Palladium nanoparticles supported on MOF-5: A highly active catalyst for a ligand- and copper-free Sonogashira coupling reaction

Article abstract of DOI:10.1016/j.apcata.2010.08.045

Palladium nanoparticles supported on MOF-5 (Pd/MOF-5), have been prepared by a chemical method at room temperature. MOF-5 and Pd/MOF-5 were characterized by X-ray diffraction, N₂ sorption, thermogravimetric analysis, scanning electron microscopy, and transmission electron microscopy. The catalyst consists of highly dispersed palladium nanoparticles (about 3-6 nm) on MOF-5 with a high surface area (533 m²/g). It exhibits efficient catalytic activity for the Sonogashira coupling reaction between aryl iodides and terminal acetylenes without the assistance of ligand and copper.

Full text of DOI:10.1016/j.apcata.2010.08.045

Applied Catalysis A: General 388 (2010) 196–201
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Palladium nanoparticles supported on MOF-5: A highly active catalyst for a
ligand- and copper-free Sonogashira coupling reaction
Shuixia Gao, Nan Zhao, Mouhai Shu∗, Shunai Che
School of Chemistry and Chemical Engineering, State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, 800 Dongchuan Rd, Shanghai 200240, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 June 2010
Received in revised form 23 August 2010
Accepted 24 August 2010
Available online 18 September 2010
Palladium nanoparticles supported on MOF-5 (Pd/MOF-5), have been prepared by a chemical method at
room temperature. MOF-5 and Pd/MOF-5 were characterized by X-ray diffraction, N2 sorption, thermo-
gravimetric analysis, scanning electron microscopy, and transmission electron microscopy. The catalyst
consists of highly dispersed palladium nanoparticles (about 3–6 nm) on MOF-5 with a high surface area
(533 m2/g). It exhibits efficient catalytic activity for the Sonogashira coupling reaction between aryl
iodides and terminal acetylenes without the assistance of ligand and copper.
Keywords:
Metal organic frameworks
Pd nanoparticles
© 2010 Elsevier B.V. All rights reserved.
Heterogeneous catalysis
Sonogashira coupling
1. Introduction
high surface area [38–41] opened the scope for its application in gas
storage [40], separation [42,43], and catalysis [44–48]. As a support
Heterogeneous catalysis has become ever more attractive due
to its advantages in the industrial application, such as the ease of
the separation of the product and the recovery of the catalyst [1,2].
Among various heterogeneous catalysts, metal nanoparticles [3,4]
have played a leading role in hydrogenation [5–7], hydroformyla-
tion [8], and carbonylation [9]. Especially, palladium nanoparticles
(PdNPs) have been used extensively for the formation of C–C bonds
[10,11]. It is a well-known fact that PdNPs without stabilization are
prone to aggregate and lose their catalytic activity [12]. Therefore,
the search for suitable supports is still an attractive challenge in the
field of heterogeneous catalysis.
In recent decades, many inert solid materials such as
mesoporous solids (MCM-41, SBA-15, SBA-16, mesoporous
silica–carbon, mesoporous silica) [13–20], polymers (polyaniline,
poly(N-vinyl-2-pyrrolidone), dendrimers) [21–25], carbon poly-
morphs (active carbon, carbon nanotubes, graphene) [26–29], and
metal oxides (Fe3O4, Al2O3) [30,31] and their variants [32–37] have
been successfully applied as catalyst supports. Less attention has
so far been directed towards metal organic frameworks (MOFs),
the polymeric materials resulting from the coordination between
transition metal centers and chelating organic ligands.
for catalysts, MOF-5 has so far been used in hydrogenation [49],
polymerization [50], and isomerization reactions [51]. However,
its utilization in C–C bond coupling reactions is not well explored.
