One-pot hydrothermal synthesis of Pd/Fe3O4 nanocomposite in HEPES buffer solution and catalytic activity for Suzuki reaction

Article abstract of DOI:10.1016/j.materresbull.2015.02.042

The Pd/Fe₃O₄ nanocomposite integrates versatile Pd nanocatalysts with magnetic separation, and has great potential in fine chemical and pharmaceutical synthesis. Its preparation usually involves multi-steps. Herein it was prepared via a facile one-pot hydrothermal synthesis in 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) buffer solution with the assistant of polyvinylpyrrolidone (PVP). HEPES plays multi-functions, particularly as a ligand to enhance the oxidation of Fe²⁺ to Fe³⁺ and as a buffer to control the pH value at slightly basic conditions (ca. 7.4) for the formation of crystalline Fe₃O₄ phase via Fe²⁺/Fe³⁺ co-precipitation. PVP works as a dispersant to prevent the particle from aggregation. The obtained Pd/Fe₃O₄ nanocomposite comprised uniform Pd nanoparticles (ca. 5 nm) deposited on Fe₃O₄ nanocrystals (ca. 15 nm). It exhibited excellent catalytic activity and stability for various Suzuki coupling reactions, and could be efficiently recovered with a magnet and recycled for at least 10 cycles without losing catalytic activity.

Full text of DOI:10.1016/j.materresbull.2015.02.042

Accepted Manuscript
Title: One-pot hydrothermal synthesis of Pd/Fe3O4
nanocomposite in HEPES buffer solution and catalytic activity
for Suzuki reaction
Author: Shaozhong Li Wei Zhang Fengxi Chen Rong Chen
PII:
S0025-5408(15)00127-0
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.materresbull.2015.02.042
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11-9-2014
9-2-2015
16-2-2015
Please cite this article as: Shaozhong Li, Wei Zhang, Fengxi Chen, Rong Chen,
One-pot hydrothermal synthesis of Pd/Fe3O4 nanocomposite in HEPES buffer
solution and catalytic activity for Suzuki reaction, Materials Research Bulletin
http://dx.doi.org/10.1016/j.materresbull.2015.02.042
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One-pot hydrothermal synthesis of Pd/Fe3O4 nanocomposite in HEPES buffer solution
and catalytic activity for Suzuki reaction
Shaozhong Li, Wei Zhang, Fengxi Chen*, Rong Chen*
School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Wuhan,
430073, PR China
* Corresponding authors. R. Chen, E-mail address: rchenhku@hotmail.com
F. Chen, Email address: fxchen@mail.wit.edu.cn
Tel.: (+86) 13659815698; Fax: (+86)27 8719 4580.
Graphical abstract
Highlights
 Pd/Fe3O4 nanocomposite was made by a facile one-pot hydrothermal method.
 In-situ formed Pd/Fe3O4 had excellent activity for various Suzuki reactions.
 In-situ formed Pd/Fe3O4 was magnetically recoverable and recycled for ten runs.
Abstract: The Pd/Fe3O4 nanocomposite integrates versatile Pd nanocatalysts with magnetic
separation, and has great potential in fine chemical and pharmaceutical synthesis. Its
preparation usually involves multi-steps. Herein it was prepared via a facile one-pot
hydrothermal synthesis in 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES)
buffer solution with the assistant of polyvinylpyrrolidone (PVP). HEPES plays
multi-functions, particularly as a ligand to enhance the oxidation of Fe2+ to Fe3+ and as a
buffer to control the pH value at slightly basic conditions (ca. 7.4) for the formation of
crystalline Fe3O4 phase via Fe2+/Fe3+ co-precipitation. PVP works as a dispersant to prevent
the particle from aggregation. The obtained Pd/Fe3O4 nanocomposite comprised uniform Pd
1

nanoparticles (ca. 5 nm) deposited on Fe3O4 nanocrystals (ca. 15 nm). It exhibited excellent
catalytic activity and stability for various Suzuki coupling reactions, and could be efficiently
recovered with a magnet and recycled for at least 10 cycles without losing catalytic activity.
KEYWORDS:
A. magnetic materials; B. chemical synthesis; C. X-ray diffraction; D. catalytic properties
1. Introduction
Palladium (Pd) nanoparticles exhibit remarkable catalytic activities for carbon-carbon
cross-coupling reactions (e.g. Suzuki, Heck and Sonogashira) [1-3], and have great potential
in the synthesis of drugs and fine chemicals. However, they face inherent problems in catalyst
agglomeration and separation, which usually result in the decrease in catalytic activity and
life time. In order to overcome these drawbacks, many efforts have been devoted to
magnetically recoverable Pd nanocatalysts [4-6], which combine the advantages of high
catalytic activity of Pd nanocatalysts and easy separation of magnetic materials.
Magnetite (Fe3O4) is often used as support to make magnetic nanocatalysts due to its
intrinsic magnetic property, good chemical stability and low toxicity. In the literature,
synthesis of Pd/Fe3O4 nanocatalysts usually involves hazardous organic solvents and
chemicals, harsh reaction conditions and multi-steps, e.g., deposit the Pd source onto the
pre-formed Fe3O4 support followed by decomposition or reduction of Pd precursors [7-13].
Therefore, it remains a great challenge to develop facile and rapid methods for the synthesis
of Pd/Fe3O4 nanocatalysts [14].
In the present work, the Pd/Fe3O4 nanocomposite was prepared via a facile one-pot
2

