In situ generation of supported nano-palladium on La2O 3: An efficient and recyclable catalyst for heck cross-coupling reaction

Article abstract of DOI:10.1080/15533174.2012.760594

The Pd/La₂O₃ catalyst was simply prepared from readily available reagents. The catalyst exhibited high activities in the Heck cross-coupling reaction and could be reused for several times without activity loss. In addition, the catalyst was characterized by the transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), powder X-ray diffraction (XRD), and inductively coupled plasma (ICP). Supplemental materials are available for this article. Go to the publisher's online edition of Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry to view the supplemental file. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/15533174.2012.760594

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In Situ Generation of Supported Nano-Palladium on
La O : An Efficient and Recyclable Catalyst for Heck
2
3
Cross-Coupling Reaction
a
a
a
a
Zhao-Hao Li , Li-Ping Xue , Pan-Pan Shang & Bang-Tun Zhao
a
College of Chemistry and Chemical Engineering , Luoyang Normal University , Luoyang , P.
R. China
Accepted author version posted online: 12 Jul 2013.Published online: 25 Aug 2013.
To cite this article: Zhao-Hao Li , Li-Ping Xue , Pan-Pan Shang & Bang-Tun Zhao (2013) In Situ Generation of Supported
Nano-Palladium on La O : An Efficient and Recyclable Catalyst for Heck Cross-Coupling Reaction, Synthesis and Reactivity in
2
3
Inorganic, Metal-Organic, and Nano-Metal Chemistry, 43:10, 1476-1479, DOI: 10.1080/15533174.2012.760594
To link to this article: http://dx.doi.org/10.1080/15533174.2012.760594
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Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 43:1476–1479, 2013
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533174.2012.760594
In Situ Generation of Supported Nano-Palladium on La O :
2
3
An Efficient and Recyclable Catalyst for Heck
Cross-Coupling Reaction
Zhao-Hao Li, Li-Ping Xue, Pan-Pan Shang, and Bang-Tun Zhao
College of Chemistry and Chemical Engineering, Luoyang Normal University, Luoyang, P. R. China
popular catalysts in the Heck reaction, have been immobilized
catalyst was simply prepared from readily avail- on various supports,[
3–5]
used in ionic liquids,
[6,7]
and microen-
The Pd/La
2
O
3
able reagents. The catalyst exhibited high activities in the Heck capsulated in polymeric coating.[
8,9]
cross-coupling reaction and could be reused for several times with-
Generally, these heterogeneous Pd catalyst systems exhibit
high catalytic activity and selectivity in the Heck reaction. How-
ever, they are not always as reactive as homogeneous catalysts
with ligands. In view of the practical and industrial applications,
the developing a ligandless, recyclable new heterogeneous cat-
alyst system to dispense with the use of expensive, toxic, air-
sensitive ligands for the Heck reactions catalyzed by palladium
out activity loss. In addition, the catalyst was characterized by
the transmission electron microscopy (TEM), X-ray photoelectron
spectroscopy (XPS), powder X-ray diffraction (XRD), and induc-
tively coupled plasma (ICP).
Supplemental materials are available for this article. Go to the
publisher’s online edition of Synthesis and Reactivity in Inorganic,
Metal-Organic, and Nano-Metal Chemistry to view the supplemen-
tal file.
catalysts is highly desirable. Basic metal oxide La2O3 as pre-
ferred promoters has been widely reported.[
10,11]
However, its
use as a support for catalytic applications is not well explored
in the Heck reaction. In this respect, we report for the first time
the Heck reaction of aryl halides on heterogeneous palladium
nanoparticles directly supported basic metal oxide La2O3.
2 3
Keywords catalyst, Heck reaction, La O , nano-palladium
INTRODUCTION
The palladium-catalyzed Heck cross-coupling reaction be-
tween aryl halides and alkenes is a very convenient and ex-
cellent method for the formation carbon–carbon bonds at un-
EXPERIMENTAL
[1,2]
substituted vinylic positions.
This reaction has been used in
Materials and Physical Measurements
the pharmaceutical industry for the synthesis biologically active
compounds, natural product, and conjugated polymers. In the
past few years, the Heck reaction catalyzed by palladium com-
plexes along with phosphine ligands in homogeneous system
has made great progress. However, as with all homogeneous
catalysts, there is separation, recovery, reuse of expensive pal-
ladium catalysts, and reduction of palladium contamination of
the isolated products problems. These problems are of environ-
mental and economic concern for the pharmaceutical industry
where large volumes of hazardous wastes and high production
costs are of great importance. To overcome these problems as
previously mentioned, palladium complexes, which are the most
La2O3 support was prepared through the precipitation
method of La(NO3)3 with NH3·H2O at pH 10.5, followed by
recovery of the solid powder with filtration, washing, drying
◦
and then calcinations at 500 C. PdCl2 (AR) was obtained from
Shanghai July Chemical Co. Ltd. (China) and used as received.
Solvents were obtained from Sinopharm Chemical Reagent Co.
Ltd (China) and purified prior to use. Other chemicals were pur-
chased from Sigma-Aldrich (China) or Alfa Aesar (China) and
used without further purification or drying. Containing 2 wt%
Pd/La2O3 catalyst was prepared by the modification of a lit-
[12]
erature preparation of Pd/TiO2 : impregnation of the La2O3
powder using a 0.1 M acidified solution of PdCl2, then the sam-
◦
ple was evaporated, dried, and calcined in air at 500 C for 3 h.
X-ray photoelectron spectroscopy (XPS) measurements were
analyzed on coated alumina with a physical electronics PHI-
Quantum 2000 Scanning ESCA Microprobe (Physical Electron-
ics Company, USA) with Al Kα radiation. Inductively coupled
plasma (ICP) analyses were performed on an Ultima 2 analyzer
Received 18 November 2011; accepted 16 December 2012.
The authors acknowledge the Natural Science Foundation of China
No. 21172105) and the Foundation of the Education Department of
Henan Province (No. 2011A150021) for the financial support.
Address correspondence to Li-Ping Xue, College of Chemistry and
Chemical Engineering, Luoyang Normal University, Luoyang 471022,
P. R. China. E-mail: lpxue@lynu.edu.cn
(
(Jobin Yvon, France). Powder X-ray diffraction (XRD) patterns
were recorded on a Rigaku D/Max RA diffractometer (Japan)
1476

