A titania-supported highly dispersed palladium nano-catalyst generated via in situ reduction for efficient Heck coupling reaction

Article abstract of DOI:10.1016/j.molcata.2010.06.004

Supported metal catalysts have been applied in many current industrial processes. The dispersion of metal particles on solid supports has been proven to affect the performance of catalysts greatly. In this study, a titania-supported palladium catalyst prepared by a simple pH-controlled adsorption method is efficient and recyclable for the Heck coupling reaction of aryl halides and alkenes. The pH-controlled adsorption method results in high dispersion of palladium species on titania surface. During the reaction, palladium nanoparticles are in situ generated via reduction on the surface of titania. TEM images indicate that the nanoparticles are nearly monodisperse in size. Recycling studies have shown that the catalyst can be readily recovered and reused several times without significant loss of catalytic activity.

Full text of DOI:10.1016/j.molcata.2010.06.004

Journal of Molecular Catalysis A: Chemical 328 (2010) 93–98
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
A titania-supported highly dispersed palladium nano-catalyst generated via in
situ reduction for efficient Heck coupling reaction
Zhaohao Lia,b, Jing Chena, Weiping Sua,∗, Maochun Honga
a State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, The Chinese Academy of Sciences,
Yangqiao West Road, Fuzhou, Fujian 350002, China
b Graduate University of Chinese Academy of Sciences, Beijing 100039, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 April 2010
Received in revised form 2 June 2010
Accepted 2 June 2010
Available online 9 June 2010
Supported metal catalysts have been applied in many current industrial processes. The dispersion of
metal particles on solid supports has been proven to affect the performance of catalysts greatly. In this
study, a titania-supported palladium catalyst prepared by a simple pH-controlled adsorption method is
efficient and recyclable for the Heck coupling reaction of aryl halides and alkenes. The pH-controlled
adsorption method results in high dispersion of palladium species on titania surface. During the reaction,
palladium nanoparticles are in situ generated via reduction on the surface of titania. TEM images indicate
that the nanoparticles are nearly monodisperse in size. Recycling studies have shown that the catalyst
can be readily recovered and reused several times without significant loss of catalytic activity.
© 2010 Published by Elsevier B.V.
Keywords:
Palladium
Titania
Adsorption method
Heck reaction
Heterogeneous catalysis
1. Introduction
als [22]. Traditionally, supported palladium catalysts are typically
prepared by impregnation these solid supports with a palladium
The cross-coupling reaction of aryl or vinyl halides with alkenes,
that is, Heck reaction, is among the most commonly used methods
for C–C bond formation and widely applied to organic and fine-
chemical synthesis [1–4]. In the past few years, the development of
ligands has provided highly active homogeneous palladium cata-
lysts for Heck reactions under mild conditions and enabled using a
wide range of aryl halides [1,4]. However, homogeneous processes
suffer from the problems concerning separation from reaction mix-
ture, reuse of expensive palladium catalysts. In addition, most of
ligands for Heck reactions are undesirable in the industrial chem-
istry because of their toxicity, high price and air-sensitivity.
In principle, the use of supported palladium catalysts could
address some of these problems mentioned above. Thus, the prac-
tical applications of Heck reactions have greatly driven the need for
the development of recyclable and efficient heterogeneous palla-
dium catalysts [5–10]. In this regard, supported palladium catalysts
exhibit particularly promising performances for the Heck reaction.
A variety of solid materials as supports have been investigated, for
example, active carbon [11], mesoporous silica [12], metal oxides
[13–16], zeolites [17,18], magnetic materials [19], hydrotalcites
[20], hydroxyapatite [21], and organic–inorganic hybrid materi-
salt aqueous solution, followed by drying or calcination in air and
reduction. The resultant palladium particles are usually randomly
dispersed on the surface of support with broad size distribution,
which may cause a decrease in catalytic performance. Therefore,
preparation of supported palladium catalysts with high dispersion
and narrow size distribution of palladium particles becomes par-
ticularly desirable for the improvement in catalytic performance.
Very recently, a highly dispersed palladium catalyst supported
on ordered mesoporous silica–carbon composites was reported to
be highly active and reusable in the Heck reaction [10] probably
because the high dispersion of palladium species on the surface of
support could prevent formation of catalytically deactivated palla-
dium black and keep all active palladium species available for the
reaction [9]. The increase in catalytic activity and reusability for
highly dispersed catalyst would allow use of low palladium load-
ing for achieving efficient turnover and thereby reduce the cost of
products in large-scale processes. In addition, a simple and effi-
cient absorption method has appeared as an attractive approach
to highly dispersed palladium on ␥-Al2O3 [23], in which palladium
content can be tuned by varying pH value of aqueous solution and
palladium(II) concentrations in the absorption stage.
