Pd nanoparticles in the thermoregulated ionic liquid and organic biphasic system: An efficient and recyclable catalyst for heck reaction

Article abstract of DOI:10.1007/s10562-012-0919-9

A thermoregulated ionic liquid and organic biphasic system composed of ionic liquid [CH₃(OCH₂CH₂)₁₆N ⁺Et₃][CH₃SO₃ ^-] (IL _PEG750) with IL_PEG750-stabilized Pd nanoparticles and organic solvents, which allows for not only a highly efficient homogeneous catalytic reaction, but also an easy biphasic separation and reuse of catalyst attributed to the thermoregulated phase-transition property of the system, was employed for Heck reaction. The IL_PEG750-stabilized Pd nanoparticle catalyst exhibited high efficiency, more importantly, after reaction the Pd nanoparticle catalyst could be separated from products by simple phase separation and recycled for six times without evident loss of activity. Graphical Abstract: A thermoregulated ionic liquid and organic biphasic system containing [CH₃(OCH₂CH₂)₁₆N ⁺Et₃][CH₃SO ₃ ^- ] (IL_PEG750)-stabilized Pd nanoparticles was applied to Heck reaction for the first time. The Pd nanoparticle catalyst exhibited high efficiency and good recyclability.[Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-012-0919-9

Catal Lett (2013) 143:200–205
DOI 10.1007/s10562-012-0919-9
Pd Nanoparticles in the Thermoregulated Ionic Liquid
and Organic Biphasic System: An Efficient and Recyclable
Catalyst for Heck Reaction
Yan Zeng
Ying Song
•
Yanhua Wang
Jingyang Jiang
•
Yicheng Xu
Zilin Jin
•
•
•
Received: 19 July 2012 / Accepted: 29 September 2012 / Published online: 12 October 2012
Ó Springer Science+Business Media New York 2012
Abstract A thermoregulated ionic liquid and organic
biphasic system composed of ionic liquid [CH (OCH CH )
2 16
liquid biphasic system, for example, the use of fluorous/
organic biphasic system [3, 4], thermoregulated polyethylene
glycol (PEG) biphasic system [5], thermoregulated phase-
transfer aqueous/organic biphasic system [6, 7] and ionic
liquid biphasic system [8, 9].
3
2
?
N Et ][CH SO ] (ILPEG750) with ILPEG750-stabilized Pd
-
3
3
3
nanoparticles and organic solvents, which allows for not only a
highly efficient homogeneous catalytic reaction, but also an
easy biphasic separation and reuse of catalyst attributed to the
thermoregulated phase-transition property of the system, was
employed for Heck reaction. The ILPEG750-stabilized Pd
nanoparticle catalyst exhibited high efficiency, more impor-
tantly, after reaction the Pd nanoparticle catalyst could be
separated from products by simple phase separation and recy-
cled for six times without evident loss of activity.
Ionic liquids with negligible vapour pressure and high
thermal stability have gained increasing attention as
promising reaction media or stabilizers for transition-metal
nanoparticles. However, in most cases, the reaction sys-
tems involving ionic liquids are either monophase allowing
for rapid transformation or biphase facilitating separation
of product from catalyst [10–18]. Therefore, to search for
new catalytic systems that might combine the advantages
of both homogeneous and heterogeneous catalysis will
have a significant impact.
Keywords Thermoregulated ionic liquid and organic
biphasic system Á Palladium Á Nanoparticle Á Catalyst
recycling Á Heck reaction
Very recently, our group has developed an effective
approach for Rh nanoparticle catalyzed hydrogenation and
hydroformylation of olefins in a thermoregulated ionic liquid
1
Introduction
and organic biphasic system [19, 20], wherein [CH
-
3
?
(
OCH CH ) N Et ][CH SO ](ILPEG750)wasemployedas
2 2 16 3 3 3
Solubletransition-metalnanoparticles incatalysis havedrawn
much attention due to their high efficiency and unique prop-
erties. However, very similar to traditional homogeneous
catalysts, one of the main disadvantages of soluble nanopar-
ticlecatalystsistheproblemofseparationthecatalystfromthe
products [1, 2]. To deal with this problem, a range of methods
has been reported, with most studies focusing on the liquid/
ionic liquid. This ionic liquid exhibits very unique solubility in
organic solvent depending on the temperature. Namely,
ILPEG750 is immiscible with the mixed toluene and n-heptane
at room temperature, but becomes homogeneous when the
temperature is increased to a certain point. Consequently, the
reaction proceeds virtually in a homogeneous system under
heating, and upon cooling to room temperature, separates into
a biphasic system composed of ionic liquid phase containing
Rh catalyst and organic phase containing the products. Such a
process provides both the advantages of classic monophasic
and biphasic system, i.e., highly catalytic efficiency and good
recyclability.
