Pd/PdO nanoparticles supported on carbon nanotubes: A highly effective catalyst for promoting Suzuki reaction in water

Article abstract of DOI:10.1016/j.cattod.2015.02.026

We report that the presence of PdO nanoparticles can enhance the catalytic performance of Pd catalyst for the Suzuki reaction in water. Heterogeneous Pd/PdO nanoparticles supported on multi-walled carbon nanotubes (CNTs), weakly oxidized multi-walled carbon nanotubes (WCNTs) and strongly oxidized multi-walled carbon nanotubes (SCNTs) catalysts are synthesized by a one-pot gas–liquid interfacial plasma (GLIP) method using Pd(NO₃)₂·2H₂O as precursor. Among these synthesized catalysts, the Pd/PdO supported on WCNTs (Pd/PdO/WCNTs) catalyst exhibits the highest catalytic activity during the Suzuki reaction in water. Moreover, the Pd/PdO catalyst shows higher catalytic activity during the Suzuki reaction than Pd catalyst. The as-prepared catalyst displayed remarkable activity toward challenging substrates such as heteroaryl halides and ortho-substituted aryl halides as well as aryl chlorides using low Pd loading in good yields. Since the catalyst exhibits extremely low solubility in organic solvent, the product can be simply extracted with ethyl acetate while the catalyst remained in the aqueous phase. The catalyst can be simply and efficiently used for ten consecutive runs without significant decrease in activity.

Full text of DOI:10.1016/j.cattod.2015.02.026

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Pd/PdO nanoparticles supported on carbon nanotubes: A highly
effective catalyst for promoting Suzuki reaction in water
a
a
a
b
a
a
Fan Yang , Cheng Chi , Sen Dong , Chunxia Wang , Xilai Jia , Liang Ren ,
a
a
a,∗
Yunhan Zhang , Liqiang Zhang , Yongfeng Li
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing, Changping 102249, China
a
b
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Analytical Chemistry for Living Biosystems, Institute of Chemistry, the Chinese
Academy of Sciences, Beijing 100190, China
a r t i c l e i n f o
a b s t r a c t
Article history:
We report that the presence of PdO nanoparticles can enhance the catalytic performance of Pd catalyst
for the Suzuki reaction in water. Heterogeneous Pd/PdO nanoparticles supported on multi-walled car-
bon nanotubes (CNTs), weakly oxidized multi-walled carbon nanotubes (WCNTs) and strongly oxidized
multi-walled carbon nanotubes (SCNTs) catalysts are synthesized by a one-pot gas–liquid interfacial
plasma (GLIP) method using Pd(NO3)2·2H2O as precursor. Among these synthesized catalysts, the Pd/PdO
supported on WCNTs (Pd/PdO/WCNTs) catalyst exhibits the highest catalytic activity during the Suzuki
reaction in water. Moreover, the Pd/PdO catalyst shows higher catalytic activity during the Suzuki reaction
than Pd catalyst. The as-prepared catalyst displayed remarkable activity toward challenging substrates
such as heteroaryl halides and ortho-substituted aryl halides as well as aryl chlorides using low Pd load-
ing in good yields. Since the catalyst exhibits extremely low solubility in organic solvent, the product can
be simply extracted with ethyl acetate while the catalyst remained in the aqueous phase. The catalyst
can be simply and efficiently used for ten consecutive runs without significant decrease in activity.
Received 17 November 2014
Received in revised form 14 February 2015
Accepted 16 February 2015
Available online xxx
Keywords:
Pd/PdO nanoparticles
Carbon nanotubes
Gas–liquid interfacial plasma
Water medium
Suzuki reaction
©
2015 Elsevier B.V. All rights reserved.
1
. Introduction
and selectivity. Therefore, it is a major issue to solve this problem
9–13].