It seemed to us that palladium nanoparticles (PdNPs) in con-
junction with MOFs offer some promise in this respect. PdNPs are
usually prepared by chemical vapor deposition [52–54] from Pd
precursors in the presence of hydrogen gas or by classical meth-
ods like impregnation [55] or co-precipitation [56]. These methods,
however, involve multiple steps, specific conditions and compli-
cated operations. The large-scale application of MOF-supported
PdNPs therefore required improved methods for their preparation.
This paper describes such a preparation of PdNPs supported on
MOF-5 and their application in ligand- and copper-free Sonogashira
coupling reactions.
2. Experimental
2.1. Synthesis of the catalyst
The preparation of MOF-5. Terephthalic acid (5.10 g, 30 mmol)
and Zn(NO3)2·6H2O (12.16 g, 40 mmol) were dissolved in
DMF(400 mL). Freshly distilled triethylamine (16.0 g, 160 mmol)
was added dropwise into the above solution under strong stirring
at room temperature [57]. The mixture was stirred for 3 h at room
temperature. The precipitate was isolated by filtration, washed
with DMF (2× 40 mL) followed by dry ether (2× 40 mL), and dried
under vacuum. MOF-5 was obtained as white powder.
A typical member of the MOFs, MOF-5, consists of Zn4O-clusters
and benzene-1,4-dicarboxylate linkers forming an open cubic net-
work. Its facile synthesis, thermodynamic stability up to 400 ◦C and
∗
Corresponding author. Tel.: +86 21 5474 7241; fax: +86 21 5474 1297.
E-mail address: mhshu@sjtu.edu.cn (M. Shu).
0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.08.045

S. Gao et al. / Applied Catalysis A: General 388 (2010) 196–201
197
The preparation of Pd/MOF-5. PdCl2 (0.53 g, 3.0 mmol) and NaCl
(0.60 g, 10.0 mmol) were dissolved in DMF (300 mL), and a clear
orange solution was obtained. The freshly prepared MOF-5 (10.0 g)
was added to the solution, and the mixture was stirred for 10 min.
Then hydrazine hydrate (4 mL, excess) was added dropwise to the
suspension under vigorously stirring, and the mixture turned to
gray immediately. After stirring for 10 min, the solid was isolated
by filtration, washed with DMF followed dry ether (2× 40 mL), and
dried under vacuum. Pd/MOF-5 was obtained as gray powder, and
kept under nitrogen atmosphere. The palladium loading was 3 wt%.
Caution: Freshly prepared Pd/MOF-5 is a self-ignitable material
in air. Dry Pd/MOF-5 should be kept under nitrogen atmosphere
and handled with care.
with TMS as the internal standard unless otherwise specified. Thin
layer chromatography (TLC) was run on 2 cm × 5 cm silica plate.
Column chromatography was run on silica gel (200–300 mesh).
3. Results and discussion
3.1. Characterizations of MOF-5 and Pd/MOF-5
The wide-angle Powder XRD Patterns for MOF-5 and Pd/MOF-
5 (Fig. 1) indicate that the basic lattice structure of MOF-5 was
well maintained after Pd loading. The sharp peaks show good
crystallinity of both materials. However, the diffraction peaks of
Pd/MOF-5 became lower and wider due to the slight change of the
structural regularity of MOF-5 after Pd loading. The characteristic
peak of Pd (1 1 1) at 2Â = 40.1◦ was indistinguishable due to the low
Pd loading amount and relatively small well-dispersed palladium
nanoparticles which is in consonance with earlier reports [18].
The N2 sorption isotherms (Fig. 2) were measured at 77K.
MOF-5 exhibited a Brunauer–Emmett–Teller (BET) surface area
of 871 m2/g, including micropore area of 671 m2/g and external
surface area of 200 m2/g. Pd/MOF-5 gave a BET surface area of
533 m2/g, including micropore area of 370 m2/g and external sur-
face area of 163 m2/g. The decrease in surface area is mainly due
to the reduction of micropore surface area. The pore size distribu-
tions calculated by density functional theory (DFT) prove that the
micropores of MOF-5 range from 1 to 2 nm and the maximum pores
are at 1.4 nm. The sizes of Pd nanoparticles range from 3 to 6 nm as
estimated by TEM image (Fig. 3). The results conclude that the Pd
nanoparticles are too large to get into the porous passage of MOF-5
and attached on the surface of the support materials.
