Nanosized Pd assembled on superparamagnetic core-shell microspheres: Synthesis, characterization and recyclable catalytic properties for the Heck reaction

Article abstract of DOI:10.1016/j.cclet.2014.05.003

A series of magnetically recyclable Pd/Fe₃O₄@γ-Al₂O₃ catalysts were synthesized using the superparamagnetic Fe₃O₄@γ-Al₂O₃ core-shell microspheres as the supporter and nano-Pd particles assembled on γ-Al₂O₃ shell as the active catalytic component. The structure of the catalysts was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), N₂ adsorption-desorption and vibrating sample magnetometer (VSM). The catalytic activity and the recyclability properties of the catalysts for the Heck coupling reaction with aryl bromides and the olefins were investigated. The results show that the microspheres of the magnetic Pd/Fe₃O₄@γ-Al₂O₃ catalysts were about 400 nm and the nano-Pd particles assembled on γ-Al₂O₃ shell were about 3-4 nm in size. The saturation magnetization (MS) of the magnetic catalysts was sufficiently high to allow magnetic separations. In the Heck coupling reactions, the magnetic Pd/Fe₃O₄@γ-Al₂O₃ catalysts exhibited good catalytic activity and recyclability. With Pd/Fe₃O₄@γ-Al₂O₃ (0.021 mol%) catalyst, the bromobenzene conversion and product yield reached about 96.8% and 91.2%, respectively, at 120 °C and in 14 h. After being recycled for six times, the conversion of bromobenzene and the recovery of the catalyst were about 80% and 90%, respectively. The nano-Pd particles were kept well dispersed in the used Pd/Fe₃O₄@γ-Al₂O₃ catalysts.

Full text of DOI:10.1016/j.cclet.2014.05.003

G Model
CCLET 2981 1–6
Chinese Chemical Letters xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
1
2
Original article
3
4
5
6
Nanosized Pd assembled on
superparamagnetic core–shell microspheres:
Synthesis, characterization and recyclable catalytic
properties for the Heck reaction
7
8
Q1 Hao Yang, Da Shi, Sheng-Fu Ji *, Dan-Ni Zhang, Xue-Fei Liu
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China
A R T I C L E I N F O
A B S T R A C T
Article history:
4 2 3
A series of magnetically recyclable Pd/Fe O @g-Al O catalysts were synthesized using the super-
3
Received 31 January 2014
Received in revised form 18 April 2014
Accepted 29 April 2014
Available online xxx
paramagnetic Fe
assembled on -Al
characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), N
3
O
4
@
g
-Al
shell as the active catalytic component. The structure of the catalysts was
adsorption–
2 3
O core–shell microspheres as the supporter and nano-Pd particles
g
2 3
O
2
desorption and vibrating sample magnetometer (VSM). The catalytic activity and the recyclability
properties of the catalysts for the Heck coupling reaction with aryl bromides and the olefins were
Keywords:
investigated. The results show that the microspheres of the magnetic Pd/Fe
were about 400 nm and the nano-Pd particles assembled on -Al shell were about 3–4 nm in size.
The saturation magnetization (MS) of the magnetic catalysts was sufficiently high to allow magnetic
separations. In the Heck coupling reactions, the magnetic Pd/Fe -Al catalysts exhibited good
catalytic activity and recyclability. With Pd/Fe -Al (0.021 mol%) catalyst, the bromobenzene
3 4 2 3
O @g-Al O catalysts
Nano-Pd
g
2 3
O
Superparamagnetic
Core–shell
3
O
4
@
g
2 3
O
Catalyst
Heck reaction
3
O
4
@
g
2 3
O
conversion and product yield reached about 96.8% and 91.2%, respectively, at 120 8C and in 14 h. After
being recycled for six times, the conversion of bromobenzene and the recovery of the catalyst were about
80% and 90%, respectively. The nano-Pd particles were kept well dispersed in the used Pd/Fe
3 4 2 3
O @g-Al O
catalysts.
ß 2014 Sheng-Fu Ji. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
9
1
0
1. Introduction
However, due to their high specific surface areas and magnetic 25
properties, magnetic nanoparticles are often easily self-agglomer- 26
11
12
13
14
15
16
17
18
19
20
21
22
23
24
The Heck reaction catalyzed by the noble metal Pd is one of the
most important coupling reactions in organic syntheses [1,2].
