A new method for the preparation of microcapsule-supported palladium catalyst for Suzuki coupling reaction

Article abstract of DOI:10.1016/j.molcata.2010.02.029

In this paper, a new method for the preparation of microcapsule-supported palladium catalyst for Suzuki coupling reaction was described. The highly monodispersed cross-linked polystyrene microcapsules containing phosphine ligand were synthesized by the se

Full text of DOI:10.1016/j.molcata.2010.02.029

Journal of Molecular Catalysis A: Chemical 323 (2010) 16–22
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
A new method for the preparation of microcapsule-supported
palladium catalyst for Suzuki coupling reaction
Ying Liua, Xiujuan Fenga,∗, Decai Baob, Kaixiao Lia, Ming Baoa,∗
a State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, China
b School of Science of Chemistry and Food, Bohai University, Jinzhou 121000, China
a r t i c l e i n f o
a b s t r a c t
Article history:
In this paper, a new method for the preparation of microcapsule-supported palladium catalyst for Suzuki
coupling reaction was described. The highly monodispersed cross-linked polystyrene microcapsules con-
taining phosphine ligand were synthesized by the self-assembling of phase separated polymer (SaPSeP)
method using diphenyl(4-vinylphenyl)phosphine and divinylbenzene as a monomer and cross-linking
agent, respectively, and 2,2ꢀ-azobisisobutyronitrile (AIBN) as an initiator within the droplets of oil-
in-water (O/W) emulsions, which were prepared by using the Shirasu porous glass (SPG) membrane
emulsification technique. The microcapsule-supported palladium catalyst was obtained by treating the
microcapsules with a solution of Pd2(dba)3 in dichloromethane. This supported palladium catalyst exhib-
ited high catalytic activity for the Suzuki coupling reaction of various aryl bromides and can be reused
several times without the loss of activity.
Received 25 November 2009
Received in revised form 1 February 2010
Accepted 23 February 2010
Available online 3 March 2010
Keywords:
Microcapsule-supported palladium catalyst
Phosphine ligand
Suzuki coupling reaction
Recycle
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
To resolve the above mentioned problem, Ley [6] and Kobayashi
[7] have carried out independent ground-breaking work using
In catalytic organic synthesis, polymer-supported transition
metal catalysts offer the following advantages over the usual
homogeneous catalysts: easy recovery and potential recycling
of expensive catalysts, simplified product purification and the
microencapsulation techniques. However it was demonstrated, by
way of a three-phase test, that commercially available PdEnCat
catalysts act as heterogeneous sources, or reservoirs, for soluble,
catalytically active species [8]. This behavior of PdEnCat catalysts
may lead to the leaching of active catalyst species. By the microen-
capsulation and cross-linking of polymer chains, Kobayashi and
co-workers used phosphinated polymers to prepare polymer incar-
cerated (PI) palladium catalysts. In the cross-linking process, some
phosphines were oxidized to the corresponding phosphine oxides,
and HSiCl3 reduction was needed to produce phosphinated PI Pd
[9]. In addition, the size of microcapsules and the existing mode
(inside or outside) of phosphine ligands could not be controlled
when this method was employed.
Here we wish to report a new method for the preparation of a
microcapsule-supported palladium catalyst for the Suzuki coupling
reaction. The principle of the preparation is schematically described
in Scheme 1. At first, monodispersed size-controllable microcap-
sules containing phosphine ligands were prepared by the Shirasu
porous glass (SPG) emulsification technique [10] using diphenyl(4-
vinylphenyl)phosphine and divinylbenzene as a monomer and
cross-linking agent, respectively. Palladium was then attached to
the interior of the microcapsules. These newly formed microcap-
sules acted as microreactors in which the catalysis occurred. Even
if palladium species dissociate from one phosphine ligand, it may
attach to another phosphine ligand inside of the microcapsule.