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-012-0919-9) contains supplementary
material, which is available to authorized users.
Y. Zeng Á Y. Wang (&) Á Y. Xu Á Y. Song Á J. Jiang Á Z. Jin
State Key Laboratory of Fine Chemicals, Dalian University of
Technology, Dalian 116024, People’s Republic of China
e-mail: yhuawang@dlut.edu.cn
Heck reaction, since its discovery in the early 1970s, has
become one of the most versatile and useful approaches in
synthetic chemistry. The catalysts most often used for this
1
23

Pd Nanoparticles in the Thermoregulated Ionic Liquid
201
reaction are either heterogeneous Pd catalysts fixed to solid
supports or homogeneous Pd complex catalysts [21–28].
The search for more efficient catalytic systems that might
combine the merits of both homogeneous and heteroge-
neous catalysis is still a tough challenge for Heck reaction.
Encouraged by our previous successful applications of
thermoregulated ionic liquid and organic biphasic system
for Rh nanoparticle catalyzed hydrogenation and hydro-
formylation of olefins, Herein, we report for the first time
the preparation of ILPEG750-stabilized Pd nanoparticles, and
their application as efficient and recyclable catalysts in
thermoregulated ionic liquid and organic biphasic system
for Heck reaction.
2.3 TEM Images of Pd Nanoparticle Catalyst
The ionic liquid ILPEG750 solution containing the Pd
nanoparticle catalyst was diluted with ethanol. Then, a drop
of the solution was placed onto a carbon-coated copper
grid, and dried at ambient temperature. The transmission
electron microscopy (TEM) images were taken with a
2
Philips Tecnai G 20 TEM at an accelerating voltage of
120 kV.
2.4 General Procedure for Heck Reaction
As a standard procedure, a mixture of aryl iodide
(
1 mmol), vinylic substrate (2 mmol), base (3 mmol),
-
3
ILPEG750 (0.2 g, containing 1 9 10 mmol Pd), toluene
(2.5 g), n-heptane (0.2 g), o-xylene (0.1 g, as the internal
standard) and a magnetic bar were added in a glass tube
equipped with a reflux condenser. The mixture was stirred
at the designated reaction temperature for the desired time.
After reaction on cooling to room temperature, the organic
phase containing products and residual reactants were
separated by simple decantation and analyzed by GC. The
lower ILPEG750 phase containing ILPEG750-stabilized Pd
nanoparticle catalyst was directly reused to catalyze the
Heck reaction of the next run. Pure product was gained by
column chromatography.
2
Experimental
2
.1 General Remarks
The ILPEG750 was prepared according to our previous paper
[29]. PdCl (99.8 %) was purchased from Evonik Degussa
2
(
Germany). Iodobenzene (99 %), bromobenzene (99 %),
chlorobenzene (99 %), 4-fluoroiodobenzene (AR), 2-fluoro-
iodobenzene (99 %), 4-bromoiodobenzene (98 %), 4-iodo-
anisole (97 %), 2-iodoanisole (99 %), 4-iodonitrobenzene
(
98 %) and p-bromoacetophenone (98 %) were supplied by
Shanghai Aladdin Reagent Company. 2-Nitroiodobenzene
97 %) was supplied by Alfa Aesar Company. Other chemi-
(
cals were obtained commercially and used without any further
purification. Gas chromatographic analyses were performed
on a Tianmei 7890 GC instrument equipped with a
3 Results and Discussion
Pd nanoparticles in ionic liquids can be prepared by a series
of methods, such as thermal or ultrasonic decomposition of
5
0 m 9 0.25 mmOV-101columnand anFIDdetector(N as
2
a carrier gas). ICP-AES analyses of Pd were carried out on
Optima 2000 DV (Perkin Elmer, USA), the detection limit of
Pd is 0.003 lg/mL. The X-ray diffraction patterns of Pd
nanoparticles were recorded by D/Max-2400 diffractometer
with Cu Ka radiation (k = 0.1541 nm). The equipment was
operated at 40 kV and 100 mA with scan range between 20°
and 100°. UV–vis measurement of ILPEG750-stabilized Pd
nanoparticles was performed on a UV-2100 UV/VIS spec-
Pd salts and complexes, reduction with H or hydride, or
2
reductive elimination of a palladacycle by reaction with a
diene [30–35]. In our earlier work, ionic liquid ILPEG750
has been proved to be an excellent medium for preparation
and stabilization of Rh nanoparticles [19, 20]. For this
reason, ILPEG750 was chosen as a stabilizer for preparation
of Pd nanoparticles. ILPEG750-stabilized Pd nanoparticles
were prepared by the simple reduction of PdCl
the presence of ILPEG750. Figure 1a shows the UV–visible
spectra of PdCl in ILPEG750 medium before and after
with H in
2
2
1
trophotometer. H NMR (400 MHz) spectra wererecorded on
Bruker Avance 400 M using TMS as an internal standard.
