Palladium nanoparticles immobilized on core-shell magnetic fibers as a highly efficient and recyclable heterogeneous catalyst for the reduction of 4-nitrophenol and Suzuki coupling reactions

Article abstract of DOI:10.1039/c4ta04919e

In this study, a novel core-shell magnetic fibrous nanocatalyst, Pd/Fe₃O₄@SiO₂@KCC-1 with easily accessible active sites and a convenient recovery by applying an external magnetic field, was successfully developed. Fe₃O₄@SiO₂@KCC-1 was functionalized with amino groups which act as robust anchors so that the palladium nanoparticles (Pd NPs) with an average diameter of about 4 nm were well-dispersed on the fibers of Fe₃O₄@SiO₂@KCC-1 without obvious aggregation. The synthesized Pd/Fe₃O₄@SiO₂@KCC-1 nanocatalyst exhibited excellent catalytic activity in the reduction of 4-nitrophenol by sodium borohydride, and the Suzuki cross coupling reactions of aryl chlorides with aryl boronic acids due to the easy accessibility of the active sites. Furthermore, the Pd/Fe₃O₄@SiO₂@KCC-1 nanocatalyst was conveniently recovered by a magnet and could be reused for at least five cycles without significant loss in activity, thus confirming its good stability. Therefore, the abovementioned approach based on core-shell magnetic fibrous Fe₃O₄@SiO₂@KCC-1 provided a useful platform for the fabrication of Pd NPs based catalysts with easy accessibility, superior activity and convenient recovery.

Full text of DOI:10.1039/c4ta04919e

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Materials Chemistry A
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PAPER
Palladium nanoparticles immobilized on core–shell
magnetic fibers as a highly efficient and recyclable
heterogeneous catalyst for the reduction of
4-nitrophenol and Suzuki coupling reactions†
Cite this: DOI: 10.1039/c4ta04919e
a
a
Xuanduong Le, Zhengping Dong, Yansheng Liu,a Zhicheng Jin,a Thanh-Do Huy,b
*
b
a
*
Minhdong Le and Jiantai Ma
In this study, a novel core–shell magnetic fibrous nanocatalyst, Pd/Fe3O4@SiO2@KCC-1 with easily
accessible active sites and a convenient recovery by applying an external magnetic field, was successfully
developed. Fe3O4@SiO2@KCC-1 was functionalized with amino groups which act as robust anchors so
that the palladium nanoparticles (Pd NPs) with an average diameter of about 4 nm were well-dispersed
on the fibers of Fe3O4@SiO2@KCC-1 without obvious aggregation. The synthesized Pd/
Fe3O4@SiO2@KCC-1 nanocatalyst exhibited excellent catalytic activity in the reduction of 4-nitrophenol
by sodium borohydride, and the Suzuki cross coupling reactions of aryl chlorides with aryl boronic acids
due to the easy accessibility of the active sites. Furthermore, the Pd/Fe3O4@SiO2@KCC-1 nanocatalyst
was conveniently recovered by a magnet and could be reused for at least five cycles without significant
loss in activity, thus confirming its good stability. Therefore, the abovementioned approach based on
core–shell magnetic fibrous Fe3O4@SiO2@KCC-1 provided a useful platform for the fabrication of Pd
NPs based catalysts with easy accessibility, superior activity and convenient recovery.
Received 18th September 2014
Accepted 6th October 2014
DOI: 10.1039/c4ta04919e
www.rsc.org/MaterialsA
crucial issue for their further application. One solution for Pd
NP stabilization involves the use of appropriate support mate-
rials. Various supports, such as carbon,14,15 Al2O3,16,17 meso-
1. Introduction
In the catalytic area, noble metal nanoparticle (NMNPs) based
heterogeneous catalysts have attracted signicant attention due
to the high catalytic efficiency in numerous liquid-phase cata-
lytic processes.1–3 In particular, palladium nanoparticles (Pd
NPs) based catalysts have been extensively studied because of
the superior catalytic performance toward diverse reactions
such as the Suzuki coupling,4,5 Heck C–C coupling,6 degradation
of pollutants,7 hydrogenation,8 and fuel cells.9 The catalytic
activity of Pd NPs is strongly dependent on the active atoms on
the surface, which are usually related to the surface properties.10
Therefore, smaller Pd NPs tends to show a superior catalytic
activity as it has a higher surface-to-volume ratio.11,12 As the size
of Pd NPs reduce, the surface energy of the Pd NPs increase,
thus leading to the tendency of inter-particle aggregation. The
aggregation of Pd NPs reduce the catalytic efficiency of the
catalysts.13 Therefore, the stability of the Pd NPs is another
porous silica,18 and core–shell magnetic mesopores,19 have been
used for the immobilization of Pd NPs owing to their excellent
stability, high surface areas, and robust surface chemistry. All of
the abovementioned materials have large specic surface areas,
which lead to the great dispersion of the NPs, thus preventing
the aggregation of the NPs and improving the activity of the
catalyst system. However, for all of the reported catalysts, the
poor accessibility to the active sites leads to a reduction of the
mass transfer effect. Silica supports with easily accessible high
surface areas and not due to the pores, therefore, are highly
desirable. Vivek Polshettiwar reported the synthesis of brous
silica nanospheres (KCC-1) by the microwave assisted hydro-
thermal technique and our laboratory research group reported
synthesis of KCC-1 by the hydrothermal method.20–22 Further-
more, Liu et al. reported the fabrication of core–shell magnetic
brous Fe3O4@SiO2@KCC-1, which have both the unique
properties of KCC-1 and the physical properties of magnetic
materials.23 Therefore, Fe3O4@SiO2@KCC-1 is extremely useful
in the design of supported catalysts because of the unique
physical properties such as easily accessible active sites, high
surface area, reusability and superparamagnetism.