GENERATION OF SUPPORTED NANO-PALLADIUM ON La2O3
1477
with Cu Kα radiation. Morphology of catalyst was analyzed on
TABLE 2
a JEOL 2010 transmission electron microscope (TEM, Japan). Heck reaction between aryl halides and alkenes in the presence
a
GC analysis was measured by a Varian 430 Gas Chromatograph
2 wt% Pd/La2O3 catalyst
(
USA) using a column packed with DC-200 stationary phase.
H and C NMR spectra were recorded using a Bruker Avance
1
13
400 spectrometer (Germany) using CDCl3 as solvent.
Time Temperature Yield
Typical Heck Reaction
Iodobenzene (0.2 mmol), methyl acrylate (0.3 mmol),
sodium acetate (0.3 mmol), Pd/La2O3 (1 mol%), and DMA
R1
COCH3 CO2Me
R2
(h)
( C)
◦
(%)
Entry
X
1
2
3
4
5
6
7
8
I
I
I
Br
10
10
10
24
24
24
10
10
120
120
120
140
140
140
120
120
93
97
85
80
56
79
90
87
2
mL were added to a 25 mL flask equipped with a magnetic
H
OCH3
H
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
stirring bar under nitrogen atmosphere. The reaction mixture
◦
was stirred at 120 C for 10 h. After the reaction, the reaction
mixture was cooled to room temperature, separated, and puri-
fied by column chromatography (SiO2, ethyl acetate/hexane).
Unless otherwise stated, product yields from the Heck reac-
tion were determined by column chromatography. The product
was dried under vacuum, weighted, and characterized by NMR
spectroscopy.
Br OCH3
Br CH3
I
I
t
H
H
CO2 Bu
Ph
aReaction conditions: 0.2 mmol aryl halides, 0.3 mmol alkenes,
.3 mmol NaOAc, 1 mmol% Pd/La , 2 mL DMA; isolated yields;
0
2 3
O
1
13
All the products were characterized by H NMR and C NMR.
RESULTS AND DISCUSSION
In order to investigate the catalytic property of the 2 wt%
Pd/La2O3 catalyst, Heck reaction of the cross-coupling of
methyl acrylate with iodobenzene was employed. In the Heck
reaction, the base was used to neutralize the acid ensuing from
the exchange of a hydrogen atom with an aryl group. The change
the previous base, the catalytic activity of the system greatly
increased with the use of NaOAc as a base (entry 5). When the
organic base Et3N was used in the reaction, a moderate yield
(64%) was obtained. DMA was found to be more effective than
other solvents such as DMF, DMSO, and NMP (entries 5–8).
The oxidative conditions seriously inhibited the desired reac-
tion, as indicated by the reaction performed under air or oxygen
atmosphere (entries 9 and 10).
We examined a wide array of aryl halides and alkenes to ex-
tend the scope of this catalytic system. The results are presented
in Table 2, a range of aryl iodides and aryl bromides could
undergo Heck reaction to produce exclusively trans products
of product yield with different base under the same conditions
◦
(
120 C, 10 h) is shown in Table 1. When inorganic base NaOH
was used in the reaction, it provided a poor yield (17%) (entry
). The higher product yields (65%, 75%) were obtained with
1
the aid of K3PO4, K2CO3 (entries 2 and 3). Compared with
TABLE 1
Influence of various bases of the Heck reaction of iodobenzene
with methyl acrylate in the presence of 2 wt% Pd/La2O3 as
a
catalyst
Entry
Base
Solvent
Yield (%)b
1
2
3
4
5
6
7
8
NaOH
K3PO4
K2CO3
Et3N
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
DMA
DMA
DMA
DMA
DMA
DMF
NMP
DMSO
DMA
DMA
17
65
75
64
97
87
85
71
23
5
c
9
d
1
0
a
Reaction conditions: 0.2 mmol PhI, 0.3 mmol methyl acrylate,
◦
b
0
.3 mmol base, 2 mL solvent, 1 mol% Pd/La O , 120 C, 10 h. Isolated
yield. Under air. Under O
2 3
c
d
2
, GC yield.
FIG. 1. XPS spectra of Pd/La2O3 catalyst after the first run.

