Preparation of a titania-supported highly dispersed palladium nano-catalyst and its application in Suzuki and Heck coupling reactions

Article abstract of DOI:10.1002/aoc.2966

In this work, palladium complexes nanoparticles in titania are prepared by a pH-controlled adsorption and without pH-controlled adsorption method. This method results in high-dispersion palladium on the titania surface. We demonstrate the use of the titania-supported palladium as an efficient catalyst for Suzuki and Heck reactions of a representative range of aryl bromides and chlorides. The reusability of catalyst was tested, and deactivation process of the catalyst was not observed after four recycles. The catalysts were characterized by FT-IR, NMR, elemental analysis, field emission scanning electron microscopy, transmission electron microscopy and X-ray diffraction. Copyright

Full text of DOI:10.1002/aoc.2966

Full Paper
Received: 27 September 2012
Revised: 10 December 2012
Accepted: 13 December 2012
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.2966
Preparation of a titania-supported highly
dispersed palladium nano-catalyst and its
application in Suzuki and Heck coupling
reactions
Kazem Karami*, Mahlagha Bahrami Shehni and Nasser Rahimi
In this work, palladium complexes nanoparticles in titania are prepared by a pH-controlled adsorption and without pH-controlled
adsorption method. This method results in high-dispersion palladium on the titania surface. We demonstrate the use of the
titania-supported palladium as an efficient catalyst for Suzuki and Heck reactions of a representative range of aryl bromides
and chlorides. The reusability of catalyst was tested, and deactivation process of the catalyst was not observed after four
recycles. The catalysts were characterized by FT-IR, NMR, elemental analysis, field emission scanning electron microscopy,
transmission electron microscopy and X-ray diffraction. Copyright © 2013 John Wiley & Sons, Ltd.
Keywords: titania; Suzuki reaction; Heck reaction; adsorption method; nano-catalyst
in large-scale processes. TiO2 usually shows pH-dependent
Introduction
surface charges when immersed in aqueous solution because of
the existence of Ti-OH on the surface.[18] The absorption of TiO2
The transition metal-catalyzed cross-coupling reaction for the
formation of carbon–carbon bonds has recently appeared as a
modern method in organic synthesis.[1–3]The palladium-catalyzed
coupling of aryl halide with arylboronic acid has evolved into one
of the most important and successful methods for forming
biaryls.[4,5] In recent years, the development of ligands has
afforded highly active palladium catalysts for Suzuki reactions.[6]
However, homogeneous catalytic systems have their own
shortcomings. For example, the separation processes are usually
complex.[7,8] Theoretically, supported palladium catalysts having
high activity offer several noteworthy practical advantages in
synthetic and industrial chemistry. Among those, easy separation
of the catalyst from the desired reaction product and easy recovery
and reuse of the catalyst are important.[9] An assortment of solid
materials as supports has been used to support palladium species.
Several MCM-41-supported ligand-free Pd as efficient catalysts for
Heck reactions of aryl bromides with styrene and methyl acrylate
under aerial conditions have been reported.[10] More recently,
Oyamada et al. have studied the hydrogenation reaction of C–C
double and triple bonds in a range of substrates using a
continuous-flow system with polysilane-supported palladium/
alumina (Pd(PSi/Al2O3)) hybrid catalyst.[11] Also, Polshettiwar
et al.[12] and Modak et al.[13] demonstrated the activity of several
silica-supported Pd catalysts in a variety of coupling reactions
(e.g. Suzuki, Heck, Sonogashira). Moreover, Ambulgekar and
co-workers carried out the Heck reaction of iodobenzene with
methyl acrylate using Pd/C as catalyst in the presence of
ultrasound at ambient temperature.[14] In addition, polymeric
materials as supports have been successfully utilized in synthesizing
catalysts for coupling reactions.[15–17] The highly dispersed catalyst
increases catalytic activity and reusability, which would allow
use of low palladium loading for reducing the cost of products
relies on an environment of different pH value. In the present
study, we report the preparation of highly dispersed palladium
complexes in TiO2 by the adsorption method for the Suzuki and
Heck coupling reactions. Palladium complex 1 was reported in
our previous work[19] and 2 was synthesized by the method
reported in our previous paper[20] and well characterized.
Furthermore, we indicate that the heterogeneous catalyst 2/TiO2
can be used in Suzuki and Heck reactions utilizing various aryl
halides and recovered and reused four times without loss of activity
in the Suzuki reaction of bromobenzene with phenylboronic
acid. The dispersed palladium complexes in TiO2 are shown
in Fig. 1.