aGansu Provincial Engineering Laboratory for Chemical Catalysis, College of
Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR
China. E-mail: dongzhp@lzu.edu.cn; majiantai@lzu.edu.cn; Fax: +86 931 891 2582;
Tel: +86 931 891 2311
bFaculty of Technical Physics and Chemistry, Le Quy Don Technical University, Hanoi,
Vietnam
Recently, the transformation of harmful organic waste into
reusable compounds with low toxicity in aqueous solutions
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ta04919e
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under mild conditions has become an extremely important area
of study for chemists. As is well known, 4-nitrophenol (4-NP), a
well-characterized industrial pollutant that has been listed on
the “Priority Pollutant List” by the U.S. Environmental Protec-
tion Agency, is harmful to aquatic life and human health in
terms of its toxicity, potential carcinogenicity, and mutage-
nicity.24–26 The use of the catalytic reduction process for the
disposal of 4-NP is expected to be the most efficient, green, and
economical method.27,28 In addition, considerable work has
been recently directed toward the Suzuki cross coupling reac-
tion because it is the most powerful and widely used method for
the synthesis of biaryls, which are important compounds in
pharmaceutical, agrochemical, natural product, and advanced
materials chemistry.29–31 Due to the inherent advantages, several
heterogeneous catalysts for the Suzuki cross coupling reaction
have been developed. Particular attention is paid to the use of
aryl chlorides as substrates because they are much cheaper and
readily available than the usually used aryl bromides and
iodides. Therefore, the catalytic activity of heterogeneous cata-
lysts for the Suzuki cross coupling of aryl chlorides is consid-
erably challenged. Till date, heterogeneous examples for the
Suzuki cross coupling reaction of less reactive aryl chlorides are
quite rare.32
Based on the aforementioned considerations, herein, we
report the synthesis of a novel core–shell magnetic brous Pd/
Fe3O4@SiO2@KCC-1 nanocatalyst system, which has the
following advantages (Scheme 1): rst, the Pd/Fe3O4@SiO2@-
KCC-1 nanocatalyst exhibits high accessibility and excellent
catalytic activity for the reduction of 4-NP and the Suzuki cross
coupling reactions of aryl chlorides with aryl boronic acids;
second, the catalyst could be easily recovered from the reaction
mixture by a magnet because of the inner magnetic Fe3O4 core;
third, the as-prepared catalyst has a long-life, because it exhibits
high reusability without a visible decrease in catalytic
performance aer ve cycles. Therefore, this study on the
2. Experimental section
2.1. Materials
Tetraethoxysilane
(TOES),
3-aminopropyltriethoxysilane
(APTES), and Pd(II) acetate were purchased from Aladdin
Chemical Co., Ltd. 4-NP, 1-pentanol, urea, and aryl halide were
purchased from Lanzhou Aihua Chemical Company. Aryl
boronic acids were obtained from Shanghai Chemical Reagent
Co. Ltd. Organic solvents used were of analytical grade and did
not require further purication.
2.2. Synthesis of Fe3O4 and Fe3O4@SiO2 core–shell
microspheres
The Fe3O4 NPs and Fe3O4@SiO2 core–shell microspheres were
synthesized according to a previously reported method.33 FeCl3
(2.6 g, 16 mmol), trisodium citrate (1.0 g, 3.4 mmol), and
sodium acetate (NaAc) (4.0 g, 48.8 mmol) were dissolved in
ethylene glycol (80 mL) with magnetic stirring. The obtained
yellow solution was then transferred and sealed into a Teon-
lined stainless steel autoclave. The autoclave was heated at 200
ꢀC for 10 h, and then allowed to cool to room temperature. The
black products were isolated by strong magnetic suction, and
washed with ethanol and deionized water several times.
The Fe3O4@SiO2 nanospheres were prepared through a
versatile solution sol–gel method as follows: the Fe3O4 NPs (0.7
g) was dispersed to a three-neck round-bottom ask charged
with absolute ethanol (140 mL), deionized water (40 mL), and
ammonia solution (3 mL, 25%) by ultrasonication for 1 h. Then
the mixture was mixed for uniformity under mechanical stirring
for 15 min at room temperature. Subsequently, 2.0 mL TEOS
was added drop wise and the mixture was allowed to proceed for
12 h under strong mechanical stirring. The resultant core–shell
Fe3O4@SiO2 nanosphere product was separated by strong
magnetic suction, followed by washing with ethanol and
deionized water several times.
Pd/Fe3O4@SiO2@KCC-1 nanocatalyst system provides
a
useful platform for exploring environmentally friendly Pd
NPs based catalysts with superior activity, convenient
recovery, and reusability.
