Palladium supported on zinc oxide nanoparticles: Synthesis, characterization, and application as heterogeneous catalyst for Mizoroki-Heck and Sonogashira reactions under ligand-free and air atmosphere conditions

Article abstract of DOI:10.1016/j.apcata.2014.02.002

In this paper, a novel Palladium (Pd) supported on ZnO nanoparticles was readily synthesized and characterized by using X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and BET specific surface area measurement. The total amount of palladium particles on ZnO was determined by induced coupled plasma (ICP) analysis and atomic absorption spectroscopy (AAS) which is 9.8 wt%. Percentage of accessible Pd as active catalyst is also estimated to 2.72% based on the thermogravimetric (TG) analysis. Nano Pd/ZnO was found as a new, novel, and excellent heterogeneous catalyst for ligand-free C-C bond formation through the Mizoroki-Heck and Sonogashira reactions under air atmosphere without the use of any Ar or N₂ flow. The catalyst can be recovered and recycled several times without marked loss of activity.

Full text of DOI:10.1016/j.apcata.2014.02.002

Applied Catalysis A: General 475 (2014) 477–486
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Applied Catalysis A: General
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Palladium supported on zinc oxide nanoparticles: Synthesis,
characterization, and application as heterogeneous catalyst for
Mizoroki–Heck and Sonogashira reactions under ligand-free and air
atmosphere conditions
Mona Hosseini-Sarvari∗, Zahra Razmi, Mohammad Mehdi Doroodmand
Department of Chemistry, Shiraz University, Shiraz 71454, Islamic Republic of Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
In this paper, a novel Palladium (Pd) supported on ZnO nanoparticles was readily synthesized and char-
acterized by using X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron
microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and BET specific surface area measurement.
The total amount of palladium particles on ZnO was determined by induced coupled plasma (ICP) anal-
ysis and atomic absorption spectroscopy (AAS) which is 9.8 wt%. Percentage of accessible Pd as active
catalyst is also estimated to 2.72% based on the thermogravimetric (TG) analysis. Nano Pd/ZnO was found
as a new, novel, and excellent heterogeneous catalyst for ligand-free C–C bond formation through the
Mizoroki–Heck and Sonogashira reactions under air atmosphere without the use of any Ar or N2 flow.
The catalyst can be recovered and recycled several times without marked loss of activity.
© 2014 Elsevier B.V. All rights reserved.
Received 18 November 2013
Received in revised form 25 January 2014
Accepted 4 February 2014
Available online 12 February 2014
Keywords:
Heterogeneous Catalysis
Mizoroki–Heck Reaction
nanoparticles
Pd/ZnO
Sonogashira reaction
1. Introduction
desirable. For this reason Pore et al. [7] in 2012 reported a novel
hydrophobic fluorous ionic liquid approach for the ligand-free Heck
It is well-known that catalytic Mizoroki–Heck and Sonogashira
reactions are two powerful methodologies for the generation of
C–C bonds particularly in the coupling of aryl halides with ter-
minal alkenes and alkynes for the generation of di-substituted
alkenes and alkynes derivatives. These compounds are important
materials for the production of natural products, biologically active
molecules, fine chemicals, pharmaceuticals, and useful materials
[1–4].
Among the various coupling reactions, the Mizoroki–Heck reac-
tion is generally carried out in polar solvents such as DMF,
N,N-DMAC, CH3CN, and phosphine ligands [5], which are normally
required to stabilize catalytically active palladium species. This can
also be achieved by employing salt additives such as ammonium
and phosphonium chlorides and bromides [6]. To overcome the
drawbacks of the using phosphin ligands, ligand-free Heck ary-
lation conditions are clearly on the rise in view of industrial and
pharmaceutical needs. Developing a new green solvent system
under ligand-free condition for the C–C bond formation is highly
reaction. Also this approach showed some advantages, the method
was laborious (using 2 mmol triethyl amine as organic base). Bha-
gat et al. [8] also reported a methodology for solvent-free one-pot
Heck reaction catalyzed by novel palladium(II) complex-via green
approach, but this method suffered from difficulty and long reac-
tion time of catalyst formation.
Pd-catalyzed reactions are undesired in the synthesis of phar-
maceutically active compounds and bioactive molecules because
of the high cost of this metal. Therefore, the efficient recovery of
the catalyst, good reusability, minimum possibility for leaching of
the metal species, and minimum uses of Pd catalysts remained a
scientific challenge of economic and environmental perspective.
