Facile synthesis, characterization and application of highly active palladium nano-network structures supported on electrospun carbon nanofibers

Article abstract of DOI:10.1039/c4ra07632j

Palladium nano-network structures supported on electrospun carbon nanofibers (Pd-NNSs-ECNFs) were successfully prepared through a novel K₂Pd^IICl₄/K₄Fe^II(CN)₆ cyanogel method. The Pd-NNSs have the features and properties of small particle size, low dimensionality, quantum effects and high stability. SEM, TEM and XPS were used to characterize the Pd-NNSs-ECNFs. The Pd-NNSs-ECNFs were used as a catalyst in different types of Suzuki coupling reactions to evaluate the catalytic abilities. The results showed that the heterogeneous catalyst (Pd-NNSs-ECNFs) had a high catalytic activity (high yields to the products) to the Suzuki coupling reactions in an environmentally friendly solvent system (ethanol/H₂O). The catalyst can be recycled by filtration, and reused seven times without losing the catalytic activity. This research opens new applications of Pd-NNSs-ECNFs in the area of green chemistry. This journal is

Full text of DOI:10.1039/c4ra07632j

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Facile synthesis, characterization and application of
highly active palladium nano-network structures
supported on electrospun carbon nanofibers†
Cite this: RSC Adv., 2014, 4, 42732
*
Xinwen Peng, Wan Ye, Yichun Ding, Shaohua Jiang, Muddasir Hanif, Xiaojian Liao
*
and Haoqing Hou
Palladium nano-network structures supported on electrospun carbon nanofibers (Pd-NNSs-ECNFs) were
successfully prepared through a novel K2PdIICl4/K4FeII(CN)6 cyanogel method. The Pd-NNSs have the
features and properties of small particle size, low dimensionality, quantum effects and high stability. SEM,
TEM and XPS were used to characterize the Pd-NNSs-ECNFs. The Pd-NNSs-ECNFs were used as a
catalyst in different types of Suzuki coupling reactions to evaluate the catalytic abilities. The results
showed that the heterogeneous catalyst (Pd-NNSs-ECNFs) had a high catalytic activity (high yields to the
products) to the Suzuki coupling reactions in an environmentally friendly solvent system (ethanol/H2O).
The catalyst can be recycled by filtration, and reused seven times without losing the catalytic activity.
This research opens new applications of Pd-NNSs-ECNFs in the area of green chemistry.
Received 25th July 2014
Accepted 29th August 2014
DOI: 10.1039/c4ra07632j
www.rsc.org/advances
prepared Pd-NNSs had high electrocatalytic activity and good
electrochemical self-stability.24
1. Introduction
In this work, we highlight the novel Pd nano-network
structures supported on electrospun carbon nanobers
(Pd-NNSs-ECNFs) as catalyst for Suzuki coupling reactions.
Cyanogels method for the preparation of the Pd-NNSs-ECNFs
catalyst is emphasized. The structures of Pd-NNSs-ECNFs were
characterized by SEM, TEM and XPS. The obtained Pd-NNSs-
ECNFs catalyst showed good activity, highly efficient and cyclic
utilization as heterogeneous catalysts for the Suzuki coupling
under the environmentally friendly solvents.
Palladium (Pd) nanoparticles have received intense interest
because of their fascinating properties and potential applica-
tions in diverse elds such as hydrogen storage,1 organic
synthesis,2,3 fuel cells,4 sensors5 and electrochemical detection,
etc.6 Due to the excellent catalytic activities, Pd nanoparticles
have been widely used in organic synthesis and electro-
catalysis.7–13 Pd nanoparticles could be supported on a number
of materials such as carbon materials,14,15 polymers,16–18 and
mesoporous inorganic materials.19,20 The heterogeneous Pd
nanoparticles show good catalytic abilities and the supported
structure is benet for the recovery and recycle use. Compared
with the traditional Pd nanoparticles, palladium nano-network
structures (Pd-NNSs) have features and properties of small
particle size, low dimensionality, quantum effect and high
stability.21–23 Pd-NNSs could be produced through a hydrogel
method.24–27 The hydrogels were prepared by mixing the K2RCl4
(R ¼ Pd/Pt) and Kx[M(CN)n] (n ¼ 4, 6; M ¼ Co, Fe, Ru, Os, Ni, Cr)
in polymer solutions and the mixture of the coordination
polymers was also called cyanogels. Cyanogels were a special
class of three-dimensional double-metal cyanides for
preparing the Pd-NNSs and other metal or alloys nano-
structures. Zhang et al. had fabricated Pd-NNSs by using
NaBH4 to reduce the K2PdIICl4/K4FeII(CN)6 cyanogel.24 The
2. Experimental
2.1. Materials
Polyacrylonitrile (PAN, Mw
¼
150 000) and anhydrous
N,N-dimethylformamide (DMF, 99.8%) were purchased from
Sigma-Aldrich. Pd/C catalysts (wetted with ca. 55% water) con-
taining 10 wt% of palladium, K2PdCl4 (99%), K4Fe(CN)6 (98.5%)
and NaBH4 (98%) were purchased from Alfa Aesar. Potassium
carbonate (K2CO3, 99%), ethanol (99.5%), phenylboronic acid
(99%), 4-nitrobenzene, 4-chlorobenzene (99%), 4-iodobenzene
(97%), 4-bromobenzene (99%), 4-nitrophenylboronic acid
(98%), 4-methoxyphenylboronic acid (98%) and 4-methox-
ybenzene (98%) were obtained from J&K chemical Ltd.