It is well known that the palladium catalyzed Suzuki reaction is
[
Platinum group metals, especially Pd, have been widely used as
catalysts in organic synthesis, and the reactions are generally per-
formed in organic media [1]. With the notion of green chemistry,
organic chemists are strongly encouraged to develop safer proto-
cols [2]. Among the 12 principles that govern green chemistry, the
two principles viz. the use of superior catalysts and green solvents
aroused considerable interest in the recent times [3–8]. The het-
erogeneous metal nanocatalysts provide considerable advantages
like high atom economy, easy isolation of products, efficient recov-
ery and recyclability of catalysts. In addition, due to stringent and
growing environmental regulations, chemical industries prefer to
use water for the purpose of replacing toxic organic solvents as
the reaction medium. However, most organic substrates dissolve
poorly in water, which results in many developed heterogeneous
catalytic systems suffer from the drawbacks of low catalytic activity
one of the most powerful and convenient approaches for the syn-
thesis of biaryl derivatives which are the structural elements of
numerous natural products, agrochemicals, pharmaceuticals and
polymers [14–16]. Nowadays, the homogeneous palladium cat-
alysts used in aqueous media Suzuki reaction have been well
developed [17–21]. However, from the viewpoint of green chem-
istry, it is highly desirable to develop reusable heterogeneous
palladium catalysts which can be suitable to catalyze the reactions
in aqueous media. In order to improve the poor water solubility
of the organic substrates, numerous attempts have been made,
including synthesizing water soluble ligands [22,23], adding sur-
factants and phase-transfer agents as a promoter [24–26]. The
heterogeneous Pd catalyzed Suzuki reaction in aqueous solu-
tion has only emerged in these years. Although many inert solid
materials such as mesoporous solids (MCM-41, SBA-15 or related
mesoporous silicas) [27,28], polymers [29,30], and metal oxides
[
31] have been successfully applied in catalyst supports, most of
these supports are synthetic materials and require laborious and
time-consuming efforts for their synthesis as well as surface func-
tionalization which makes the catalyst easily dispersed in water.
∗ Corresponding author. Tel.: +86 01089739028; fax: +86 10 89739028.
E-mail address: yfli@cup.edu.cn (Y. Li).
http://dx.doi.org/10.1016/j.cattod.2015.02.026
0
920-5861/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: F. Yang, et al., Pd/PdO nanoparticles supported on carbon nanotubes: A highly effective catalyst for
promoting Suzuki reaction in water, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.02.026

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2
The carbon nanotubes have been normally used as a material sup-
port for the dispersion and stabilization of metal nanoparticles
because of their large chemically active surface, unique physical
properties, inherent size, hollow geometry and stability at high
temperatures [32–36]. Moreover, the carbon nanotubes have been
while 1 g SCNTs were treated with an oxidative acid mixture
◦
(3:1, vol/vol 96% H SO /78% HNO ) 100 ml at 110 C for 100 min.
2
4
3
The resulting mixture was diluted with ice water. After that, the
obtained suspension was centrifuged at 4000 rpm for 10 min and
washed with distilled water for several times until the pH of the
mixture reached 7. Finally, the precipitate obtained was dried in a
vacuum oven for 12 h before use.
mass produced by several organizations, including “Carbon Mul-
TM
tiwall Nanotubes” of Hyperion Company, “CoMoCAT
Process at
SWeNT” of the University of Oklahoma [37], “HiPCO Process” of
Rice university [38], and “Nano Agglomerate Fluidized” process of
Tsinghua University [39,40]. As is known to all, the oxidized carbon
nanotubes (OCNTs) are more easily dispersed in the water thanks to
their large amount of carboxyl and hydroxyl groups on the surface
of CNTs, if the OCNTs were used as support to fabrication heteroge-
neous catalysts to catalyze the reaction in water, which could show
high reactivity due to the hydrophilic property of the supporter.