2.2. Catalytic reactions
General procedure for the Pd/MOF-5-catalyzed ligand- and copper-
free Sonogashira coupling reaction. A 100 mL Schlenk flask was
charged with Pd/MOF-5 (100 mg) and a certain base, and purged
with nitrogen. The solvent (20 mL) and acetylene (3.3 mmol) were
added followed by iodobenzene (3.0 mmol). The flask was main-
tained at 80 ◦C in an oil bath and was stirred for the appropriate
time. The reaction mixture was cooled to the room temperature,
the solid was removed by filtration and washed with methanol
(2× 10 mL). The filtrate was collected and the solvent was removed
under vacuum. The residue was extracted with dichloromethane
(2× 20 mL). The solution was washed with brine (20 mL), dried over
sodium sulfate, and concentrated under a reduced pressure. The
resulted crude material was purified via silica gel chromatography
using hexane as the eluent.
The reuse of the catalyst. After the first run of the coupling reac-
tion between phenylacetylene and iodobenzene, the catalyst was
isolated by centrifuge, and washed with methanol (2× 10 mL).
The catalyst was transferred to a 100 mL Schlenk flask, then base
(4.5 mmol), phenylacetylene (3 mmol) and iodobenzene (3 mmol)
were added, and the mixture was heated under stirring for a certain
time. The product was isolated following the procedure as the first
run and the catalyst was collected and used for the next cycle.
The SEM images of MOF-5 and Pd/MOF-5 showed no signifi-
cant changes in surface morphology and structure (Fig. S1). The gap
between MOF-5 grains is conductive for attachment of palladium
nanoparticles. A close vision about Pd/MOF-5 was provided by the
TEM image (Fig. 3), and Pd nanoparticles exist as black dots ranging
from 3 to 6 nm, which are easily distinguished from the surround-
ing gray regions of MOF-5. The PdNPs were well dispersed on the
outside surface and no obvious aggregation was observed, whereas
unsupported Pd NPs aggregate immediately and easily self-ignite
[58]. These results are also in accordance with the XRD analysis
(Fig. 1). The electron diffraction spectroscopy further proved the
existence of C, O, Zn, and Pd in the analysis area.
The TGA curves (Fig. 4) of MOF-5 and Pd/MOF-5 demonstrated
the superior thermodynamic stability of the materials up to 400 ◦C.
The materials were completely stable without any weight loss
under 100 ◦C. For both materials, the first weight loss occurred in
the range of 100–300 ◦C related to the loss of the embedded solvent
DMF. The weight loss observed was 11.4% for MOF-5 and 16.8% for
Pd/MOF-5, the difference is due to the different solvent content
by Pd loading. The second weight loss is between 410 and 530 ◦C
2.3. Characterizations
Powder X-ray diffraction (XRD) patterns were recorded using a
Rigaku X-ray diffractometer D/MAX-2200/PC equipped with CuKa
radiation (40 kV, 20 mA) at the rate of 4.0◦/min over the range of
3–60◦ (2Â). The nitrogen adsorption–desorption isotherms were
measured at 77 K with a Quantachrome Nova 4200E porosimeter.
The microscopic morphological features were observed by using a
scanning electron microscopy (SEM, JEOLJSM-7401F). An accelerat-
ing voltage, 1 kV (resolution: ≈1.4 nm) was chosen for all samples.