Therefore, the microcapsule-supported palladium catalyst is very
possibility of carrying out the desired transformation in
a
continuous-flow system. Therefore, polymer-supported catalysts,
which are often considered as heterogenized homogeneous cat-
alysts, have attracted considerable attention over the past years
[1–5]. The methods for the preparation of polymer-supported cat-
alysts can broadly be separated into two groups: in the first, a
polymer is prepared by first using a monomer bearing ligand
moiety, before the catalyst is anchored to the polymer through
a complexation reaction between the transition metal atom and
ligand moiety on the polymer. In the second, a polymer is modi-
fied by a ligand, and then the catalyst is attached to the polymer
through the method mentioned above. Transition metal complexes
of polymer-supported catalysts are usually linked to the polymer
chain, and protrude into the reaction solvent. This leads to the loss
of catalyst activity when they are recovered by filtration and recy-
cled due to the leaching of active catalyst species from polymeric
support.
∗
Corresponding authors. Tel.: +86 411 39893862; fax: +86 411 39893687.
E-mail addresses: xiujuan.feng@163.com (X. Feng),
mingbao@dlut.edu.cn (M. Bao).
1381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.02.029

Y. Liu et al. / Journal of Molecular Catalysis A: Chemical 323 (2010) 16–22
17
Scheme 1. Schematic illustration for the preparation of the microcapsule-supported palladium catalyst.
stable, and can be reused several times without loss of activ-
ity.
methanol, acetone, dichloromethane and methanol successively,
and dried in vacuum. The white powder of the microcapsule parti-
cles was obtained in 52% yield (1.87 g).
2. Experimental
2.3. Preparation of microcapsule-supported palladium catalyst
2.1. Reagents and materials
A solution of Pd2(dba)3 in CH2Cl2 was added to the micro-
capsules (1.0 g) swelled in the dry CH2Cl2 (10 mL). The mixture
obtained was then stirred at room temperature under N2 atmo-
sphere for 3 h. This microcapsule-supported palladium catalyst was
obtained by ligand-exchange reaction. The content of Pd in the
microcapsules was determined by inductively coupled plasma (ICP)
after the microcapsules were filtrated and washed with CH2Cl2
(10 mL ×2) as well as dried in a vacuum.
Polyvinyl alcohol (PVA, Mn ≈ 1170) and sodium dodecylsul-
fate (SDS) were used as received. Divinylbenzene (DVB) was
washed with 10% sodium hydroxide and water, dried with
CaCl2, and then distilled from calcium hydride before being
used. 2,2ꢀ-Azobisisobutyronitrile (AIBN) was recrystallized from
ethanol. Bromobenzene (ABCR, 99%), 2-bromotoluene (Acros,
99%), 4-bromoacetophenone (Alfa Aesar, 98%), 4-bromoanisole
(ABCR, 99%), 4-bromophenol (Acros, 97%), 1-bromo-2,4,5-
2.4. General procedure for Suzuki coupling reaction
trifluorobenzene
(ABCR,
99%),
1-bromo-4-nitrobenzene
(Aldrich, 99%), 4-bromo-N,N-dimethylaniline (Aldrich, 97%), 4-
fluorobromobenzene (ABCR, 98%), 3-bromopyridine (Acros, 99%),
4-bromobenzaldehyde (Alfa Aesar, 99%), 2-bromobenzaldehyde
(Alfa Aesar, 98%), 4-bromo-2-fluorobenzaldehyde (Acros, 97%),
1-bromonaphthalene (ABCR, 98%), Pd2(dba)3, phenylboronic acid
and K2CO3 were used in their commercially available form. Shirasu
porous glass (SPG) membrane was obtained from the Fujikin
incorporation of Japan. Double-distilled deionizing water was used
in the present experiments. Other analytical reagents were used
as received.
Aryl bromides (0.5 mmol), phenylboronic acid (1.5 equiv.),
K2CO3 (1.5 equiv.), microcapsule-supported Pd(0) catalyst contain-
ing 1 mol% Pd catalyst, and IPA (2 mL) were added to a Schlenk
reactor. The mixture was put in a preheated oil bath at a given
temperature of 80 ◦C and stirred for 2–5 h. After completion of the
reaction, the mixture was cooled to room temperature, and the cat-
alyst was collected by filtration, washed with acetone, water and
ethanol, then dried under vacuum and reused. The organic filtrate
was evaporated under vacuum to give a crude product. The crude
product was further purified by flash chromatography using silica
gel (EtOAc/Petroleum) to give the desired coupling product.
