Modification of silica using piperazine for immobilization of palladium nanoparticles: A study of its catalytic activity as an efficient heterogeneous catalyst for Heck and Suzuki reactions

Article abstract of DOI:10.1007/s13738-012-0188-y

An efficient heterogeneous palladium catalyst system has been developed based on immobilization of Pd nanoparticles on silica-bonded N-propylpiperazine (SBNPP) substrate. SBNPP substrate can stabilize the Pd nanoparticles effectively so that it can improve their stability against aggregation. Also, grafted piperazine species onto the silica backbone prevents the removing of Pd nanoparticles from the substrate surface. It seems that the high recyclable capability of Pd-SBNPP catalysts is resulted from these two characteristics. Transmission electron microscopy (TEM) of catalyst is shown the size of Pd nanoparticles in Pd-SBNPP average of 20 nm. Furthermore, X-ray photoelectron spectroscopy (XPS) of Pd-SBNPP is shown the presence of Pd(0) in the structure of this catalyst. Overall, TEM, XPS and XRD experiments strongly suggested that Pd nanoparticles were formed and immobilized on silica-functionalized piperazine. The catalytic activity of this catalyst was investigated in the Heck and Suzuki reactions. The catalyst could be recycled several times without appreciable loss in catalytic activity.

Full text of DOI:10.1007/s13738-012-0188-y

J IRAN CHEM SOC (2013) 10:527–534
DOI 10.1007/s13738-012-0188-y
ORIGINAL PAPER
Modification of silica using piperazine for immobilization
of palladium nanoparticles: a study of its catalytic activity
as an efficient heterogeneous catalyst for Heck and Suzuki
reactions
•
Khodabakhsh Niknam Maryam Sadeghi Habibabad
•
•
•
Abdollah Deris Farhad Panahi
Mohammad Reza Hormozi Nezhad
Received: 9 August 2012 / Accepted: 8 November 2012 / Published online: 4 December 2012
Ó Iranian Chemical Society 2012
Abstract An efficient heterogeneous palladium catalyst
system has been developed based on immobilization of
Pd nanoparticles on silica-bonded N-propylpiperazine
(SBNPP) substrate. SBNPP substrate can stabilize the Pd
nanoparticles effectively so that it can improve their sta-
bility against aggregation. Also, grafted piperazine species
onto the silica backbone prevents the removing of Pd
nanoparticles from the substrate surface. It seems that the
high recyclable capability of Pd-SBNPP catalysts is
resulted from these two characteristics. Transmission
electron microscopy (TEM) of catalyst is shown the size of
Pd nanoparticles in Pd-SBNPP average of 20 nm. Fur-
thermore, X-ray photoelectron spectroscopy (XPS) of Pd-
SBNPP is shown the presence of Pd(0) in the structure of
this catalyst. Overall, TEM, XPS and XRD experiments
strongly suggested that Pd nanoparticles were formed and
immobilized on silica-functionalized piperazine. The cat-
alytic activity of this catalyst was investigated in the Heck
and Suzuki reactions. The catalyst could be recycled sev-
eral times without appreciable loss in catalytic activity.
Keywords Heterogeneous palladium nanoparticles Á
Silica-bonded N-propylpiperazine Á Heck reaction Á
Suzuki reaction Á Aryl halides Á Alkenes
Introduction
The wide application of metal-catalyzed reactions in aca-
demic studies and in the industry piqued the interest of
many researchers to improve the synthetic methods for
excellent performing of these processes [1–4]. Among
many different existing metal-catalyzed strategies for C–C
bond formations, palladium catalyzed protocols are mostly
considered due to the unique characteristics of palladium
[5–10]. Considering the importance of palladium reactions
and the cost of palladium metal, different homogeneous
and heterogeneous palladium catalyst systems have been
developed [11–13]. In respect to the homogeneous cata-
lysts, two different types of Pd catalytic systems are
reported: (i) using palladium salt with ligands to be formed
in in situ Pd-complex in reaction condition, and (ii) using a
pre-prepared complex of Pd. It is noteworthy that, in both
systems often phosphorous and nitrogen-containing ligands
have been used [14]. However, homogenous systems have
two major problems; first, they cannot be reused and,
therefore, they are not economical. Second, many ligands
are toxic and air sensitive, so it is not a green and stable
system [15]. In order to have the ability to recover
homogeneous catalysts, a complex of palladium metal with
the related ligands is deposited onto the surface of solid
supports, thus forming heterogeneous catalysts [16, 17].