2
reduction. There was no obvious absorption peak of the
Pd (II) ion at 430 nm after reduction which indicated
2
.2 Preparation of ILPEG750-Stabilized Pd
Nanoparticles
that PdCl has been reduced. In addition, the ILPEG750-
2
stabilized Pd catalysts were analyzed by X-ray diffraction
(XRD). The XRD pattern (Fig. 1b) with diffraction lines at
2h = 40.12, 46.70, 68.36, 82.23 and 86.56° indicated the
formation of face-centered cubic (fcc) Pd (0).
In a typical experiment, a mixture of PdCl2 (5.8 mg,
.0328 mmol) and ILPEG750 (6.0 g, 6.56 mmol) was added
0
in a 75 mL stainless steel autoclave and stirred under
hydrogen (4 MPa) at 70 °C for 2 h. Then, the reactor was
cooled to room temperature and depressurized. The color
of the solution turned to be black, indicating the formation
of Pd nanoparticle catalyst.
Further analysis by TEM revealed that the Pd nanopar-
ticles thus obtained display a homogeneous distribution in
ILPEG750 with an average particle size of 3.0 ± 0.4 nm in
diameter (Fig. 2).
123

2
02
Y. Zeng et al.
Fig. 1 a UV–visible absorption
spectra of PdCl
2
in ILPEG750
before and after reduction (left);
b X-ray diffraction pattern of
ILPEG750-stabilized Pd
nanoparticles (right)
Fig. 2 TEM image and particle
size histogram of ILPEG750
stabilized Pd nanoparticles
based on 100 randomly
-
(
selected particles)
The thermoregulated phase-transition property of the
Table 1 Miscibility temperature of the ILPEG750 and organic biphasic
system with ILPEG750-stabilized Pd nanoparticles
ILPEG750 and organic biphasic system with ILPEG750
-
stabilized Pd nanoparticles has been confirmed (see Fig. S1
in the Supporting Information). At room temperature, the
lower ionic liquid phase containing Pd nanoparticles was
immiscible with the upper organic phase consisting of
toluene and n-heptane (a in Fig. S1). Interestingly, when
the system was heated gradually to a certain temperature
Entry
Toluene
(g)
n-Heptane
(g)
Miscibility
temperature
o
(
C)
1
2
3
4
5
6
7
2.0
2.0
2.0
2.0
2.0
2.4
3.0
0.2
0.3
0.4
0.5
0.6
0.6
0.6
40
70
100
120
160
130
90
(
miscibility temperature), the biphasic system merged into
a clear single phase (b in Fig. S1). Finally, by decreasing
the temperature to room temperature, the biphasic system
recovered completely to its original form as a (c in Fig. S1).
Moreover, the miscibility temperature that the system
changed from biphase to monophase can be tuned by
regulating the compositions of organic solvents (Table 1).
It can be seen from Table 1 that increasing the amount
of n-heptane will increase the miscibility temperature
The amount of ILPEG750 is 0.5 g, ILPEG750/Pd = 200:1 (molar ratio)
Namely, for a reaction whose product is soluble in organic
phase, one can make the reaction proceed in a homoge-
neous phase under heating. When the reaction is com-
pleted, the product and the catalyst are collected easily
from the organic and ionic liquid phase, respectively, just
by cooling the reaction system to room temperature. The
lower ionic liquid phase containing catalyst can be directly
recycled without further purification or activation.