2.2. Preparation of monodispersed hollow microcapsules
The emulsification procedure was carried out by utilizing a
hydrophilic SPG membrane emulsification kit with average pore
size of 2.8 ␮m. Prior to emulsification, the membrane was soaked in
deionized water containing a small amount of SDS for over 30 min
after ultrasonic treatments. Then, the SPG membrane was installed
into the SPG membranes module as a filter media. The dispersed
phase consisting of 4-styryldiphenylphosphine (SDPP, 10 mmol,
2.88 g), DVB (2.5 mmol, 0.72 g), AIBN (3 wt% of monomers) and
toluene (2.4 mL) was poured into a pressure tight vessel, and
allowed to permeate into the continuous phase containing PVA
(1.0 wt%), SDS (0.3 wt%) and deionized water (60 mL) through the
hydrophilic membrane under nitrogen gas pressure.
2.5. Three-phase test
Silica gel supported aryl bromide was prepared according to the
literature procedure [11]: a solution of 3-aminopropyl triethoxysi-
lane (5.1 mL, 22.0 mmol) in dry THF (10 mL) was added dropwise to
a solution of 4-bromobenzoyl chloride (4.83 g, 22.0 mmol) and tri-
ethyl amine (3.2 mL, 22.0 mmol) in dry THF (50 mL) at −15 ◦C under
nitrogen atmosphere. The resulting mixture was warmed to room
temperature, and stirred for 2 h. Then the solid was removed via
filtration and the solvent was removed under vacuum at room tem-
perature, giving 8.5 g of the desired product, 4-bromobenzamide
3-propyltriethoxysilane as a beige solid.
The obtained emulsion was transferred into a three-neck bot-
tle glass that had a condenser, a magnetic stirrer and a nitrogen
inlet nozzle. Emulsion polymerization was carried out for 24 h at
70 ◦C under nitrogen atmosphere. After the polymerization had fin-
ished, the reaction mixture was cooled to room temperature and
methanol was added for the precipitation of the polymer particles.
The polymer particles were then washed with deionized water,
A mixture of 4-bromobenzamide 3-propyltriethoxysilane (8.5 g,
21.9 mmol) thus obtained and pyridine (2.3 mL, 29.4 mmol) was
added dropwise to a suspension of silica (2.0 g) in dry toluene
(50 mL) under nitrogen atmosphere. The resulting mixture was
refluxed for 24 h. And then the suspension was filtered and Soxhlet
extracted with dichloromethane for 24 h. The resulting solid was
dried under vacuum at room temperature, giving 2.4 g of white

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Y. Liu et al. / Journal of Molecular Catalysis A: Chemical 323 (2010) 16–22
powder, and the content of aryl bromide was 1.64 mmol/g deter-
mined by elemental analysis.
A solution of 4-bromobenzamide (0.25 mmol), phenylboronic
acid (0.38 mmol, 1.5 equiv.), and K2CO3 (0.38 mmol, 1.5 equiv.) in
IPA (2 mL) was stirred in the presence of microcapsule-supported
Pd catalyst (1.0 mol% Pd) and silica gel supported aryl bromide
(192 mg) at 80 ◦C for 4 h. And then the supernatant was analyzed by
GC and the solid was separated by filtration, washed with ethanol,
and further extracted with dichloromethane. The solid was then
hydrolyzed with 2 M KOH in ethanol/water (1.68 g, 10 mL of EtOH,
5 mL of H2O) at 90 ◦C for 3 days. The resulting solution was neutral-
ized with aqueous HCl (10 wt%), extracted with dichloromethane
followed by ethyl acetate, concentrated, and the resulting mixture
was analyzed by 1H NMR.
2.6. General characterization
Photographs of microcapsules were taken by a TE2000-S (Nikon
Eclipse) optical microscope. The scanning electron microscope
(SEM) measurements were conducted by using a JSM-6700 III (JEOL
Co., Tokyo, Japan) electron microscopy. The ratio of phosphorus to
carbon on the surface of microcapsules was determined by SEM
with an EDX (JEM-6700, Oxford INCA) attachment. The transmis-
sion electron microscope (TEM) measurements were conducted
using a Tecnai G22oS-Twin electron microscopy at an acceleration
voltage of 200 kV. A small drop of a suspension of microcapsules
in ethanol was deposited onto a carbon-coated copper grid and
dried at room temperature. The ratio of phosphorus to carbon in the
whole microcapsule was determined by TEM with an EDX (Noran
Instru.) attachment. The diameter distribution of the microcapsule
particles was determined by a LSTM 100Q laser diffraction particle
size analyzer. The amount of Pd was obtained by inductively cou-
pled plasma (ICP) atomic emission spectrometry (Optima 2000 DV).