In the recent years, there is considerable attention on
preparation of heterogeneous palladium catalyst based on
nanometer scale palladium species [18, 19]. Palladium
nanoparticles on a suitable support have the ability to be
Electronic supplementary material The online version of this
article (doi:10.1007/s13738-012-0188-y) contains supplementary
material, which is available to authorized users.
K. Niknam (&) Á M. S. Habibabad Á A. Deris Á F. Panahi
Department of Chemistry, Faculty of Sciences,
Persian Gulf University, 75169 Bushehr, Iran
e-mail: khniknam@gmail.com; niknam@pgu.ac.ir
M. Reza Hormozi Nezhad
Department of Chemistry, Sharif University of Technology,
11155-9516 Tehran, Iran
M. Reza Hormozi Nezhad
Institute for Nanoscience and Nanotechnology (INST),
Sharif University of Technology, 9161 Tehran, Iran
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J IRAN CHEM SOC (2013) 10:527–534
Silica-bonded N-propylpiperazine (SBNPP)
recoverable and ligand free green catalytic systems with
improved catalyst characteristics [11, 17–21]. In hetero-
geneous Pd nanoparticle systems, the catalyst properties
are improved due to high surface-to-volume ratio and
consequently large fraction of metal atoms at the surface
and also strong synergistic interaction between noble metal
nanoparticles and the support [22–31]. Usually, palladium
nanoparticle catalysts are prepared from a palladium salt on
a substrate or stabilizer in the presence of a reducing agent
[32, 33]. Steric, electrostatic, and electrosteric factors are
responsible for the stabilization of PNPs during their
synthesis.
To a magnetically stirred mixture of 3-chloropropylsilica
(3 g) in xylene (110 mL), piperazine (1.3 mmol) was
added and heated at 60 °C–70 °C, in presence of triethyl-
amine as proton abstractor, under continuous stirring and a
dry nitrogen atmosphere for 48 h. Then, the mixture was
filtered and washed with dichloromethane (3 9 10 mL)
and ethanol (3 9 10 mL). After drying in oven N-propyl-
silica piperazine (SBNPP) was obtained as white powder
(3.4 g) [34].
Pd nanoparticles on silica-bonded N-propylpiperazine
(PNP-SBNPP)
Silica is an important solid surface for heterogeneous
catalyst because of its abundant availability, high stability
and the fact that organic groups can adjoin to the silica
surface with strong connection to generate catalytic sites.
With this view that silica gel surface is highly porous and
can trap the palladium nanoparticles and hence stabilize
them (prevent their aggregation), this substrate is used for
supported palladium nanoparticles [25]. Laying the func-
tional groups on the silica was shown more tendentious
to stabilize the Pd nanoparticles and catalyst recovery
[22–30].
To a mixture of silica-bonded N-propylpiperazine (PNP-
SBNPP) (1 g) in absolute ethanol (10 mL), palladium
acetate (0.15 g, 0.67 mmol) was added and stirred 24 h at
room temperature. Then, the mixture was filtered and
washed with ethanol (3 9 10 mL). After drying in vacuum
oven corresponding Pd nanoparticles on silica-bonded
N-propylpiperazine (PNP-SBNPP) was obtained as dark
solid (1.1 g).
General procedure for Heck reaction
Experimental
To a mixture of aryl halide (1 mmol), alkene (1.2 mmol),
and Na2CO3 (2.5 mmol) in 3 mL DMF, Pd nanoparticles
on silica-bonded N-propylpiperazine (PNP-SBNPP)
(0.035 g, 1.97 mol %) was added and heated on oil bath at
120 °C for the time specified in Table 2. The reaction was
followed by TLC. After completion of the reaction, the
mixture was cooled down to room temperature and filtered,
and the remaining was washed with dichloromethane
(2 9 10 mL) to separate catalyst. After extraction dichlo-
romethane from water, the organic phase dried over
Na2SO4. Evaporation of the solvent gave products. For
further purification if needed, the products passed through a
short column of silica gel using n-hexane as eluent.