2.3. Synthesis of Fe3O4@SiO2@KCC-1 core–shell
microspheres
The Fe3O4@SiO2@KCC-1 core–shell microspheres were
synthesized according to the previously reported method
(Scheme 1).23 Fe3O4@SiO2 (0.25 g) was dispersed in an aqueous
solution (30 mL) containing urea (0.3 g) to form solution A
under ultrasonication for 1 h. CPB (0.5 g) was added to 0.75 mL
of n-pentanol and 30 mL cyclohexane to from solution B.
Solution A was added to solution B under stirring at room
temperature. Then 1.25 g TEOS was added drop wise to the
abovementioned solution. The resulting mixture was continu-
ally stirred for 1 h at room temperature and then placed into a
120 ꢀC environment for 5 h, thus inciting the reaction. Aer the
reaction was completed, the mixture was allowed to cool to
room temperature, and the Fe3O4@SiO2@KCC-1 core–shell
microspheres were isolated by strong magnetic suction, then
washed with deionized water and acetone, and dried overnight
in a drying oven at 40 ꢀC. This material was then calcined at
ꢀ
Scheme 1 Preparation of the Pd/Fe3O4@SiO2@KCC-1 nanocatalyst.
550 C for 5 h in air.
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mixtures were analyzed by gas chromatography (GC) or
GC-mass spectrometry (GC-MS).
2.4. Preparation of the Pd/Fe3O4@SiO2@KCC-1 nanocatalyst
The aminopropyl functionalized Fe3O4@SiO2@KCC-1 nano-
composite Fe3O4@SiO2@KCC-1-NH2 was synthesized by the
following procedure (Scheme 1): In a 100 mL round-bottom
ask, Fe3O4@SiO2@KCC-1 (0.6 g) and APTES (0.15 g) were
introduced into 50 mL toluene by ultrasonication for 1 h. The
mixture was reuxed for 12 h under a nitrogen protection. Aer
cooling to room temperature, the solution was isolated by
strong magnetic suction, and the solid was washed several
times with ethanol to remove the remaining non-supported
The recovery and reuse of the Pd/Fe3O4@SiO2@KCC-1
nanocatalyst is described below. A mixture of 1-iodo-4-nitro-
benzene (0.5 mmol), phenyl boronic acid (0.75 mmol), K2CO3
(1 mmol), EtOH (5.0 mL), and Pd/Fe3O4@SiO2@KCC-1 nano-
catalyst (0.2 mol% Pd) was stirred at 80 ꢀC under air. Aer
cooling to room temperature towards the end of the reaction,
the Pd/Fe3O4@SiO2@KCC-1 nanocatalyst was separated by a
powerful magnet. The separated catalyst was washed with
ethanol and water, dried under vacuum at room temperature,
and reused. The reaction mixtures were analyzed by GC or
GC-MS. This procedure was repeated ve times.
ꢀ
APTES. It was then dried in a drying oven at 40 C overnight,
from which the Fe3O4@SiO2@KCC-1-NH2 nanocomposites were
obtained.
The Pd/Fe3O4@SiO2@KCC-1 nanocatalyst was synthesized
by the following procedure. A 100 mL round-bottom ask was
charged with 0.5 g Fe3O4@SiO2@KCC-1-NH2 nanocomposite,
0.1 g Pd(OAc)2, 30 mL deionized water, and 20 mL acetonitrile,
aer which it was ultrasonically dispersed for 30 min. The
obtained suspension was stirred at room temperature for 2 h.
Subsequently, a fresh NaBH4 solution (0.2 M, 10 mL) was added
into the abovementioned suspension. Aer 2 h of reduction, the
product was isolated by strong magnetic suction, washed
repeatedly with deionizeꢀd water and ethanol, and dried over-
night in a vacuum at 30 C.
2.7. Characterization
The powder X-ray diffraction (XRD) spectra were obtained by a
Rigaku D/max-2400 diffractometer using Cu-Ka radiation in the
2q range of 10–90ꢀ. Fourier transform infrared (FT-IR) spectra
were collected by using KBr pellets on a Nicolet Nexus 670 FT-IR
spectrometer equipped with a deuterated triglycine sulfate
pyroelectric detector. Transmission electron microscopy (TEM)
images were obtained on a Tecnai G2 F30, FEI, USA. The UV-Vis
measurement was conducted with a UV2800PC UV-Vis spec-
trophotometer. The magnetic measurement of Fe3O4@SiO2@
KCC-1 and Pd/Fe3O4@SiO2@KCC-1 was performed using a
quantum design vibrating sample magnetometer (VSM) at room
temperature in an applied magnetic eld sweeping from ꢁ15 to
15 kOe. The reaction conversion was estimated using GC (P.E.
Auto System XL) or GC-MS (Shimadzu QP2010S).
2.5. General procedure for the reduction of 4-NP
The reduction of 4-NP by NaBH4 was chosen as a model reaction
to investigate the catalytic performance of the Pd/Fe3O4@
SiO2@KCC-1 nanocatalyst. In short, 2.8 mL of deionized water
and 30 mL of 4-NP (0.01 M) were mixed with 0.2 mL of a freshly
prepared aqueous NaBH4 solution (0.5 M), resulting in the
formation of a deep yellow solution. Then, 5 mL Pd/Fe3O4@
SiO2@KCC-1 nanocatalyst (5 mg mLꢁ1) was added to the
resulting solution, and the reaction was allowed to proceed
until the solution became colorless. The reaction conversion
was determined by ultraviolet visible spectroscopy (UV-Vis).