Chemistry and Chemical Engineering College, Jiangxi Normal University, Nanchang,
Jiangxi 330022, P.R. China. E-mail: s.jiang19830913@gmail.com; haoqing@jxnu.
edu.cn; Tel: +86 791 88120740
2.2. Preparation of electrospun carbon nanobers (ECNFs)
ECNFs were prepared by carbonizing the electrospun PAN
nanobers. PAN nanobers were prepared according to
previous report in our group.12 10 wt% PAN/DMF solution was
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra07632j
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electrospun to nanobers in an electric eld of 100 kV mꢀ1, by Hz respectively on a Bruker Avance-400 spectrometers. FT-IR
applying a 25 kV voltage to a 25 cm gap between the spinneret spectra were recorded by Perkin-Elmer SP one FT-IR. Elemental
and the collector. The carbonized electrospun nanobers were analyses were taken by a Perkin-Elmer model 2400 CHN analyses.
prepared in high temperature furnace with following steps: (1) Inductively coupled plasma atomic emission spectroscopy (ICP-
PAN nanobers were heated to 230 ꢁC (1 ꢁC minꢀ1) and AES) (Optima 7300DV, Perkin-Elmer) was employed to measure
annealing for 2 h in air for the pre-oxidation; (2) heating up to the concentrations of Palladium. The quantity of Pd-NNSs-ECNFs
800 ꢁC at a heating rate of 5 ꢁC minꢀ1 and annealing for 30 min was measured by the way of ICP (Varian, 725-ES).
for the carbonization.
In order to improve the hydrophilicity, the ECNFs were
activated by mixing the ECNFs and nitric acid (40 vol%) in a
3. Results and discussion
round bottom ask under ultrasonic for 5 min and then heated
3.1. Preparation of Pd-NNSs-ECNFs
at 90 C for 2 h.28
ꢁ
The activation of ECNFs by nitric acid solution (40 vol%) for 2 h
at 90 ꢁC improved their hydrophilicity29,30 and so that the
aqueous solution could easily permeate into the carbon nano-
ber mat. The amount of Pd-NNSs-ECNFs could be tuned by
controlling the pumping cyanogel time (Fig. 1). The amount of
Pd-NNSs inside of electrospun carbon nanober mat was in
proportional to the pumping time. The quantity of Pd-NNSs
supported on ECNFs were measured by the Inductively Coupled
Plasma (ICP) method. 0.5 g Pd-NNSs-ECNFs were put in 60 mL
aqua regia, then vigorously stirring for 60 min at room
temperature. Then the solution was diluted, and the quantity of
palladium was measured by ICP.
2.3. Synthesis of Pd-NNSs-ECNFs
10 mL of 50 mM K2PdIICl4 aqueous solutions, 1 mL of PVP
aqueous solution (10 wt% in water) and 5 mL of 50 mM K4-
FeII(CN)6 aqueous solutions were mixed at 10–15 ꢁC. Then, a
piece of activated ECNFs was immersed into the above mixture
solution and pumped by vacuum pump for 10 min until the
ECNFs were completely wetted. Aer stood for 20 min, the
mixture was further treated by vacuum pumping for 10–110 min,
then the orange-red K2PdIICl4/K4FeII(CN)6 cyanogel was formed.
Aer that, 25 mL of 0.5 M NaBH4 solution was added to the
orange-red K2PdIICl4/K4FeII(CN)6 cyanogel and the resulting
cyanogel was stood for an additional 1 h. Finally, the mixture was
ltered, washed and dried in a vacuum oven at 50 ꢁC for 12 h.