High-resolution transmission electron microscopy (HRTEM) was
performed by JEOL JEM-3010 microscope operating at 300 kV
˚
(Cs = 0.6 mm, resolution 1.7 A). Images were recorded by means of a
CCD camera (MultiScan model 794, Gatan, 1024 × 1024 pixels, pixel
size 24 × 24 ␮m) at 50–80k magnification under low-dose condi-
tions. Thermogravimetric analysis (TGA) was performed from 40
to 800 ◦C in nitrogen atmosphere with a Perkin-Elmer TGA 2050
instrument at a heating rate of 20 ◦C/min. The concentration of
Palladium supported on MOF-5 was determined by using induc-
tively coupled plasma-atomic emission spectrometry (ICP-AES, Iris
Advantage 1000). UV–visible absorption spectra were measured
with a UNIC WFZ UV-4802H in range of 190–350 nm. XPS experi-
ments were carried out on a RBD upgraded PHI-5000C ESCA system
(Perkin-Elmer) with Mg K␣ radiation (hꢀ = 1253.6 eV) or Al K␣ radi-
ation (hꢀ = 1486.6 eV). Binding energies were calibrated by using
the containment carbon (C1s = 284.6 eV). 1H and 13C NMR spectra
were recorded on a Varian Mercuryplus-400 spectrometer in CDCl3
Fig. 1. Powder X-ray diffraction patterns of MOF-5 and Pd/MOF-5.

198
S. Gao et al. / Applied Catalysis A: General 388 (2010) 196–201
Fig. 2. Nitrogen sorption isotherms and pore size distribution curves f MOF-5 and Pd/MOF-5.
3.2. Application in ligand- and copper-free Sonogashira coupling
reactions
Initially, iodobenzene and phenylacetylene were chosen as the
substrates to investigate the solvent effect for the Pd/MOF-5 cat-
alyzed Sonogashira reaction at the presence of Na3PO4·12H2O as
the base (Table 1). Aprotic solvents such as acetonitrile and ace-
tone (entries 1 and 2) gave low yields. Protic solvents like mixed
alcohol aqueous solvent and pure alcohol were also tested. It was
found that only 13% yield was obtained by 50% iso-propanol (entry
4), and this may be due to the existence of a large amount of water
which will cause the damage of MOF-5. Among the alcohol solvents,
iso-propanol was ineffective (14%) (entry 3), tert-butanol generated
multiple product and only a little bit of diphenylacetylene was iso-
lated (7%) (entry 7), ethanol gave 49% yield in 6 h (entry 6), however
methanol resulted in full conversion and 86% yield in 3 h (entry 5).
Universally used organic base triethyl amine gave 90% conver-
sion with 31% of desired product (Table 2, entry 1). Hence we
tested the influence of various inorganic bases. Very low yield was
observed in case of NaOAc (entry 2), in contrast to it carbonate
and phosphate. The reactions employing 2.0 equivalent anhydrous
K2CO3, K3PO4·3H2O or Cs2CO3 as the base leaded to 90% yield
(entries 5–7). These bases were further tested with the change of
mole ratio to further increase the yields (entries 8–12). Surpris-
ingly, 1.5 equivalent K3PO4·3H2O gave extremely high yield (98%)
(entry 9). The catalytic performance with simple bases revealed 96%
with NaOH, and 95% with KOH (entries 13 and 14). No product was
obtained in the absence of the base (entry 15).
Fig. 3. TEM image of Pd/MOF-5.
corresponding to the decomposition of organic species. Comparing
with MOF-5, the maximum weight loss temperature of Pd/MOF-5
decreased from 510 to 469 ◦C, and the observation may be caused by
the decomposition of organic moieties facilitated by active Pd. The
2% difference (ꢁW2 − ꢁW1) in the TGA analysis between MOF-5
and Pd MOF-5 is due to the palladium loading.
The loading amount of palladium is the most critical factor,
which remarkably affects catalytic activity. As shown in Table 3,
when the Pd loading amount is 2.2 wt%, increasing Pd dose can
Table 1
Optimization of solvents.
