle, D. J. Smith, H. Terrones,
D. D´ıaz and M. Terrones, Small, 2006, 2, 346; K. Jiang, A. Eitan,
L. S. Schadler, P. M. Ajayan, R. W. Siegel, N. Grobert, M. Mayne,
M. Reyes-Reyes, H. Terrones and M. Terrones, Nano Lett., 2003, 3,
275.
9 J. Jang and J. Bae, Angew. Chem., Int. Ed., 2004, 34, 3803; J. Jang and
J. H. Oh, Adv. Mater., 2004, 16, 1650; J. Jang, B. Lim and M. Choi,
Chem. Commun., 2005, 4214; J. Jang, J. H. Oh and G. D. Stucky,
Angew. Chem., Int. Ed., 2002, 41, 4016.
In conclusion, we demonstrated the use of N-MCNPs as a
magnetically recoverable catalyst support. Well-dispersed palla-
dium nanoparticles with narrow size distribution could be
deposited on the N-MCNPs by a simple impregnation method.
The Pd/N-MCNPs displayed high catalytic activities for three
types of coupling reactions, and the magnetic property of the
support gave an opportunity for easy recovery and recycling of the
catalyst via magnetic separation. Importantly, versatile metal
nanoparticles may be decorated on the N-MCNPs, which might
be applied to catalysts, sensors, bactericides, and so forth.
This work was supported by the Brain Korea 21 program of the
Korean Ministry of Education, and the Hyperstructured Organic
10 J. Jang and H. Yoon, Adv. Mater., 2003, 15, 2088.
11 J. Jang and H. Yoon, Small, 2005, 1, 1195.
12 P. D. Stevens, G. Li, J. Fan, M. Yen and Y. Gao, Chem. Commun.,
2005, 4435; M. Kawamura and K. Sato, Chem. Commun., 2006, 4718;
S. Ko and J. Jang, Angew. Chem., Int. Ed., 2006, 45, 7564; A.-H. Lu,
W. Schmidt, N. Matoussevitch, H. Bo¨nnemann, B. Spliethoff, B. Tesche,
E. Bill, W. Kiefer and F. Schu¨th, Angew. Chem., Int. Ed., 2004, 43, 4303;
S. C. Tsang, V. Caps, I. Paraskevas, D. Chadwick and D. Thompsett,
Angew. Chem., Int. Ed., 2004, 43, 5645.
13 B. Yoon and C. M. Wai, J. Am. Chem. Soc., 2005, 127, 17174.
14 Y. Li, X. M. Hong, D. M. Collard and M. A. El-Sayed, Org. Lett.,
2000, 2, 2385.
15 S. U. Son, Y. Jang, J. Park, H. B. Na, H. M. Park, H. J. Yun, J. Lee
and T. Hyeon, J. Am. Chem. Soc., 2004, 126, 5026.
1470 | Chem. Commun., 2007, 1468–1470
This journal is ß The Royal Society of Chemistry 2007