Highly uniform Pd nanoparticles supported on g-C3N4 for efficiently catalytic suzuki-miyaura reactions

Article abstract of DOI:10.1007/s10562-015-1537-0

Pd nanoparticles were supported on the surface of g-C₃N₄ by a facile method assisted by ultrasonication. The Pd/g-C₃N₄ catalyst was characterized by N₂-adsorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The XRD and TEM analyses reveal that the Pd nanoparticles are evenly distributed on the surface of g-C₃N₄ with an average size of 4 nm. The XPS results indicate that the Pd interacts with the N of g-C₃N₄. The supported catalyst shows high efficiencies in Suzuki- Miyaura reactions with up to 99% isolated yield under mild reaction conditions. The catalyst can be easily recycled at least 3 times without any loss of activity and selectivity. The excellent performance of the catalyst in activity and reusability may be attributed to the strong interaction between g-C₃N₄ and Pd nanoparticles, and the robust nature of the CN framework, respectively.

Full text of DOI:10.1007/s10562-015-1537-0

Catal Lett
DOI 10.1007/s10562-015-1537-0
Highly Uniform Pd Nanoparticles Supported on g-C N
3
4
for Efficiently Catalytic Suzuki–Miyaura Reactions
1
2
3
1
Xiangyang Su
•
Ajayan Vinu
•
Salem S. Aldeyab
•
Lin Zhong
Received: 26 February 2015 / Accepted: 19 April 2015
Ó Springer Science+Business Media New York 2015
Abstract Pd nanoparticles were supported on the surface
of g-C N by a facile method assisted by ultrasonication.
supported catalyst shows high efficiencies in Suzuki–
Miyaura reactions with up to 99 % isolated yield under
mild reaction conditions. The catalyst can be easily recy-
cled at least 3 times without any loss of activity and se-
lectivity. The excellent performance of the catalyst in
activity and reusability may be attributed to the strong
interaction between g-C N and Pd nanoparticles, and the
3
4
The Pd/g-C N catalyst was characterized by N -adsorp-
2
3
4
tion, X-ray diffraction (XRD), transmission electron mi-
croscopy (TEM), and X-ray photoelectron spectroscopy
(
XPS). The XRD and TEM analyses reveal that the Pd
nanoparticles are evenly distributed on the surface of
g-C N with an average size of 4 nm. The XPS results
3
4
robust nature of the CN framework, respectively.
3
4
indicate that the Pd interacts with the N of g-C N . The
3
Graphical Abstract
4
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-015-1537-0) contains supplementary
material, which is available to authorized users.
&
Lin Zhong
zhonglin@scu.edu.cn
1
2
3
College of Chemical Engineering, Sichuan University,
Chengdu 610065, Sichuan, China
Australian Institute for Bioengineering and Nanotechnology,
University of Queensland, Brisbane, QLD 4072, Australia
Petrochemical Research Chair, Department of Chemistry,
King Saud University, Riyadh, Saudi Arabia
123

X. Su et al.
Keywords Pd nanoparticles Á g-C N Á Suzuki–Miyaura
Li and Antonietti et al. firstly reported the Suzuki cou-
pling reaction over the supported Pd catalysts on the
surface of g-C N by light-mediated catalyst activation.
3
4
reaction Á Heterogeneous catalysis
3
4
They found that under photo irradiation and very mild
conditions, the stimulated electron transfer from g-C N
1
Introduction
3
4
nanorods to Pd nanoparticles plays an important role in
activating the substrates of the Suzuki coupling reactions
with a high activity and selectivity [21]. Very recently,
Fu and Wang et al. developed a photodeposition method
to prepare uniformly dispersed Pd nanoparticles on
g-C N [22]. The nanocomposite shows superior catalytic
The Suzuki–Miyaura reaction is one of the most important
organic transformations for constructing carbon–carbon
bond [1]. Over the decades, the Suzuki–Miyaura reaction
with Pd-based catalyst has achieved great progress and
been applied extensively in polymer science and fine che-
micals and pharmaceutical industries [2, 3]. However, the
reaction is still facing some challenges such as difficulty in
separating the product and recycling high-cost Pd species
3
4
activity in the Suzuki–Miyaura coupling reactions.
However, the activity of the catalyst decreased remark-
ably from 100 % to about 85 % after three recycles [22].
Although no explanation for the activity decrease was
presented in the paper, it is believed that the declined
activity may be due to the week interaction between Pd
[
4, 5]. Heterogenization of Pd-based catalyst can provide a
simple but efficient method to overcome the drawback
though the activity in heterogeneous system is usually
lower than that of homogeneous competitor [6–8]. Re-
cently, supported Pd nanoparticles have demonstrated the
capacity of catalyzing the Suzuki–Miyaura reaction which
aroused great interest from the industry [9, 10]. However,
the development of novel supported Pd catalyst with a high
activity and reusability remains very challenging and
timely [11–17]. The so-called g-C N which is the most
nanoparticles and g-C N as the photoreduction method
3 4
used in this process to support metal nanoparticles nor-
mally results in moderate or even weedy metal-support
interaction [23]. Herein, we prepared the Pd nanoparti-
cles supported on g-C N by a facile ultrasonic-assisted
3
4
solution reduction method. It was found that the sup-
ported catalyst exhibits a high activity and selectivity in
the Suzuki–Miyaura reaction under mild reaction condi-
tions. Moreover, the catalyst can be straightforwardly
reused for at least three times without any loss of ac-
tivity and selectivity. The strong interaction between the
3
4
stable allotrope of carbon nitride has generated widespread
attention for its applications in materials science and cat-
alysis [18]. For example, the recent researches have shown
that g-C N is a unique support for metal nanocatalysts
3
4
with excellent catalytic performance [19, 20]. Neverthe-
less, the work involving Pd nanoparticles supported on
g-C N for Suzuki–Miyaura reaction is very rare [21, 22].
Pd nanoparticles and the g-C N support may be at-
3 4
tributed for the outstanding performance and the stability
of the catalyst.
3
4
1
23