For these reasons, palladium nanoparticles, colloidal palladium
species, and heterogeneous palladium catalytic systems on a sup-
port (including organic, polymers, or inorganic supports) have been
made [9–12]. However, these heterogeneous palladium catalysts
often suffer from problems such as low catalytic efficiency degra-
dation, difficult synthetic procedures [9l], and the leaching of the
metal species [10f].
On the other hand, for another important cross coupling reaction
namely Sonogashira reactions, the use of phosphine ligands for less
reactive bromide and chloride substrates under new green solvent
∗
Corresponding author: Tel.: +98 711 6137169; fax: +98 711 6460788.
E-mail address: hossaini@shirazu.ac.ir (M. Hosseini-Sarvari).
http://dx.doi.org/10.1016/j.apcata.2014.02.002
0926-860X/© 2014 Elsevier B.V. All rights reserved.

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M. Hosseini-Sarvari et al. / Applied Catalysis A: General 475 (2014) 477–486
systems is common. Recently much attention has been attracted to
the use of copper complexes and salts as the catalysts or cocatalysts
for the Sonogashira coupling reaction [13]. These systems actually
suffer from huge drawbacks such as formation of homo-coupling
products, use of organic solvent, high temperature, phosphine lig-
ands, and oxidizing agent or air [14]. For example Huang et al. [15],
reported an aqueous phase synthesis of palladium tripod nano-
structures for Sonogashira coupling reactions under 2.5 mol % of
CuI as cocatalyst in water without any homo-coupling products.
But in this method 5 mol % PPh3 and 2.0 equiv. KOH as a strong
base was used. Borja and coworkers [16] reported a methodology
for Sonogashira coupling reaction under copper- and phosphine-
free conditions; however, this method was limited due to difficulty
and longer reaction time of catalyst formation by a sol-gel process.
Among the important II–VI semiconductors, ZnO has been
widely studied because of its fundamental properties and poten-
tial uses in devices such as gas sensors, solar cells, resonators,
field-effect transistors, and as a catalyst [17]. In recent years, the
modification of ZnO with noble metals such as Ag, Au, Pt, and Pd
has attracted significant attention [18]. As a noble metal, palladium,
whose ionic radius (0.080 nm) is close to that of Zn2+ (0.074 nm), has
been widely used in the industry catalysis, especially for methanol
synthesis [19]. However, modifying ZnO with palladium has been
applied in the area of catalytic reaction [19,20]. To the best of our
knowledge, there are a few studies on C–C coupling reactions with
ZnO modified by palladium [21,22]. Kim and Choi [21] reported
a method for preparing ZnO-supported Pd (Pd/ZnO) and pd-M
(M = Cu, Ni, and Ag) nano particles (Pd-M/ZnO) by ꢀ-irradiation,
and their catalytic efficiencies were evaluated in hydrogenation
and Suzuki reactions. Also, these catalysts showed some advan-
tages, but either the method for the preparation of the catalyst
was laborious (using large amount of nano powder ZnO, using
nitrogen gas to remove oxygen from the reaction vessel, using the
ꢀ-ray irradiation, and using organic solvent such as MeOH) or the
Suzuki C–C coupling reactions were limited only to iodohalides and
phenylboronic acid and EtOH as solvent. Indeed, the molar ratio of
iodohalid: phenylboronic acid: Pd/ZnO catalyst was 1:2:1.3, and
the loaded amount of the palladium on ZnO was estimated to 19.4
wt%.
microscopy (SEM) was performed at accelerating voltage of 25 kV.
The amount of palladium nanoparticles supported on ZnO was
measured by inductively coupled plasma (ICP, Varian, Vista-pro)
and atomic absorption spectroscopy (AAS, Varian Model Spectra
AA 220 [Mulgrave, -Vic, -australia]). The size of the synthesized
nanoparticles was also confirmed by a Philips CM10 TEM instru-
ment. X-ray photoelectron spectroscopy (XPS) measurements were
conducted with a XR3E2 (VG Microtech) twin anode X-ray source
using AlK␣ = 1486.6 eV). A lab-made thermogravimetric analyzer
(TGA) [23] was also adopted for studying both the interaction
behavior of CO (Linde, 99.99%) as a selective probe molecule with
palladium nanoparticles and thermal stability of Pd-supported
ZnO nanoparticles after interacted with CO molecules. The specific
surface areas (SSABET; [m2 g−1]) of the nanopowders were deter-
mined with the nitrogen adsorption measurement, applying the
BET method at 77 K (BELsorp-mini II). The porous structural param-
eter used in this paper was taken from Barret–Joyner–Halenda
(BJH) data. 1H and 13CNMR spectra were obtained on a Bruker DPX
250 MHz instrument.
2.2. Preparation of Pd/ZnO nanoparticles
Pd/ZnO catalyst was prepared by coprecipitation (CP) method.
