Mesoporous palladium-copper ferrites as highly efficient and magnetically separable catalysts for Suzuki coupling reaction

Article abstract of DOI:10.1016/j.materresbull.2015.09.013

Mesoporous Cu-Pd ferrites were prepared by using MCM-41 and SBA-15 mesoporous molecular sieves as hard templates and well characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), ICP-atomic emission spectroscopy, N₂ adsorption-desorption and X-ray photoelectric spectroscopy (XPS). Compared with samples prepared by the sol-gel auto-combustion methods, the as-prepared mesoporous ferrites have shown uniform pore size and much higher specific surface areas. The samples showed a remarkably high activity in connection with the Suzuki coupling reaction, especially Cu_0.99Pd_0.01Fe₂O₄ prepared by using MCM-41 as template obtained an excellent TOF as high as 36.9 × 10³ h^-1 and no obvious decrease in the activity was observed after ten consecutive runs.

Full text of DOI:10.1016/j.materresbull.2015.09.013

Accepted Manuscript
Title: Mesoporous palladium-copper ferrites as highly
efficient and magnetically separable catalysts for Suzuki
coupling reaction
Author: Jinhui Tong Lingdi Su Lili Bo Xiaodong Cai
Qianping Zhang Qizhao Wang
PII:
S0025-5408(15)30115-X
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.materresbull.2015.09.013
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9-8-2015
8-9-2015
Please cite this article as: Jinhui Tong, Lingdi Su, Lili Bo, Xiaodong Cai, Qianping
Zhang, Qizhao Wang, Mesoporous palladium-copper ferrites as highly efficient and
magnetically separable catalysts for Suzuki coupling reaction, Materials Research
Bulletin http://dx.doi.org/10.1016/j.materresbull.2015.09.013
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Mesoporous palladium-copper ferrites as highly efficient and
magnetically separable catalysts for Suzuki coupling reaction
Jinhui Tong [a, b] *, Lingdi Su [a, b], Lili Bo [c], Xiaodong Cai [a, b], Qianping Zhang [a, b], Qizhao Wang [a, b]
[a] Key Laboratory of Eco-Environment-Related Polymer Materials, Ministry of Education, PR China.
[b]
Key Laboratory of Gansu Polymer Materials, College of Chemistry and Chemical Engineering,
Northwest Normal University, Lanzhou 730070, PR China.
[c] College of Science, Gansu Agricultural University, Lanzhou 730070, P. R. China.
Corresponding author Fax: (+) 86-931-7971533; e-mail: jinhuitong@126.com (Jinhui Tong)
Graphical abstract
Highlights
 Mesoporous Cu-Pd ferrites were prepared by a facile, mild and
environment-friendly route
 The catalysts have shown great efficiencies in Suzuki coupling reaction and up
to 36.9×103 h-1 TOF can be obtained.
 The catalyst can be magnetically separated easily and have shown prominent
recyclability.
Abstract
Mesoporous Cu-Pd ferrites were prepared by using MCM-41 and SBA-15
mesoporous molecular sieves as hard templates and well characterized by X-ray
diffraction (XRD), transmission electron microscopy (TEM), ICP-atomic emission

spectroscopy, N2 adsorption-desorption and X-ray photoelectric spectroscopy (XPS).
Compared with samples prepared by the sol-gel auto-combustion methods, the
as-prepared mesoporous ferrites have shown uniform pore size and much higher
specific surface areas. The samples showed a remarkably high activity in connection
with the Suzuki coupling reaction, especially Cu0.99Pd0.01Fe2O4 prepared by using
MCM-41 as template obtained an excellent TOF as high as 36.9×103 h-1 and no
obvious decrease in the activity was observed after ten consecutive runs.
Keywords: A. Magnetic materials，B. Chemical synthesis，B. Magnetic properties, C.
Transmission electron microscopy, D. Catalytic properties
1. Introduction
The Suzuki coupling reaction is one of the most important reactions in processes
[5-8]
of forming carbon-carbon bonds [1-4], especially for the preparation of biaryls
.
Generally, the Suzuki coupling reactions are catalyzed by homogeneous
Pd-containing catalysts [9-11]. However, separation of the expensive catalyst is often
problematic in these homogeneous systems. Moreover, aggregation and precipitation
of palladium metal granules in the homogeneous systems always leads to the loss of
activity of the catalysts. Therefore, heterogeneous catalysts are highly desirable,
especially in large-scale synthesis, from both environmental and economic aspects.
A most important strategy to find catalysts is supporting the homogeneous one on
a substrate material, such as Pd (II)-Schiff base complex supported by multi-walled
carbon nanotubes [12], Pd catalysts supported by functionalized mesoporous SBA-15
[14]
[13], silica
and terpolymers [15], etc. Recently, magnetic nanoparticles (MNPs)