As a typical metal oxide, TiO2 usually shows pH-dependent sur-
face charges when it is immersed in aqueous solution because of
the existence of Ti–OH on the surface. The absorption property of
TiO2 greatly changes with environment of different pH value. In
∗
Corresponding author. Fax: +86 591 83771575.
E-mail address: wpsu@fjirsm.ac.cn (W. Su).
1381-1169/$ – see front matter © 2010 Published by Elsevier B.V.
doi:10.1016/j.molcata.2010.06.004

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Z. Li et al. / Journal of Molecular Catalysis A: Chemical 328 (2010) 93–98
this paper, we report the high catalytic performance of the highly
dispersed Pd on TiO2 catalyst prepared by the adsorption method
for the Heck cross-coupling reaction of aryl halides and alkenes.
The method represents a simple and efficient approach to highly
dispersed catalysts. Moreover, we demonstrate that this heteroge-
neous catalyst can be readily recovered and reused several times
without significant loss of activity.
Scheme 1. Heck reaction between bromobenzene and methyl acrylate.
wavelength range of 300–800 nm were measured by a Lambda-
900 spectrophotometer with an integrating sphere, and BaSO4 was
used as a reference sample.
2. Experimental
Unless otherwise stated, product yields from the Heck reac-
tion were determined by column chromatography. GC analysis was
measured by a Varian 430 Gas Chromatograph using a 3-m length
column packed with DC-200 stationary phase. 1H and 13C NMR
spectra were recorded using a Burker Avance 400 spectrometer
using CDCl3 as solvent.
2.1. General
Commercially Titania P-25 (TiO2, 80% anatase 20% rutile, BET
surface area: 53 m2 g−1) was provided by Degussa. PdCl2 (AR)
was obtained from Shanghai July Chemical Co. Ltd. and used
as received. Solvents were obtained from Sinopharm Chemical
Reagent Co. Ltd and purified prior to use. Other chemicals were
purchased from Sigma–Aldrich or Alfa Aesar and used without
further purification or drying. Water used in the experiments
was deionized (DI) and doubly distilled prior to use. Unless
otherwise stated, all experiments were performed in nitrogen
atmosphere.
2.4. Catalytic reactions
In a typical reaction, 0.26 wt% Pd/TiO2-ads catalyst (0.10 g,
0.5 mol% Pd relative to bromobenzene), bromobenzene (0.5 mmol),
methyl acrylate (0.6 mmol), and KOAc (0.6 mmol) were added to a
pressure tube, followed by addition of DMA 2 mL under nitrogen
atmosphere. The tube was sealed and the mixture was allowed
to stir in a preheated oil bath at 140 ◦C for 24 h. After the reac-
tion was completed, the solid catalyst was separated by centrifuge,
washed with water to remove base and salt and finally with ace-
tone to remove adsorbed organic substrate, followed by drying at
120 ◦C and activated at 250 ◦C under vacuum (2 Torr) for 2 h prior to
being reused. The filtrate was diluted by water followed by extrac-
tion with ethyl acetate (3× 10 mL). The combined organic phase
was washed with water (2× 10 mL) and brine (10 mL), and dried
over anhydrous Na2SO4. The solvent was removed and the resul-
tant residue was purified by column chromatography (SiO2, ethyl
acetate/hexane gradient). The product was dried under vacuum,
weighted and characterized by 1H and 13C NMR spectroscopy. To
study heterogeneity of the Heck reaction, a hot filtration test was
also adopted. After 5 h of reaction time the 0.26 wt% Pd/TiO2-ads
solid catalyst was filtered by hot filtration under nitrogen atmo-
sphere and the hot filtrate was further reacted with fresh KOAc
(0.6 mmol) at 140 ◦C for additional 10 h. After the reaction, the sam-
ples of the reaction mixture were analyzed by GC and ICP.
2.2. Catalyst preparation
Pd/TiO2 catalysts prepared by an adsorption method (Pd/TiO2-
ads) were prepared by the following procedures: 2.0 g of TiO2 was
added to 60 mL aqueous solution of PdCl2 (0.03 mol/L), of which
different pH was adjusted by 2 M HCl aqueous solution. The sus-
pension was then vigorously stirred for 2 h at room temperature.