1478
Z.-H. LI ET AL.
FIG. 2. TEM images of 2 wt% Pd/La2O3 catalyst after the first run and its statistics histogram.
in moderate to excellent yields. The best results were obtained or leached Pd species, contrasting experiments have been inves-
in the reaction with the highly reactive aryl iodides. A slower tigated under the same conditions obtained from the filtrate and
reaction was observed with aryl bromides, but increasing the wash solution of catalyst. After similar purification processes,
reaction time from 10 h to 24 h provided the desired product the products from Heck reactions were obtained in only ca. 5%
(entries 4, 5, 6). Furthermore, electron-rich 4-bromanisole could isolated yield. Analysis of the final reaction mixtures for dis-
be coupled in moderate yield (entry 6). Concerning the varia- solved palladium showed that an extremely low amount of Pd
2
t
tion of the alkenes, as expected, alkenes (R = CO2 Bu, Ph) also (0.6 ppm) had leached during the reaction by ICP. These results
gave better yields (entries 7, 8). clearly suggest the reaction approximately proceed in heteroge-
XRD analysis indicated that the peaks from the fresh 2 wt% neous conditions during the reaction, although we can not rule
Pd/La2O3 solid sample catalyst and after the first run are similar out the homogeneous process.[
14–16]
At present, the true mech-
in profile, and no palladium-containing species were observed anism regarding our catalysts is still unclear and related studies
due to low amount of Pd content or its high dispersion (S.1). are underway in our laboratory.
Although no Pd species could be detected by XRD, the pres-
ence of Pd species in this solid sample was clearly evidenced
by XPS. Figure 1 shows the XPS spectra of the region corre-
sponding to the binding energy range of 331–347 eV, which
CONCLUSION
In summary, we have developed a simple and efficient method
included Pd 3d5/2 and Pd 3d3/2 peaks. The peaks located around for recyclable Pd-supported basic La2O3 and demonstrated their
3
3
35.2 eV and 340.7 eV were assigned to the Pd 3d5/2 and Pd activities in Heck cross-coupling reaction. This heterogeneous
d3/2 level in the Pd(0) metallic form in agreement with literature catalyst is stable and robustness in recycling without noticeable
[13]
report. Before reaction, no Pd particles could be discerned in loss of activity.
the TEM images for the 2 wt% Pd/La2O3 solid sample, suggest-
ing that Pd species was likely highly dispersed on the surface
SUPPLEMENTARY MATERIALS
of La2O3. However, inspection of the TEM images of isolated
wt% Pd/La2O3 catalyst samples after the first run indicated the
Powder X-ray diffraction patterns of Pd/La2O3 catalyst.
2
involvement of Pd nanoparticles. Figure 2 displays the TEM im-
age and corresponding size distribution of the isolated catalyst.
Evidently, highly dispersed spherical metallic Pd nanoparticles
with the size of 10 nm on the La2O3 surface were observed.
Recyclability is an essential advantage of heterogeneous cat-
alysts over homogeneous ones. Hence, we tested the recycla-
bility of this catalyst in the Heck reaction of iodobenzene with
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