.
Entry
Solvent
t (h)
Yield (%)a
7
Conversion (%)
1
2
3
4
5
6
7
MeCN
acetone
iPrOH
3
3
6
3
3
6
3
31
58
>99
83
>99
>99
>99
10
14
13
86
49
7
50%iPrOHb
MeOH
EtOH
tBuOH
a
Isolated yield.
iPrOH:H2O = 1:1.
b
Fig. 4. TGA of MOF-5 and Pd/MOF-5 under nitrogen atmosphere.

S. Gao et al. / Applied Catalysis A: General 388 (2010) 196–201
199
Table 2
Effect of base on Sonogashira reaction.
.
Entry
Base
Equivalent
t (h)
Yield (%)a
31b
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Et3N
NaOAC
Na2CO3
1.5
2.0
2.0
2.0
2.0
2.0
2.0
1.5
1.5
1.0
1.5
1.5
1.5
1.5
0
3
4
4
3
3
3
3
3
3
3
3
3
3
3
24
3c
31d
86
90
90
90
85
98
84
95
68
96
95
0e
Na3PO4·12H2O
K2CO3
K3PO4·3H2O
Cs2CO3
K2CO3
K3PO4·3H2O
K3PO4·3H2O
Cs2CO3
Fig. 5. The plot of yield/time.
Na3PO4·12H2O
NaOH
KOH
None
UV–vis spectrophotometer was used to determine the optimum
reaction time by detecting the concentration of the reagent and
the product during the reaction process. Because of that the char-
acteristic absorption peaks of iodobenzene and phenylacetylene
are overlapped (Fig. S2), the absorption of diphenylacetylene was
chosen as response to the progress of the reaction (Fig. S3). The
product shows the strongest absorption at 278 nm. After drawing
the standard absorption curve of diphenylacetylene at 278 nm, the
corresponding yield at different reaction time was calculated as
shown in Fig. 5. The highest yield 98% was obtained at 180 min
whereas at 210 min no significant change in yield was observed.
The yield at the start point was observed to be 8% rather than zero,
this may be due to the initiation of reaction before the temperature
reached 80 ◦C. (We recorded the time when the temperature of the
reaction system reached 80 ◦C.)
On the basis of these results, we employed the following opti-
mized reaction conditions: 3 mmol of aryl iodide, 3.3 mmol of
alkyne, 100 mg 3 wt% of Pd/MOF-5, 4.5 mmol of K3PO4·3H2O, and
20 mL of methanol as the solvent. These conditions were applied
for various aryl iodides and terminal acetylenes to assess the scope
towards a wide range of aryl iodides bearing either electron-
withdrawing or electron-donating functional groups (Table 5,
entries 1–8) with excellent yields. Comparing with electron-rich
aryl iodides, electron-deficient para-substituted of aryl iodides
showed highest activity in the process of coupling with pheny-
lacetylene (entries 5, 7, and 8), whereas ortho-substituted aryl
iodides gave lower isolated yield than para-substituted ones,
this is possibly due to steric effect (entries 2 and 5). It is note
worthy that 3,5-bis(trifluoromethyl)iodobenzene furnished the
desired product in lower isolated yield than 4-(trifluoromethy)
iodobenzene mainly owing to the fact that the former has
less electron-withdrawing inductive effect (entries 5 and 6). In
case of aryl acetylenes, electron-deficient para-substituted sub-
strates process better coupling than electron-rich ones (entries
9-12). In addition, the coupling of aliphatic acetylenes with aryl
iodides has also been achieved successfully (entry 13). Cross
couplings of para-substituted substrates with various electron-
withdrawing or electron-donating functional groups were also
investigated to further study of the electronic effect. Cou-
pling reactions between iodobenzene with electron-withdrawing
groups and phenylacetylene with electron-donating groups gave
desired product almost quantitatively (entries 14–19). When
iodobenzene bearing electron-donating groups and phenylacety-
lene with electron-withdrawing groups were used as substrates,
the yields are lower (entries 20 and 21). However, the
yields were significantly dropped when both were used with
electron-withdrawing groups or electron-donating groups (entries
22–24).
a
Isolated yield, conversion is 100% unless otherwise stated.
b
c
Conversion: 90%.