The thermal stability of the microcapsules was studied by thermal
gravimetric analysis (TGA) using a TGA/SDTA851e (Mettler-Toledo
Co., Sweden) TGA instrument in temperatures ranging from 0 to
600 ◦C with a heating rate of 10 ◦C/min. The 1H NMR spectra were
recorded on a Bruker Avance II 400 MHz spectrometer using CDCl3
as the solvent.
Fig. 1. Energy-dispersive X-ray (EDX) analysis spectra of the surface of the micro-
capsules (upper), and the entirety of the microcapsules (lower).
by EDX combined with transmission electron microscope, Fig. 1,
lower) [15]. The optical micrograph (OM, after swelled for 7 h in
toluene), scanning electron microscope (SEM) images and diame-
ter distribution of the microcapsule particles are shown in Figs. 2–4.
As shown in Figs. 2 and 3, the size of the prepared microcapsules
was uniform. The average diameter and the coefficient of varia-
tion (CV) of the microcapsule particles are 7.78 ␮m and 15.42%,
respectively. Fig. 5 shows the transmission electron microscope
(TEM) images of the microcapsules after drying under vacuum,
3. Results and discussion
3.1. Synthesis and characterization of the
microcapsule-supported palladium catalyst
The highly monodispersed cross-linked polystyrene micro-
capsules containing phosphine ligand were synthesized by the
self-assembling of phase separated polymer (SaPSeP) method
[12,13] using diphenyl(4-vinylphenyl)phosphine and divinylben-
zene as a monomer and cross-linking agent, respectively, and AIBN
as an initiator within the droplets of oil-in-water (O/W) emulsions,
which were prepared by using the SPG membrane emulsification
technique.
During the preparation of microcapsules, we found that in
this method of microcapsule preparation, the solvent plays an
important role in getting the phosphine ligand inside the microcap-
sule. Therefore, the influence of solvent in the organic phase was
investigated by using different solvents such as dichloromethane,
hexadecane, dodecanol, and toluene. Among the solvents investi-
gated, toluene gave the lowest ratio of phosphorus to carbon on
the surface of the microcapsule (0.89%) determined by energy-
dispersive X-ray (EDX) spectrometry combined with scanning
electron microscope (Fig. 1, upper) [14], which indicated that 90%
of phosphine ligands existed inside the microcapsule (ratio of phos-
phorus to carbon on the whole microcapsule was 8.90% determined
Fig. 2. Optical micrograph (400× magnification) of microcapsules containing phos-
phine ligand.

Y. Liu et al. / Journal of Molecular Catalysis A: Chemical 323 (2010) 16–22
19
Fig. 3. SEM image of microcapsules containing phosphine ligand.
Fig. 6. The weight loss curve of the microcapsules determined by TGA.
Table 1
Solvent effect on the Suzuki coupling reaction of 4-bromoanisole with phenyl-
boronic acid catalyzed by microcapsule-supported palladiuma.
Entry
Solvent
Time (h)
Yield (%)b
1
2
3
4
5
6
7
Toluene
Toluene/IPA(1/1)
IPA
Ethanol
1,2-Dioxane
DME
2
2
2
4
4
4
2
64
91
99
63
32
51
96
DMF
a
Reaction conditions: 4-bromoanisole (0.5 mmol), phenylboronic acid (1.5 equiv.),
K2CO3 (1.5 equiv.), 1 mol% Pd catalyst (12.5 mg), IPA (2 mL), 80 ◦C.
b
Isolated yield.