Homogeneous and heterogeneous Pd catalysts are often used in
the Heck reaction. However, homogeneous catalysts are used in
less than one fifth of industrial applications due to the facts that the
homogeneous catalysts are easily lost after reaction, difficult to
separate from the reaction system and certainly costly [3].
Generally, Pd catalysts are initially synthesized as heterogeneous
catalysts followed by loading onto active carbon [4], metal oxides
[5], zeolites [6], polymers [7], or clay [8].
2
ated [11]. Therefore, SiO , C, and metal oxides, polymers are 27
usually used to modify the magnetic nanoparticles and to obtain 28
magnetic nanoparticles with special surface properties that still 29
keep their dispersiveness [12–15].
2 3
g-Al O has been used as 30
supporter, adsorbent, and catalyst, owing to its low cost, good 31
chemical stability, high surface area, acidic sites, and controllable 32
synthetic process [16–19]. In the catalytic industry,
ideal supporter. Particularly, the catalysts that are formed by 34
loading the noble metal on the -Al were used in organic 35
2 3
g-Al O is an 33
g
2 3
O
reactions [19–22]. It is still difficult, however, to separate the 36
catalyst from the liquid phase after the reaction. In our previous 37
research, we found an easy way to synthesize monodispersed 38
Magnetic catalysts have the incomparable advantages over the
traditional catalysts because of their high activity, magnetic
recyclability, and reusability. Research on the magnetic nano-
catalysts for the Heck reaction had been reported earlier [9,10].
Fe
that if we could coat the Fe
structures, and then load Pd onto the Fe
3
O
4
[23], which had uniformed diameter. Thus we hypothesized 39
with -Al to obtain core–shell 40
-Al nanospheres, 41
3
O
4
g
2 3
O
3
O
4
@g
2 3
O
the resulting catalysts would combine their advantages of high 42
activity, excellent mesoporous structure, magnetic recyclability, 43
*
Corresponding author.
E-mail address: jisf@mail.buct.edu.cn (S.-F. Ji).
and reusability.
44
http://dx.doi.org/10.1016/j.cclet.2014.05.003
1
001-8417/ß 2014 Sheng-Fu Ji. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Please cite this article in press as: H. Yang, et al., Nanosized Pd assembled on superparamagnetic core–shell microspheres: Synthesis,
characterization and recyclable catalytic properties for the Heck reaction, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/
j.cclet.2014.05.003

G Model
CCLET 2981 1–6
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H. Yang et al. / Chinese Chemical Letters xxx (2014) xxx–xxx
45
46
47
48
49
50
51
52
53
In this paper, we illustrated the construction of magnetically
recyclable Pd/Fe -Al nanocomposites possessed the
core–shell structure. To evaluate the activity and the stability of
the Pd/Fe -Al nanocomposites, the Heck coupling reac-
tions were chosen as the model reaction. Results showed that the
catalyst could be easily separated from the reaction system by
employing an external magnetic field, because of the super-
substrates, and time were investigated individually. The catalyst
was collected by an external permanent magnet, and the product
was analyzed by gas chromatography (GC). For recycling, the
collected catalyst was washed with tetrahydrofuran and separated
by an external permanent magnet, then dried under vacuum at
60 8C for 12 h. Then the catalysts were utilized for another run of
catalytic testing. Each time, catalyst loss was measured by
precision electronic balance.
104
105
106
107
108
109
110
111
3
O
4
@
g
2 3
O
3
O
4
@g
2 3
O
3 4
paramagnetic behavior of Fe O . Furthermore, it can be reused for
several cycles with sustained selectivity and activity.