hydrothermal synthesis in HEPES buffer solution.
HEPES represents
HOCH2CH2
N
N
CH2CH2SO3H
4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (
), which is
low-toxic and widely used as a buffer in cell culture. The obtained Pd/Fe3O4 nanocomposite
exhibited excellent catalytic performance for various Suzuki coupling reactions, and could be
efficiently recovered with a magnet and recycled for at least 10 cycles without losing its
catalytic activity.
2. Experimental
The Pd/Fe3O4 nanocomposite was prepared via one-pot hydrothermal synthesis as
follows: 0.0133 g of PdCl2·2H2O (0.075 mmol), 0.16 g of polyvinylpyrrolidone (PVP, M.W.
=55,000) and 0.0149 g FeCl2·4H2O (0.075 mmol) were added into the solution of 10 mL of
HEPES buffer solution (200 mmol/L, pH 7.40) in 2 mL of deionized water. After sonication
for 2 min, the whole mixture was transferred to a 30 mL Teflon-lined stainless-steel autoclave
o
and heated at 100 C for 8 h. After cooling down to room temperature (r.t.), black particles
were collected with a magnet, washed with deionized water and ethanol, and dried in a
desiccator at r.t. All chemicals were of analytical grade from Sigma-Aldrich.
To investigate the influence of halide ions on the synthesis of Pd/Fe3O4 composites, the
same experimental procedure as described above was used except that 2 mL of deionized
water was replaced with 2 mL of NaX (X = Cl, Br and I) aqueous solution (1 mol/L).
Suzuki reaction was carried out as follows: aryl halide (ArX, X=Br, I) (1.0 mmol),
phenylboronic acid (1.5 mmol), tetrabutylammonium bromide (TBAB, 1 mmol), K2CO3 (2
mmol), and Pd/Fe3O4 nanocatalyst (1.5 mg, containing 0.5 mmol % Pd with regard to ArX)
3

were added into 9 mL of ethanol/H2O (v/v=1:2) solution. The whole mixture was refluxed at
86 oC for 0.5-1.5 h, and then cooled down to r.t. The product was extracted with diethyl ether
for three times, and then dried with anhydrous Na2SO4. After being concentrated in a rotary
evaporator, the product was finally purified by column chromatography using diethyl
1
ether/petroleum ether as eluent, and identified by comparing their H NMR spectra with
standard data. For comparison, carbon-supported Pd nanoparticles (Pd/C) with 10 % of Pd
content purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) were also
used as catalysts for Suzuki reaction.
Powder X-ray diffraction (XRD) was carried out on a Bruker AXS D8 Discover (Cu
Kα = 1.5406 Å). Transmission electron microscopy (TEM) images and selected area electron
diffraction (SAED) patterns were recorded on a Philips Tecnai G2 20 electron microscope
operating at 200 kV. The energy-dispersive X-ray (EDX) analysis was performed on an
Oxford Instruments INCA with a scanning range from 0 to 20 keV. The Pd content in
Pd/Fe3O4 nanocatalyst was analyzed by inductively coupled plasma mass spectrometry
(ICP-MS, Agilent ICP-MS 7500ce). Magnetic properties were investigated on a SQUID
magnetometer (Quantum Design MPMS) at 298 K.
3. Results and discussion
3.1. Characterization of Pd/Fe3O4 nanocomposite
The XRD pattern of Pd/Fe3O4 nanocomposite showed diffraction peaks of both Pd (fcc)
and magnetite crystals, which were well indexed in Fig. 1 according to standard diffraction
data of Pd (JCPDS 05-0681) and Fe3O4 (JCPDS 75-0033). No other peak was observed,
4