(
Table 1, entries 1–5), while increasing the amount of
toluene will lead to the decrease of miscibility temperature
(
Table 1, entries 5–7).
The above-mentioned thermoregulated phase-transition
process would provide a simple, atom-efficient, and envi-
ronmentally benign way for various Pd-catalyzed reactions
when the reaction procedure is appropriately designed.
1
23

Pd Nanoparticles in the Thermoregulated Ionic Liquid
203
Table 2 Effects of different bases on the Heck reaction of iodo-
benzene with methyl acrylate
Entry
Base
Conversion (%)
1
2
3
4
5
Et
KOH
CO
NaOH
Na CO
3
N
89
34
K
2
3
22
17
2
3
\1
Reaction conditions: 1 mmol of iodobenzene, 2 mmol of methyl
acrylate, 3 mmol of the base, 0.1 mmol % Pd relative to the amount
of iodobenzene, 80 °C, 9 h. Conversions were calculated from GC
analyses
Fig. 4 The recycling efficiency of ILPEG750-stabilized Pd nanoparti-
cle catalyzed Heck reaction of iodobenzene with methyl acrylate
To investigate the catalytic activity of ILPEG750-stabi-
lized Pd nanoparticles in the thermoregulated ionic liquid
and organic biphasic system, the Heck reaction of iodo-
benzene with methyl acrylate was chosen as a model
reaction. The reactions were run in air. Considering that a
base is needed for Heck reactions, the effects of commonly
The recycling efficiency of ILPEG750-stabilized Pd
nanoparticles in the thermoregulated ionic liquid and
organic biphasic system was also investigated. After
reaction, the upper organic phase containing product was
separated from the lower ionic liquid phase containing
catalyst by simple phase separation. And the lower catalyst-
containing ionic liquid phase was directly used in the next
reaction run. Under the reaction conditions (1 mmol of
used Et N and several inorganic bases on the reaction of
3
iodobenzene with methyl acrylate were examined and the
results were listed in Table 2. It can be seen that using
organic liquid base Et N as the base is better than those of
3
iodobenzene, 2 mmol of methyl acrylate, 3 mmol of Et N,
3
inorganic bases due to their different solubilities in the
0
1
.1 mmol % Pd relative to the amount of iodobenzene,
reaction system. Therefore, Et N was chosen as the base
3
00 °C, 5 h), the recovered ILPEG750-stabilized Pd nano-
for the following Heck reactions.
particle catalyst exhibited highly catalytic efficiency
without evident loss of activity after the catalyst was
recycled for six times (Fig. 4). Comparison of the TEM
images of Pd nanoparticles between newly prepared and
after six recycles (Fig. 2 vs. Fig. S2 in the Supporting
Information) showed an increase of particle size from
original 3.0 nm to 6.0 nm. This can be ascribed to the
Ostwald ripening process, which introduced two mecha-
nisms for the growth of nanoparticles [36]. ICP-AES
analysis of the upper colourless organic phase separated
from the first run indicated that the leaching of Pd was
Figure 3 presents the effects of reaction time and tem-
perature on the Heck reaction of iodobenzene with methyl
acrylate. The results demonstrated that prolonging the
reaction time or increasing the temperature will all benefit
the product yield. Under the conditions of 100 °C, 5 h, the
product yield was 96 %. It is worth noting that the product
yield increased sharply from 36 % (70 °C, 10 h) to 90 %
(
80 °C, 10 h). This may be due to the fact that the reaction
system changed from biphase to monophase (the misci-
bility temperature of the system is 80 °C). In addition, the
selectivity to the trans-product is 100 %.
0
.5 wt.%. However, from the second run on, the leaching
of Pd was lower than the minimum detectable amount
0.003 lg/mL). The highly stability of ILPEG750-stabilized
(
Pd nanoparticle catalyst might result from the electrosteric
stabilization of ILPEG750, which combines both the elec-
trostatic and steric effects together in one molecular
structure. To confirm this hypothesis, experiments only
using tetraoctylammonium bromide possessing the elec-
trostatic effect or CH (OCH CH ) OH possessing the
3
2
2 16
steric effect as stabilizers for preparation of Pd nanoparti-
cles were performed. As expected, for both cases
agglomeration and formation of Pd black occurred.