To a mixture of palladium nitrate (0.027 g/mL) and zinc nitrate
(0.267 g/mL) solutions, aqueous solution of sodium carbonate (1 M)
was added at room temperature to produce a final pH of 8. Then
it was aging for 2 h at 70–80 ◦C and the precipitates were filtered,
washed several times with distilled water and absolute ethanol,
dried at 80 ◦C overnight and then calcinated at 723 K for 2 h.
2.3. General procedure for Mizoroki–Heck coupling reaction of
aryl halides with styrene
A mixture of arylhalide (1 mmol), styrene (1 mmol), K2CO3
(1 mmol), and nano Pd/ZnO (0.009 g, which contains 832 × 10−8
mol% of Pd which was determined by ICP) in H2O (1 mL) was placed
in a 25 mL round bottom flask. In the case of the substrates which
are insoluble in water, a mixture of H2O/EtOH (1:1) was used as
solvent. The mixture was stirred at 90 ◦C. After the reaction was
finished, the reaction mixture was cooled to the room tempera-
ture, diluted with ethyl acetate (5 mL), and the slurry was stirred
at room temperature to ensure removal of the product from the
surface of the catalyst. Then it was centrifuged to separate the cata-
lyst. The centrifugate was washed with water (2 × 5 mL), dried over
anhydrous sodium sulfate, further concentrated under reduced
pressure, and purified by column chromatography on silicagel to
give the desired product.
Herein, in this paper, we focus on preparation and full char-
acterization of Pd/ZnO, which appears to be highly active catalyst
for the Mizoroki–Heck and Sonogashira reactions. The goal of the
work is the synthesis of Pd/ZnO by co-precipitation method with
palladium supported on ZnO (palladium loading is 9.8 wt%) for the
C–C bond formations under ligand-free and air atmosphere condi-
tions.
2. Experimental
2.1. Materials and instruments
2.4. General procedure for Mizoroki–Heck coupling reaction of
aryl halides with ethyl acrylate
Chemical materials were purchased from Fluka, Aldrich, Alfa
Aesar, and Merck. The progress of the reactions was followed with
TLC using silicagel SILG/UV 254 plates or by GC using a Shimadzu
gas chromatograph (GC-10A) instrument with a flame ionization
detector using a column of 15.0% carboxwax 20.0 M chromosorb-w
acid-washed 60–80 Å mesh size diameter. Evaporation of solvents
was performed at reduced pressure, with a Buchi rotary evaporator.
Column chromatography was carried out on short columns of sil-
ica gel 60 (70–230 Å mesh size diameter) in glass columns (2–3 cm
diameter) using 15–30 g of silica gel per one gram of crude mixture.
IR spectra were run on a Shimadzu FTIR-8300 spectrophotometer.
Power X-ray diffract meter with Cu K␣ (␭ = 1.54178 Å) radiationwas
used. The morphology of the products was determined by using
Leica Cambridge, model s360, version V03.03. Scanning electron
To a 25 mL round bottom flask with a mixture of arylhalide
(1 mmol), ethyl acrylate (1 mmol), K2CO3 (1 mmol), and nano
Pd/ZnO (0.009 g), 1 mL of DMF was added. In the case of the sub-
strates which are insoluble in water, a mixture of H2O/EtOH (1:1)
was used as solvent. The resulting solution was stirred at 100 ◦C
in an oil bath. After the reaction was finished, it was cooled to
the room temperature, and DMF was removed under reduced
pressure. The residue was diluted with ethyl acetate (5 mL), and
centrifuged to separate the catalyst. The centrifugate was washed
with water (2 × 5 mL), dried over anhydrous Na2SO4, filtered, and
concentrated. Further purification was achieved by column chro-
matography.

M. Hosseini-Sarvari et al. / Applied Catalysis A: General 475 (2014) 477–486
479
2.5. General procedure for Sonogashira reaction
A mixture of arylhalide (1 mmol), phenylacetylene (1 mmol), CuI
(2 mol%), K2CO3 (1 mmol), and nano Pd/ZnO (0.009 g) in H2O (1 mL)
was stirred at 90 ◦C for the appropriate time. In the case of the sub-
strates which are insoluble in water, a mixture of H2O/EtOH (1:1)
was used as solvent. The progress of the reaction was monitored by
TLC or GC. The separation of the catalyst from the reaction mixture
and the work-up process is similar to the section 2.3.