indicative of high purity of Pd/Fe3O4 nanocatalyst.
TEM images of Pd/Fe3O4 nanocomposite (Fig. 2) revealed that spherical Pd nanoparticles
of ca. 5 nm were well-dispersed on the surface of Fe3O4 nanoparticles of ca. 15 nm. Its SAED
pattern (Fig. 2a inset) featured many bright spots, indicating high crystallinity of the product.
In consistent with the XRD result, the SAED pattern was features of both Pd (fcc) and Fe3O4
crystals. The elemental composition of selected areas in TEM images was also determined by
EDX spectrum (Fig. S1 in supporting information), confirming the presence of palladium,
iron and oxygen. The Pd content in the Pd/Fe3O4 nanocomposite was 33.8 wt%, as
determined by ICP-MS analysis.
The room-temperature magnetization curve of the Pd/Fe3O4 nanocomposite (Fig. 3)
showed typical superparamagnetic characteristics, i.e., zero coercivity and no remanence on
hysteresis. The saturated magnetization value was 41.3 emug-1. These magnetic properties are
ideal for magnetic separation and recycling of the Pd/Fe3O4 nanocomposite. It can be easily
separated from the solution with a normal magnet (see Fig. 3 inset), while avoid aggregation
after removing the external magnet.
3.2. Effects of reaction components on the synthesis of Pd/Fe3O4 nanocomposite
HEPES: The addition of HEPES is crucial for the one-pot formation of Pd/Fe3O4
nanocomposite
via
hydrothermal treatment
of the
reaction
mixture
of
FeCl2-PdCl2-HEPES-PVP-H2O. Under otherwise identical conditions except the absence of
HEPES, no crystalline iron oxide phase except Pd crystals was formed (XRD shown in Fig.
S2). The Pd crystals were formed via 2Fe2+ + Pd2+  2Fe3+ + Pd. HEPES may play
multi-functions [15]: Firstly, it may reduce the redox potential of Fe3+/Fe2+ by forming the
5

iron-HEPES complex, thus enhancing the oxidation of Fe2+ to Fe3+ by Pd2+ ions in the
solution along with the inadvertent air. Such enhancing effects have been reported in Fenton
oxidation processes for degradation of organic pollutants in waste water treatment with the
assistance of different organic ligands (e.g., EDTA [16, 17], 5-sulfosalicylic acid [18] and
citrate [19]). For example, the formation of Fe-EDTA complex reduces the standard redox
potential of Fe3+/Fe2+ from the conventional level of 0.77 V to 0.17 V [16], which is much
lower than the standard redox potential of Pd2+/Pd (0.915 V). Secondly, HEPES is a buffer to
control the pH value of the reaction mixture at ca. 7.4. Fe3O4 was formed by Fe2+/Fe3+
co-precipitation under such slightly basic conditions, i.e., Fe2+ + 2Fe3+ + 8OH-  Fe3O4 +
4H2O. Thirdly, HEPES acts as an anchoring agent for depositing Pd nanoparticles onto Fe3O4
by multiple functional groups (e.g., sulfonate, hydroxyl and amine groups or their derivatives).
This is important for the formation of well-dispersed and stable Pd/Fe3O4 nanocomposites,
similar to the preparation of Pd nanoparticles supported on Fe3O4/amino acid nanocomposite
[13].
PVP: PVP works as a dispersant in our synthesis system. Without the addition of PVP,
Pd/Fe3O4 composite with minor γ-Fe2O3 (PDF No. 4-755) impurity was produced, as shown
by XRD in Fig. 4. However, the resulting particles were severely agglomerated, as shown by
TEM images in Fig. 5.
Influence of halide ions: It has been reported that various Pd nanostructures (e.g.,
nanosheets, nanobars, nanorods, concave tetrahedra, and a series of single-crystalline
rhombic dodecahedral, cubic, and octahedral nanocrystals) could be synthesized with the
assistance of halide ions, particularly Br- and I-, due to the selective interaction of halides
6

with different crystal facets that alters their surface energies and growth rate [20-22].
Therefore, the influence of halide ions was investigated in this work by adding 2 mmol of
NaX (X= Cl-, Br- and I-) during the synthesis of Pd/Fe3O4 nanocomposites.
As observed in Fig. 6, the Pd/Fe3O4 nanocomposite synthesized with the addition of
NaBr contained more uniform and well-dispersed Pd nanoparticles (ca. 5 nm) (b1-b3 in Fig.
6). In contrast, the Pd/Fe3O4 nanocomposites synthesized with the addition of NaCl and NaI
contained Pd nanoparticles with broader size distributions (5-10 nm and 5-20 nm,
respectively). Some Pd nanorods and bigger polyhedral nanoparticles were also observed in
Fig. 6c. The non-uniform shape and size distribution were unbeneficial for the uniform
dispersion of Pd particles on the surface of Pd/Fe3O4, which was clearly revealed in Fig.6.
In comparison with the Pd/Fe3O4 nanocomposite synthesized in the absence of NaX, the
Pd/Fe3O4r nanocomposite synthesized with the addition of NaBr has lower Pd content (23.9
wt% vs. 33.8 wt%). This could be due to the interaction of Br- with Pd nanoparticles, which
prevents Pd from depositing on the surface of Fe3O4.
3.3. Catalytic performance of Pd/Fe3O4 nanocomposite
As shown in Table 1, the Pd/Fe3O4 nanocomposite exhibited excellent catalytic activity
for Suzuki cross-coupling reactions of phenylboronic acid with various aryl bromides (entries
1-3) and aryl iodides (entries 4–5). The isolated yield of the corresponding coupling products
o
reached up to 92-98 % after reaction for 0.5-3 h at 86 C. In comparison, carbon-supported
Pd nanoparticles of similar particle size (~5 nm, Fig. S3) showed inferior catalytic activity for
all reactions studied, particular for deactivated p-bromophenol (isolated yield: 93% vs. 30%
7