Moreover, there are some evidences to show that the PdO
nanoparticles could enhance the catalytic activity of Pd nanopar-
ticles [41–43], and the synthesis of Pd/PdO nanoparticles is still
challenging due to the uncontrollable procedure for the oxidation of
Pd nanoparticles [44–47]. Most recently, our group focused on the
fabrication of metal nanoparticles by using the GLIP method which
shows several advantages, such as ultra-high density, high pro-
cess rate, preparation of nanomaterials in large scale, avoiding use
of reduction reagents, reaction at room temperature without stir-
ring, providing medium reduction conditions and emission of argon
along with glow [35,36,48–50]. Based on this background, in this
work we report a facile one-pot GLIP synthesis of Pd/PdO nanopar-
ticles on various kinds carbon nanotubes, including CNTs, WCNTs
and SCNTs under low argon pressure by using Pd(NO ) ·2H O as
2.3. Fabrication of the Pd/PdO nanoparticles decorated CNTs
The Pd/PdO/CNTs catalyst was prepared by a GLIP method with
Pd(NO3)2·2H2O, CNTs at room temperature for 10 min. The glow
plasma was generated between the top flat stainless steel (SUS) and
the bottom ionic liquid electrode. Ar gas was introduced and used
as the plasma-forming gas. VDC = 200–230 V was applied to a stain-
less steel electrode in gas phase for the generation of an Ar plasma,
where the discharge current I was fixed to 0.01 A and the Ar gas was
introduced up to a pressure of 140 Pa. The palladium salt dissolved
in 1 ml ionic liquid 1-butyl-3-methylimidazolium tetrafluorobo-
rate ([bmim]BF4), 48 mg CNTs were added to the stainless steel
reactor, then the palladium solution was added to the reactor incu-
bation 15 min. For the formation of Pd nanoparticles, electrons
were irradiated toward the ionic liquid for t = 10 min, then the mix-
ture was sonicated in ethanol to remove the excess impurities and
extracted from the ionic liquid by a centrifuge process. Pd/PdO
nanoparticles decorated CNTs, WCNTs and SCNTs (Pd/PdO/CNTs,
Pd/PdO/WCNTs and Pd/PdO/SCNTs) were prepared, and the respec-
tive materials were called Pd-n, (n = 1, 2, 3).
3
2
2
Pd precursor, as seen in Fig. 1. Additionally, the GLIP method syn-
thesized Pd materials were utilized as catalyst to study the Suzuki
reaction in water, and the Pd/PdO/WCNTs catalyst exhibits more
catalytic activity than Pd/WCNTs, which could be ascribed to the
existence of PdO species. To the best of our knowledge, it is the
first time for us to report that the PdO species in the presence of
Pd nanoparticles can enhance the reactivity of Suzuki reactions in
water.
2.4. Pd-2 catalyzed Suzuki reaction
The Pd-2 catalyst corresponding to a percentage of palladium of
0.1 mmol% with respect to 4-bromoacetophenone was used during
the reaction process. 4-Bromoacetophenone (0.5 mmol, 100 mg)
and phenylboronic acid (0.6 mmol, 73 mg) were mixed together in
a pressure vial. After that, 1 ml H2O and 2 equiv. diisopropylamine
◦
were added in the vial, then the mixture was stirred at 90 C, and the
reaction progress was monitored by Thin Layer Chromatography
(
TLC). After completion of the reaction, the product was extracted
2
. Experimental
with ethyl acetate, and the aqueous phase containing the Pd-2
catalyst was washed with miliQ-water for elimination of the salt.
Later on, the aqueous phase containing the Pd-2 loaded with the
reactants and base is used in the next run. The organic extracts
were dried with anhydrous magnesium sulfate, filtered and evap-
orated to dryness. At last, the mixture was purified with a silica
gel chromatography to afford 1-([1,1-biphenyl]-4-yl) ethanone
2.1. General information
All chemicals and solvents were purchased from commercial
suppliers. Transmission electron microscopy (TEM, Tecnai G2, F20)
combined with an energy dispersive X-ray spectroscopy (EDS) at
an acceleration voltage of 200 kV was used to measure the size,
morphology, size distribution and element content of Pd catalysts.