Fig. 4. Diameter distribution of microcapsules containing phosphine ligand.
in dichloromethane to obtain the microcapsule-supported palla-
dium catalyst by ligand-exchange reaction. Inductively coupled
plasma (ICP) analysis indicated that the ratio of Pd was 2.17 wt%,
and the EDX determination shows that no Pd existed on the sur-
face of microcapsule, which clearly indicated the Pd was completely
immobilized inside the microcapsule.
which clearly indicate that the microcapsules prepared possess a
hollow structure. The thermal stability of the microcapsules was
also studied by thermal gravimetric analysis (TGA) in tempera-
tures ranging from 0 to 600 ◦C with a heating rate of 10 ◦C/min
(Fig. 6). The weight loss occurred from the temperature of around
200 ◦C that means the microcapsules are stable under the tempera-
ture of 200 ◦C. The microcapsules were then treated with Pd2(dba)3
3.2. Suzuki coupling reaction catalyzed by
microcapsule-supported palladium catalyst
The Suzuki-type cross-coupling reaction between aryl bromides
and phenylboronic acid was chosen to evaluate the catalytic activ-
ity and stability of the microcapsule-supported palladium catalyst
[16,17]. As is known, the solvent has a strong influence on the
C–C bond forming reaction catalyzed by polymer-supported tran-
Scheme 2. Three-phase test in the Suzuki reaction with microcapsule-supported
Pd.
Fig. 5. TEM images of the microcapsules after drying under vacuum.

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Y. Liu et al. / Journal of Molecular Catalysis A: Chemical 323 (2010) 16–22
Table 2
Suzuki coupling reaction of aryl bromides with phenylboronic acid catalyzed by microcapsule-supported palladiuma.
Entry
Aryl bromide
Time (h)
Product
Yield (%)b
1
2
2
3
92
67
3
4
3
2
81
99
5
3
88
6
7
3
5
85
15
8
2
97
9
3
3
98
82
10
11
12
3
2
58
71

Y. Liu et al. / Journal of Molecular Catalysis A: Chemical 323 (2010) 16–22
21
Table 2(Continued).
Entry
Aryl bromide
Time (h)
Product
Yield (%)b
13
3
5
88
87
14
a
Reaction conditions: aryl bromide (0.5 mmol), phenylboronic acid (1.5 equiv.), K2CO3 (1.5 equiv.), 1 mol% Pd catalyst (12.5 mg), IPA (2 mL), 80 ◦C. The reaction progress was
monitored by TLC.
b
Isolated yield.
sition metal catalysts [18,19]. The solvent was initially screened
for the model reaction of 4-bromoanisole with phenylboronic acid,
the results of which are shown in Table 1. Because polystyrene
swells easily in toluene, the toluene was first utilized as a solvent,
and 64% isolated yield of 4-methoxybiphenyl was obtained (entry
1). To enhance the solubility of base, K2CO3, a mixture of toluene
and isopropanol (IPA) was subsequently examined, an excellent
yield was obtained (91%, entry 2). Pleasingly, when the IPA was
used as a solvent in the absence of toluene, an excellent yield was
also obtained (99%, entry 3). The solvents, ethanol, 1,2-dioxane
and 1,2-dimethoxyethane (DME), gave low yields (63%, 32% and
51%, respectively; entries 4–6) despite the reaction time being pro-
longed to 4 h. As expected, an excellent yield was also obtained
when the N,N-dimethylformamide (DMF) was used as a solvent
(96%, entry 7). Because IPA is readily available and inexpensive with
low toxicity [20,21], it was employed as a solvent in the subsequent
study.
Suzuki reactions of a wide range of aryl bromides with phenyl-
boronic acid were tested, the catalysis results of which are
summarized in Table 2. The reaction of bromobenzene with phenyl-
boronic acid proceeded smoothly to offer biphenyl in 92% yield
(entry 1). However, the 2-methyl phenyl bromide gave only mod-
erate reaction yield (67%, entry 2), due to the steric hindrance
of ortho-methyl group. The properties of substitutes at the para
position of the aromatic ring did not have any influence on the
reaction yields. The coupling reactions of phenyl bromides bearing
electron-donating groups or electron-withdrawing groups on para
position with phenylboronic acid were completed within 2–3 h
and gave good to excellent yields (81–99%, entries 3–6, 8–10).
Only 15% yield of 2-phenylbenzaldehyde was obtained when the
2-bromobenzaldehyde was treated with phenylboronic acid (entry
7). Fluorinated substrates, 4-bromo-3-fluorobenzaldehyde and 1-
bromo-2,4,5-trifluorobenzene, were also examined, and moderate
reaction yields were obtained (58% and 71%, respectively, entries
11 and 12). A good yield was obtained from the reaction of
1-bromonaphthalene with phenylboronic acid (88%, entry 13).