The reduction reactions were amplied 40ꢂ to characterize
the reusability of the Pd/Fe3O4@SiO2@KCC-1 nanocatalyst. The
catalyst was recovered using a powerful magnet, washed with
water and ethanol and dried in a vacuum at room temperature
before being used in the next catalytic run. The 4-NP catalytic
reduction process was carried out within 5 min for each cycle.
This procedure was repeated ve times.
3. Results and discussion
3.1. Catalyst preparation and characterization
As shown in Scheme 1, Fe3O4@SiO2@KCC-1 was rst func-
tionalized with APTES, then the aminopropyl groups acted as
adsorption centers for Pd2+. When the NaBH4 was added to the
solution, the Pd2+ ions were reduced and immobilized on the
bres of Fe3O4@SiO2@KCC-1.
Fig. 1a shows the FT-IR spectra of Fe3O4@SiO2@KCC-1
(black line) and Fe3O4@SiO2@KCC-1-NH2 (red line). The
adsorption peaks at 1096 and 805 cmꢁ1 correspond to
the antisymmetric and symmetric stretching vibrations of the
Si–O–Si bond in the oxygen–silica tetrahedron, respectively. The
peaks at 467 and 563 cmꢁ1 correspond to the Si–O and Fe–O
stretching, respectively. The strong peak at 3424 cmꢁ1 shows
the large number of Si–OH groups, which are proven to be
2.6. General procedure for the Suzuki cross coupling
reaction
Aer optimizing the reaction conditions (Table 2), Suzuki advantageous for the modication of APTES on the Fe3O4@
coupling reactions were performed by placing the aryl halide SiO2@KCC-1 surface by hydrogen bonds.4 The adsorption peak
(0.5 mmol), phenyl boronic acid (0.75 mmol), K2CO3 (1.0 at 2926 cmꢁ1 corresponds to the –CH stretching. In the FT-IR
mmol), EtOH (5.0 mL), and Pd/Fe3O4@SiO2@KCC-1 nano- spectrum of Fe3O4@SiO2@KCC-1-NH2, the peak around 3400
catalyst (0.2 mol% Pd) in a 10 mL round bottom ask. The cmꢁ1 represents the adsorption of the –OH and –NH2 groups.
mixture was stirred at 80 ꢀC for the required time under air. The nitrogen, hydrogen, and carbon contents were 0.70%,
Aer the completion of the reaction, the mixture was cooled to 0.86%, and 3.20%, respectively, measured by elemental anal-
room temperature and the Pd/Fe3O4@SiO2@KCC-1 nano- ysis. The FT-IR spectra and elemental analysis data revealed
catalyst was separated by a powerful magnet. The reaction that the APTES was successfully graed on the
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Fig. 2 (a) and (b) TEM images of Fe3O4@SiO2@KCC-1, (c) TEM
images of Pd/Fe3O4@SiO2@KCC-1 (inset image: size distribution
histogram and HRTEM image of the Pd NPs crystal structure in detail
on Fe3O4@SiO2@KCC-1).
in Fig. 2a, Fe3O4@SiO2@KCC-1 possesses the core of Fe3O4 NPs
and silica bres. The average diameter of Fe3O4@SiO2@KCC-1
Fig. 1 (a) FT-IR spectra of Fe3O4@SiO2@KCC-1 and Fe3O4@SiO2@ and the Fe3O4 core were 350 nm and 100 nm, respectively. The
KCC-1-NH2, (b) the XRD of Fe3O4@SiO2@KCC-1 and Pd/Fe3O4@
SiO2@KCC-1.
TEM images shown in Fig. 2b further conrm that the distance
between the two bers is about 18 nm (Fig. 2b). The larger
distance between the bers leads to the easy loading of the Pd
NPs on Fe3O4@SiO2@KCC-1, resulting in the signicant
Fe3O4@SiO2@KCC-1 surface, thus enabling them to act as increase in the accessibility of the active sites on the
robust anchors for metal NPs.
Pd/Fe3O4@SiO2@KCC-1 nanocatalyst. The TEM images of the
The XRD patterns of the Fe3O4@SiO2@KCC-1 and Pd/Fe3O4@ Pd/Fe3O4@SiO2@KCC-1 nanocatalyst are shown in Fig. 2c. The
SiO2@KCC-1 samples are shown in Fig. 1b. The broad peak Pd NPs were remarkably dispersed on the surface of the core–
between 20ꢀ and 30ꢀ corresponds to amorphous silica, indicating shell magnetic brous Fe3O4@SiO2@KCC-1. The Pd NPs have a
that the silica was successfully coated on the surface of the Fe3O4 narrow size distribution with a mean particle size of about
core. The XRD pattern of the Pd/Fe3O4@SiO2@KCC-1 shows the 4 nm (Fig. 2c, inset). From the inset picture of Fig. 2c, it can be
characteristic peaks of magnetite NPs. The sharp and strong peaks seen that the resolved lattice fringes of the Pd (111) planes
conrm that the products are well crystallized.