X-ray diffraction (XRD, Bruker D8 Advance Germany) was applied
to characterize the crystal structure of the hybrid materials, and the
data were collected on a Shimadzu XD-3A diffractometer using Cu
K␣ radiation. The X-ray photoelectron spectroscopy (XPS, Thermo
Fisher K-Alpha American with an Al K␣ X-ray source) was used to
measure the elemental composition of samples. The amounts of
Pd were determined by inductively coupled plasma optical emis-
sion spectrometer (ICP-OES). 1H NMR and C NMR spectra were
recorded on JNM-LA300FT-NMR for checking the final product from
the Suzuki reaction.
(
97 mg, 99%) as a white solid.
3
. Results and discussions
Initially, the supports are prepared by using CNTs as a start-
ing material for the preparation of the catalyst. The WCNTs and
SCNTs are prepared as described in Section 2. The FTIR spectra of
CNTs, WCNTs and SCNTs are compared in Fig. 2a. The spectra of
WCNTs and SCNTs indicates the intensive bands at wavenumbers
13
−
1
3
400, 1750–1550, 1385, 1300–950 cm , which can be indexed to
OH, C O, C C, C C, etc. As a result of oxidation, the relative
increase and partial separation of bands in the 3400, 1750, 1635
O
−
1
and 1565 cm wave region indicates the increase of the amount
of OH, C O and C O function groups, and the relative intensity
of SCNTs is stronger than WCNTs. These results are also confirmed
by XPS results, as shown in Fig. 2b. The oxygen content of MCNTs,
WCNTs and SCNTs were 2.7, 8.5 and 18.4 wt%, respectively.
What’s more, all kinds of Pd-n catalysts are examined by TEM,
as shown in Fig. 3. The morphologies of the WCNTs are not changed
compared with the original CNTs, but the length of the SCNTs
2.2. Fabrication of the OCNTs
The CNTs used in our experiments were purchased from Beijing
Cnano Technology Limited (Purity: >95%, Average length: 10 ␮m,
Average diameter: 11 nm. As control, WCNTs were prepared by
slight oxidation of 1 g purified CNTs with cooled piranha solutions
◦
(
4:1, vol/vol 96% H SO /30% H O ) 100 ml at 25 C for 100 min,
2
4
2
2
Please cite this article in press as: F. Yang, et al., Pd/PdO nanoparticles supported on carbon nanotubes: A highly effective catalyst for
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Fig. 1. Schematic illustration of synthesis of Pd/PdO nanoparticles supported on CNTs by a GLIP method.
Table 1
measured results from the SAED pattern. These results indicate that
the Pd-n have face-centered crystalline structure.
Pd/PdO nanoparticle size (nm)a and Pd loading on supports (wt.%).b
Sample
Pd-1
Pd-2
Pd-3
In order to confirm the chemical element states of the Pd
nanoparticles, another powerful technique XPS for studying and
identifying of Pd catalysts are shown in Fig. 5. The results indi-
cate that the Pd-n catalysts are composed of Pd, C, and O, which
in agreement with the EDX measurement. The peaks at 284.17 and
Pd-n size (nm)
Pd loading (wt.%)
3.4
8.7
3.1
9.5
5.1
9.1
a
Average size obtained from the size distribution histogram.