After the adsorption, the powdery solid was separated by filtration
and washed for several times with a large amount of water until
no Cl− could be detected in the filtrate by AgNO3 aqueous solu-
tion, followed by drying at 120 ◦C overnight and calcination in air at
500 ◦C for 3 h. The resulting light yellow solid powder was cooled to
room temperature and stored in a vacuum desiccator. For compar-
ison, the 0.26 wt% Pd/TiO2 catalyst was prepared by a conventional
wet impregnation method (Pd/TiO2-imp). TiO2 was added into a
PdCl2 aqueous solution without pH adjustment and the mixture
was evaporated to dryness at 60 ◦C. The resultant powdery solid
was further dried at 120 ◦C and finally calcined in air at 500 ◦C for
3 h. The resulting solid powder was cooled to room temperature
and stored in a vacuum desiccator.
3. Results and discussion
2.3. Characterization
3.1. Heck reaction catalyzed by catalysts with different palladium
content
Inductively coupled plasma (ICP) analyses of the content of Pd
in each solid catalyst sample and the filtrate after the reaction were
performed on an Ultima 2 analyzer (Jobin Yvon). Powder X-ray
diffraction (XRD) patterns were recorded on a Rigaku D/Max RA
diffractometer with Cu K␣ radiation at 40 kV and 40 mA with a
scan speed of 2◦ min−1 and a range of 2ꢀ from 10◦ to 80◦. The mor-
phology, structure and EDX spectra of catalyst were analyzed on
a JEOL 2010 transmission electron microscope (TEM) with energy
dispersive X-ray (EDX) spectrometer. The sample for TEM analy-
sis was prepared by placing a drop of the suspension in ethanol
onto a continuous carbon-coated copper TEM grid and dried at
room temperature under atmospheric pressure. For the sample,
more than 100 particles from different parts of the grid were used
to determine the particle size distribution. X-ray photoelectron
spectroscopy (XPS) measurements were analyzed on coated alu-
mina with a physical Electronics PHI-Quantum 2000 Scanning ESCA
Microprobe using Al K␣ radiation (1846.6 eV) as X-ray source to
investigate the oxidation state of Pd before and after reaction. Dif-
fuse reflectance UV–Vis reflectance spectra of the samples in the
Initially, Heck reaction of bromobenzene and methyl acrylate
was chosen as a model reaction (Scheme 1) with 0.53 wt% Pd/TiO2-
ads catalyst that was prepared when the pH value of aqueous
solution of PdCl2 was 2. Bases, solvents, and loadings of the cat-
alyst were screened to optimize reaction conditions (Table 1). The
effect of bases in the reaction was observed. Alkali carbonates gave
low or moderate yields (entries 1–3) and K3PO4 gave a lower yield
(entry 4). When organic base Et3N was used, the moderate product
yield (39%) was obtained (entry 7). Gratifyingly, the use of KOAc
dramatically improved the yield (79%) (entry 6). DMA was found to
be more effective than other solvents such as DMF, DMSO and NMP
(entry 6 and entries 11–13). Reducing the amount of the catalyst
loading from 0.5 mol% to 0.25 mol% or 0.1 mol% led to decrease in
yields (entries 8 and 9). The oxidative conditions seriously inhib-
ited the desired reaction, as indicated by the reaction performed
under air or oxygen atmosphere (entries 14 and 15). In addition,
the time dependence of the yield under the optimized condition
is shown in Fig. 1. It is clear that the reaction of bromobenzene

Z. Li et al. / Journal of Molecular Catalysis A: Chemical 328 (2010) 93–98
95
Table 1
Optimization of the Heck reaction of bromobenzene and methyl acrylate using
0.53 wt% Pd/TiO2-ads catalyst.a.
Entry
Pd (mol%)b
Base
Solvent
Yield (%)c
40
1
2
3
4
5
6
7
8
9
10
11
12
13
14d
15e
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.25
0.1
0
0.5
0.5
0.5
0.5
0.5
Na2CO3
K2CO3
Cs2CO3
K3PO4
NaOAc
KOAc
Et3N
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMF
DMSO
NMP
DMA
DMA
45
10
23
70
79
39
69
50
0
54
42
70
35
5
a
Reaction conditions: bromobenzene (0.5 mmol), methyl acrylate (0.6 mmol),
base (0.6 mmol), solvent (2 mL), 24 h, 140 ◦C, under N2.
Fig. 2. Effect of pH value and Pd content on catalytic properties of the Pd/TiO2
catalyst for Heck reaction (optimized conditions).
b
The ratio of absolute amount of Pd loading to bromobenzene.
Isolated yields.