Chemicals and reagents
Chemicals were purchased from Fluka, Merck and Aldrich
Chemical Companies. The products were characterized by
comparison of their spectral and physical data with those
1
reported in literature. For recorded H-NMR spectra we
were using Bruker Avance (300 MHz or DRX 500 MHz)
or Bruker Ultrashield (400 MHz) in pure deuterated CDCl3
solvent with tetramethylsilane (TMS) as internal standards.
X-ray diffraction (XRD, D8, Advance, Bruker, axs), FT-IR
spectroscopy (Shimadzu FT-IR 8000 spectrophotometer),
and SEM instrumentation (SEM, XL-30 FEG SEM, Phi-
lips, at 20 kV) were employed for characterization of cat-
alyst. Surface spectroscopy analyses of the catalyst were
performed in an ESCA/AES system. This system is
equipped with a concentric hemispherical (CHA) electron
energy analyzer (Specs model EA10 plus) suitable for
X-ray photoelectron spectroscopy (XPS). Melting points
determined in open capillary tubes in a Barnstead Elec-
trothermal 9100 BZ circulating oil melting point apparatus.
The reaction monitoring was accomplished by TLC on
silica gel PolyGram SILG/UV254 plates. Column chro-
matography was carried out on columns of silica gel 60
(70–230 mesh). All the products were characterized with
IR, NMR and the known compounds compared with those
reported in literature [8–32].
General procedure for Suzuki cross-coupling reaction
To a mixture of aryl halide (1 mmol), aryl boronic acid
(1.2 mmol), and Na2CO3 (2.5 mmol) in 2 mL ethanol, Pd
nanoparticles on silica-bonded N-propylpiperazine (PNP-
SBNPP) (0.03 g, 1.69 mol %) was added and refluxed for
the time specified in Table 3. The reaction was followed by
TLC. After completion of the reaction, the mixture was
cooled down to room temperature and filtered, and the
remaining was washed with dichloromethane (3 9 10 mL)
to separate catalyst. After extracting dichloromethane from
water, the organic phase dried over Na2SO4. Evaporation
of the solvent gave products. For further purification if
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J IRAN CHEM SOC (2013) 10:527–534
529
Scheme 1 The synthetic rout
for preparation of PNP-SBNPP
OH
OH
O
(MeO)3Si
Cl
SiO2
O Si
O
Cl
SiO2
Toluene, Reflux, 18 h
OH
NH
HN
Et3N
Xylene, Reflux
O
O
Pd(OAc)2
EtOH, r.t., 24 h
SiO2
O Si
O
N
SiO2
O Si
N
N
Pd0
NH
n
O
needed, the products passed through a short column of
support with a relatively good monodispersity. Also, the
histogram revealing the size distributions of Pd nanopar-
ticles on SBNPP substrate is shown in Fig. 3 and is pro-
posed according to the data obtained from the TEM image.
For Pd-SBNPP catalyst, the average sizes of Pd nano-
particles are obtained, 20 nm.
silica gel using n-hexane as eluent.
Results and discussion
Catalyst preparation and characterization
The microscopic features of the catalyst were observed
with SEM (Fig. 4). In this figure, we can see morphology
of the silica substrate. It can be seen that the Pd nanopar-
ticles with a spherical morphology are well immobilized to
the SBNPP substrates.
As shown in Scheme 1, immobilized palladium nanoparti-
cles on piperazine-modified silica (Pd-SBNPP) are prepared
in a three-step process. First, activated silica [35] was reacted
with 3-chloropropyl trimethoxysilane in refluxing toluene to
produce 3-chloropropylsilica (3-CPS). Second, 3-CPS was
treated with piperazine to generate the corresponding sub-
strate. Finally, palladium acetate is reduced on SBNPP sub-
strate using ethanol as green reducing agent [21] (Scheme 1).