Conversion: 67%.
Conversion: 87%.
Conversion: <5%.
d
e
Table 3
Effects of the loading amount of palladium on Sonogashira reaction.a
Entry
X (wt%)
Dose (Pd, mg/mmol)
t (h)
Yield (%)b
1
2
3
4
2.2
2.2
3.0
3.5
0.7
1.1
1.0
1.2
4
3
3
3
82
90
98
98
a
Product and yields for Sonogashira coupling between phenylacetylene and
iodobenzene catalyzed by Pd/MOF-5 (base, 1.5 equivalent of K3PO4·3H2O; solvent,
methanol; temp., 80 ◦C).
b
Isolated yield, conversion is 100% unless otherwise stated.
improve the yield to 90% (entries 1 and 2). However, raising the
loading amount from 2.2 to 3.0 wt% even with lower dose of Pd, the
yield can significantly increase to 98% (entry 3). It indicated that
increasing the Pd loading is more effective than raising Pd dose.
However, there is no apparent improvement observed with higher
loading 3.5 wt% (entry 4).
A significant effect of the reaction temperature was found in
the reaction (Table 4). Raising the reaction temperature from 20
to 80 ◦C significantly shortened the reaction time (entries 1–4) and
gave better yields. A further temperature increase to 100 ◦C can
accelerate the reaction rate in a shorter reaction time (entries 5
and 6), but did not give a better yield after 3 h (entry 7).
Table 4
Effects of reaction temperature on Sonogashira reaction.a
Entry
T (◦C)
t (h)
Yield (%)b
1
2
3
4
5
6
7
20
40
60
24
24
3
11c
12d
58
80
3
98
80
1
38e
70
100
100
1
3
96
a
Product and yields for Sonogashira coupling between phenylacetylene and
iodobenzene catalyzed by Pd/MOF-5 (3 wt% of Pd) (base, 1.5 equivalent of
K3PO4·3H2O; solvent, methanol).
b
Isolated yield, conversion is >99% unless stated.
Conversion: 75%.
Conversion: 89%.
Conversion: 71%.
c
d
e

200
S. Gao et al. / Applied Catalysis A: General 388 (2010) 196–201
Table 5
Table 6
Sonogashira coupling of aryl iodides with terminal acetylenes.
Investigation into the reuse of Pd/MOF-5.a
.
.
Entry
Run
t (h)
Yield (%)b
Entry
R
Rꢀ
Product
t (h)
Yield (%)a
1
2
3
4
5
1
2
3
4
5
3
3
3
5
5
98
94
60
61
49c
1
2
3
4
5
4-Me
2-MeO
4-MeO
2-CF3
4-CF3
3,5-CF3
4-Ac
4-NO2
H
H
H
H
H
4-NO2
4-Ac
4-CF3
4-NO2
4-Ac
4-CF3
4-Me
4-MeO
4-Me
4-MeO
4-Ac
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
3b
3c
3d
3e
3f
3g
3h
3i
3b
3d
3j
3k
3l
3m
3n
3o
3p
3q
3r
3
5
3
3
3
4
3
3
3
3
5
6
3
3
3
3
3
90
92
94
83
>99
a
After each run, the catalyst was isolated by centrifuge, washed with methanol,
and dried for the next cycle.
6
90
98
>99
83
85
90
87
92
99
99
99
99
99
99
85
92
60
31c
54d
7
b
8b
9
Isolated yield, conversion is 100% unless otherwise stated.
c
Conversion: 91%.