Experimental
Materials and Techniques
All chemicals such as commercial titania P-25, solvent and other
chemicals were purchased from Merck and Aldrich. Conversions
were monitored using an Agilent 6890N gas chromatograph
equipped with a capillary HP-5+ column, based on aryl halides.
The column properties were: 30 m length, 0.32 mm internal
diameter and 0.25 mm film thickness. A Hitachi S4160 field
*
Correspondence to: Kazem Karami, Department of Chemistry, Isfahan University
of Technology, Isfahan 84156/83111, Iran. Email: karami@cc.iut.ac.ir
Department of Chemistry, Isfahan University of Technology, Isfahan 84156/
83111, Iran
Appl. Organometal. Chem. (2013)
Copyright © 2013 John Wiley & Sons, Ltd.

K. Karami et al.
1.5 mmol base, 8 ml methanol, 0.02 g 1/TiO2 or 2/TiO2 as catalyst.
The mixture was stirred in a pre-heated oil bath. After the
reactions were completed, the solid catalyst was separated by
centrifuge, washed with water several times to eliminate base
and solid, washed with acetone to eliminate adsorbed organic
substrate and dried at 100ꢀC. The product of the reaction blend
was analyzed by GC. Also, the products were characterized by
comparing their melting points, 1H and 13C NMR spectra with
those found in the literature.[21]
Figure 1. The structures of dispersed palladium complexes.
emission scanning electron microscope was employed to
show nano dimensions. Powder X-ray diffraction (XRD) patterns
were recorded on a Philips X’Pert diffractometer with Cu Ka
radiation at 40 kV and 30 mA, with a scan speed of 2ꢀ minÀ1
and a range of 2θ from 5ꢀ to 80ꢀ. The morphology, structure
and energy-dispersive spectroscopy (EDS) of catalyst were
analyzed on a JSM2100F transmission electron microscope
with an EDS spectrometer. The sample for transmission electron
microscopy (TEM) was prepared by placing a drop of the
suspension in ethanol on to a continuous carbon-coated copper
TEM grid and drying at room temperature under atmospheric
pressure.
Results and Discussion
XRD Pattern
Powder XRD analysis confirmed that the TiO2 nanoparticles were
in the anatase phase in comparison with standard diffractions
from JCPDS card 21-1272. In the case of 1/TiO2, the XRD pattern
indicated that the amount of metallic Pd was less than 3%,
verifying complex 1 coated on the TiO2 (Fig. 2).
Field Emission Scanning Electron Microscopy (FESEM)
Microstructure
The FESEM images showed catalysts with nanoparticles. The
surface morphology of the catalysts has also been investigated.
It was indicated that palladium complex 1 was agglomerated
on the TiO2, which was synthesized without pH adsorption
method (Fig. 3a). But in the case of 2/TiO2, it was verified that pH
adsorption method played an important role in the distribution of
nanoparticles (Fig. 3b).
Catalyst Preparation
Synthesis of palladium complexes
Synthesis of complex 1 has been reported previously..[19] Besides,
complex 2 was prepared by the method reported in our earlier
work.[20]
2: Yield 93%; m.p. 180–184. Elemental analysis calcd for
C25H23NIPPd: C, 49.90; H, 3.85; N, 2.33. Found: C, 49.79; H, 3.76; N,
2.41%. IR (KBr, cmÀ1); a (N-H) 3223, s (N-H) 3161. 1HNMR (300MHz,
ppm, CDCl3); d = 4.02 (brs, 2H, CH2), 4.23(brs, 2H, NH2), 6.37
Transmission Electron Micrsoscopy (TEM) Microstructure and
Energy-Dispersive Spectroscopy (EDS)
3
(m, 2H, C6H4), 6.82 (t, 1H, C6H4, JHH = 6 Hz), 6.95 (d, 1H, C6H4,
3JHH = 6 Hz), 7.30 (m, 9H, 3C6H5), 7.69 (m, 6H, 3C6H5). 13CNMR
(100 MHz, CDCl3, ppm); Caliphatic {d =49.2}, Caromatic {d = 120.05, 120.29,
To study the possible reason for the different catalytic performance
of the catalysts, the surface morphology of the catalysts was also
characterized by TEM. A series of TEM figures were taken from
1/TiO2 and 2/TiO2 catalysts before use in the Suzuki and Heck
reactions, and are presented in Fig. 4(a) and (b) respectively,
which indicate size distribution and EDS spectra of the
isolated catalyst. Evidently, highly dispersed spherical palladium
complex nanoparticles of size 5–20 nm on the TiO2 surface
were observed.