3.2. Structural characterization
The structure and morphology of the Pd-NNSs-ECNFs were
characterized by the scanning electron microscopy (SEM, Fig. 2)
2.4. Catalysis of Pd-NNSs-ECNFs for Suzuki coupling
The obtained Pd-NNSs-ECNFs were used as catalysts for Suzuki
coupling reactions. The reaction procedure is described as
follows, and all reactions were carried out under nitrogen
atmosphere. 7 mL of the ethanol, 7 mL of the deionized water,
1.5 mmol NaOH, 1 mmol of the aryl halide, 1.25 mmol phe-
nylboronic acid, 0.5 mol% Pd-NNSs/ECNFs were placed in a 50
ꢁ
mL ask and heated at 80 C for 10 h with stirring (500 rpm).
And then the above mixture was ltered at room temperature.
The ltrate was extracted with ethyl acetate for three times and
the organic phase was added anhydrous Na2SO4 to absorb
residual water. Finally, the solution was ltered and concen-
trated in vacuum. The products were obtained by column
chromatography using ethyl acetate/petroleum ether. The lter
cake (Pd-NNSs-ECNFs) was washed with distilled water for
recycle use.
Fig. 1 The quantity change of Pd-NNSs supported on CNFs with
pumping cyanogel time.
2.5. Characterizations
X-ray photoelectron spectra (XPS) were recorded on an Escalab-
MKII spectrometer with a MoKa X-ray as excitation source.
Scanning electron microscope (SEM) was performed on SEM,
TESCAN vega 3. The transmission electron microscope (TEM)
was conducted by a JEOL 2000 transmission electron micro-
scope operating at 200 kV. The samples for TEM characteriza-
tion were prepared by placing a small piece of ultrathin
Pd-NNSs-ECNFs mat on the copper grid and dropping a drop of
ethanol on the sample for a good “adhesive” between the mat
and the grid. Proton and carbon nuclear magnetic resonance
spectra (1H and 13C-NMR) were measured at 400 MHz and 100 Fig. 2 SEM images of the Pd-NNSs-ECNFs.
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Fig.
3 TEM images of the Pd-NNSs supported on ECNFs with
increasing resolution (a to d).
and transmission electron microscopy (TEM, Fig. 3). The
Pd-NNSs were located inside of the ECNFs so that they were
difficult to drop out. Pd-NNSs were seen to be dispersed in the
carbon nanobers in Fig. 3. The transmission electron micro-
scopic (TEM) images (Fig. 3) showed Pd-NNSs were composed of
small spherical Pd-NPs (about 5 nm). These small primary NPs
are interconnected to one another to form larger secondary 3D
networks nanostructures with abundant pores.
Fig. 4 XPS spectra of Pd-NNSs: total spectrum (a) and enlarged Pd 3d
spectrum (b) supported on the ECNFs.
3.3. XPS spectra of Pd-NNSs
Scheme 1 Suzuki coupling reactions of aryl halide with phenylboronic
X-ray photoelectron spectroscopy (XPS) measurements were acid, with sodium hydroxide in ethanol/water, catalysed by Pd-NNSs-
ECNFs respectively.
shown in Fig. 4a and deconvoluted XPS spectra of the Pd-NNSs
were plotted in the Fig. 4b. XPS spectra showed a big peak at 285
eV for the Cls and two small peaks at 334 eV and 342 eV, for the
Pd 3d regions.24 The spectra also indicated that the PdII was
Table 1 Suzuki–Miyaura coupling of 4-chlorobenzene catalysed by
successfully reduced to the metallic Pd0. XPS spectra further
indicated that very little Pd-NNSs are loaded on the surface of
the carbon nanobers and mainly embedded in the carbon
nanobers.24 This happens because the K2PdIICl4/K4FeII(CN)6
cyanogel could intrude inner bers and aer reduction Pd-NNSs
on the outer surface could easily wash away leaving behind the
rmly embedded Pd-NNSs inside the bers.
Pd-NNSs-ECNFsa
Entry
Base
Temperature
Time
Yieldb
1
2
3
4
5
6
7
K2CO3
K3PO4
Cs2CO3
Na2CO3
NaOH
KOH
80 ꢁ
80 ꢁ
80 ꢁ
80 ꢁ
80 ꢁ
C
C
C
C
C
10 h
10 h
10 h
10 h
10 h
10 h
10 h
65%
45%
56%
41%
91%
92%
93%
3.4. Catalytic characteristics of Pd-NNSs-ECNFs
Suzuki coupling reaction catalysed by Pd is an important and
powerful strategy for the formation of carbon–carbon (C–C)
bonds.31,32 In this study, in order to evaluate the catalytic
activity, the Pd-NNSs-ECNFs were used as catalysts in the Suzuki
(C–C coupling) reaction between aryl halide and phenylboronic
acid, as shown in the Scheme 1.