2.4. Characterizations
112
5
4
5
2. Experimental
The X-ray diffraction (XRD) pattern was collected on a D/Max
113
114
115
116
117
118
119
120
121
122
123
124
125
126
5
2.1. Synthesis of Fe
3
O
4
@g
2
-Al O
3
2500 VB 2+/PC diffractometer (Rigaku, Japan) with Cu–K
a
˚
irradiation (
l
= 1.5418 A, 200 kV, 50 mA) with 2
u
values between
5
5
5
5
6
6
6
6
6
6
6
6
6
6
7
7
7
7
7
7
7
7
7
7
8
8
8
8
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
Fe
3
O
4
was synthesized according to
a
Á6H
slightly modified
O (10.8 g), NaAc
108 and 808. Transmission electron microscopy (TEM) was
performed with a JEOL (JEM-2100) transmission electron micro-
solvothermal method [23]. Briefly: FeCl
3
2
(28.8 g) and cetyltrimethyl ammonium bromide (CTAB, 0.014 g)
were dissolved in 400 mL of glycol under stirring. The obtained
homogeneous yellow solution was transferred into a Teflon-lined
stainless-steel autoclave. The autoclave was sealed and heated at
200 8C under 400 rpm stirring speed. After heating for 12 h, the
autoclave was cooled naturally to room temperature. The obtained
black magnetic particles were separated with a permanent
magnet, washed with ethanol six times, and dried in vacuum at
60 8C for 24 h.
scope (JEOL, Japan) operated at 200 kV accelerating voltage. The N
2
adsorption–desorption analysis was conducted with an ASAP
2020 M automatic specific surface area and aperture analyzer
(MICROMERITICS, USA). Magnetic properties of the samples were
measured using a vibrating sample magnetometer (VSM; Lake
Shore Model 7400, USA) under magnetic fields up to 18 kOe. The Pd
loading amount was determined by inductively coupled plasma
mass spectrometry (ICP-MS, SPECTRO ARCOS EOP; SPECTRO
Analytical Instruments GmbH, Germany).
3 4
The obtained Fe O particles (0.1 g) were dispersed in an
aluminum isopropoxide (AIP) ethanol solution (60 mL, 0.016 mol/
L) under ultrasonication. After 30 min, the solution was transferred
to a three-neck flask (250 mL) and stirred for 12 h at 45 8C to obtain
3. Results and discussion
127
128
the Fe
3
O
4
particles whose surface was saturated with AIP.
3.1. The structure of the catalyst
Subsequently, 50 mL of ethanol/water (5/1, V/V) was added into
the solution, and the mixture was allowed to stir for another 1 h to
complete the hydrolysis of AIP. Then the mixture was transferred
to a Teflon-sealed autoclave and heated at 80 8C for 20 h. The
obtained particles were separated with a permanent magnet,
washed several times with deionized water and ethanol, and
then dried in vacuum at 50 8C for 12 h. After that the products were
Fig. 1 shows the crystallinity and phase composition of the
samples by X-ray powder diffraction (XRD). The peaks of all
samples could be indexed to face center cubic magnetite phase
129
130
131
132
133
134
135
136
137
138
139
140
141
142
(Fe
45.98, 66.78 could be indexed to the characteristic diffraction peaks
of -Al (2 = 37.68, 45.78, 66.68; PDF No. 10-425), which
indicates that both Fe and -Al had been obtained [24].
Since the peak at 2 = 37.68 was so close to the Fe characteristic
diffraction peak that they could not be distinguished by the XRD.
Fig. 1 shows that the Pd/Fe -Al nanocomposites displayed
the characteristic diffraction peaks indexed to the Pd (0), which
suggested the presence of Pd (0) on Pd/Fe -Al composites.
3 4
O ; JCPDS No. 19-0629). The extra weak diffraction peaks at
g
2
O
3
u
put into a tube furnace and the system was purged with N
Then the tube furnace was heated from room temperature to
500 8C (1 8C/min) under the N (5 mL/min) ambience, and kept at
500 8C for 4 h. After cooling to room temperature naturally, the
Fe -Al was collected.
2
.
3
O
4
g
2 3
O
u
3 4
O
2
3
O
4
@
g
2 3
O
3
O
4
@g
2 3
O
3
O
4
@g
2 3
O
8
4
2.2. Preparation of Pd/Fe
3
O
4
@g
2
-Al O
3
The intensity became stronger as the concentration of Pd
increased.
85
86
87
88
89
90
91
92
93
94
95
96
97
98
0.1 g of the obtained Fe
a 2.5 mL PdCl
tion. After 30 min, the solution was diluted to 100 mL and
transferred to a three-neck flask, stirred for 12 h under 25 8C,
separated by a permanent magnet, and finally dried in vacuum at
100 8C for 12 h. The obtained products were put into a tube
3
O
4
@
g
-Al
2 3
O particles was dispersed in
2
aqueous solution (0.6 mg/mL) under ultrasonica-
furnace, which was purged with H
heated from room temperature to 200 8C (1 8C/min) under the H
(5 mL/min) ambience, and kept at 200 8C for 3 h. After cooling to
room temperature, Pd/Fe -Al (1.5 wt%) was collected.