Under air.
c
d
e
Under O2, GC yield.
with methyl acrylate took about 24 h for completion. There was an
induction period of about 3 h, after which the reaction proceeded
rapidly during the following period.
Scheme 2. Heck reaction between aryl halides and alkenes.
Considering that the catalytic properties of supported catalysts
depend on their preparation conditions, we further evaluated the
catalytic performance of Pd/TiO2 catalysts obtained at different pH
values in a fixed 0.03 mol/L PdCl2 aqueous solution. Palladium con-
tent in the Pd/TiO2 can be regulated by varying pH value of PdCl2
aqueous solution. When the pH value was lower than the identical
electric point (IEP = 6.25) of TiO2 [24], the surface would be trans-
may account for the high catalytic performances of the sample with
an appropriate Pd content. The relatively lower yield at a much
lower Pd content (<0.26 wt%) probably suggests that the interac-
tion existing between Pd species and TiO2 might cause the decrease
in catalytic performance. The use of the 0.26 wt% Pd/TiO2-ads cat-
alyst led to exclusive formation of trans-methyl cinnamate in 86%
isolated yield.
+
ferred to = Ti–OH2 and the absorption of Pd could easily occur
on the surfaces via coulombic interactions. As shown in Fig. 2, Pd
content on TiO2 increased when the pH value was increased. Inter-
estingly, the catalyst containing 0.26 wt% palladium, which was
prepared when the pH value was 1.5, exhibited the best activ-
ity for the Heck reaction of bromobenezene with methyl acrylate
under a fixed ratio of Pd/bromobenzene using optimized condi-
tions while catalysts containing higher Pd content were less active.
It is reasonable to speculate that a higher Pd content may lead to
the agglomeration of Pd species after the calcination or during the
reaction. Thus, highly dispersed Pd species on the surface of TiO2
3.2. Heck reaction of aryl halides with alkenes catalyzed by
0.26 wt% Pd/TiO2
The scope of the reaction (Scheme 2) was explored under the
optimized reaction conditions with 0.26 wt% Pd/TiO2-ads catalyst.
As shown in Table 2, a range of aryl iodides, aryl bromides and
even activated aryl chlorides could undergo Heck reaction to pro-
duce exclusively trans products in moderate to excellent yields.
Aryl halides with electron-withdrawing substituents in the para
positions reacted smoothly (entries 1, 7, 9, 11, and 12), while
Table 2
Heck reaction of aryl halides with alkenes catalyzed by 0.26 wt% Pd/TiO2-ads
catalyst.a.
Entry
X
R1
R2
R3
Time (h)
Yield (%)b
1
2
3
4
5
6
7
8
9
10
11c
12c
13c
I
I
CH3CO
OCH3
H
CH3
H
OCH3
COOEt
H
CH3CO
H
COOEt
CH3CO
H
H
H
H
H
CH3
H
H
H
H
H
H
H
H
Benzene
COOMe
COOMe
COOMe
COOMe
COOMe
COOMe
Phenyl
10
10
24
24
24
24
24
24
24
24
36
36
36
96
93
86
65
56
50
94
92
95
83
64
60
27
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Phenyl
COOi-Bu
COOMe
Phenyl
COOMe
a
Reaction conditions: aryl halide (0.5 mmol), alkene (0.6 mmol), base (0.6 mmol),
Pd/TiO2 (0.5 mol%), DMA (2 mL), 140 ◦C, under N2.
b
Isolated yields.
Fig. 1. Time of yield plot for the Heck reaction in the presence of 0.53 wt% Pd/TiO2-
ads as catalyst.
c
150 ◦C, Addition TBAB (0.3 mmol).

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Z. Li et al. / Journal of Molecular Catalysis A: Chemical 328 (2010) 93–98
Fig. 3. XPS spectra of 0.26 wt% Pd/TiO2-ads catalyst (A) before the reaction and (B)
after the induction period.
aryl halides with electron-donating substituents were less reac-
tive (entries 4–6). In addition, ortho-substituted aryl bromides also
provided the desired product (entry 5). We were pleased to find
that this catalyst system can couple activated aryl chlorides in the
presence of 0.3 mol tetrabutylammonium bromide (TBAB), finish-
ing a synthetically useful yield (entries 11 and 12). Deactivated
chlorobenzene also participated in this reaction, albeit in a substan-
tially comparable yield (entry 13). With respect to alkenes, styrene
and i-butyl acrylate served as suitable coupling partners (entries 1,
8–10, and 12). It should be noted that, the reaction could be scaled
up, as exemplified by the coupling reaction of bromobenzene with
methyl acrylate 5.0 mmol scale that gave a yield comparable to that
on 0.5 mmol scale.