80 ꢁC
80 ꢁ
NaOMe
C
a
Reaction conditions: 4-chlorobenzene (1 mmol), phenylboronic acid
(1.5 mmol), base (3.0 mmol), deionized water (8 mL), ethanol (8 mL)
b
and 0.5 mol% Pd-NNSs-ECNFs, 80 ꢁC, under N2. Determined by GC
In Suzuki coupling reaction, the various bases have big
inuence on the efficiency.11 Therefore, we rstly optimized the
analysis with an internal standard.
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Table 2 Suzuki–Miyaura reactions of various aryl halides catalysed by Pd-NNSs-ECNFsa
Entry
Time
Yieldb (%)
Product
Catalyst recycled
1
2
10 h
10 h
91–86%
97–92%
7 times
7 times
3
10 h
10 h
10 h
10 h
10 h
10 h
99–96%
96–91%
98–93%
<3%
7 times
7 times
7 times
—
4
5
6c
7d
8e
32%
—
95–90%
7 times
9f
10 h
97–92%
7 times
a
Reaction conditions: aryl halide (1.0 mmol), phenylboronic acid (1.5 mmol), NaOMe (3.0 mmol), ethanol (8 mL), water (8 mL), and 0.5 mol%
b
c
d
Pd-NNSs-ECNFs, 80 ꢁC, under N2. Determined by the GC-analysis with an internal standard. Pd(OAc)2 (0.5 mol% Pd) as catalyst. Pd/C (0.5
mol% Pd) as catalyst. e 4-Nitrophenylboronic acid instead of phenylboronic acid. 4-Methoxyphenylboronic acid instead of phenylboronic acid.
f
experimental conditions of using different bases. The high
Generally, heterogeneous catalysts easily lose their catalytic
yields were obtained when using NaOH, KOH, or NaOMe as activity due to extensive leaching of the active metal species. To
base (Table 1, entry 5–7) and NaOH was the most efficient (high our surprise, the Pd-NNSs-ECNFs can be recycled and reused
yield and cost effective) base in Suzuki coupling reaction with seven times without losing its catalytic activity (Table 2, entry
ethanol/water used as the solvent.
1–5). Simple ltering could retrieve the catalyst from the reac-
Secondly, Pd-NNSs-ECNFs were used as catalysts for Suzuki– tion pot. Obviously, the excellent self-stability of Pd-NNSs
Miyaura reactions of various aryl halides (Table 2). The enhanced the catalytic stability.
Pd-NNSs-ECNFs showed good catalytic activity to different aryl
halides. It should be noted that the low-active 4-chlorobenzene
also could be coupled in good yield (Table 2, entry 1). For the
4. Conclusions
high-active aryl bromides and aryl iodides, higher yields were
obtained (Table 2, entry 2–5).
Palladium nano-network structures supported on the electro-
spun carbon nanobers (Pd-NNSs-ECNFs) were successfully
prepared by a novel K2PdIICl4/K4FeII(CN)6 cyanogel method. The
structures of Pd-NNSs-ECNFs were characterized by the SEM,
TEM and XPS. The Pd-NNSs-ECNFs catalyst was successful
applied in Suzuki coupling reaction and showed high catalytic
activity in an environmentally friendly solvent system (C2H5OH/
H2O). The catalyst could be recycled and reused many times
without losing its catalytic activity. We anticipate that the
Pd-NNSs-ECNFs would be applied in various elds and provide
new opportunities in green chemistry area.
The unreduced PdII (Pd(OAc)2) was also used as catalyst for
the coupling reaction (Table 2, entry 6). However, C–C bond
formation was not observed, indicating that palladium in the
reduced form is the active site for the Suzuki–Miyaura coupling
reaction.
A commercial Pd/C catalyst (Alfa Aesar, 0.5 mol% Pd) was
also applied for the comparison purpose for the same coupling
reaction (Table 2, entry 7). The yield was only 32% of the cross-
coupled product by using Pd/C in the coupling of 4-chlor-
obenezene with phenylboronic acid. The high catalytic activity
of Pd-NNSs-ECNFs may be ascribed to the large active surface
area, which originated from the particular 3D networks struc-
ture24 and plentiful pores of electrospun carbon nanobers mat.
Such a structure ensured the high dispersion of Pd active sites
and facilitates the diffusion of the reactants in the pores.
Acknowledgements
This Work is supported by the National Natural Science Foun-
dation of China (Grant no. 21374044), the Academic and
Technical Leader Plan of Jiangxi Provincial Main Disciplines
(Grant no. 050008), The Major Special Projects of Jiangxi
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Provincial Department of Science and Technology (Grant no. 14 C. Pham-Huu, N. Keller, L. J. Charbonniere, R. Ziessel and
20114ABF05100) and the Technology Plan Landing Project of
Jiangxi Provincial Department of Education.
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