The PNP–SBNPP catalysts were studied using the fol-
lowing techniques: FT-IR spectroscopy was used for the
characterization of SBNPP (Fig. 1), transmission electron
microscopy (TEM) (Fig. 2), scanning electron microscopy
(SEM) (Fig. 4), powder X-ray diffraction (XRD) (Fig. 5),
and X-ray photoelectron spectroscopy (XPS) (Fig. 6).
The FT-IR transmission spectrums of 3-chloropropyl-
silica, SBNPP, and PNP-SBNPP are shown in Fig. 1. The
FT-IR of PNP-SBNPP the characteristic absorption at
3444.6 cm-1 is associated with the free -OH site on silica,
while stretching frequency in 3419.6 cm-1 is related to
free -OH in 3-chloropropylsilica. Stretching frequency in
2966.3 cm-1 related to C–H bond vibrations, strong bond
in 1000–1200 cm-1 presumably because of stretching
frequency C–O and C–N bonds that overlap with asym-
metric stretching frequency of Si–O–Si bonds and two
peaks in 1095.5 and 794.6 cm-1 related to asymmetric
stretching and symmetric stretching of Si–O–Si bond,
respectively, while stretching frequency in 1097.4 and
802.3 cm-1 is related to Si–O-Si in 3-chloropropylsilica.
TEM images of PNP-SBNPP catalyst (Fig. 2) show that
the Pd nanoparticles with near spherical morphology are
assembled onto the silica-bonded N-propylpiperazine
The crystalline structure of the PNP-SBNPP catalyst is
displayed by the XRD pattern depicted in Fig. 5. The XRD
Fig. 1 FT-IR of 3-chloropropylsilica, SBNPP, and PNP-SBNPP for
comparison
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J IRAN CHEM SOC (2013) 10:527–534
pattern of the PNP-SBNPP catalyst also shows palladium
nanoparticles on the silica surface. The strongest peaks of
the XRD pattern correspond to SiO2, and other peaks are
indexed as the (111), (200), (220), and (311) planes of the
palladium nanoparticle [35].
Figure 6 shows the X-ray photoelectron spectroscopy
(XPS) spectrum of Pd (3d) envelope confirming the
metallic Pd particles. XPS Pd (3d5/2) and Pd (3d3/2) peaks
were observed at binding energy 335 and 340.1 eV,
respectively. Such binding energy values are similar to that
reported for Pd metals in the literature [35–37].
The BET surface area using nitrogen adsorption iso-
therms at the temperature of liquid nitrogen which gave the
results of as,BET 1.71 m2 g-1 and the total pore volume
0.1653 cm3 g-1 (see supplementary material).
Fig. 2 Transmission electron microscopic (TEM), which show the
image of Pd nanoparticle on SBNPP support
Fig. 3 Histogram representing the size distribution of Pd nanopar-
ticles on SBNPP substrate
Fig. 4 Scanning electron microscopy (SEM) of PNP-SBNPP
(magnifications: 1 mm, 100 lm and 1 lm)
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J IRAN CHEM SOC (2013) 10:527–534
531
Fig. 5 XRD of PNP-SBNPP
Table 1 Optimization reaction between bromobenzene and ethyl
acrylate in the presence of PNP-SBNPP as a catalyst at various
conditions
O
Br
PNP-SBNPP
OEt
OEt
+
Solvent, Base
Temperature
O
Entry Solvent Base
Catalyst
loading (g)
Temp Time Yielda
(°C)
(h)
%
1
DMF
DMF
DMF
DMF
DMF
DMF
DMF
Na2CO3 0.0
120
120
120
120
120
120
120
24
24
8
0
2
Na2CO3 0.01
Na2CO3 0.035
Na2CO3 0.05
K2CO3 0.035
Cs2CO3 0.035
60
90
89
70
30
10
80
30
Trace
54
77
55
3
4
4
5
8
6
8
Fig. 6 The X-ray photoelectron spectroscopy (XPS) of PNP-SBNPP
7
Et3N
0.035
8
8
Toluene Na2CO3 0.035
Reflux 10
Reflux 10
Reflux 10
Reflux 10
To confirm the Pd content, the supported catalyst was
treated with concentrated HCl and HNO3 to digest the Pd
species and then analyzed by ICP analysis. For this purpose,
100 mg of catalyst was digested in 50 mL of HNO3 (10 %
w/w) and then atomic emission of solution was determined
which resulted in a Pd content about 6.0 % w/w.