supports, especially spinel ferrites have attracted much attention because the magnetic
catalysts can be simply separated by applying external magnetic fields. Several
magnetic ferrites have been used as supports for palladium nanoparticles, such as
copper ferrite, zinc ferrite and cobalt ferrite [16-19]. Although the catalysts have shown
high catalytic activities and excellent recyclabilities, Pd nanoparticles may leach
unavoidably during the reaction and separation processes for such supported catalysts
prepared relying on re-impregnating methods. Copper ferrite nanoparticles have also
been widely used in catalytic and magnetic fields [19-23]. However, to the best of our
knowledge, no Cu-Pd ferrite solid solutions have been used as catalysts for the Suzuki
coupling reaction.
In this work, we prepared mesoporous Pd-doped copper ferrites with high specific
surface areas using molecular sieves as templates. The synthesized materials have
shown prominent catalytic activities and recyclabilities in connection with the Suzuki
coupling reaction.
2. Experimental
2.1. Materials and equipments
All chemicals were of analytical grade and used without further purification. XRD
patterns of the synthesized materials were collected using a PANalytical X，Pert Pro
diffractometer with Cu Kα radiation. TEM micrographs were obtained using a Hitachi
H-600 microscope. BET surface area measurements were performed on a
Micromeritics ASAP 2010 instrument at liquid nitrogen temperature. The Pd content
was determined by inductively coupled plasma (ICP) on a Perkin-Elmer ICP/6500

1
13
atomic emission spectrometer. H-NMR and C-NMR spectra were recorded on a
Mercury 400 MHz nuclear magnetic resonance instrument.
2.2. Synthesis of the samples
Mesoporous spinel Cu-Pd ferrite was synthesized according to the route reported
[24]
in the literature
as follows: 2.5 g of the nitrate salts in stoichiometric proportion
were pre-mixed together with 1.0 g of the selected mesoporous silica powder and
ground in an agate mortar in the presence of 10 ml of n-hexane to yield a
homogeneous mixture. The resulting mixture was subsequently dispersed in 30 ml of
n-hexane and stirred overnight under reflux at 70 oC. After cooling, the solid products
were recovered by filtration, dried in air at 70oC and then calcined at 600 oC for 5 h.
Finally, the silica template was selectively removed by treating the powders three
times with NaOH (2 M, 24 h at room temperature). Cu0.99Pd0.01Fe2O4 was prepared by
using SBA-15 and MCM-41 as templates and designated as CPFO-S and CPFO-M
respectively. For comparison, solid Cu0.995Pd0.005Fe2O4, Cu0.99Pd0.01Fe2O4 and
Cu0.98Pd0.02Fe2O4 ferrite nano-powders were synthesized according to the sol-gel
[25]
auto-combustion route described in our previous work
CPFO-A0, CPFO-A, and CPFO-A1 respectively.
2.3. Suzuki coupling reaction process
and designated as
All Suzuki coupling reactions were carried out in a 10 mL round bottom flask
equipped with a reflux condenser. In a typical experiment, 0.5 mmol of aryl halide,
0.6 mmol of phenylboronic acid, 1.0 mmol of base and catalyst, 1.0 mL solvents were
added to the flask. The resulting mixture was stirred in an oil bath at the specified

temperature for desired time. The reaction was monitored by thin layer
chromatography (TLC). After completion of the reaction, the reaction mixture was
cooled to room temperature and the catalyst was recovered by external magnet and
washed with ethanol. The reaction mixture was diluted with water and extracted with
ethyl acetate for three times. The organic layers were combined, dried over MgSO4,
filtered and evaporated under reduced pressure. The residue was purified by column
chromatography on silica gel to give the pure products. The identities of the products
were confirmed by 1H-NMR and 13C-NMR spectra.
3. Results and discussion
3.1. Characterization of the samples
Fig. 1. The XRD patterns of the samples
The XRD patterns of the samples are shown in Fig. 1, which are consistent with
the standard pattern of face-centered cubic CuFe2O4 (JCPDS NO.34-0425) [26, 27]. The
eleven peaks at 18.3°, 29.8°, 34.6°, 35.9°, 37.1°, 41.6°, 43.8°, 58.0°, 62.0°, 63.8°and
74.7° can be ascribed to the reflection of (101), (112), (103), (211), (202), (004), (220),
(321), (224), (400) and (413) diffractions of CuFe2O4. The diffraction peaks in