By changing the volume of PdCl added, we synthesized a series of
Pd/Fe -Al catalysts with different Pd loadings in 1.5%,
2
. Then the tube furnace was
2
3
O
4
@g
2 3
O
2
3
O
4
@g
2 3
O
3.0%, 4.5%, 6.0%, respectively. They were marked as Pd-1, Pd-2,
Pd-3, Pd-4, respectively.
9
9
2.3. Catalytic reaction
100
101
102
103
3 4 2 3
For the Heck reactions, 10 mg of Pd/Fe O @g-Al O catalyst,
arylhalide, olefin, dodecane (as an internal standard substance),
and base were added to the solvent. The reaction was carried out
under reflux condition. The effects of Pd content, solvents,
3 4 2 3
Fig. 1. Wide-angle XRD patterns of Pd/Fe O @g-Al O with different Pd loading.
Please cite this article in press as: H. Yang, et al., Nanosized Pd assembled on superparamagnetic core–shell microspheres: Synthesis,
characterization and recyclable catalytic properties for the Heck reaction, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/
j.cclet.2014.05.003

G Model
CCLET 2981 1–6
H. Yang et al. / Chinese Chemical Letters xxx (2014) xxx–xxx
3
3 4 3 4 2 3 3 4 2 3
Fig. 2. SEM images of Fe O (a) and Fe O @g-Al O (b); TEM images of Fe O @g-Al O (c, d); TEM images of Pd-1 (e), Pd-2 (f), Pd-3 (g), Pd-4 (h).
143
144
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147
148
149
150
151
152
153
154
155
156
157
158
Fig. 2 shows typical SEM and TEM images of Fe
Pd/Fe -Al . It was easily observed, as in Fig. 2a–d, that the
diameters of the as-synthesized Fe nanoparticles were about
300 nm with a narrow size distribution, and that the thickness of
the -Al shell was about 40 nm. If observed carefully we could
find some porous structures, which resulted from the stacking of
smaller -Al particles on the outer space of the -Al shell.
Fig. 2e–f illustrated the TEM images of different loading of Pd/
Fe -Al (Pd-1, Pd-2, Pd-3, Pd-4). One could see from the
3
O
4
@
g
-Al
2
O
3
and
3 4 2 3
characteristics of Pd/Fe O @g-Al O with different Pd loading 159
3
O
4
@
g
2
O
3
were shown in Table 1. The isotherms of all the samples are type III 160
(definition by IUPAC). The appearance of type 3-H hysteresis loops 161
in isotherms was indicated because there was no presence of 162
3 4
O
g
2
O
3
orderly mesoporous structures in Fe
particles. After Pd was loaded, the BET surface area decreased 164
compared with Fe -Al magnetic particles. As the Pd 165
3
O
4
@g
-Al
2
O
3
magnetic 163
g
2
O
3
g
2
O
3
3
O
4
@g
2 3
O
loading increased, the decrease of BET surface area became more 166
significant. However, the pore volume remained constant, 167
indicating that the Pd clusters were loaded on the surface of the 168
3
O
4
@g
2 3
O
figure that there were some small Pd clusters, which stacked by
smaller Pd nano-particles with the average diameter of 3–4 nm on
Fe
3
O
4
@g
2 3
-Al O [25], which was consistent with the TEM results. 169
3 4 2 3
the surface of Fe O @g-Al O . As the Pd loading increases, the
clusters tend to increase. However, excess Pd loading may lead to
Pd cluster agglomerating.
Nitrogen adsorption–desorption isotherms of all the samples
are illustrated in Fig. 3, and the textural and structural
Magnetic properties of the samples were investigated by a 170
vibrating sample magnetometer (VSM) at room temperature with 171
the applied magnetic field ranging from À18,000 to 18,000 Oe. As 172
illustrated in Fig. 4, all samples were superparamagnetic nano- 173
particles. Compared with Fe
intensity of the -Al @ Fe
successively decreased. Even so, the MS of the Pd/
was adequately high to allow its easy separation from the reaction 177
3
O
4
, the saturation magnetization (MS) 174
and Pd-1, Pd-2, Pd-3, Pd-4 175
-Al @Fe 176
g
2
O
3
3 4
O
g
2
O
3
3 4
O
system.