All compounds were known and characterized by comparison
of their physical and spectroscopic data with the data already
described in the literature.
2.6. General procedure for studying the interaction of CO as probe
molecule with Pd-supported ZnO
To study the interaction between CO as a selected probe
molecule with ZnO and Pd/ZnO nanoparticles, a constant quan-
tity (0.50 mg) of the synthesized nanoparticles was individually
inserted to the analyzing cell of the TG analyzer. The sample
was then heated to ∼120 ◦C inside N2 atmosphere with temper-
ature ramp of ∼5.0 ◦C min−1 to desorb any previously adsorbed
species such as water molecule or any volatile organic compounds.
After aging the sample at this temperature for ∼10 min, it was
annealed to the room temperature with temperature ramp equal
to -2.0 ◦C min−1, followed by changing the atmosphere of the ana-
lyzing cell from N2 to CO via purging a laminar flow of CO gas
with 20 mL min−1 flow rate. Also, the thermal stability of the CO-
interacted Pd/ZnO nanoparticles was studied using the TG analyzer
during enhancing the temperature of the sample with 2.0 ◦C min in
N2 atmosphere.
3. Results and Discussion
3.1. Characterization of Pd/ZnO nanoparticles
In order to evaluate the content of Pd supported on ZnO
nanoparticles, the catalyst was analyzed by ICP and AAS analyzers.
The results revealed that, the content of Pd-doped on ZnO was 9.84
and 9.80% (w/w) as measured by ICP and AAS, respectively.
Figure 1, shows the XRD patterns of pure and 9.84 wt% Pd
supported on ZnO nanoparticles calcined at 723◦K. All diffraction
peaks could be indexed to two crystalline phases. The XRD features
appeared at 2␪ = 31.8◦, 34.5◦, 36.3◦, and 47.6◦, which were due to the
diffractions of (100), (002), (101), and (102) planes of the wurtzite
structure ZnO (JCPDS Card File No. 36-1451), while the broad and
very small peak (due to the low content of Pd) appear at a bragg’s
angle of 41.2◦ originated from the diffraction of (111) planes, agree
well with the face-centered cubic (fcc) morphology of palladium
(JCPDS Card File No. 05-0681), respectively. The diffraction inten-
sity of Pd-doped ZnO is not as strong as that of the pure sample. This
can be attributed to the effect of Pd doping. No characteristic peaks
from other impurities are detected in the patterns demonstrating
that the sample has high phase purity. It suggests that the obtained
product is Pd/ZnO sample. Furthermore, the average crystallite size
of the unmodified and 9.84 wt% palladium-modified ZnO, evalu-
ated by the Scherre’r formula [24], are about 21–23 nm, indicating
that the ZnO crystal does not change a little before and after palla-
dium modification. Calculating the size particles of Pd by XRD was
impossible because of very small amounts of Pd loaded on ZnO so
the peak of Pd(0) is not strong.
Figure 1. XRD pattern of (a) nano ZnO and (b) 9.84 wt% of Pd modified ZnO calcined
at 723 k.
ZnO which was determined by TEM is in agreement with the results
obtained from XRD.
From the SEM image of nano Pd/ZnO in Figure 2c, we could
observe the morphology of the Pd/ZnO nanoparticles, composed
of spherical and cubic particles. The morphology showed rather a
dispersion of particles, with diameters ranging between 20–25 nm.
While the SEM image provide morphology and estimated particle
sizes, TEM image can reveal internal structure and a more accurate
measurement of particle size and morphology. It should be noted
that, due to various reasons such as the same morphology and size
distribution of each ZnO and Pd nanoparticles, partially the same
contrast of the electron beam through the ZnO and Pd nanoparti-
cles during the TEM analysis, and finally, due to the phenomenon
such as relative coagulation of the synthesized nanoparticles, no
significant difference was observed between the morphology and
structure of ZnO and Pd nanoparticles during characterization by
TEM (Figure 2c), even after enhancing the contrast by Au sputtering.
The surface area, average pore volume, and pore size of nano
Pd/ZnO sample is given in Table 1. The diameter of particles can
be calculated by dBET 6/(SSABET × ꢁsamples 26 nm, where ꢁsamples
are the density of ZnO (ꢁZnO = 5.61 g/cm3) [25,26] and the density
of palladium (ꢁPd = 12.02 g/cm3) [27], which was compared with
the average crystalline sizes (dXRD ave = 23 nm) calculated by the
The XRD results are in agreement with observations from TEM.