After characterization of catalyst, its catalytic activity is
tested in two important cross-coupling reactions: Heck and
Suzuki.
9
THF
H2O
Na2CO3 0.035
Na2CO3 0.035
10
11
12
13
CH3CN Na2CO3 0.01
DMF
DMF
Na2CO3 0.035
Na2CO3 0.035
140
80
10
48
Reaction conditions: bromobenzene (1 mmol), ethyl acrylate
(1.2 mmol) and base (2.5 mmol) in solvent (3 mL)
a
Isolated yield
Pd-SBNPP catalyzed reactions
not take place in the absence of catalyst even after 24 h at
120 °C (Table 1, entry 1). The optimized amount of cata-
lyst was 0.035 g (1.97 mol % of Pd). By decreasing the
catalyst to 0.01 g (0.56 mol % of Pd), the yield of product
decreased and the time of reaction was increased (Table 1,
entry 2), while the increase of catalyst to 0.05 g
(2.82 mol % of Pd) did not have more effect on the
Heck reaction
We tested various conditions to optimize the amount of
PNP-SBNPP catalyst on the Heck reaction between bro-
mobenzene and ethyl acrylate (Table 1). This reaction does
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J IRAN CHEM SOC (2013) 10:527–534
Y
reaction yield (Table 1, entry 4). In addition, different
conditions such as solvent, base and temperature were
optimized and the result was shown in Table 1.
X
PNP-SBNPP (0.035 g)
Na2CO3 (2.5 mmol)
DMF (3 mL), 120 o
C
+
Y
R
R
(1 mmol)
(1.2 mmol)
Thus, the following conditions were selected; aryl halide
(1 mmol), ethyl acrylate (1.2 mmol), Na2CO3 (2.5 mmol),
and PNP-SBNPP catalyst (0.035 g, 1.97 mol %) in 3 mL
DMF at 120 °C (Scheme 2 and Table 2).
Y = COOEt, Ph
Scheme 2 Investigation the Heck reaction catalyzed by PNP-
SBNPP
Table 2 The reaction of ethyl acrylate or styrene with aryl halides in the presence of PNP-SBNPP as catalyst (Heck reaction) at 120 °C
Entry
Aryl halide
Y
Product
Time (h)
Yield (%)a
1
COOEt
8
90
Br
Cl
COOEt
COOEt
2
3
COOEt
COOEt
15
20
62
45
Cl
COOEt
MeO
NC
MeO
NC
4
5
COOEt
COOEt
8
90
50
Br
COOEt
COOEt
16
Me
Me
Br
Me
Me
Br
Me
Me
6
7
8
Ph
Ph
Ph
15
24
15
78
54
84
Cl
Br
NC
NC
9
Ph
Ph
Ph
Ph
15
24
24
24
75
70
45
48
Br
N
N
10
11
12
Cl
Cl
Cl
O2N
MeO
Me
O2N
MeO
Me
Reaction conditions: aryl halide (1 mmol), alkene (1.2 mmol), Na2CO3 (2.5 mmol), catalyst (0.035 g), DMF (3 mL) at 120 °C
Isolated yield
a
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J IRAN CHEM SOC (2013) 10:527–534
533
The generality of this protocol was applied for coupling
of various alkenes (including ethyl acrylate and styrene
derivatives) with aryl halides (Table 2).
bromobenzene providing an excellent yield (91 %) of
desired product. The reaction also worked well with less
reactive chlorobenzene giving 81 % yield. As it was clearly
shown in Table 3, the GC-yield of by-product confirms this
point that these catalyst systems are more effective, and so
homocoupling of boronic acid was obtained poorly.
Electron-withdrawing groups such as nitro, and donating
groups such as methyl, methoxy, and methylthio were well
tolerated under the present catalytic systems (Table 3,
entries 3–4, 5–8). 2-iodothiophene and 4-bromoisoquino-
line were reacted with phenyl boronic acid in the presence
of PNP-SBNPP catalyst under optimized conditions to give
the corresponding product in high yield (Table 3, entries 9
and 10).