Finally, 3-bromopyridine, a brominated aromatic heterocycle, was
tested, and the desired coupling product was obtained in 87% yield
(entry 14).
2, 98%). Again, the catalyst was recovered and used repeatedly.
Even when the catalyst was reused for the 11th time, the cou-
pling reaction of 4-bromoacetophenone with phenylboronic acid
still proceeded smoothly to give the desired product in excellent
yield (run 11, 96%), and the proportion of Pd on the recovered
catalyst was 1.83 wt% determined by ICP analysis. When the cat-
alyst was reused for the 12th time, a slightly reduced yield was
obtained (run 12, 82%). These results clearly indicate that the
microcapsule-supported palladium catalyst is very stable and can
be used repeatedly.
3.4. Three-phase test
In an attempt to ascertain whether the catalysis occurred inside
or outside of the microcapsules, a three-phase test was utilized
(Scheme 2). This test, developed by Rebek and co-workers [22,23]
and used by Corma [24,25], Davies [26], and others [27–29], is
considered to be a definitive test for the presence of catalyti-
cally active homogeneous metal species. In our case, if the catalyst
remains immobilized inside of the microcapsules, no transforma-
tion should be observed for the anchored aryl bromide, because the
solid substrate could not enter the microcapsule through the pores
of microcapsule to access the catalyst species. If homogeneous Pd is
released outside of the microcapsules, then the supported aryl bro-
mide can be converted to product, a biaryl compound. When the
three-phase test was performed under standard conditions (K2CO3,
PhB(OH)2, IPA, 1.0 mol% Pd, 80 ◦C, 5 h), 4-bromobenzoic acid was
recovered in 92% GC yield upon cleavage from the support, and
its coupling product, 4-phenylbenzoic acid, was observed in only
4% GC yield, which was considered that due to the small amount
Table 3
Recycled use of the microcapsule-supported Palladium catalyst for Suzuki reaction
of 4-bromoacetophenone with phenylboronic acida.
Run
Yield (%)b
Run
Yield (%)b
3.3. Recycled use of the microcapsule-supported palladium
catalyst
1
2
3
4
5
6
97
98
98
92
98
92
7
8
9
10
11
12
94
92
96
98
96
82
The stability of microcapsule-supported palladium catalyst was
then investigated, the results of the repeated use of the catalyst
for the coupling reaction of 4-bromoacetophenone with phenyl-
boronic acid are summarized in Table 3. Using the fresh catalyst,
97% yield was obtained (run 1). The use of the recovered cata-
lyst also produced an excellent yield of the coupling product (run
a
Reaction conditions: 4-bromoacetophenone (0.5 mmol), phenylboronic acid
(1.5 equiv.), K2CO3 (1.5 equiv.), 1 mol% Pd catalyst (12.5 mg), IPA (2 mL), 80 ◦C, 2 h.
b
Isolated yield.

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Y. Liu et al. / Journal of Molecular Catalysis A: Chemical 323 (2010) 16–22
of leaching Pd. The coupling product, 4-biphenylcarboxamide,
derivered from the soluble 4-bromobenzamide substrate was
obtained in 92% isolated yield. These results clearly indicate that
the microcapsule-supported Pd catalyst is very stable and almost
all of the catalysis occurred inside of the microcapsules. Further
evidence was provided by a hot filtration experimentation. After
the supported catalyst was filtrated off from the hot reaction mix-
ture, the desired Suzuki coupling reaction almost could not proceed
under the same conditions.
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In conclusion, we present a new method for the preparation
of supported palladium catalyst for C–C bond forming reaction. In
the microcapsule-supported palladium catalyst prepared, the Pd
species were anchored inside of the microcapsules, which there-
fore acted as microreactors. It was considered that the substrates
entered the microcapsule through its pores and were transformed
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lized within the microcapsule. The products then came back out
through the pores. Our study on the catalyst reuse suggests that
the microcapsule-supported palladium catalyst was very stable and
can be reused at least 11 times without loss of catalytic activity.
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supported palladium catalyst as a catalyst are now in progress.
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We are grateful to the National Natural Science Foundation
of China (20572010) for financial support. This work was partly
supported by the Program for Changjiang Scholars and Innovative
Research Teams in Universities (IRT0711).
Appendix A. Supplementary data
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