From Figure 2a and 2b the average particle size of nano ZnO and
nano Pd/ZnO was shown to be ∼25 nm. As can be seen, the shape
of nanoparticles is spherical and cubic at random. The size of nano

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M. Hosseini-Sarvari et al. / Applied Catalysis A: General 475 (2014) 477–486
fundamental parameter approach (Rietveld method) [24] based
on the half-maximum widths of Sherrer’s equation.
It is well known that material surface composition and chemical
states are very important through the process of catalytic reaction.
The surface composition and chemical states can be determined
by means of XPS spectrum according to the characterizing binding
energies of different elements on material surfaces [28]. The XPS
spectra of 9.8 wt% palladium modified ZnO was obtained. Figure 3
shows the Pd3d, Zn2p, and O1s XPS spectra. From Figure 3a, it could
be seen that the XPS signals were weak due to its low content of
palladium, and the binding energy (BE) of the Pd3d5/2 XPS peak was
+2
at about 335.5 eV. In general, the binding energy of Pd(3d5/2) is
about 337.0 eV according to the binding energy handbook of the
XPS instrument [29]. Thus, the binding energy of Pd3d5/2 appeared
at 335.5 eV, indicating that Pd exists mainly as the form of zero-
valence Pd on ZnO surface. From Figure 3b, it can be seen that
the peak position of Zn 2p3/2 in Pd/ZnO sample is about 1022.0 eV
indicating that Zn is in the formal Zn+2 valance state [30]. The XPS
spectrum of O1s is shown in Figure 3c. The peak centered at 530.2 eV
is closely associated with the lattice oxygen (OL) of ZnO, that it is
attributed to the contribution of Zn–O in ZnO crystal lattice [31].
The peak at about 532.5 eV is attributed to the oxygen of surface
hydroxyl (OH) groups resulting from the chemisorbed water [32].
The FTIR spectrum of nano Pd/ZnO in KBr matrix is shown in
(Figure 4). There is a broad band at 3425 cm−1 corresponding to
the vibration mode of water OH group indicating the presence of
small amount of water adsorbed on the ZnO nano crystal surface.
The band at 1650 cm−1 is due to the OH bending of water. The
adsorption band at 460 cm−1 is the stretching mode of ZnO [17f].
In this study, to estimate the quantity of Pd-incorporated ZnO
crystal as well as to determine the percentage of accessible Pd
as active catalyst, TG analyzer was selected as detection system
according to the recommended procedure. Figure 5 clearly shows
the trace (diagram of weight percentage vs. time) for each ZnO
and Pd/ZnO nanoparticles. As clearly shown, the quantity of CO
interacted with Pd was easily estimated according to the difference
between the weight percentage of CO interacted with each ZnO and
Pd/ZnO nanoparticles, which was estimated to 4.3 0.1% (Figure 5).
For further reliability about this value, the thermal stability of
Pd/ZnO nanoparticles was also investigated in detail. According to
the thermogram (Figure 6), a significant weight loss to 1.6 0.1%
at temperature to ∼95 ◦C is correlated to the desorption of H2O,
organic species, and the CO-adsorbed Pd/ZnO. Whereas the weight
loss (4.2 0.1%) at ∼ 170 ◦C clearly points to the percentage of CO
interacted with the Pd nanoparticles, which is in good agreement
with the result obtained from the trace (Figure 5). Considering pal-
ladiumocene, i.e., Pd(CO)6 as thermodynamically stable species,
this result simply points to the percentage of accessible Pd, which
is estimated to be 2.72 0.01%. Consequently, from total amount of
Pd, supported on ZnO crystal (9.84%), analyzed by the ICP and AAS,
∼7.12% has been incorporated with the ZnO crystal and ∼2.72%
is accessible for playing a role as catalyst during the synthesis of
organic compounds.
Figure 2. (a) TEM image of ZnO; (b) TEM and (c) SEM images of the nano Pd/ZnO.
Table 1
Results of BET surface area measurements for nano Pd/ZnO.
Surface area
BET surface area
(m2.g−1
40.61
38.18
)
BJH adsorption
cumulative surface
area of pores (m2.g−1
)
3.2. Catalytic application of nano Pd/ZnO
Pore volume
Single point adsorption
total pore volume of
0.24
0.19
3.2.1. Mizoroki–Heck coupling reaction catalyzed by nano Pd/ZnO
After successful preparation and characterization of nano
Pd/ZnO, its catalytic activity was first examined in the
Mizoroki–Heck reactions. This choice was based on the fact
that such a reaction provides a powerful tool for the C–C bond
formation from arylhalides and alkenes.