Calculated by ICP.
b
5
33.17 eV indicate that the chemical components are C1s and O1s,
respectively. A small peak corresponding to Pd3p is observed at
562.08 eV. The Pd catalysts show two peaks for Pd3d5/2 and Pd3d3/2
which are split into two types of Pd electronic states (Pd0 and
PdO) centered at 335.97, 337.87, 341.17 and 343.37 eV, as shown
in Fig. 5b. Fig. 5c–e shows the XPS-peak-imitating analysis of Pd-1,
Pd-2, Pd-3 catalysts, and the content percentage of PdO are 59%,
73% and 63%, respectively. The XPS results suggest that the Pd cat-
alyst contains Pd and PdO active species, and the synergistic action
of these parts may play an important role in highly active catalytic
species.
becomes shorter, which is due to the introduction of nitric acid
in the oxidation system. It is found that the mean particle size of
Pd-2 is 3.1 nm, smaller than 3.4 nm and 5.1 nm for Pd-1 and Pd-3
(Table 1), as shown in Fig. S2. This is probably because that CNTs lack
the defect to stabilize the Pd nanoparticles, whereas the length of
the SCNTs were reduced, leading to the decrease of the CNTs surface
area, which result in the larger particle size of Pd/PdO nanoparti-
cles on the surface of CNTs and SCNTs compared with the WCNTs.
The Pd loading of Pd-1, Pd-2 and Pd-3 are 8.7, 9.5 and 9.1 wt.%,
respectively (Table 1). Furthermore, the Pd/PdO nanoparticles of
Pd-2 was examined by high-resolution TEM (HRTEM), as seen in
Fig. 3d, where the interfinger distances of 0.225 nm, 0.195 nm and
The above mentioned results demonstrate that Pd catalysts
have been successfully prepared, and the water-medium Suzuki
reactions are employed to evaluate the catalytic activity of the
as-prepared catalysts. Table 2 listed the performance of the het-
erogenous Pd catalysts in water-medium Suzuki reactions with
4-bromoanisole and phenylboronic acid under different conditions.
Although the catalysts have different Pd loadings, the very low
Pd content in each reaction is fixed at 0.0005 mmol by adjusting
the addition amount of catalyst. We preliminary test the perfor-
mance of Pd-2 catalyst in the Suzuki reaction of 4-bromoanisole
and phenylboronic acid in different bases as a model reaction, and
the reactions were conveniently carried out in a macro-vial reac-
0.34 nm are found, corresponding to the (1 1 1), (2 0 0) lattice plane
of the face-centered-cubic (fcc) Pd and shells separation of CNTs,
respectively.
Additionally, SAED pattern also provides quick and easy crystal
orientation information of the obtained Pd-2, as shown in Fig. 4a.
The lattice spacing measured from the diffraction rings are 0.35,
0.23, 0.21, 0.20, 0.17, 0.14, 0.12 and 0.11 nm, which corresponds
to the reflections of C(0 0 2), Pd(1 1 1), C(1 0 0), Pd(2 0 0), C(0 0 4),
Pd(2 2 0), C(1 1 0) and Pd(3 1 1), respectively. Furthermore, XRD pat-
tern of pd-n are indicated in Fig. 4b which shows the characteristic
◦
tor at 90 C. Our interest is to optimize the base which shows the
◦
◦
◦
◦
◦
◦
diffraction peaks at 25.9 , 40.2 , 42.6 , 46.5 , 68.1 and 82.0 , cor-
responding to C(0 0 2), Pd(1 1 1), C(1 0 0), Pd(2 0 0), Pd(2 2 0) and
Pd(3 1 1), respectively. The lattice spacing of the C and Pd are also
calculated by the Bragg’s formula, which was well-matched to the
highest efficiency in Pd-n catalytic Suzuki reaction system by using
water as solvent for water endows the reaction with green and safe
properties. Previous studies have showed that bases had remark-
able influences on the reactivity of the Suzuki reaction in water.
Fig. 2. (a) FT-IR spectra of CNTs (black curve), WCNTs (red curve) and SCNTs (green curve); (b) XPS spectra of CNTs (black curve), WCNTs (red curve) and SCNTs (green curve).
(
For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Please cite this article in press as: F. Yang, et al., Pd/PdO nanoparticles supported on carbon nanotubes: A highly effective catalyst for
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Fig. 3. TEM images of Pd-1 (a), Pd-2 (b) and Pd-3 (c), HRTEM image of Pd-2 (d).