2
122.63, 123.76, 132.89, 137.10, 134.18, 134.30 (Co, PPh3, JPC = 16),
1
125.71, 126.03, 129.71, 130.05, 135.54, 135.66 (Ci, PPh3, JPC = 40)}.
31PNMR (300 MHz, CDCl3, ppm): 42.21.
Nano-catalyst preparation
2/TiO2 catalyst was prepared according to the adsorption
method by following the literature.[18] 0.68 g TiO2 was added to
60 ml aqueous solution of 2 (0.01 mol lÀ1). pH was adjusted 1.5
by 2 M HCl aqueous solution and the suspension was vigorously
stirred for 6 h at room temperature. By filtration of the suspension
after adsorption, the powdery solid was separated and washed
with a large amount of water several times. Then powdery solid
was dried at 100ꢀC. To prepare 1/TiO2 this process was done
using 1 without pH adjustment.
EDS provides an elemental analysis of the samples,
showing the presence and relative distribution of the present
Catalytic Reactions
In the Suzuki reaction, 0.02 g (0.12 mol% Pd) 1/TiO2 or 2/TiO2
as heterogeneous catalyst, 0.5 mmol aryl halide, 0.75 mmol
phenylboronic acid and 1.5 mmol base were added to a balloon,
followed by addition of methanol 8 ml. The mixture was stirred in
a pre-heated oil bath. In the Heck reaction, a round-bottom flask
was charged with 0.5 mmol aryl halide, 0.75 mmol alkene,
Figure 2. Powder XRD profile of 1/TiO2 nanoparticle.
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Appl. Organometal. Chem. (2013)

A titania-supported Pd nano-catalyst and its application
1/TiO2 was not as satisfactory as complex 1 in the similar reaction
in homogeneous media.[22] Referring to our previous report,[22]
1 did not show high catalytic activity in homogeneous media.
Also, the reaction of bromobenzene with phenylboronic acid
(Table 1, entry 7) and iodobenzene with styrene (Table 2,
entry 1) using 1/TiO2 did not proceed well. Hence we stopped
applying this catalyst in the coupling reactions. For catalytic
activity of complex 2, several Suzuki reactions of reactive
and unreactive aryl bromides were carried out (Table 1, entries
10–14). It was revealed that 2/TiO2 had better performance
than 2 in the coupling reactions (Table 1). The results verified
that, apart from the rate of reactivity of complexes 1 and 2,
the pH-controlled adsorption method played an important role
in the high catalytic performance of 2/TiO2 in the coupling
reactions.
(a)
The reactivity of 2/TiO2 was also tested in Suzuki reactions of
aryl chlorides with phenylboronic acid. Surprisingly, in spite of
using sterically hindered aryl chlorides, bearing electron-
donating groups, the yields of the reactions were promising
(Table 1, entries 8–10). 4-bromoacetophenone and 2-bromotoluene
did not react using only TiO2 as catalyst (Table 1, entries 1 and 5).
For further investigations, the efficiency of 1/TiO2 and 2/TiO2
was also studied in Heck reactions (Table 2). Similar to the Suzuki
reaction, the catalytic performance of 1/TiO2 in the Heck reaction
of iodobenzene with styrene was poor (Table 2, entry 1).
However, relating to the low C-I bond strength compared with
C-Br, iodobenzene was used instead of bromobenzene.
The coupling of styrene and methyl acrylate with a variety of
aryl halides was also examined using 2/TiO2 as the catalyst. Good
to excellent yields were gained, apart from the substituents of
aryl halides.
(b)
Figure 3. SEM image of (a) 1/TiO2 and (b) 2/TiO2.
The results demonstrate that in this case the Heck reaction
could be utilized with aryl bromides and chlorides having
different functional groups: cyano, amino, methyl, etc. (Table 2,
entries 2–14). In addition, there was high catalytic activity in the
reaction of 2-chlorotoluene with styrene using catalyst 2/TiO2
(Table 2, entry 12). With regard to alkenes, styrene served as an
appropriate coupling partner (Table 2, entries 2, 4–6, 8–10, 12
and 13). Furthermore, based on Li et al.’s report,[18] it is concluded
that PdO and PdCl2 will not work in the Heck reaction as
efficiently as 2/TiO2 under the reaction conditions employed in
this work.
elements. The EDS spectrum, Fig. 4(a) (1/TiO2) and (b) (2/TiO2),
obtained for palladium complex/TiO2 catalyst indicates successful
deposition of the palladium complex nanoparticles on the
titania surface.