Pd-doped samples were shifted 0.02-0.08° to lower 2θ values as compared to the
un-doped samples. This implies that Pd was doped in the lattice of CuFe2O4 and
formed ferrite solid solutions.
Fig. 2. The TEM images of a) MCM-41; b) SBA-15; c) CPFO-M; d) CPFO-S;
e) CPFO-A
Fig. 3. The EDX images of a) CPFO-M based on the image c in Fig. 2; b) CPFO-S
based on the image d in Fig. 2
The morphologies of the samples and the two template molecular sieves were
characterized by TEM (Fig. 2). TEM images of MCM-41 and SBA-15 (Fig. 2a, Fig.

2b) show highly ordered mesoporous structures, which are in agreement with the
literature [28, 29]. Fig. 2c and Fig. 2d show representative TEM images of CPFO-M
and CPFO-S obtained from the replication of MCM-41 and SBA-15 respectively. The
two replicas clearly present extended domains of the ordered 3D pore structure. Fig.
2e is a representative TEM image of the solid samples CPFO-A0, CPFO-A and
CPFO-A1. The three samples show an irregular morphology and the particle size
distribution is broad (10-50 nm), concentrated in 25-30 nm primarily.
The EDX characteristics of CPFO-M and CPFO-S based on the corresponding
TEM images in Fig. 2 have shown in Fig. 3. The results confirmed the occurrence of
homogeneous distributions of Cu, Fe, Pd, O and trace of residual Si.
Fig. 4. Porous textures of the samples (a) The nitrogen adsorption-desorption
isotherms and (b) pore size distribution curves of the samples
Fig. 4 presents the nitrogen adsorption-desorption isotherms and corresponding
pore size distribution plots of the as-prepared samples. Those of the samples
CPFO-A0 and CPFO-A1 are similar with that of the sample CPFO-A. So they are
not presented tediously. The BET surface area, pore volumes and pore diameters of
the samples are listed in Table 1. The N2 adsorption-desorption isotherms of the

samples (Fig. 4a) exhibit typical type IV behavior with an H3-type hysteresis loop
according to the IUPAC classifications indicating that all the samples have porous
structure. The pore size distributions plots of the samples CPFO-S and CPFO-M
indicate narrow mesopores distributions of ~7.7 nm and ~3.6 nm respectively, while
that of the sample CPFO-A indicates broad piled pores distributions of ~14 nm (Fig.
4b). The results have shown that the sample CPFO-M possess the highest BET
surface areas of 167.2 m2·g-1 and the smallest pore size of 3.6 nm. Pd contents of the
samples measured by ICP–atomic emission spectroscopy are also listed in Table 1. As
can be seen that CPFO-M contains the lowest Pd content, only 0.14 wt.%.
Fig. 5 shows the X-ray photoelectron spectroscopy (XPS) data for the catalyst. The
wide spectra (Fig. 5a) of the catalysts reveal the predominant presence of copper, iron,
oxygen, and carbon. The surface content of Pd was examined by high-resolution
X-ray photoelectron spectroscopy (XPS). As shown in Fig. 5b, The Pd-3d spectra
revealed a negligible surface palladium content on the catalysts (below the detection
limit). We have noticed the fact that XPS signals were noisy and tried to increase the
number of scans, which however did not lead to any improvement unless for those
samples with higher elemental contents.

Fig. 5. XPS spectra for the samples: (a) wide spectra, (b) Pd 3d
3.2. Catalysis tests
The catalytic performances of the as-prepared catalysts were evaluated in the
Suzuki coupling reaction of 4-nitro-bromobenzene and phenylboronic acid and the
results have been listed in Table 2. The results have shown that no reaction occurred
in the absence of catalyst (entry 1). It is clear that the mesoporous samples have
shown much higher catalytic activities than the solid ones and the ferrites supported
Pd catalysts reported in the references [17, 30] probably due to their higher surface
areas and mesoporous structures (entries 2-6). Especially, the catalyst CPFO-M has
shown the highest activity and as high as 98% yield and a 24.83×103 h-1 TOF can be
o
obtained after 0.5 h reaction at 70 C (entry 4). As for the solid catalysts, they have
shown different activities in the coupling reaction between 4-methoxybromobenzene
and 4-chlorophenylboronic acid mainly due to their different Pd contents since they
have similar structures. The results have shown that the yield of the product increased
with increasing Pd content (entries 7-9). The catalyst CPFO-A1, which contained the
highest Pd content of 0.87 wt%, has obtained the highest yield (97%) and the lowest
TOF (170 h-1) (entry 9). Based on above facts, CPFO-M was chosen as the catalyst to