3 4 2 3
.2. Catalytic performance of Pd/Fe O @g-Al O
178
3
179
The Heck reaction of bromobenzene with styrene was carried 180
out as a model reaction to evaluate the catalytic ability of the 181
Table 1
Textural and structural characteristics of Pd/Fe
loading.
3 4 2 3
O @g-Al O with different Pd
Catalyst
Palladium (wt%)
Textural and structural
characteristics
S
BET
2
V
BJH
3
Dv
(m /g)
(cm /g)
(nm)
Fe
Fe
3
O
O
4
4
–
468.6
179.3
166.8
153.9
138.6
124.5
0.35
0.26
0.24
0.23
0.23
0.25
2.3
5.9
5.7
5.8
5.8
5.7
3
@
g-Al
2
O
3
–
Pd-1
Pd-2
Pd-3
Pd-4
1.5
3.0
4.5
6.0
2 3 4 2 3
Fig. 3. N adsorption–desorption isotherms of Pd/Fe O @g-Al O with different Pd
loading.
Please cite this article in press as: H. Yang, et al., Nanosized Pd assembled on superparamagnetic core–shell microspheres: Synthesis,
characterization and recyclable catalytic properties for the Heck reaction, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/
j.cclet.2014.05.003

G Model
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H. Yang et al. / Chinese Chemical Letters xxx (2014) xxx–xxx
rate of bromobenzene became faster. The conversion of bromo-
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
benzene increased slowly with a low Pd/bromobenzene molar
ratio (Fig. 5a, 0.007%) and needed 24 h to finish the reaction,
whereas a higher Pd/bromobenzene molar ratio (Fig. 5c, 0.021%)
needed 14 h to finish the reaction. Furthermore, an increase in the
Pd/bromobenzene molar ratio could not help to decrease the
reaction time (Fig. 5d). For the Pd atom utilization, the conversion
of Pd-1–Pd-4 was 60.5%, 81.7%, 96.8%, 99.3% in 14 h, respectively.
The conversion of bromobenzene rose to about 18% when the Pd
content increased 1.5 wt%, and it decreased with the increased Pd
content, which means that the dominant factor was changed from
the number of active sites to the mass transfer rate of the
substrates [26]. In addition, increasing the Pd content led to serious
Pd clusters agglomerating (Pd-4) judged by the TEM image, which
suggests that the a Pd content of Pd-3 (4.5 wt%) was adequate to
catalyze the reaction.
Table 2 show the Heck activity of bromobenzene and styrene
catalyzed by Pd-3 catalysts under different reaction conditions. As
we can see from entry 1 to entry 4, temperature was a key factor for
the Heck reaction. As the temperature rose, the conversion of
bromobenzene increased; when the temperature reached to
3 4 2 3
Fig. 4. Magnetization curves of Pd/Fe O @g-Al O with different Pd loading. (a)
Fe O , (b) Fe O @g-Al O , (c) Pd-1, (d) Pd-2, (e) Pd-3, (f) Pd-4.
3 4 3 4 2 3
120 8C, a 96.8% conversion of bromobenzene was achieved
with a high yield of 91.2%. High temperature enabled more
molecule activated, thus increasing the reaction rate. For the
purposes of energy saving and prolonging the life of the catalyst,
lower reaction temperatures are more desirable, thus 120 8C was
the best reaction temperature for the Pd-3 catalyst. It was also
found that inorganic base was more effective than organic base,
2 3 2 3 3
and Na CO was the ideal base among Na CO , NaOAc, and Et N
tested (entry 4 to entry 6)
Table 3 shows a comparison of the activity and reaction
conditions of supported Pd catalysts in the Heck reaction of
bromobenzene with styrene that had been published in the
literature. As we can see from the table, the diameter of Pd was
the key factor. Decreasing the diameter of the catalyst means
more active sites can be exposed to the reaction system.
Comparison with the TOF value of different catalyst revealed that
the Pd-3 catalyst had a higher TOF value at a lower reaction
temperature (120 8C), shorter reaction times (14 h), and lower Pd
molar content (0.021 mol%) as compared to other supported
3 4 2 3
Fig. 5. Time-dependent Heck activities of Pd/Fe O @g-Al O catalysts with
different Pd loading.