According to Table 2, ethyl acrylate was found to cou-
ple smoothly with bromobenzene and chlorobenzene pro-
viding a moderate yield of the desired product (entries 1
and 2). The electron-donating groups on the aryl ring
decreased the reaction rate as well as the yield of product
(Table 2, entry 3). Sterically hindered mesityl bromide was
treated with ethyl acrylate under optimized conditions
providing 50 % yield of the desired product (Table 2, entry
5). Bromobenzene and chlorobenzene derivatives were also
successfully coupled with styrene and the electron-with-
drawing and electron-donating groups on the aryl ring with
good- to high-yields (Table 2, entries 6–12). It should be
noted that all products obtained exclusively with an (E)-
configuration.
Recycling of the PNP–SBNPP catalyst for the Heck
and Suzuki reactions
Suzuki reaction
The possibility of recycling the PNP-SBNPP catalyst was
examined using the reaction of bromobenzene with ethyl
acrylate under optimized conditions. Upon completion, the
reaction mixture was filtered and the catalyst was washed
with dichloromethane. The recycled catalyst could be
reused five times without any treatment (Table 4).
The possibility of recycling the PNP-SBNPP catalyst in
Suzuki reaction was examined using the reaction of
4-nitro-bromobenzene with phenylboronic acid under
optimized conditions. When the reaction was completed
(confirmed by TLC), the reaction mixture was filtered and
the catalyst was washed with dichloromethane. The recy-
cled catalyst could be reused six times without any treat-
ment (Table 5).
The scope of this methodology was extended to the Suzuki
reaction. Initially the reaction of bromobenzene with
phenylboronic acid was studied as a model reaction in the
presence of PNP-SBNPP as catalyst. The optimized con-
ditions for Suzuki reaction were as follows: aryl halide
(1 mmol), aryl boronic acid (1.2 mmol), Na2CO3
(2.5 mmol), PNP-SBNPP (0.03 g, 1.69 mol %) in 2 mL of
ethanol at reflux conditions. The efficiency of this system
was further extended for coupling of various aryl halides
having different steric and electronic properties with
phenylboronic acid. The results are summarized in Table 3.
Phenylboronic acid was found to couple smoothly with
Table 3 The reaction of phenyl boronic acid or 4-ethylphenyl boronic acid with aryl halides in the presence of PNP-SBNPP as catalyst.
Reaction conditions: aryl halide (1 mmol), boronic acid (1.2 mmol), Na2CO3 (2.5 mmol), water (2 mL) at 80 °C
Ar
X
PNP-SBNPP (0.03 g)
Ar-B(OH)2
+ Ar-Ar
+
R
R
Na2CO3 (2.5 mmol)
EtOH (3 mL), Reflux
by-product
(1 mmol)
(1.2 mmol)
Entry
X
R
Ar
Time/h
Yield (%)a
Yield of by-product (%)
1
2
3
4
5
6
7
8
9
10
a
Br
Cl
Br
Cl
Cl
Cl
Br
Cl
H
Ph
1
91
81
92
73
62
68
84
61
87
85
–
–
H
Ph
4
4-NO2
4-NO2
4-OMe
4-Me
4-SMe
4-OMe
-
Ph
1
–
Ph
8
8
Ph
8
18
15
8
Ph
5
Ph
4
4-Et-Ph
Ph
8
10
5
2-Iodothiophene
4-Bromoisoquinoline
0.8
1.5
-
Ph
5
Isolated yield
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J IRAN CHEM SOC (2013) 10:527–534
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Entry
Yield of product (%)
Recovery of PNP–SBNPP (%)
fresh
90
89
87
87
86
85
[99
99
1
2
3
4
5
98
97
97
96
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Entry
Yield of product (%)
Recovery of PNP–SBNPP (%)
fresh
91
90
90
89
88
87
87
[99
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1
2
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4
5
6
99
98
97
97
96
Reaction conditions: PNP–SBNPP (0.06 g), Na2CO3 (5.0 mmol),
EtOH (5 mL), bromobenzene (2.0 mmol) and ethyl acrylate
(2.4 mmol)
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In conclusion, this work shows that palladium nanoparti-
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without noticeable loss of reactivity.
Acknowledgments We are thankful to Persian Gulf University
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