Inorganic bases such as NaOH, KOH, CsF, NaOAc and K PO ·H O
(entries 7–10). However, under the same reaction conditions, the
Pd-1 and Pd-3 show relative lower reactivity compared with Pd-2
(entries 11 and 12), the Pd-2 catalyst could afford the product in 94%
by prolongation the reaction time to 4 h (entry 13). According to the
results, two main reasons are possibly related to the better catalytic
activity of Pd-2. First, the support influences the catalytic activity
of Suzuki reaction in water. MCNTs tend to show more hydropho-
bic property, which leads to the low catalytic activity of catalyst
in water. SCNTs show more hydrophilic property, which makes
the contact between catalyst and the starting material difficult. By
3
4
2
are soluble in water and have been successfully used in efficient
ligand free catalytic systems, which demonstrate low activity in
present protocol, respectively. To our surprise, the K CO shows a
2
3
higher efficiency than above bases, providing a positive result of a
3% isolated yield of the cross-coupling product in 1 h (entries 1–6).
7
Subsequently, a series of organic bases have been examined. Tri-
ethylamine, N,N-diisopropylethylamine and diethylamine are all
inefficient, but isopropylamine is the most efficient base in the
present catalytic system, which provides a 77% isolated yield in 1 h
Fig. 4. (a) SAED image of Pd-2; (b) XRD spectra of Pd-1 (black curve), Pd-2 (red curve) and Pd-3 (green curve). (For interpretation of the references to color in this figure
legend, the reader is referred to the web version of this article.)
Please cite this article in press as: F. Yang, et al., Pd/PdO nanoparticles supported on carbon nanotubes: A highly effective catalyst for
promoting Suzuki reaction in water, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.02.026

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Fig. 5. (a) XPS spectra of Pd-1 (black curve), Pd-2 (red curve) and Pd-3 (green curve); (b) high-resolution XPS Pd3d image of Pd-1 (black curve), Pd-2 (red curve) and Pd-3
green curve); XPS-peak-imitating analysis of Pd-1 (c); Pd-2 (d) and Pd-3 (e). (For interpretation of the references to color in this figure legend, the reader is referred to the
(
web version of this article.)
comparison, WCNTs show moderate hydrophilic and hydrophobic
property, which enables the organic compounds to gather on the
catalyst surface easily without decreasing the catalytic activity in
water. Second, the small nanoparticle size probably contributes to
the better catalytic activity of the Pd-2. The XRD results illustrate
that all the synthesized hybrid materials show the same diffraction
peaks and crystalline lattice which show almost the same catalytic
activity center, and the same amount of Pd salt is used to decorate
the MCNTs, WCNTs and SCNTs. It means that the smaller parti-
cle size can produce more catalytic activity center, leading to that
the Pd-2 shows the better catalytic activity than Pd-1 and Pd-3.
It is worth to mention that the Pd-2 catalyst shows much higher
catalytic activity than Pd/WCNTs which is Pd nanoparticles dec-
orated WCNTs synthesized by the GLIP method using Pd(OAc)2
as a precursor (entry 14). The morphology and particle size of
Pd/WCNTs is similar to Pd-2 (Fig. S1), and it is clearly indicated
that the PdO active species show stimulative action in the current
catalytic system.
With the optimized reaction conditions in hand, we have fur-
ther studied the generality of the cross-coupling reactions between
aryl halides and aryboronic acids in the presence of 0.1 mol%
◦
Pd-2 and 2 equiv. of diisopropylamine at 90 C in water. The
results are shown in Table 3. Various 4-substituted aryl bromides,
bearing either electro-donating or electro-withdrawing groups,
Table 2
Suzuki coupling reaction in water under different conditions.a
Entry
Cat.