Cross-Coupling Reactions
In the beginning, the Suzuki reaction of bromobenzene and
phenylboronic acid was chosen as a model reaction to test the
catalysts 1/TiO2 and 2/TiO2. Derived from our previous results,[22]
we chose MeOH as the solvent, Na2CO3 and K3PO4.3H2O as the
bases and 65ꢀC as the reaction temperature. Since the reactions
were not completed in 1 h, the reaction time was increased up
to 7 h. To evaluate the role of the pH-controlled adsorption
method, 2/TiO2 was prepared by this method and 1/TiO2 was
prepared without this method.
To assess the catalytic reactivity of 2/TiO2, we performed a
number of Suzuki reactions of phenylboronic acid with several
aryl bromides (Table 1, entries 1–6). Aryl bromides with
electron-withdrawing substituents reacted smoothly (Table 1,
entries 1–3). Considering the steric effect, aryl bromides with
ortho substituents, e.g. 2-bromotoluene and 2- bromoaniline,
provided the corresponding products in high yields (Table 1,
entries 5 and 6). Conversely, in the presence of 1/TiO2, Suzuki
reaction of bromobenzene with phenylboronic acid did not
proceed well (Table 1, entry 7). This result, with respect to the
reaction conditions, showed that the catalytic reactivity of
For further studies, more Suzuki reactions of aryl bromides
were carried out in the presence of 1/TiO2 (Table 3). These
reactions were much more difficult than those when 2/TiO2
was applied.
Also, to generalize the scope of the reactions, phenylboronic
acid derivatives were utilized. The results are shown in Table 4.
In order to contrast, palladium complexes 1 and 2 were used
instead of PdCl2 in this work. In comparison with the catalyst
reported previously,[18] the catalyst 2/TiO2 showed reactivity at
lower reaction temperature and in shorter reaction times. In
particular, aryl chlorides reacted much more smoothly (Table 2,
entries 10–14).
It is well known that metal catalyst supported on titania
exhibits the ’strong metal–support interaction’ (SMSI) phenomenon.
This is due to the decoration of the metal surface by an
electron transfer between the support and the metals.[23,24]
Thus, owing to the charge transfer from Ti species to Pd, it
can be inferred that the reaction mechanism begins with a
reduction step in which the Pd( Ι) complex is converted into
the Pd(0) catalyst.
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K. Karami et al.
a
b
Figure 4. TEM image and EDS spectra of nano-catalysts (a) 1/TiO2 and (b) 2/TiO2.
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A titania-supported Pd nano-catalyst and its application
Table 1. The effect of catalyst on Suzuki reaction of aryl halides with phenylboronic acida
Entry
R
X
Base
Yield (%)b
1
p-CH3CO
p-CN
p-CF3
H
Br
K3PO4.3H2O
K3PO4.3H2O
K3PO4.3H2O
Na2CO3
94, 0c
95
2
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Br
Br
Br
Br
3
96
4
90
80, 0c
5
o-CH3
o-NH2
H
K3PO4.3H2O
K3PO4.3H2O
Na2CO3
6
76
7
30d
73
8
H
Na2CO3
9
o-CH3
o-NH2
p-CH3CO
H
K3PO4.3H2O
K3PO4.3H2O
K3PO4.3H2O
Na2CO3
64
10
11
12
13
14
61
68e
64e
52e
48e
o-CH3
o-NH2
K3PO4.3H2O
K3PO4.3H2O
aReaction condition: bromobenzene (0.5 mmol), phenylboronic acid (0.75 mmol), catalyst (0.12 mol% Pd), Na2CO3 (1.5 mmol), MeOH (8 ml).
bIsolated yields (average of two experiments).
cOnly TiO2 was used as catalyst.
d1/TiO2 was used as catalyst.
eComplex 2 was used as catalyst.