An attempt to develop C–C bond formations by Heck reaction,
the cross coupling of 1-(4-bromophenyl)ethanone (1 mmol) and
styrene (1 mmol) in the presence of nano Pd/ZnO, was studied
in various parameters such as amount of catalyst, base, and
pores (cm3.g−1
)
BJH adsorption
cumulative volume of
pores (cm3.g−1
)
Pore size
Mean pore diameter
(nm)
Pore size distribution
(nm)
24.36
1.88

M. Hosseini-Sarvari et al. / Applied Catalysis A: General 475 (2014) 477–486
481
Figure 4. FTIR spectrum of nano Pd/ZnO.
Figure 5. Trace showing the interaction of CO as molecular probe with ZnO and
Pd/ZnO nanoparticles.
Figure 6. Thermogram showing the interaction of CO as molecular probe with
Pd/ZnO nanoparticles at N2 atmosphere.
solvent. The experimental results are summarized in Scheme 1,
and Tables 2–4.
The results of the initial screen for the amounts of nano Pd/ZnO
are displayed in Table 2. As it can be seen from entries 1–5, by
increasing the amount of catalyst from 0.0005 to 0.01 g, the yield of
desired product increased, and the reaction time decreased. There
were no differences in yield and reaction time between 0.009 g
(entry 4) and 0.01 g (entry 5) of the catalyst. Therefore, because of
lower amount of Pd loading, we chose 0.009 g of the nano Pd/ZnO
for further experiments.
Optimization with this catalyst was completed with different
bases and solvents. Based on the results outlined in Table 3, it is
Figure 3. XPS spectra of (a) Pd3d, (b) Zn2p, and (c) O1s spectrum of 9.84 wt%, nano
Pd/ZnO.
Scheme 1. Mizoroki–Heck reaction of 1-(4-bromophenyl)ethanone and styren by
nano Pd/ZnO catalyst.

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M. Hosseini-Sarvari et al. / Applied Catalysis A: General 475 (2014) 477–486
Table 2
Then we tried to study the Heck reaction between aryl halides
and ethyl acrylate in order to show the originality and generality of
nano Pd/ZnO catalyst. By using the similar conditions for the Heck
reaction described above in water as solvent, a trace amount of the
product was observed. Therefore, we screened different solvents
and bases. Finally we found that for this reaction by using DMF
(1 mL) as solvent and K2CO3 (1 mmol) as base at 100 ◦C, after 3 h,
98% of the desired product was obtained. Therefore, on the basis
of optimized reaction conditions, the coupling between a range of
aryl halides and ethyl acrylate were carried out (Table 6).
Similar to the reaction between aryl halides and styrene
the reactivity of aryl bromides and chlorides with electron-
withdrawing substituent (Table 6, entries 4,8–10) was higher than
those with electron donating substituent (Table 6, entries 2,6).
Sterically hindered aryl iodides and chlorides were also found to
react smoothly with ethyl acrylate providing excellent yields of the
desired products. (Table 6, entries 5, 11).
Optimization of the amount of nano Pd/ZnO in Heck reaction.a
Entry
1
2
3
4
5
Nano Pd/ZnO (g)
Time (h)
0.0005
22
0.001
20
0.005
20
0.009
17
98
0.01
16
98
Conversion %b
95
95
97
a
Reaction conditions: 1-(4-Bromophenyl)ethanone (1 mmol), styrene (1 mmol),
nano Pd/ZnO, H2O (1 mL), and K2CO3 (1 mmol) at 90 ◦C. b Determined by GC.
Table 3
The effect of base on Heck coupling reaction.a
Entry
Base
mmol
Time (h)
Conversion %b
1
2
3
4
5
6
7
8
K2CO3
K2CO3
Na2CO3
K3PO4
KF.2H2O
KOH
1
0.5
1
1
1
1
1
1
17
17
20
18
20
18
20
20
98
95
98
98
90
95
70
65
Cs2CO3
NaOAC
3.3. Sonogashira coupling reaction catalyzed by nano Pd/ZnO
a
Reaction conditions: 1-(4-Bromophenyl)ethanone (1 mmol), styrene (1 mmol),
Impressed with the results for the Mizoroki–Heck coupling
reaction, we further explored the application of the nano Pd/ZnO
catalyst in another important C–C bond forming reaction, namely
Sonogashira cross-coupling of arylhalides with phenylacetylene. As
a model study, we first chose to study the effect of CuI salt on the
efficiency of the nano Pd/ZnO catalyzed Sonogashira cross-coupling
reaction of 1-(4-bromophenyl)ethanone and phenylacetylene.