Pd-based catalysts such as Pd loaded on mesoporous silica or TiO
5]. The activity of Pd on carbon nanofiber [27] was higher than
that of the Pd-3 catalyst, but carbon nanofiber was not as effective
as Fe -Al in terms of the separation from the reaction
2
[
182
183
184
185
186
187
188
189
190
191
192
Pd/Fe
Pd content, temperature, base, and solvent on the reaction. Time-
dependent Heck reaction activity of the Pd/Fe -Al
catalysts with different Pd loadings is shown in Fig. 5, with
reaction conditions: 10 mg of Pd/Fe -Al catalyst, bromo-
benzene (20 mmol), styrene (30 mmol), dodecane (10 mmol, as
internal standard substance), Na CO (24 mmol), NMP (N-methyl-
2-pyrrolidone, 40 mL), and reaction temperature (120 8C). It can
be observed from Fig. 5 that the conversion of bromobenzene
increased to as much as 100% given sufficient reaction time. The
difference is that with the increase of Pd content, the conversion
3
O
4
@
g
-Al
2
O
3
. We systematically investigated the effects of
3
O
4
@g
2 3
O
systems, stability, and recyclability.
3
O
4
@g
2 3
O
3.3. Relationship between the solvents, substrates and the catalytic
performance of the catalyst
238
239
3
O
4
@g
2 3
O
2
3
The condition of Heck reaction was rigorous, requiring an
argon atmosphere to avoid the Pd leaching while maintaining
the high activity in an anhydrous environment, and phosphine
ligands to stabilize the catalyst, but the phosphine ligands are
toxic and should be avoided. The solvents were often DMF
240
241
242
243
244
Table 2
a
Heck reaction activities of Pd-3 catalysts under different reaction conditions
Entry
Temperature (8C)
Base
Conversion (%)
Yield (%)
Selectivity (%)
1
2
3
4
5
6
90
100
110
120
120
120
Na
Na
Na
Na
2
2
2
2
CO
CO
CO
3
3
3
3
19.7
47.7
75.2
96.8
82.6
94.6
17.2
43.9
70.5
91.2
76.9
88.2
87.3
92.0
93.8
94.2
93.1
93.2
CO
b
3
Et N
NaOAc
a
Reaction system: 20 mmol bromobenzene, 30 mmol styrene, 24 mmol base, 10 mmol dodecane, 40 mL NMP; reaction time: 14 h; catalyst: 0.01 g Pd-3 (4.5 wt%).
b
3
Et N: triethylamine.
Please cite this article in press as: H. Yang, et al., Nanosized Pd assembled on superparamagnetic core–shell microspheres: Synthesis,
characterization and recyclable catalytic properties for the Heck reaction, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/
j.cclet.2014.05.003

G Model
CCLET 2981 1–6
H. Yang et al. / Chinese Chemical Letters xxx (2014) xxx–xxx
5
Table 3
Comparison of the activity and reaction conditions of supported Pd catalysts in the Heck reaction of bromobenzene with styrene published in the literature.
a
b
Catalyst
Pd diameter (nm)
Catalyst (mol% Pd)
Temperature (8C)
Time (h)
Yield (%)
TOF
Reference
Pd/mesoporous silica
ꢀ10
5
0.1
170
140
120
130
120
48
24
10
24
14
82
17.1
7.7
[28]
Pd/TiO
Pd/carbon nanofiber
Fe –NH –Pd
2
0.5
92
[5]
2–3
8.9
3–4
0.01
5
95
950.0
0.8
[27]
3
O
4
2
96
[29]
Pd-3
0.021
91.2
310.2
This paper
a
Moles of Pd/moles of bromobenzene.
b
À1
À1
molproduct mol Catalyst
h .
Table 4
Heck reaction activities of Pd-3 catalysts under different reaction conditions.
a
Entry
Solvent
Conversion (%)
Selectivity (%)
Yield (%)
1
2
3
4
NMP
96.8
98.7
94.2
92.1
94.2
94.4
91.2
90.9
94.2
94.4
DMF
DMSO
DMAc
100.0
100.0
a
Reaction system: 20 mmol bromobenzene, 30 mmol styrene, 24 mmol base,
0 mmol dodecane, 40 mL solvent; reaction time:12 h; catalyst: 0.01 g Pd-3
1
(4.5 wt%).