Highly Uniform Pd Nanoparticles Supported on g-C
3
N
4
for Efficiently Catalytic Suzuki–Miyaura…
2
Experimental
.1 Catalyst Preparation
The g-C N was synthesized using urea as carbon and ni-
The X-ray photoelectron spectroscopy (XPS) was recorded
on Kratos XSAM800 (Al Ka, 1486.69 eV) and the
charging effect was corrected by the C1 s peak at
2
284.6 eV. The Pd loading of the catalysts was determined
by SPECTRO ARCOS inductively coupled plasma atomic
emission spectroscopy (ICP-AES). Proton nuclear mag-
1
netic resonance ( H NMR) was carried out on Bruker
3
4
trogen source and SiO nanosphere as a hard template [24].
2
Briefly, 5 g of 40 % SiO solution (Ludox HS40, 12 nm)
2
was added into 10 g of urea solution and then the mixture
was heated at 90 °C under stirring. After the evaporation of
water, a white solid was collected and then carefully
grinded. Then, the power was transferred into a covered
quartz boat and heated to 550 °C in a tubular furnace at a
heating rate of 3 K min in air. The heating at 550 °C was
retained for 4 h to complete the polymerization of urea. A
yellow solid was gathered and put into a 2 M NaOH so-
400 MHz NMR spectrometer to confirm the structure of
the Suzuki–Miyaura reaction products.
2.3 Catalytic tests
-
1
For a typical reaction, the mixture of bromobenzene
(0.5 mmol, 78.5 mg), phenylboronic acid (0.55 mmol,
61 mg) and 4 mg catalyst was added into a pressure-re-
sisting reaction flask. The flask was sealed after the addi-
lution to remove the SiO template at room temperature.
2
After 24 h, a light yellow solid was filtered and washed
with water and ethanol several times. Finally, the obtained
g-C N was dried at 70 °C in a vacuum oven overnight.
tion of K CO (2 mmol, 276 mg) and 4 ml H O–ethanol
2 3 2
(v/v = 1:1). The reaction was then carried out at 80 °C for
2 h. After the substrate bromobenzene was converted
completely (monitored by the TLC analysis), the catalyst
was separated by a short column (silica gel). The product
was purified by column chromatography (silica gel, pet-
roleum ether/acetic ether). The structures of the products
3
4
The supported Pd catalyst was synthesized by an ultra-
sonic-assisted solution method [25]. 0.2 g g-C N was
3
4
dispersed into 20 ml water and ultrasonicated for 15 min.
Then, 4 ml PdCl (1 % w/w) solution was added into the
2
1
mixture. After further ultrasonication for another 15 min.
were further confirmed by H NMR.
1
0 ml NaBH solution was added into the mixture to re-
4
duce the Pd(II) to Pd(0). After filtration, the black solid was
washed with water and ethanol fully. Then, the catalyst was
dried at 70 °C in a vacuum oven overnight.
2.4 Catalyst Recycling Experiment
4-Bromoanisole (0.5 mmol, 93.5 mg) and phenylboronic
acid (0.55 mmol, 61 mg) and 4 mg catalyst were added
into a pressure-resisting reaction flask, followed by the
addition of K CO (276 mg, 2 mmol) and 4 ml H O–
2
.2 Materials and Characterization
2
3
2
ethanol (v/v = 1:1). The reaction flask was then sealed and
heated at 80 °C for 2.5 h. Then, the reaction mixture was
cooled to room temperature and transferred into a cen-
trifuge tub. After centrifuged at 5000 rpm for 15 min, the
supernatant in the tube was carefully removed and the re-
maining solid was washed with water and ethanol. The
procedure of centrifuge and washing were repeated three
times. The separated catalyst was then used for the next
reaction cycle.
LUDOX HS-40 colloidal silica was obtained from Sigma
Aldrich. 2-Bromotoluene (98 %), 4-bromoanisole (97 %),
4
-bromobenzaldehyde (95 %) and 4-methoxyphenyl-
boronic acid were purchased from J&K Scientific Ltd.
Iodobenzene (99 %) and bromobenzene (99 %) were de-
livered by Chengdu Best Reagent Co., Ltd. Urea, PdCl2
(
(
59 %), phenylboronic acid (98 %) and chlorobenzene
99 %) were provided by Chengdu Kelong Chemical Co.,
Ltd. All the chemicals are commercially available and were
used as-received.
The N adsorption–desorption isotherms and pore di-
2
3 Results and Discussion
ameter distribution were measured by Quantachrome In-
struments Quadrasorb-SI. Scanning electron microscope
Nitrogen adsorption–desorption experiments were per-
formed on g-C N and Pd/g-C N (Table 1; Fig. 1). All
(
SEM) was conducted on JEOL JSM-7500F. The trans-
3
4
3 4
mission electron microscopy (TEM) was performed on
Tecnai G2 F20 S-TWIN at an acceleration voltage of
samples showed a type IV/H3 adsorption isotherms which
indicated the existence of mesopores in the samples. For
g-C N , which was synthesized by a template method [24],
2
00 kV. Samples for TEM measurement were prepared by
3
4
placing a small drop of a colloid suspension in ethanol on a
carbon-coated copper grid. X-ray diffraction (XRD) data
was collected on Philips X’Pert MPD diffractometer at a
the templating effect is surprisingly limited. Most of the
pore sizes in g-C N are concentrated at about 5.6 nm in-
3
4
stead of 12 nm, the particle size of the silica template.
However, the g-C N yield (8 wt%) is about twice that of
-
1
scanning rate of 0.03° s in the 2h range from 10° to 80°.
3
4
123