in entry 3). This implied that there were some synergistic effects between supported Pd
nanocatalysts and Fe3O4. Similar synergistic effects between Pd and iron oxide for CO
oxidation have been recently reported due to strong metal–support interactions in the
literature [23]. It would be reasonable to expect stronger metal–support interactions in our Pd/
Fe3O4 nanocatalysts since both components were formed in one pot with intimate contact
with each other. The stronger metal–support interactions prevent Pd nanoparticles from
aggregating into inactive Pd black during reaction, which is crucial to form stabile and
recyclable magnetic nanocatalysts.
Sukuzi coupling reaction of 4-iodophenyl methyl ether with phenylboronic acid was used
as a model reaction to test the recyclability of the Pd/Fe3O4 nanocatalyst under the same
reaction conditions. The used Pd/Fe3O4 nanocatalyst was first separated and recovered from
the reaction mixture with a magnet, then washed with deionized water (4 mL×3) and ethanol
(4 mL×3), and finally dried in vacuum for 24 h before re-use. As depicted in Fig. 7, the
isolated yield of product was still 88% after ten reaction cycles (i.e., the ninth re-use of
Pd/Fe3O4 nanocatalyst). This demonstrated that the Pd/Fe3O4 nanocatalyst was magnetically
recoverable and had excellent catalytic activity and stability.
4. Conclusions
A facile one-pot hydrothermal method has been developed to synthesize the Pd/Fe3O4
nanocomposite in HEPES buffer solution (200 mmol/L, pH 7.40) with the assistant of PVP.
The addition of HEPES was crucial for the formation of Fe3O4 while PVP prevented the
particles from aggregation. The obtained Pd/Fe3O4 nanocomposite consisted of uniform Pd
8

nanoparticles (ca. 5 nm) deposited on Fe3O4 nanocrystals (ca. 15 nm). It exhibited excellent
catalytic activity and stability for various Suzuki coupling reactions, and could be efficiently
recovered with a magnet and recycled for at least 10 times without losing its catalytic
activity.
Acknowledgements
This work was supported by High-tech Industry Technology Innovation Team Training
Program of Wuhan Science and Technology Bureau (2014070504020243), National Natural
Science Foundation of China (21171136), International Cooperation Program of Hubei
Province (2011BFA021) and Natural Science Foundation of Hubei Province (2014CFB795).
9

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Table 1 Suzuki coupling reactions of phenylboronic acid with various aryl halides over
different Pd nanocatalysts.
Isolated yield (%)
Pd/Fe3O4 Pd NPs
Time
(h)
Entry
1
Substrates
Products
1.5
1.5
92
94
85
40
2
3
4
5
3
93
98
95
30
94
92
0.5
0.5
13

Figures captions:
Fig. 1. XRD pattern of Pd/Fe3O4 nanocomposite.
Fig. 2. TEM images of Pd/Fe3O4 nanocomposite. Inset in Fig. 2a is the SAED pattern.
Fig. 3. Magnetization curve of Pd/Fe3O4 nanocomposite at 298 K.
Fig. 4. XRD pattern of Pd/Fe3O4 composite prepared in the absence of PVP. Asterisk-marked
peaks were assigned to minor γ-Fe2O3 impurity (PDF No. 4-755) in the product.
Fig. 5. TEM images of the Pd/Fe3O4 composite prepared in the absence of PVP.
Fig. 6. TEM images and corresponding particle size distributions of Pd/Fe3O4
nanocomposites synthesized with the addition of different NaX salts. (a1-a3) NaCl, (b1-b3)
NaBr and (c1-c3) NaI.
Fig. 7. Recyclability of the Pd/Fe3O4 nanocomposite as catalyst for Suzuki coupling reaction
of phenylboronic acid with 4-iodophenyl methyl ether.
Fig. 1.
14

Fig. 2.
Fig. 3.
15

Fig. 4.
16

Fig. 5.
17

Fig. 6.
18

Fig. 7.
19