When the reaction was performed with 2 mol% of CuI and nano
Pd/ZnO (0.009 g) in the presence of K2CO3 as a base in water
at 90 ◦C for 10h, the expected cross-coupling product of phenyl-
ethynylbenzene was afforded in 98% yield (Table 7, entry 6).
For further optimization of the reaction conditions, the
decreasing yields of the coupling product was observed by
decreasing the added amounts of CuI and nano Pd/ZnO catalyst
(Table 7, entries 1–5). For a blank test (Table 7, entry 7), a signifi-
cantly lower 10% yield of the cross-coupling product was obtained
while the same reaction condition was carried out in the absence
of the nano Pd/ZnO catalyst. To evaluate the effect of the solvent
(Table 7, entries 6, 8–12), results showed that H2O is the best choice.
For comparison on the efficiency of base in this reaction (Table 7,
entries 6, 13–15) K2CO3 was found to be the most effective and
therefore, 1 mmol of K2CO3 as an inexpensive and readily available
inorganic base was used in this study.
On the basis of the optimized reaction conditions, the coupling
reactions between a range of arylhalides (I, Br, and Cl) and phenyl
acetylene were carried out in the presence of nano Pd/ZnO. By
using this protocol, the coupling reaction of various functional-
ized arylhalides with phenyl acetylene can give the corresponding
arylated alkynes in good to excellent yields (Table 8). Similar to
Heck reaction, it has been found that, aryliodieds with a variety
of electron-rich and electron-poor substituent proceeded, giving
the coupling products in good to excellent yields (Table 8, entries
1–6). The reactivity of aryl bromides and chlorides with electron-
withdrawing substituent (Table 8, entries 8–12) was higher than
those with electron donating substituent (Table 8, entry 7). Inter-
estingly, no significant steric effects were observed in this study,
because sterically hindered ortho-substituted iodobenzene also
provided good yields with phenylacetylene (Table 8, entries 5, 12).
Finally, in order to show the merit of this catalytic method, we
compared our obtained results with similar data published by other
groups which used other catalysts (Table 9).
nano Pd/ZnO (0.009 g), H2O (1 mL), and base at 90 ◦C.b Determined by GC.
evidence that K2CO3, Na2CO3, and K3PO4 are superior bases to the
others. Consequently, because K2CO3 is an inexpensive and readily
available inorganic base, we chose it for further investigations. Also,
the selection of solvent was important for the present method.
Results (Table 4) show that nano Pd/ZnO is a very efficient and
suitable catalyst for such coupling reaction in water. We then tried
this reaction with catalyst in other solvent. The use of protic polar
solvents, such as EtOH and ethane-1,2-diol, which are commonly
used in Pd catalyzed coupling reactions, were not effective in the
present method (Table 4, entries 2 and 3). Use of DMSO and DMF
provides long reaction times and the reactions were completed in
22 and 19 h, respectively (Table 4, entries 4 and 5). Use of toluene
and CH2Cl2 is not successful and only 10% conversion yields were
observed (Table 4, entries 7 and 8).
On the basis of the optimized reaction conditions, the coupling
reactions between a range of aryl halides (I, Br, and Cl) and styrene
were carried out in the presence of nano Pd/ZnO (0.009 g), K2CO3
(1 mmol), and H2O (1 mL) at 90 ◦C. As shown in Table 5, aryl iodides
with a variety of electron-rich and electron-poor substituent pro-
ceeded smoothly, giving the coupling products in good to excellent
yields. The reactivity of aryl bromides and chlorides with electron-
withdrawing substituent (Table 5, entries 8–11) was higher than
those with electron donating substituent (Table 5, entry 7). Fur-
thermore, steric hindrance due to the ortho substituents on the aryl
iodides and chlorides affected the reaction progress. Thus 2-methyl
iodobenzene and 1-chloro-2-(trifluoromethyl) benzene needed a
higher reaction time than 4-substitued iodo and chloro benzenes
(Table 5, entries 5, 11).