optimize the reaction conditions.
3.3. Optimization of the Reaction Conditions
The Suzuki coupling reaction of 4-nitrobromobenzene and phenylboronic acid
was selected as the model reaction to optimize the reaction conditions and the results
are listed in Table 3. Among the investigated solvents, mixture of ethanol and water
with 1:1 volume ratio was found the best one and the highest yield (about 98%) was
obtained (entry 4). The base is considered to be crucial in the Suzuki-coupling
reaction. Among the investigated bases, K2CO3 was found to be the top one under our
reaction conditions and the highest yields about 98% was obtained (entry 4). The
effect of the catalyst amount was also investigated and the results have shown that the
yield increased from 97% to 99% while the TOF decreased from36.9×103 to 18.8×103
h-1 when the catalyst amount increased from 2.0 mg to 4.0 mg (entries 4, 10, 11). In
pursuit of higher TOF, 2.0 mg catalyst was employed in the following investigation.
3.4. Extension of the Substrate Scope
Based on above investigations, the general applicability of the catalyst CPFO-M in
the coupling of various aryl halides and arylboronic acids was investigated under the
optimized conditions. The results are summarized in Table 4. As can be seen, the
catalyst has shown prominent catalytic activities for aryl iodides and bromides
including not only activated ones with electron-withdrawing substituents (entries 3, 4),
but also deactivated ones with electron-donating substituents (entries 5-14), and

moderate yields (59-99%) and high TOF of up to 36.9×103 h-1 were obtained. The
Coupling reactions in which one coupling partner contains a nitrogen donor are
typically difficult to accomplish [31]. It is therefore noteworthy that the catalyst
CPFO-M successfully promoted the coupling reactions of 4-bromoaniline with
phenylboronic acid and up to 90% yield was obtained (entry 7). Unfortunately, for
aryl chlorides, which are much more difficult to activate than aryl iodides and
bromides, only low yields were obtained even under prolongation of the reaction time
(entries 15–18). Therefore, much more effort is needed to develop new catalysts
which are more effective in connection with the Suzuki coupling reactions of
unreactive aryl chlorides.
3.5. Recyclability of the catalyst
The recycling performance of the catalyst was investigated using the reaction of
4-nitrobromobenzene and phenylboronic acid. As shown in Fig. 6, the catalyst could
be reused ten times with a slight loss in yield from 98% to 87%. The decrease could
be mainly attributed to unavoidable loss of the catalyst during the process of
collection. The results confirm that the catalyst has good stability and recyclable
applicability for the Suzuki coupling reaction under our experimental conditions.

Fig. 6. The reuse of the catalyst
4. Conclusions
Mesoprous Cu-Pd ferrites with high specific surface areas and pore volumes were
successfully synthesized by using mesoporous MCM-41 and SBA-15 molecular
sieves as templates. The catalysts have shown excellent catalytic performances in the
Suzuki coupling reaction, especially the one replicated from MCM-41, and up to
36.9×103 h-1 TOF has been obtained. What’s more, the catalyst can be magnetically
separated easily and exhibited excellent reusability. Expectedly, the catalyst can be
applied in large-scale industrial synthesis.
Acknowledgments
The authors are grateful to the National Natural Science Foundation of China
(51302222, 21363021), Natural Science Foundation of Gansu Province
(1308RJYA017), and Program for Changjiang Scholars and Innovative Research
Team in University (IRT1177) for financial support.