4-Me-Ph
4-MeO-Ph
4-F-Ph
4-C5H11-Ph
C5H11
4-MeO-Ph
4-MeO-Ph
4-MeO-Ph
4-Me-Ph
4-Me-Ph
4-Me-Ph
4-F-Ph
4-F-Ph
4-Me
10
11
12
13
14b
15
16
17b
18
19
20
21
22
23
24
that aggregation of Pd nanoparticles was found at certain places. It
can be concluded that the deactivation of the catalyst is mainly due
to oxidation of Pd and the aggregation of the Pd nanoparticles.
4. Conclusions
3
3
A highly active heterogeneous catalyst, Pd/MOF-5, has been
prepared, characterized and successfully applied in ligand- and
copper-free Sonogashira coupling reactions. The high thermo-
dynamic stability and specific surface area of MOF-5 meet the
demands for utilization as a catalyst support. The cubic network
and porous structure of MOF-5 provide a good dispersion of the
PdNPs which are highly active, as exemplified in the Sonogashira
coupling of halobenzenes with phenylacetylene which is applica-
ble to a wide range of substrates with good electronic and steric
tolerance. The catalyst can be reused for several times, and the
economy and facility of its preparation provide the possibility for
its industrial application.
3s
3t
3u
3v
3w
12
12
24
24
24
4-MeO
4-F-Ph
a
Isolated yield, conversion is 100% unless otherwise stated.
5 mL of toluene was added in the solvent due to the poor solubility of 4-
iodonitrobenzene in methanol.
b
c
Conversion: 86%.
Conversion: 93%.
d
Usually in copper assisted Sonagashira reaction major by-
product is terminal diyne due to Glaser coupling [59,60]. Similar
observations have been made in copper-free system while the
by-product diyne is formed in trace amount. In some reactions con-
versions are much higher compared to yield due to oxidation of
iodobenzene, it is note worthy that all the conversions are calcu-
lated with respect to iodobenzene. The oxidized by-products are
found to be complicated for structural deduction.
Bromobenzene and chlorobenzene gave no desired product
under the prior optimized conditions. Under severe reaction con-
ditions (i.e. 4.0 equivalent of KOH, 100 ◦C, and 7 wt% Pd loading),
54% yield was obtained. However, our catalytic system is not active
enough to achieve the coupling between inert chlorobenzene and
phenylacetylene.
Acknowledgments
This work was supported by the National Natural Science Foun-
dation of China (Grant No. 20951001), the 973 project of China
(2007CB209701 and 2009CB930402) and the Scientific Research
Foundation for the Returned Overseas Chinese Scholars of State
Education Ministry.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.apcata.2010.08.045.
To test the heterogeneity of catalyst, the concentration of pal-
ladium species in the hot reaction solution after 1 h reaction was
detected by ICP-AES, and only 2.1 ppm of Pd was observed in the
solution. The observation suggested a negligible Pd leaching and
confirmed the heterogeneous process similar to the Pd/C system
[61]. The recyclability of the catalyst is shown in Table 6. The activity
of the recycled catalyst did not changed significantly in the second
run (entry 2, 94%), but decreased significantly from the third run
(entries 4–6), the yield can be slightly increased (entries 4 and 5)
by prolonging the reaction time. In order to find out the reasons
for deactivation of catalyst, the ICP-AES analysis was performed
to the cold filtrations after the first, third and fifth run. After each
run Pd leaching amount was less than 1.0 ppm. It is found that the
self-ignitable nature and deactivation of catalyst is due to the oxi-
dation. The XPS studies (Fig. S4) on the recycled catalyst confirm
the existence of Pd(II) which was not observed in the freshly pre-
pared catalyst [18,62]. This indicates that the Pd(0) in the catalyst
was oxidized to Pd(II) upon the exposure of the catalyst to air dur-
ing the catalytic cycles. Furthermore TEM image (Fig. S5) revealed
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