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
(N,N-dimethylformamide), DMAc (dimethylacetamide), NMP
(N-methyl-2-pyrrolidone), DMSO (dimethyl sulfoxide), etc. In
the coupling reaction, the organic groups of the substrates varied
from hydrophilic groups to hydrophobic groups or from electron-
withdrawing groups to electron donating groups. So the nature of
the solvents and substrates can influence the catalytic perfor-
mance of the catalyst. Commonly, a catalyst had a high activity
only in a specific solvent [5,30]. In contrast, our Pd-3 catalyst was
suitable for most of the common solvents with a high conversion
and yield, as shown in Table 4.
The effects of substituted groups in substrates were also
examined. As illustrated in Table 5, the conversion of substrates
with electron withdrawing groups in the para-position, at entry 1
to entry 2, was much higher than the substrates with electron
donating groups, at entry 4 to entry 5. This result was the same as
the other supported Pd catalysts result, which could be explained
by the fact that the electron-withdrawing groups facilitate the
oxidative addition during the Heck reaction [31]. When the olefin
changed from styrene to straight-chain olefins, such as butyl
acrylate or methyl acrylate, the conversion of bromobenzene
decreased under the same reaction conditions, as shown at entry 6
to entry 7.
Fig. 6. Recycling of the Pd-3 catalyst in Heck reaction.
important factors. Here our Pd-3 catalyst was reused for six times, 270
each time after reaction the catalyst was separated from the 271
reaction system by a permanent magnet, and the collected catalyst 272
was washed with tetrahydrofuran and dried under vacuum at 273
60 8C for 12 h; catalyst loss was measured by precision electronic 274
balance, then the catalyst was used in the next cycling under the 275
same reaction conditions and giving 94.2%, 89.4%, 89.1%, 84.7% and 276
78.6% isolated yield for the second, third, fourth, fifth and sixth 277
runs, respectively, as shown in Fig. 6.
278
Fig. 6 also shows that the weight loss of the catalyst was only 2% 279
or less in each regeneration process, and the total weight loss of the 280
catalyst was only 10% after six cycles. A baseline loss may be 281
unavoidable in the regenerate process and the Pd leaching, which 282
is the reason that the catalyst activity dropped each cycle. The 283
conversion of bromobenzene decreased as the catalyst weight 284
decreased, but remained above 80% with selectivity remaining at 285
100% after being used for six times. After use, the catalyst was 286
dissolved in concentrated nitric acid and analyzed by inductively 287
coupled plasma mass spectrometry (ICP-MS) to determine the 288
2
67
3 4 2 3
3.4. Stability and reusability of Pd/Fe O @g-Al O
2
2
68
69
For practical application in the Heck reaction, the shelf life of
the heterogeneous catalysts and their reusability are very
Table 5
a
Heck reaction of various aryl bromides with different olefins catalyzed by Pd-3.
R1
Br
+
R1
Catalyst Base
Solvent
HBr.
+
R
R
1
Entry
R
R
Conversion (%)
Yield (%)
1
2
3
4
5
6
7
COCH
3
C
C
C
C
C
6
6
6
6
6
H
H
H
H
H
6
6
6
6
6
100.0
100.0
88.8
67.2
60.2
79.7
83.6
95.8
96.1
82.4
65.5
58.6
75.5
78.5
NO
H
2
OCH
3
NH
H
2
COOi-Bu
COOCH
H
3
a
Reaction system: 20 mmol halohydrocarbon, 30 mmol olefin, 24 mmol base,
1
0 mmol dodecane, 40 mL NMP; reaction time:12 h; catalyst: 0.01 g Pd-3 (4.5 wt%).
Fig. 7. TEM images of the Pd-3 catalyst recycled for six times in Heck reaction.
Please cite this article in press as: H. Yang, et al., Nanosized Pd assembled on superparamagnetic core–shell microspheres: Synthesis,
characterization and recyclable catalytic properties for the Heck reaction, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/
j.cclet.2014.05.003

G Model
CCLET 2981 1–6
6
H. Yang et al. / Chinese Chemical Letters xxx (2014) xxx–xxx
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Please cite this article in press as: H. Yang, et al., Nanosized Pd assembled on superparamagnetic core–shell microspheres: Synthesis,
characterization and recyclable catalytic properties for the Heck reaction, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/
j.cclet.2014.05.003