(d ¼ 0.225 nm) detected in the HRTEM image were attributed to
The Fe3O4 NPs show ve characteristic diffraction peaks at 2 the Pd nanocrystals.
theta ¼ 30.3ꢀ, 35.6ꢀ, 43.2ꢀ, 57.2ꢀ, and 63ꢀ corresponding to
The elemental composition of the Pd/Fe3O4@SiO2@KCC-1
(220), (311), (400), (511) and (440), respectively. The character- samples was determined by EDX analysis and X-ray photoelec-
istic diffraction peak at 39.8ꢀ corresponds to Pd (111). The XRD tron spectroscopy (XPS) (Fig. 3). The EDX spectrum shown in
pattern of Pd/Fe3O4@SiO2@KCC-1 demonstrates that the Pd Fig. 3a reveals that the as-prepared products contain Fe, Si, Pd,
NPs were successfully immobilized on the Fe3O4@SiO2@KCC-1 Cu, C, and O, respectively. Among these elements, Cu and C are
surface. The weight percentage of Pd in the Pd/Fe3O4@SiO2@ generally inuenced by the copper network support lms and
KCC-1 nanocatalyst, as determined by atomic absorption spec- their degree of oxidation; the Si, O, Fe and Pd signals result
troscopic analysis, was 9.01 wt%.
from the Pd/Fe3O4@SiO2@KCC-1 nanocatalyst.
The morphologies and structural features of the core–shell
XPS was performed to investigate the chemical state of the
magnetic brous Fe3O4@SiO2@KCC-1 and Pd/Fe3O4@SiO2@ surface of the obtained Pd/Fe3O4@SiO2@KCC-1 nanocatalyst
KCC-1 nanocatalysts were analyzed by TEM. The as-prepared (Fig. 3b). The XPS elemental survey scan of the surface of the
core–shell magnetic brous Fe3O4@SiO2@KCC-1 with a brous Pd/Fe3O4@SiO2@KCC-1 nanocatalyst revealed the presence of
structure was uniform and monodispersed (Fig. 2a). As is shown the Si, O, Fe, Pd, N, and C elements in the samples. As shown in
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Fig. 3 (a) EDS spectrum of Pd/Fe3O4@SiO2@KCC-1, (b) XPS spectrum of Pd/Fe3O4@SiO2@KCC-1 (inset image: high resolution spectrum of Fe
2p and Pd 3d).
the inset of Fig. 3b, the peaks observed in the XPS spectra of Fe
2p at the binding energies of 711 and 724.5 eV are characteristic
of Fe3O4. Moreover, the inset of Fig. 3b shows the XPS spectrum
of Pd 3d5/2 and Pd 3d3/2 at the binding energies of 335 and 340.5
eV, respectively.
The magnetic measurements were carried out by VSM at
room temperature. The magnetization curves measured for
Fe3O4@SiO2@KCC-1 and Pd/Fe3O4@SiO2@KCC-1 are shown in
Fig. 4. The magnetic saturation values of Fe3O4@SiO2@KCC-1
and Pd/Fe3O4@SiO2@KCC-1 are 20.4 and 12 emu gꢁ1, respec-
tively. The decrease in the saturation magnetization was
due to the presence of the APTES and Pd nanoparticles on the
Fe3O4@SiO2@KCC-1 surface. Moreover, Fig. 4 (inset image)
shows the separation–redispersion process of the Pd/Fe3O4@
SiO2@KCC-1 nanocatalyst, which demonstrates that the catalyst
is drawn from the solution to the sidewall of the vial by applying
Fig.
4 Room temperature magnetization curves of the Fe3O4@
an external magnetic eld. Therefore, the abovementioned
results indicated an easy and efficient way to separate and
recycle the Pd/Fe3O4@SiO2@KCC-1 nanocatalyst from the
solution by an external magnetic eld.
SiO2@KCC-1 and Pd/Fe3O4@SiO2@KCC-1 nanocatalysts (inset image:
the separation–redispersion process of the Pd/Fe3O4@SiO2@KCC-1
nanocatalyst by an external magnetic field).
3.2. Catalytic reduction of 4-NP
UV-Vis absorption spectroscopy. As shown in Fig. 5a, the pure
The catalytic activity of the Pd/Fe3O4@SiO2@KCC-1 nano- 4-NP solution exhibited a distinct absorption maximum at 317
catalyst was studied by the reduction reaction of 4-NP to 4-AP in nm, which shis to 400 nm in the presence of an alkali because
presence of NaBH4. The reduction process was monitored by of the formation of 4-nitrophenolate ions.34 The reduction of 4-
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transferred a surface-hydrogen species to the surface of the Pd
NPs. Simultaneously, the 4-nitrophenolate were also adsorbed
on the surface of the Pd NPs resulting in the rapid occurrence
and completion of the reduction reaction.36,37 The addition of
the Pd/Fe3O4@SiO2@KCC-1 nanocatalyst into the reaction
mixture led to the decrease in the peak intensity of the
4-nitrophenolate ion at 400 nm with a concomitant increase in
the peaks corresponding to 4-AP at 300 nm (Fig. 5b), reecting
the decay of 4-NP and formation of 4-AP. Thus, the conversion
of 4-NP to 4-AP can be monitored by tracking the changes in the
absorbance peak of the 4-nitrophenolate ions at 400 nm.38 The
absorbance peak value of 4-nitrophenolate ions at 400 nm
decreased to zero, indicating the successful conversion of 4-NP
to 4-AP. Fig. 5b shows the complete reduction of 4-NP to 4-AP
aer 275 seconds. The concentration of NaBH4 was very high
compared with that of 4-NP, and the reaction followed pseudo-
rst-order reaction kinetics.22 Therefore, we used pseudo-rst-
order kinetics to evaluate the kinetic rate constant (kapp) of the
current reaction [eqn (1)]. The reaction conversion was calcu-
lated from Ct/C0 (Fig. 5c), which was measured using the relative
intensity of the UV-Vis absorbance (At/A0) at 400 nm. Ct and C0
are the concentrations of the 4-NP at reaction time t and the
initial stage, respectively.