Base
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
9
0
1
2
3
4
Pd-2
Pd-2
Pd-2
Pd-2
Pd-2
Pd-2
Pd-2
Pd-2
Pd-2
Pd-2
Pd-1
Pd-3
Pd-2
Pd/WCNTs
KOH
NaOH
CsF
1
1
1
1
1
1
1
1
1
1
1
1
4
4
36
45
6
NaOAc
K3PO4·H2O
K2CO3
Et3N
iPr2NEt
Et2NH
iPr2NH
iPr2NH
iPr2NH
iPr2NH
iPr2NH
9
19
73
16
8
7
1
1
1
1
1
77
65
70
94
75
a
◦
Reaction conditions: 4-Bromoanisole (0.5 mmol), phenylboronic acid (0.6 mmol), Cat. Pd (0.1 mol%), and base 1 mmol in 1 ml H2O at 90 C.
Isolated yield.
b
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Fig. 6. (a) The yields for 10 cycles of the Pd-2 catalyzed Suzuki reaction; (b) the TEM image for recovery of Pd-2 catalyst after 10 cycles Suzuki reactions.
Table 3
Pd-2 catalyzed Suzuki coupling reaction.a
conditions without any phase-transfer additive (entries 12 and 13).
Further investigations are focused on the effects of the substituted
arylboronic acid, and the yield of the coupling of 4-bromoanisole
with 4-methyl boronic acid is lower than that of the cross-coupling
of 4-bromoanisole with 4-methyoxyphenyl boronic acid under the
same reaction conditions (entries 14 and 15). However, increasing
the steric effect in arylboronic acid could decrease the reactivity
ꢀ
Time (h) Yieldb (%)
Entry Ar-Br (1)
4-MeC6H4Br (1a)
Ar -B(OH)2 (2)
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
PhB(OH)2 (2a)
PhB(OH)2 (2a)
PhB(OH)2 (2a)
PhB(OH)2 (2a)
PhB(OH)2 (2a)
PhB(OH)2 (2a)
PhB(OH)2 (2a)
PhB(OH)2 (2a)
PhB(OH)2 (2a)
PhB(OH)2 (2a)
4
4
84
99
98
99
99
94
99
86
99
99
71
53
50
84
92
88
99
95
96
93
(entry 16). The cross-coupling of the hydrophilic group furnished
4-MeOC6H4Br (1b)
4-HOC6H4Br (1c)
4-CH3COC6H4Br (1d)
4-NO2C6H4Br (1e)
4-ClC6H4Br (1f)
C6H5I (1g)
C6H5Br (1h)
1-C10H7Br (1i)
3-Bromopyridine (1j)
aromatic bromide 4-bromophenol with 4-bromoanisole give the
excellent yield (entry 17). Additional studies revealed that com-
pared with electron-donating groups, 4-bromoacetophenone with
the electro-withdrawing group on the aromatic ring coupling with
aryl boronic acid show higher yields (entries 18–20).
Reusability is one of the most important features for a het-
erogeneous catalyst, which is superior to a homogenous one.