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Table 1
BET surface area, Pore volume, Pore diameter and Pd content of the samples.
Catalyst
BET
(m2·g-1)
50.3
Pore volume
(cm³·g-1)
0.14
Pore diameter
(nm)
Pd content
(wt %)
0.21
13.9
CPFO-A0
CPFO-A1
CPFO-A
CPFO-S
52.6
0.15
12.9
0.87
56.4
0.16
14.2
0.44
121.4
167.2
0.32
7.7
0.24
0.28
3.6
0.14
CPFO-M
Table 2 The Suzuki reaction of bromobenzene and phenylboronic acid catalyzed by
different catalysts a
Temperature Time Yield b TOF c/h-1
Entry
Catalyst
R1
R2
(oC)
70
(h)
0.5
0.5
0.5
0.5
12
(%)
—
39
81
98
92
95
35
95
97
（×103）
—
1
2
—
4-NO2
4-NO2
4-NO2
4-NO2
4-NO2
4-NO2
H
H
H
H
H
H
70
3.14
CPFO-A
CPFO-S
3
70
11.97
24.83
0.05f
3.80 f
0.25
4
70
CPFO-M
Pd/CoFe2O4
Pd/NiFe2O4
CPFO-A0
CPFO-A
CPFO-A1
5d
6 e
7
Reflux
90
0.25
12
4-MeO 4-Cl
4-MeO 4-Cl
4-MeO 4-Cl
80
8
80
12
0.32
9
80
12
0.17
a
Reaction condition: aryl halide (0.5 mmol); boronic acid (0.6 mmol); K2CO3 (1

mmol); solvent（EtOH/H2O (v/v=1:1) 1 mL); catalyst 3.0 mg.
b Isolated yield by column chromatography.
c TOF (turnover frequency) = mols of coupling product /mols of Pd/time.
d From the reference [14]. Reaction conditions: 0.5 mmol of aryl halide, 0.75 mmol of
boronic acid, 1.25 mmol of Na2CO3, 2 mg of Pd–CoFe2O4 (atomic percent of Pd is
1.13%), 5 mL ethanol.
e From the reference [21]. Reaction conditions: aryl halide (1 mmol), arylboronic acid
(1.2 mmol), base (2 mmol), 4 mL of 1:1 DMF/H2O and Pd/NiFe2O4 (0.1 mol%) at
90 °C.
f Based on the data from the references.
Table 3 Optimization of the reaction conditions a
c
Catalyst
TOF /h-1
（×103）
—
Entry
Solvent
Base
Yield b (%)
amount (mg)
1
2
3
3
3
3
3
3
3
3
3
2
4
Acetonitrile
Water
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
K3PO4
CsCO3
NaOH
None
Trace
Trace
18
—
3
EtOH
4.56
4
EtOH/H2O (1:1)
EtOH/H2O (1:1)
EtOH/H2O (1:1)
EtOH/H2O (1:1)
EtOH/H2O (1:1)
EtOH/H2O (1:1)
EtOH/H2O (1:1)
EtOH/H2O (1:1)
98
24.83
7.09
5
28
6
93
23.60
8.60
7
34
8
Trace
Trace
97
—
9
—
10
K2CO3
K2CO3
36.90
18.80
11
99
a
Reaction condition: 4-nitrobromobenzene (0.5 mmol); phenylboronic acid (0.6
mmol); catalyst CPFO-M; base (1 mmol); solvent(1 ml); T=70 oC; t= 0.5 h.

b Isolated yield by column chromatography.
c TOF (turnover frequency) = mols of coupling product /mols of Pd/time.
Table 4 Suzuki coupling reaction of various aryl halides and arylboronic acids a
TOF c/h-1
(×103)
34.60
8.55
Entry
X
R1
R2
Time (h)
Yield b (%)
1
2
I
H
H
H
0.5
2
91
90
97
84
98
80
90
99
72
99
94
59
98
88
57
23
28
21
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
H
H
3
4-NO2
4-COOH
4-Me
2-Me
4-NH2
4-MeO
4-OH
4-Me
4-Me
4-Me
4-MeO
4-MeO
4-CN
4-Me
H
0.5
6
36.90
2.66
4
H
5
H
5
3.72
6
H
5
3.04
7
H
6
2.85
8
H
6
3.14
9
H
4
3.42
10
11
12
13
14
15d
16d
17d
18d
4-Me
4-Cl
5-NO2
4-Me
4-Cl
4-Me
4-Me
4-Me
H
4
4.70
5
3.57
5
2.24
5
3.72
6
2.79
13
13
13
13
0.83
0.34
0.41
H
0.31
a
Reaction condition: aryl halide (0.5 mmol); boronic acid (0.6 mmol); K2CO3 (1
mmol); solvent（EtOH/H2O (v/v=1:1) 1 mL); catalyst (2 mg); T=70 oC.
b Isolated yield by column chromatography.
c TOF (turnover frequency) = mols converted/mol of active site/time. d T=90 oC