X. Su et al.
Table 1 Textural properties of
g-C and Pd/g-C
2
BET surface area (m g
-1
3
Pore volume (cm g
-1
)
Sample
)
Pore size (nm)
3
N
4
3 4
N
g-C
3
N
4
17
30
0.10
0.13
5.6
5.6
Pd/g-C
3 4
N
Fig. 1 N
2
adsorption–desorption isotherms of g-C
Pd/g-C N (h and j) (Inset: Barrett-Joyner-Halenda (BJH) pore
3
N
4
(s and d) and
3 4
size distribution profiles)
3 4 3 4
Fig. 2 XRD profiles of g-C N and Pd/g-C N
the one prepared without template [26]. After supporting
Pd nanoparticles (the Pd loading determined by ICP is 14
wt%), the pore size destitution and pore volume of the
layered platelet-like morphology. This layered structure
may generate mesopores in g-C N which are available
3
4
g-C N are nearly retained, revealing that Pd nanoparticles
3
4
sites for adsorption of nitrogen during nitrogen adsorption–
desorption experiments. The TEM images of Pd/g-C N
are mostly dispersed on the outside of the pores. The
minimal change in the nitrogen adsorption–desorption
3
4
(Fig. 3c, d clearly demonstrated that the small-sized Pd
isotherms of the g-C N further reveal that the pores of the
4
3
particles are evenly distributed on the surface of g-C N
3
4
g-C N are not altered by the supported Pd nanoparticles.
3
4
after the deposition. The average size of the nanoparticles
is about 3.9 nm which is well consistent with the value
determined by XRD. Figure 3e shows the Pd crystal planes
measured as 0.226 nm which is attributed to the lattice
spacing of (111) plane of metallic Pd [19, 25]. Thus, these
results revealed that highly uniform Pd nanoparticles can
be supported successfully on g-C N by the ultrasonic-as-
Interestingly, compared with the support, the surface area
of the catalyst Pd/g-C N increases slightly from 17 to
3
4
2 -1
0 m g . The increase may due to ultrasonic effect
3
which could exfoliate the g-C N resulting in higher sur-
3
4
face area during the synthesis of the catalyst [27].
The XRD patterns of the g-C N and Pd/g-C N are
3
4
3 4
3
4
shown in Fig. 2. Before the Pd deposition, there are two
sisted method. Moreover, the strong interactions between
Pd nanoparticles and the p bonded planar -layers and in-
completely condensed amino groups of g-C N could in-
prominent peaks for g-C N : the strong peak (002) at 27.6°
3
4
relates to the inter-planar stacking structure of g-C N ,
3
4
3
4
while the weak (100) peak at 13.3° is derived the lattice
planes parallel to the c-axis [26–28]. However, after the Pd
deposition, two broadly peaks appear at 40.0° and 46.2°
which are assigned to the (111) and (200) planes of Pd(0)
respectively [19, 29]. The average size of the Pd
nanoparticles determined by Scherrer’s equation with
measuring the half width of the (111) peak is about 3.7 nm.
duce the formation small-sized Pd nanoparticle and then
stabilize the resulted nanoparticles [19].
XPS tests were carried out to further investigate the
chemical states of the N of g-C N before and after Pd
3
4
deposition and the Pd on g-C N , which could help to
4
3
understand the interactions between N atoms and Pd atoms
23, 29]. As shown in Fig. 4a, the N1 s core level of the
g-C N has a positive shift after Pd deposition. Fig. S1
[
The morphology of g-C N before and after Pd depo-
3
4
3
4
sition was then investigated by means of SEM and TEM.
As shown in Fig. 3a, b, g-C N was formed with typically
shows that the positive shifts are about 0.1–0.3 eV for the
deconvoluted peaks of N1 s. This indicates that the N
3
4
1
23