ILPEG750-stabilized Pd nanoparticles in the thermoregu-
lated ionic liquid and organic biphasic system has shown to
be an efficient and recyclable catalyst for the Heck reaction
of iodobenzene with methyl acrylate, therefore, extension
Fig. 3 Effects of reaction time and temperature on the Heck reaction
of iodobenzene with methyl acrylate
123

2
04
Y. Zeng et al.
Table 3 Heck reactions
between aryl halides and vinylic
substrates catalyzed by
X
R2
ILPEG750-stabilized Pd
Et N
R2
R1
R1
ILPEG750-stabilized Pd
3
a
nanoparticles
X = I, Br, Cl
R1 = F, Br, OCH , COCH , NO
2
3
3
n
R2 = COOCH , COOBu , Ph
3
b
c
Entry
Aryl halides
Products
Time (h)
Yield (%)
O
I
1
2
3
4
5
6
7
8
O
5
5
89
87
71
88
89
92
79
58
O
I
O
I
12
3
O
I
O
F
F
O
I
O
3
F
F
O
O
I
O
O
3
Br
Br
I
5
O
O
O
I
O
5
O
O
O
I
9
O
5
5
62
90
NO2
NO2
a
Reaction conditions: 1 mmol
of aryl iodide, 2 mmol of
vinylic substrate, 3 mmol of
O
I
1
0
O
NO2
NO2
Et
to the amount of aryl iodide,
3
N, 0.1 mmol % Pd relative
O
Cl
Br
d
d
d
1
b
00 °C
Selectivity to the trans-
11
36
36
24
0
0
O
O
O
product is 100 %
c
Isolated yield
1
1
2
3
d
O
1
40 °C, 0.5 mmol % Pd
Br
O
71
relative to the amount of aryl
halide
H3COC
H3COC
of the Heck reaction to other substituted aryl iodides was
carried out and moderate to good yields were obtained as
shown in Table 3. All of the products were trans-isomers,
(–OCH ) attached to iodobenzene will decrease the activity
3
(Table 3, entries 7 and 8). But for iodobenzene with
electron-withdrawing –NO group, the Heck reaction is
2
1
which were identified by H NMR (see Supporting Infor-
less active than that of iodobenzene with –OCH group
3
mation). For iodobenzene, displacement of methyl acrylate
to butyl acrylate, the reaction activity remains nearly the
same (Table 3, entry 1 vs. entry 2). When styrene was used
as the starting material, the Heck reaction was less active
(Table 3, entries 7 and 9), similar result has been observed
in literature [37]. Furthermore, the positions of substitutes
play an important role in the Heck reactions. For iodo-
benzene with –OCH group, the Heck reaction is more
3
(
Table 3, entry 3). For iodobenzene with electron-with-
drawing groups (–F, –Br), the Heck reaction is more active
Table 3, entries 4–6), while electron-donating group
active when –OCH group is attached to the para position,
3
but the opposite result is obtained for –NO2 group
(Table 3, entries 7–10). This observation was similar to the
(
1
23

Pd Nanoparticles in the Thermoregulated Ionic Liquid
205
result reported in literature [28]. In addition, less active
chlorobenzene and bromobenzene instead of iodobenzene
were also investigated for Heck reaction. Unfortunately, no
expected coupling products were obtained at 140 °C for
9. Mu XD, Meng JQ, Li ZC, Kou Y (2005) J Am Chem Soc
1
27:9694
0. Dupont J, Fonseca GS, Umpierre AP, Fichtner PFP, Teixeira SR
2002) J Am Chem Soc 124:4228
1
(
11. Fonseca GS, Umpierre AP, Fichtner PFP, Teixeira SR, Dupont J
(2003) Chem Eur J 9:3263
3
6 h (Table 3, entries 11 and 12). Further investigation
1
1
1
1
2. Yuan X, Yan N, Xiao CX, Li CN, Fei ZF, Cai ZP, Kou Y, Dyson
PJ (2010) Green Chem 12:228
3. Hu Y, Yu YY, Hou ZS, Li H, Zhao XG, Feng B (2008) Adv
Synth Catal 350:2077
4. Crespo-Quesada M, Dykeman RR, Laurenczy G, Dyson PJ,
Kiwi-Minsker L (2011) J Catal 279:66
5. Sidhpuria KB, Daniel-da-Silva AL, Trindade T, Coutinho JAP
with p-bromoacetophenone as the substrate led to 71 %
Heck coupling product at 140 °C for 24 h (Table 3, entry 13).