dCt/dt ¼ ꢁkappt, or ln(Ct/C0) ¼ ln(At/A0) ¼ ꢁkapp
t
(1)
As shown in Fig. 5c, the linear correlation between ln(Ct/C0)
and reaction time (t) was conrmed through pseudo-rst-order
kinetics. We calculated the apparent kinetic rate constant kapp
to be 1.96 ꢂ 10ꢁ2 for the Pd/Fe3O4@SiO2@KCC-1 catalyzed
reductions of 4-NP. Then, kapp was normalized to the concen-
tration of Pd, deriving knor to reveal the intrinsic catalytic
activity of the catalyst (knor ¼ kapp/c(Pd)).14 We compared the
activity of Pd/Fe3O4@SiO2@KCC-1 with Pd catalysts loaded on
different supports for the reduction of 4-NP (Table 1). In Table 1,
Pd/Fe3O4@SiO2@KCC-1 shows excellent catalytic activity for the
reduction of 4-NP by NaBH4, with the highest knor (2.78 sꢁ1
mMꢁ1) among the Pd catalysts loaded on different supports:
SBA-15,39 PEDOT (conducting polymer),40 Microgel-PS (microgel
coated polystyrene),41 FG (functionalized grapheme nano-
hybrids),42 PiHP (hyper-branched polymer),43 and Al2O3.44 It is
also higher than the Pd nanoclusters45 and Pd nanocrystals46
based catalysts. The excellent catalytic activity of the
Pd/Fe3O4@SiO2@KCC-1 nanocatalyst is owed to the ease of
access of its active sites, and the low aggregation of the Pd NPs
on the Fe3O4@SiO2@KCC-1 support. In this situation, the
reactant can be easily adsorbed on the Pd NPs surface, which
allows the reduction reaction to start quickly and nish rapidly.
Fig. 5 (a) UV-Vis spectra of 4-NP before and after adding NaBH4
solution, (b) the successive reduction of 4-NP to 4-AP over the
prepared Pd/Fe3O4@SiO2@KCC-1 nanocatalyst, (c) plot of Ct/C0 and
ln(Ct/C0) versus reaction time for the reduction of 4-NP over
Pd/Fe3O4@SiO2@KCC-1.
NP to 4-AP can be obtained in the presence of NaBH4 without
any catalyst, but the time required is about 430 min.35
Furthermore, when the support Fe3O4@SiO2@KCC-1 was added
to the 4-NP and NaBH4 solution, it was found that, the color of
the reaction mixture did not change even aer 120 min, sug-
3.3. Catalyst testing for the Suzuki cross coupling reaction
gesting that the Fe3O4@SiO2@KCC-1 support was almost inac- The abovementioned results revealed that the Pd/Fe3O4@
tive for the reduction reaction (Fig. S1†). Because of this, the SiO2@KCC-1 nanocatalyst exhibited excellent properties for the
Pd/Fe3O4@SiO2@KCC-1 nanocatalyst was added in order to reduction of 4-NP. We further explored the catalytic activity of
signicantly shorten the reaction time. When the Pd/Fe3O4@ the Pd/Fe3O4@SiO2@KCC-1 nanocatalyst for the Suzuki cross
SiO2@KCC-1 nanocatalyst was added into the reaction mixture, coupling reactions. To explore the optimal reaction conditions,
the BH4ꢁ ions were adsorbed on the surface of the Pd NPs and a series of reactions was performed using several time
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Table 1 Comparison of the apparent kinetic rate constant (kapp) and normalized rate constants (knor) of Pd catalysts loaded on different supports
for the reduction of 4-NP
kapp (sꢁ1
)
knor (sꢁ1 mMꢁ1
)
Ref.