Highly Uniform Pd Nanoparticles Supported on g-C
3
N
4
for Efficiently Catalytic Suzuki–Miyaura…
Fig. 3 SEM image of g-C
nanoparticles (f)
N
3 4
(a), TEM images of g-C
N
3 4
(b) and Pd/g-C
3
N
4
(c), HRTEM images of Pd/g-C
3
N
4
(d, e) and size distribution of Pd
Fig. 4 XPS spectra of g-C
N
3 4
and Pd/g-C
N
3 4
atoms on the Pd/g-C N4 are charged more positively,
3
nanoparticles can accept electrons from the N atoms of
g-C N [23, 29]. Furthermore, it was found that most of the
which may be due to the electronic transfer from the N
atoms to Pd ones [23, 29]. Figure 4b illustrates the XPS
spectrum of the supported Pd. There is a clear doublet that
corresponds to Pd3d5/2 and Pd3d3/2 appeared in the spec-
trum. The Pd3d3/2 peak at 340.3 eV and the Pd3d5/2 peak at
3
4
surface Pd species were reduced to zerovalent form during
the solution reduction. Interestingly, the Pd(II)/Pd(0) ration
is about 0.71 which is much higher than that (about 0.14)
for the catalyst synthesized by photodeposition [22]. Such a
higher Pd(II) content may be attributed to the stronger
metal-support interaction [32] which is generated during
the Pd deposition. The stronger interaction would be much
more beneficial to the dispersion and stability of the
metallic Pd nanoparticles and the catalytic performance.
3
35.0 eV are assigned to Pd(0), while the Pd3d3/2 peak at
42.0 eV and Pd3d5/2 peak at 337.0 eV are attributed to
3
Pd(II) [19]. Interestingly, the peak at 335.0 eV assigned to
Pd(0) is more negative than that of normal metallic palla-
dium (335.2 eV) [30], which may be due to that the Pd
123