The reusability of the catalyst in the model reaction between 4-
4
4
4
4
4
4
4
4
1
1
1
1
1
1
1
1
1
1
2
3-Bromothiophene (1k) PhB(OH)2 (2a)
4
4-NO2C6H4Cl (1l)
PhB(OH)2 (2a)
PhB(OH)2 (2a)
12
12
4
4
4
4
4
4
4
bromoacetophenone and phenylboronic acid in the presence of
4-HOOCC6H4Cl (1m)
4-CH3OC6H4Br (1b)
4-CH3OC6H4Br (1b)
4-CH3OC6H4Br (1b)
4-HOC6H4Br (1c)
4-CH3COC6H4Br (1d)
4-CH3COC6H4Br (1d)
4-CH3COC6H4Br (1d)
◦
0
.2 mol% of catalyst at 90 C is further explored. To confirm the reac-
4-MePhB(OH)2 (2b)
4-MeOPhB(OH)2 (2c)
2-MeOPhB(OH)2 (2d)
4-MePhB(OH)2 (2b)
4-MePhB(OH)2 (2b)
4-MeOPhB(OH)2 (2c)
2-MeOPhB(OH)2 (2d)
tion catalyzed by solid Pd-2 rather than homogenous Pd species,
we have carried out the reaction, then the catalyst Pd-2 is removed
by filtration and the solvent is evaporated and treated with nitro-
hydrochloric acid. The Pd content has been examined by ICP-OES
analysis, and no leaching of Pd nanoparticles (less than detection
limit) is observed during the reaction. To assess the recyclability of
Pd-2, multiple coupling of 1d and 2a cycles have been carried out,
after the use of the catalyst in the above mentioned Suzuki reac-
tion, the product was simply extracted with ethyl acetate while
the catalyst remained in the aqueous phase, and then, the aqueous
phase containing the catalyst was washed several times by deion-
ized water to remove the salt. In the next stage, the aqueous phase
containing the Pd-2 was recharged with fresh coupling partners
and diisopropylamine for the next run. The results indicate that this
simple separation method can be repeated for 10 consecutive runs,
and the product is obtained high yields every time (Fig. 6a). After
the 10 cycle’s reactions, the catalyst was again examined by TEM,
and the image indicates that particle size is a little bit increased, as
shown in Fig. 6b.
a
Reaction conditions: Aryl bromide (0.5 mmol), boronic acid (0.6 mmol), Cat. Pd
◦
(
0.1 mol%), and base 1 mmol in 1 ml H2O at 90 C.
Isolated yield.
b
such as Me, OMe, OH, Cl, COCH3, provided the corre-
sponding products in good to excellent yield (entries 1–6). No
substituted aryl halide group such as iodobenzene, bromoben-
zene, 1-bromonaphthalene are successfully coupled with boronic
acid in excellent yields (entries 7–9). This catalytic system could
tolerate heterocyclic aromatic bromide, the yield of coupling
of 3-bromopyridine and 3-bromothiophene with phenyl boronic
smoothly, giving the corresponding product 99% and 71% yield,
respectively (entries 10 and 11). Encouraged by such promis-
ing results, we next turn our attention to the Suzuki reaction
of aryl chlorides. To our surprise, the present catalytic system
can activate 4-chloronitrobenzene and 4-chlorobenzoicacid for the
cross-coupling reaction with phenylboronic acid, resulting in a
4. Conclusion
5
0% and 53% isolated yields of the corresponding products in 12 h,
In summary, we have provided an environmentally-friendly
approach to the synthesis of Pd/PdO nanoparticles functionalized
CNTs, WCNTs and SCNTs by a GLIP method using Pd(NO ) ·2H O as
which is a successful example of the heterogeneous catalyst cat-
alytic Suzuki reaction of aryl chloride in pure water under aerobic
3
2
2
Please cite this article in press as: F. Yang, et al., Pd/PdO nanoparticles supported on carbon nanotubes: A highly effective catalyst for
promoting Suzuki reaction in water, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.02.026

G Model
CATTOD-9506; No. of Pages7
ARTICLE IN PRESS
F. Yang et al. / Catalysis Today xxx (2015) xxx–xxx
7
a precursor. The synthesized hybrid materials are utilized as a cata-
lyst to study the Suzuki reaction in water media. The results indicate
that the Pd/PdO nanoparticles supported on WCNTs show better
morphology and catalytic activity compared with CNTs and SCNTs
as supporters. Also, the Pd/PdO catalyst exhibits higher catalytic
activity than Pd catalyst. Moreover, a wide range of aryl halides
react with arylboronic acid can be formed to the corresponding
biphenyl derivatives, and the Pd-2 catalyst can be recovered sev-
eral times without leaching and loss of activity. Further work is in
progress to extend such kind of catalyst for other applications.
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Please cite this article in press as: F. Yang, et al., Pd/PdO nanoparticles supported on carbon nanotubes: A highly effective catalyst for
promoting Suzuki reaction in water, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.02.026