Fig. 4. TEM image, size distribution and EDS spectra of 0.26 wt% Pd/TiO2-ads catalyst
after the induction period.
To gain more information about the nature of the isolated catalyst,
a thin film of the same isolated catalyst was prepared and studied
using XPS. As shown in Fig. 3, the peaks located around 335.7 eV
and 340.9 eV were assigned to the Pd 3d5/2 and Pd 3d3/2 level in the
Pd(0) metallic form in agreement with literature report [29]. The
binding energy clearly showed the fact that the Pd nanoparticles in
our isolated catalyst contained only Pd(0) species. The above phe-
nomena support that the Heck reaction proceeds via the traditional
Pd0/Pd2+ cycle but not via the Pd2+/Pd4+ mechanism.
3.3. Characterization of the catalysts
Powder X-ray diffraction (XRD) analysis indicated that the
0.26 wt% Pd/TiO2-ads solid sample mainly comprised anatase with
minor rutile (S1), in agreement with its UV–Vis diffuse reflectance
spectrum (S2) in which the intense absorption band centered at
380 nm could be assigned to the intrinsic band gap absorption of
anatase [25]. Although no Pd species could be detected by XRD and
UV–Vis diffuse reflectance due to low amount of Pd content or its
high dispersion, the presence of Pd species in this solid sample was
clearly evidenced by X-ray photoelectron spectroscopy (XPS). Fig. 3
shows the XPS spectra of the region corresponding to the binding
energy range of 333–345 eV, which included Pd 3d5/2 and Pd 3d3/2
peaks. Two peaks located around 336.1 eV and 341.4 eV, a char-
acteristic of the electron binding energy of Pd(II) in the 3d5/2 and
3d3/2 level [26–27]. When Pd content on the surface of TiO2 was
increased, a broad band between 450 nm and 550 nm owing to a
d–d transition of Pd(II) species could be obviously observed through
UV–Vis diffuse reflectance spectrum [28].
Before reaction, for the 0.26 wt% Pd/TiO2-ads solid sample, no
PdO or Pd particles could be discerned in the TEM images, suggest-
ing that Pd species was likely highly dispersed on the surface of
TiO2. However, inspection of the TEM images of isolated 0.26 wt%
Pd/TiO2-ads catalyst samples after the induction period (3 h) in
the reaction of bromobenzene and methyl acrylate indicated the
involvement of Pd nanoparticles. Fig. 4 displays the TEM image,
corresponding size distribution and EDS spectra of the isolated
catalyst. Evidently, highly dispersed spherical metallic Pd nanopar-
ticles with the size of 5–6 nm on the TiO2 surface were observed.
It is essential to verify that the catalytic activity is caused by
solid Pd/TiO2 rather than the leached palladium species. In the
Heck reaction of bromobenzene and methyl acrylate, the 0.26 wt%
Pd/TiO2-ads solid catalyst was removed by hot filtration at 19%
GC yield under nitrogen atmosphere, and the filtrate was further
reacted with fresh KOAc at 140 ◦C for 10 h. The above treatment of
the filtrate gave no coupling product. Pd leaching was negligible
and the concentration of Pd in the filtrate was less than 0.2 ppm, as
found by ICP analysis. This observation, in connection with the fact
that the Pd nanoparticles initially generated by in situ reduction
did not change during the reaction, suggested that catalytic pro-
cess might occur on the surface of Pd nanoparticles, although we
cannot rule out the homogeneous process [30].
3.4. Reusability of the catalyst
For practical application in the Heck reaction, the lifetime of the
heterogeneous catalysts and their reusability are very important
factors.
We have carried out recycling uses of the 0.26 wt% Pd/TiO2-ads
catalyst for the Heck reaction between 4-iodoanisole and methyl
acrylate, the coupling product was obtained with a yield of 92%
in the first cycle. The solid catalyst can be separated readily from
the reaction mixture by centrifuge, washed and dried at 250 ◦C
under vacuum for 2 h. The recycled catalyst was then used in the
model reaction under the same reaction conditions giving 90%, 94%,
91%, and 89% isolated yield for the first, second, third and forth

Z. Li et al. / Journal of Molecular Catalysis A: Chemical 328 (2010) 93–98
97
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molcata.2010.06.004.
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Acknowledgements
The authors gratefully acknowledge the financial support of
the 973 Program (2006CB932903 and 2009CB939803), “The Dis-
tinguished Oversea Scholar Project”, “One hundred Talent Project”
and NSF of Fujian Province (2009J05040).
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