X. Su et al.
Typically, Suzuki–Miyaura reaction can be catalyzed by
supported catalysts with Pd(0) and/or Pd(II) species [8, 10].
For the catalyst supported with both Pd(0) and Pd(II)
species, the supported and in situ formed Pd nanoclusters
are hypothesized to be the catalytic active species for the
reaction [8]. To evaluate its catalytic performance and
stability, the catalyst Pd/g-C N was then tested for the
phenylboronic acid. All the reactions can be finished in the
reaction time range from 2.5 to 4 h with a high isolated
yield (over 95 %). However, the catalyst shows a low ac-
tivity in activating aryl chloride even with longer reaction
time and catalyst loading. Interestingly, the ICP analysis
for the hot filtration after the reaction of bromobenzene and
phenylboronic acid showed that the leaching of Pd is very
little (less than 0.5 % of the total palladium). Moreover, the
hot filtration from the reaction solution with about 50 %
conversion showed no further reactivity at the same reac-
tion temperature even for several hours. These results
indicated the supported Pd nanoparticles may interact
strongly with the support g-C N and are very stable on the
3
4
Suzuki–Miyaura reactions. (Table 2). A blank reaction of
iodobenzene and phenylboronic acid shows that the
Suzuki–Miyaura reaction did not happen in the absence of
the catalyst even after 24 h. However, to our delight, with a
small amount of the catalyst (Pd, 1 mol %), iodobenzene
was converted completely to the Suzuki–Miyaura product
within 1 h with 99 % isolated yield. When bromobenzene
was chosen as a substrate, the reaction completed smoothly
in 2 h with an isolated yield of 97 %. It is worth noting that
the isolated yield of the product can reach 63 % even the
reaction time was 10 min. Accordingly, the TOF value for
3
4
surface of the support. The remarkable stability of the
-
1
this reaction is 378 h , which is seven times higher than
the one for our previous N-free carbon-supported Pd cat-
alyst [31]. This significant improvement of the activity can
be attributable to the incorporation of N heteroatoms to the
support, resulting in enhanced metal-support interaction
[
33]. The catalyst Pd/g-C N was further tested in the
3 4
Suzuki–Miyaura reaction of aryl halides (iodobenzene and
bromobenzene) and methoxyphenylboronic acid. The re-
sults demonstrated that the catalyst exhibited high effi-
ciency in all these reactions though the reaction times are
longer than those of the reactions using phenylboronic acid
as one of the two substrates. The high activity and selec-
tivity was also found in the reactions of other aryl bromides
with electron-withdrawing and donating groups and
Fig. 5 Recycling of Pd/g-C
4-bromoanisole and 0.55 mmol phenylboronic acid, 2 mmol
CO , 4 ml H
O-ethanol (v/v = 1:1), 80 °C for 2 h
3 4
N . Reaction conditions: 0.5 mmol
K
2
3
2
3 4
Table 2 Suzuki-Miyaura reactions catalyzed by Pd/g-C N
X
R
1
R
2
Reaction time (h)
Isolated yield (%)
I
4-H
4-H
4-H
4-H
4-H
2-CH
H
1
99
99
97
97
63
93
98
99
30
I
4-OCH
3
3
2
Br
Br
Br
Br
Br
Br
H
2
4-OCH
3.5
1/6
4
a
H
H
H
H
H
3
4-OCH
4-CHO
4-H
3
2.5
3.5
24
b
Cl
2 3 2
Reaction conditions: 0.5 mmol aryl halides, 0.55 mmol phenylboronic acid, 4 mg catalyst (1 mol %), 2 mmol K CO , 4 ml H O-ethanol
(
a
v/v = 1:1), 80 °C. The reactions were monitored by TLC
The TOF is 378 h
-1
b
1
0 mg catalyst (2.5 mol %)
1
23

Highly Uniform Pd Nanoparticles Supported on g-C
3
N
4
for Efficiently Catalytic Suzuki–Miyaura…
catalyst was further confirmed by the recycling ex-
periments. It was found that the catalyst can be recycled at
least for three times without any loss of activity and se-
lectivity Fig. 5. Under the same reaction conditions, only
after the fourth recycle, the isolated yield of the product
decreases slightly from 98 to 93 %. The HRTEM images of
the recycled catalyst revealed that overwhelming majority
of the Pd nanoparticles sustain their sizes and dispersion
after five reaction cycles (Fig. S2). The insignificant loss of
the activity and selectivity in the fifth may be due to a small
part of agglomerate Pd nanoparticles (Fig. S2). Further-
more, XPS measurement was also performed on the reused
catalyst (Fig S3). As shown in Fig. S3a, there is no sig-
nificant changes in peak shapes for the N1 s after reaction,
which indicated that the support could be stable during the
reaction. However, surprisingly, the Pd(II)/Pd(0) ratio of
the reused catalyst increased to 1.5 though the chemical
shifts were approximately same for the fresh and reused
catalyst (Fig. S3b). During the reaction, the Pd(II) species
in g-C N might undergo a cycle of involving Pd(II) to
5 Supplementary Material
The Supplementary Material include the deconvoluted
XPS profiles of the g-C N before and after deposition,
3
4
1
TEM images of the catalyst after five reaction runs and H
NRM spectra of the products.
Acknowledgments We are grateful for the financial support by the
National Natural Science Foundation of China (Grant No: 20903068)
and the Specialized Research Fund for the Doctoral Program of
Higher Education (Grant No: 20090181120054). One of the authors
A. Vinu thanks Australian Research Council for the Future Fellow-
ship and the University of Queensland for the start-up grants. The
project was also financially supported by King Saud University, Vice-
Deanship of Scientific Research.
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