b
c
Samples
c(4-NP)a (mM)
c(Pd)a (mM)
Pd/SBA-15
Pd/PEDOT
Pd/microgel-PS
Pd/FG
Pd nanoclusters
Pd/PiHP
Pd nanocrystals
Pd/Al2O3
Pd/Fe3O4@SiO2@KCC-1
0.1
6.29 ꢂ 10ꢁ2
2.52 ꢂ 10ꢁ1
2.15 ꢂ 10ꢁ3
4.72 ꢂ 10ꢁ3
1.6
11.8 ꢂ 10ꢁ3
65.8 ꢂ 10ꢁ3
1.50 ꢂ 10ꢁ3
2.35 ꢂ 10ꢁ3
7.89 ꢂ 10ꢁ4
20 ꢂ 10ꢁ3
0.118
0.261
0.698
0.498
0.005
0.222
0.134
1.085
2.78
39
40
41
42
45
43
46
44
0.1
5.8 ꢂ 10ꢁ2
2
2.3
9 ꢂ 10ꢁ2
8 ꢂ 10ꢁ2
3.6 ꢂ 10ꢁ2
8.48 ꢂ 10ꢁ3
7.06 ꢂ 10ꢁ3
4.83 ꢂ 10ꢁ3
9.2 ꢂ 10ꢁ3
19.6 ꢂ 10ꢁ3
0.1
0.1
This work
a
c: concentration. b kapp: apparent rate constant. c knor: rate constant normalized to the molar concentration of Pd. Data were given or calculated in
the respective papers; some data were not obtained.
durations, solvents, bases, and temperatures to obtain the best respectively (Table 2: entries 3, 8–10). For the reactions at room
possible combination. Initially, the experiments were per- temperature, 40 ꢀC, and 60 ꢀC corresponding products were
formed using the Suzuki cross-coupling reaction of 1-iodo-4- obtained in the following yields: 54%, 78.8%, and 90.8% (Table
nitrobenzene (0.5 mmol) with phenyl boronic acid (0.75 mmol) 2: entries 11–13). The best catalytic activity of the Pd/Fe3O4@
in presence of Pd/Fe3O4@SiO2@KCC-1 (0.2% Pd) and 5 mL SiO2@KCC-1 nanocatalyst was observed using ethanol as the
ꢀ
ethanol in air at 80 ꢀC for 1 h, 2 h, and 3 h. The corresponding solvent, and K2CO3 as the base at 80 C (Table 2: entry 3).
products were obtained in the following yields: 91.0%, 94.0%, Aer screening the reaction conditions, the catalyst system
and 97%, respectively (Table 2: entries 1–3). The reaction was studied for the Suzuki cross coupling reaction of various
product with the highest yield was obtained at 3 h, therefore the aryl halides and aryl boronic acids (Table 3). As listed in Table 3,
reaction time of 3 h was chosen. As it is known, the solvent plays when the coupling of aryl iodides with aryl boronic acids pro-
a crucial role in the rate of Suzuki coupling reactions. When the ceeded at 80 ꢀC for 3 h, the corresponding products were
reactions were carried out in methyl alcohol, ethyl acetate, obtained in high yields (Table 3: entries 1–8). The Pd/Fe3O4@
deionized water, and acetone under the same conditions, the SiO2@KCC-1 nanocatalyst also revealed excellent catalytic
products were obtained in poor to moderate yields of 85.0%, activity for the coupling between aryl bromides and aryl boronic
79.6%, 27.8%, and 30%, respectively (Table 2: entries 4–7). The acids as the reaction time approached 6 h (Table 3: entries
reactions were carried out under similar conditions using 9–18). However, for the reactions of 1-bromo-2-nitrobenzene,
different bases such as K2CO3, CH3COONa, NaOH, and poor yields (Table 3, entries 13 and 18) were obtained due to the
(C2H5)3N; the yields were 97%, 80.3%, 90.1%, and 80.9%, electron-withdrawing –NO2 group in the ortho position. It is
Table 2 The effects of solvent, base and temperature on the Suzuki cross coupling of 1-iodo-4-nitrobenzene with phenyl boronic acid using the
Pd/Fe3O4@SiO2@KCC-1 nanocatalyst
Entry
Solvent
Base
Temp (ꢀC)
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
Ethanol
Ethanol
Ethanol
Methyl alcohol
Ethyl acetate
Deionized water
Acetone
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
CH3COONa
NaOH
(C2H5)3N
K2CO3
K2CO3
K2CO3
80
80
80
80
80
80
80
80
80
80
25
40
60
1
2
3
3
3
3
3
3
3
3
3
3
3
91.0
94.0
97.0
85.0
79.6
27.8
30.0
80.3
90.1
80.9
54.0
78.8
90.8
9
10
11
12
13
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Table 3 Suzuki cross coupling reactions of aryl halides with aryl boronic acids using the Pd/Fe3O4@SiO2@KCC-1 nanocatalysta
Paper
Entry
Aryl halide
Aryl boronic acid
Time (h)
Yield (%)
98.9
1
2
3
3
96.8
3
3
3
3
3
3
3
6
6
6
6
6
6
6
6
6
99.0
96.0
99.0
97.8
99.0
97.0
89.0
91.8
90.6
86.8
26.2
90.8
93.2
91.8
87.3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
6
28.1
a
Reaction condition: aryl halide (0.5 mmol), aryl boronic acid (0.75 mmol), K2CO3 (1.0 mmol), EtOH 5.0 mL, Pd/Fe3O4@SiO2@KCC-1 nanocatalyst
(0.2 mol% of Pd), and 80 ꢀC, in air. Yield was determined by GC-MS or GC analysis.