4
Conclusions
(
2011) Green Chem 13:340
16. Jansat S, Durand J, Favier I, Malbosc F, Pradel C, Teuma E,
mez M (2009) ChemCatChem 1:244
We have developed a novel biphasic system composed of
ionic liquid ILPEG750 with ILPEG750-stabilized Pd nano-
particles and organic solvents for Heck reaction. The cat-
alytic system is free of phosphines or other ligands, which
are generally expensive, toxic and unrecoverable. In addi-
tion, the reactions can be performed conveniently under the
Go
´
1
7. Zhang GP, Zhou HH, Hu JQ, Liu M, Kuang YF (2009) Green
Chem 11:1428
1
8. Cal o` V, Nacci A, Monopoli A, Cotugno P (2009) Angew Chem
Int Ed 48:6101
19. Zeng Y, Wang YH, Jiang JY, Jin ZL (2012) Catal Commun 19:70
2
0. Zeng Y, Wang YH, Xu YC, Song Y, Zhao JQ, Jiang JY, Jin ZL
2012) Chin J Catal 33:402
1. Beletskaya IP, Cheprakov AV (2000) Chem Rev 100:3009
air condition without using inert atmosphere. The ILPEG750
-
(
stabilized Pd nanoparticle catalyst exhibited high efficiency
and all the Heck coupling products are trans-isomers. More
importantly, this new catalytic system combines the
advantages of homogeneous (high efficiency) and hetero-
geneous catalysis (good recyclability).
2
22. Amatore C, Jutand A (2000) Acc Chem Res 33:314
23. Yin LX, Liebscher J (2007) Chem Rev 107:133
2
2
4. Prechtl MHG, Scholten JD, Dupont J (2010) Molecules 15:3441
5. Favier I, Madec D, Teuma E, G o´ mez M (2011) Curr Org Chem
1
5:3127
2
6. Rodr ´ı guez-P e´ rez L, Pradel C, Serp P, G o´ mez M, Teuma E (2011)
ChemCatChem 3:749
27. Phan NTS, Van der Sluys M, Jones CW (2006) Adv Synth Catal
348:609
28. Lamblin M, Nassar-Hardy L, Hierso JC, Fouquet E, Felpin FX
Acknowledgments We gratefully acknowledge the financial sup-
port provided by the National Natural Science Foundation of China
(Grant No. 21173031) and the Program for New Century Excellent
Talents in Universities of China (Grant No. NCET-07-0138).
(
2010) Adv Synth Catal 352:33
2
3
9. Tan B, Jiang JY, Wang YH, Wei L, Chen DJ, Jin ZL (2008) Appl
Organometal Chem 22:620
0. Cassol CC, Umpierre AP, Machado G, Wolke SI, Dupont J
References
(
2005) J Am Chem Soc 127:3298
3
3
1. Reetz MT, Westermann E (2000) Angew Chem Int Ed 39:165
2. Deshmukh RR, Rajagopal R, Srinivasan KV (2001) Chem
Commun 17:1544
3. Xu LJ, Chen WP, Xiao JL (2000) Organometallics 19:1123
4. Cal o´ V, Nacci A, Monopoli A, Detomaso A, Iliade P (2003)
Organometallics 22:4193
1
2
3
4
. Roucoux A, Schulz J, Patin H (2002) Chem Rev 102:3757
. Widegren JA, Finke RG (2003) J Mol Catal A 191:187
. Chechik V, Crooks RM (2000) J Am Chem Soc 122:1243
. Moreno-Ma n˜ as M, Pleixats R, Villarroya S (2001) Organome-
tallics 20:4524
3
3
5
6
. Huang TS, Wang YH, Jiang JY, Jin ZL (2008) Chin Chem Lett
3
3
3
5. Umpierre AP, Machado G, Fecher GH, Morais J, Dupont J (2005)
Adv Synth Catal 347:1404
6. Wen F, B o¨ nnemann H, Jiang JY, Lu DM, Wang YH, Jin ZL
(2005) Appl Organometal Chem 19:81
7. Luo CC, Zhang YH, Wang YG (2005) J Mol Catal A 229:7
1
9:102
. Li KX, Wang YH, Jiang JY, Jin ZL (2010) Catal Commun
1:542
1
7
8
. Li KX, Wang YH, Jiang JY, Jin ZL (2010) Chin J Catal 31:1191
. Dupont J, de Souza RF, Suarez PAZ (2002) Chem Rev 102:3667
123