Table 2. The effect of catalyst on Heck reaction of aryl halide with alkealkenesa
Entry
R1
R2
X
Base
Na2CO3
Temp. (ꢀC)/time (h)
Yield (%)b
1
H
Phenyl
Phenyl
COOMe
Phenyl
Phenyl
Phenyl
COOMe
Phenyl
Phenyl
Phenyl
COOMe
Phenyl
Phenyl
COOMe
I
65/7
30c
93
90
93
91
90
86
82
76
68
63
58
73
67
2
p-CH3CO
p-CH3CO
p-CN
p-CF3
H
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
Cl
K3PO4.3H2O
K3PO4.3H2O
K3PO4.3H2O
K3PO4.3H2O
Na2CO3
65/10
65/10
65/10
65/10
65/10
65/10
65/10
65/10
65/10
65/10
65/10
65/10
65/10
3
4
5
6
7
H
Na2CO3
8
o-CH3
o-NH2
H
K3PO4.3H2O
K3PO4.3H2O
Na2CO3
9
10
11
12
13
14
H
Na2CO3
o-CH3
p-CH3CO
p-CH3CO
K3PO4.3H2O
K3PO4.3H2O
K3PO4.3H2O
aReaction conditions: aryl halide (0.5 mmol), alkene (0.75 mmol), catalyst (0.12 mol% Pd), base (1.5 mmol), MeOH (8 ml).
bIsolated yields (average of two experiments).
c1/TiO2 was used as catalyst.
Reusability of the Catalysts
heterogeneous catalysts are significant aspects in coupling
reactions, the catalyst was isolated from the reaction combination
by centrifuge, washed and dried at 120ꢀC for 2 h. The recovered
catalyst was subsequently utilized in this reaction under similar
conditions, giving 89% yield for the second and 87% yield
In this work, catalyst 2/TiO2 was applied in the reaction of
bromobenzene with phenylboronic acid, giving 90% yield for the
first cycle (Table 1, entry 4). As the lifetime and reusability of
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K. Karami et al.
Table 3. The effect of catalyst on Suzuki reactions of aryl bromides with phenylboronic acida
Entry
R
Yield (%)b
1
2
3
4
p-CH3CO
p-CN
28
32
5
o-CH3
o-NH2
8
aReaction conditions: aryl bromide (0.5 mmol), phenylboronic acid (0.75 mmol), catalyst (0.12 mol% Pd), Base (1.5 mmol), MeOH (8 ml).
bIsolated yields (average of two experiments).
Table 4. The effect of catalyst on Suzuki reactions of aryl halides with phenylboronic acid derivativesa
Entry
R1
R2
X
Yield (%)b
1
2
3
4
5
6
p-CH3CO
p-CH3CO
o-CH3
p-CH3O
o-CH3
Br
97
89
86
80
78
62
Br
Br
Br
Cl
Cl
p-CH3O
o-CH3
o-CH3
p-CH3CO
o-CH3
p-CH3O
o-CH3
aReaction conditions: aryl halide (0.5 mmol), (R)-phenylboronic acid (0.75 mmol), catalyst (0.12 mol% Pd), base (1.5 mmol), MeOH (8 ml).
bIsolated yields (average of two experiments).
Figure 5. Recycling of 2/TiO2 catalyst in the Suzuki reaction between
bromobenzene and phenylboronic acid.
for the third and forth runs (Fig. 5). Additionally, the catalyst 2/
TiO2 was characterized by TEM after using four times. Its image
(Fig. 6) demonstrated that palladium complex 2 nanoparticles
did not agglomerate after four recoveries. Additionally, after reac-
tion of bromobenzene with phenylboronic acid (Table 1, entry 4),
the amount of palladium in the collected organic products solu-
tion was very low by ICP. Indeed, after recycling four times, leach-
ing of palladium was trace. Moreover, it can be concluded that
heterogeneous palladium is the catalytically active species.
Figure 6. TEM image of nano-catalyst 2/TiO2 after four recoveries.
All these results suggest that preparing catalyst by pH
adjustment method, as used in synthesis of 2/TiO2, results in
creating smaller particles and consequently produces catalysts
with high reactivity.
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A titania-supported Pd nano-catalyst and its application
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Conclusion
We prepared a capable palladium complex/TiO2 catalyst, 2/TiO2,
by a pH-controlled adsorption method. TEM images indicated
nanoparticle of the catalyst. This catalyst showed a high activity
and efficiency for Suzuki and Heck reactions using a range of acti-
vated and deactivated aryl halides. Also, different phenylboronic
acid derivatives were used in order to check the generality. In order
to assess, the catalyst can be simply retrieved and reused four times
without losing catalyst activity.
Acknowledgment
We thank Isfahan University of Technology for financial support.
[18] Z. Li, J. Chen, W. Su, M. Hong, J. Mol. Catal. A: Chem. 2010, 328, 93.
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Appl. Organometal. Chem. (2013)
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