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noteworthy that aryl iodides and aryl bromides containing between aryl chlorides and aryl boronic acids. Few heteroge-
electron-donating groups, such as –CH3, –OH, –NH2 were found neous catalyst systems show good activity toward aryl chlorides.
to be more active than the aryl iodides and aryl bromides con- The highly dispersed smaller Pd NPs with easily accessible
taining electron-withdrawing, such as –NO2 (Table 3).
active sites, are sustainable substrates for Suzuki cross coupling
Although the catalyst was active for aryl iodides and aryl reactions, under ligand-free conditions.49
bromides, our main objective was to design a nanocatalyst for
the Suzuki coupling of challenging substrates, that is, the
Suzuki coupling reactions of aryl chlorides with aryl boronic
3.4. Reusability of the Pd/Fe3O4@SiO2@KCC-1 nanocatalyst
acids. In our initial attempts, we used the same optimized
conditions for the aryl iodides and aryl bromides, but the
products were obtained in low yields.
For the practical application of heterogeneous systems, the
recyclability of the catalyst is an important factor. The stability
and reusability of the Pd/Fe3O4@SiO2@KCC-1 nanocatalyst
were performed by catalyzing the reduction of 4-NP and the
Suzuki coupling reaction of 1-iodo-4-nitrobenzene with phenyl
boronic acid.
As shown in Fig. 6, the recovered Pd/Fe3O4@SiO2@KCC-1
nanocatalyst exhibited almost constant catalytic activity for at
least ve successive cycles in the reduction of 4-NP and Suzuki
cross coupling reaction of 1-iodo-4-nitrobenzene with phenyl
boronic acid. Moreover, the catalyst was easily separated
magnetically from the reaction mixture, washed with ethanol
and deionized water, and nally dried for the next run. This
result suggests that the Pd/Fe3O4@SiO2@KCC-1 nanocatalyst
possesses robust stability.
Therefore, we optimized several reaction parameters,
including the solvent, reaction temperature mentioned in
previously studied reports,47,48 and found that the use of
tetra-n-butylammonium bromide (TBAB) and changing the
solvent from ethanol to (n-methyl-2-pyrꢀrolidone) NMP and
raising the reaction temperature to 140 C was necessary to
achieved good yields. Aer optimizing the reaction condi-
tions, the Pd/Fe3O4@SiO2@KCC-1 nanocatalyst was studied
for Suzuki cross coupling reactions of several aryl chlorides
with aryl boronic acids, which are summarized in Table 4.
Interestingly, the Pd/Fe3O4@SiO2@KCC-1 nanocatalyst also
revealed excellent catalytic activity for the Suzuki coupling
Table 4 The Suzuki cross coupling reactions of aryl chlorides with aryl boronic acids using the Pd/Fe3O4@SiO2@KCC-1 nanocatalysta
Entry
Aryl chlorides
Aryl boronic acid
Time (h)
Yield (%)
83.2
1
2
12
12
86.0
3
4
12
12
85.8
80
5
6
7
12
12
12
83.8
87.8
86.2
8
12
79.8
a
Reaction conditions: aryl chlorides (0.5 mmol), aryl boronic acid (0.75 mmol), K2CO3 (1.0 mmol) were added in 5 mL NMP at 140 ꢀC for 12 h in the
presence of TBAB (0.05 mmol) and Pd/Fe3O4@SiO2@KCC-1 (2.5 mol% of Pd). Yield was determined by GC-MS or GC.
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Paper
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9 E. Antolini, Energy Environ. Sci., 2009, 2, 915–931.
10 H. Lee, S. E. Habas, S. Kweskin, D. Butcher, G. A. Somorjai
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Fig. 6 The reusability of the Pd/Fe3O4@SiO2@KCC-1 nanocatalyst for
the reduction of 4-NP by NaBH4 and Suzuki cross coupling reaction of
1-iodo-4-nitrobenzene with phenyl boronic acid.
4. Conclusion
17 G. Yuan and M. A. Keane, Ind. Eng. Chem. Res., 2007, 46, 705–
715.
In this study, a novel core–shell magnetic brous Fe3O4@
SiO2@KCC-1 based Pd/Fe3O4@SiO2@KCC-1 nanocatalyst
with easily accessible active sites was successfully synthe-
sized. The catalyst could be easily recovered by applying an
external magnetic eld, and reused for next catalytic run. The
Pd/Fe3O4@SiO2@KCC-1 nanocatalyst showed excellent cata-
lytic activity for the reduction of 4-NP and the Suzuki
coupling reaction for a range of aryl bromides and iodides
with aryl boronic acids. Notably, the catalyst also displayed
good catalytic activity for the Suzuki coupling reaction of aryl
chlorides with aryl boronic acids. Moreover, the catalyst was
chemically stable and could be easily recycled for at least ve
times in the corresponding reaction without reduction in the
catalytic activity. The Pd/Fe3O4@SiO2@KCC-1 nanocatalyst
acts as a relatively green catalyst with superparamagnetism,
eco-friendly nature and convenient recovery, and is a prom-
ising candidate for Pd NPs based catalytic applications in
industrial syntheses.
18 S. Wei, Z. Ma, P. Wang, Z. Dong and J. Ma, J. Mol. Catal. A:
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The authors acknowledge the nancial support from the NSFC
(Grants 21301082) and the Natural Science Foundation of
Gansu (no. 1308RJYA028).
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