Table 4
The effect of solvents on Heck coupling reaction.a
Entry
Solvent
Time (h)
Conversion %b
1
2
3
4
5
6
7
8
H2O
EtOH
Ethane-1,2-diol
DMSO
DMF
Solvent free
Toluene
CH2Cl2
17
18
20
22
19
20
20
20
98
65
55
90
95
20
10
10
3.3.1. Catalyst recycling in Mizoroki–Heck and Sonogashira
reactions
Having established the efficacy of the Pd/ZnO nanoparticles in
the Heck and Sonogashira coupling reactions, we then investigated
a
Reaction conditions: 1-(4-bromophenyl)ethanone (1 mmol), styrene (1 mmol),
nano Pd/ZnO (0.009 g), solvent (1 mL), and K2CO3 (1 mmol) at 90 ◦C.b Determined
by GC.

M. Hosseini-Sarvari et al. / Applied Catalysis A: General 475 (2014) 477–486
483
Table 5
Mizoroki–Heck reaction of aryl halides with styrene.a
Entry
X
R
Time (h)
Yield %a
1
2
3
4
5
6
7
8
I
I
I
I
I
I
Br
Br
Br
Cl
Cl
Cl
H
17
24
22
10
24
20
23
12
17
15
10
15
95
95
95
100
90
90
90
92
98
85
100
95
4-OMe
4-NH2
4-NO2
2-CH3
4-CH3
4-OH
4-CN
4-COCH3
4-CN
4-CF3
2-CF3
9
10
11
12
aReaction conditions: arylhalide (1 mmol), styrene (1 mmol), K2CO3 (1 mmol), Pd/ZnO (0.009 g), and H2O (1 mL) at 90 ◦C.b Isolated yield.
Table 6
Mizoroki–Heck reaction of aryl halides with ethyl acrylatea.
Entry
X
R
Time (h)
Yield (%)b
1
2
3
4
5
6
7
8
I
I
I
I
I
I
Br
Br
Br
Cl
Cl
Cl
H
3.10
5
6
3
8
8
6
4
3.15
5
98
95
88
98
90
93
90
99
98
95
98
94
4-OMe
4-NH2
4-NO2
2-CH3
4-CH3
4-OH
4-CN
4-COCH3
4-CN
4-CF3
2-CF3
9
10
11
12
4
5
aReaction conditions: arylhalide (1.0 mmol), ethyl acrylate (1.0 mmol), K2CO3 (1.0 mmol), Pd/ZnO (0.009 g), and DMF (1.0 mL) at 100 ◦C.b Isolated yield.
the recyclability of the catalyst (Table 10). To clarify this issue, cat-
alytic recycling experiments were carried out using a Heck reaction
of 1-(4-bromophenyl)ethanone with styrene, and a Sonogashira
reaction of 1-(4-bromophenyl)ethanone with phenyl acetylene as
model reactions. In both reactions, after completion for the first
reaction, the catalyst could be recovered from the reaction mixture
by diluting with EtOAc and centrifugation to separate the catalyst.
The recovered catalyst was further washed with EtOAc and water
Table 7
Sonogashira cross-coupling reaction of 1-(4-bromophenyl)ethanone with phenylacetylene.a
Entry
CuI:Pd/ZnO(mol%) : (g)
Base
Solvent
Yield %b
1
2
3
4
5
6
7
8
0.5:0.0005
0.5:0.001
0.5:0.003
0.5:0.005
1:0.005
2:0.009
2:0
2:0.009
2:0.009
2:0.009
2:0.009
2:0.009
2:0.009
2:0.009
2:0.009
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K3PO4
Cs2CO3
Na2CO3
H2O
H2O
H2O
H2O
H2O
H2O
H2O
EtOH
DMSO
DMF
CH3CN
PHCH3
H2O
40
55
80
80
90
98
30
80
70
70
48
40
90
89
95
9
10
11
12
13
14
15
H2O
H2O
a
Reaction conditions: 1-(4-Bromophenyl)ethanone (1 mmol), phenylacetylene (1 mmol), nano Pd/ZnO, CuI, solvent (1 mL), and base (1 mmol) at 90 ◦C for 7 h.b Isolated yield.

484
M. Hosseini-Sarvari et al. / Applied Catalysis A: General 475 (2014) 477–486
Table 8
Sonogashira reaction of aryl halides with phenylacetylene.a
Entry
X
R
Time (h)
Yield %b
1
I
H
7
98
2
3
4
5
6
7
8
9
I
I
I
I
4-OMe
4-NH2
4-NO2
2-CH3
4-CH3
4-OH
4-CN
4-COCH3
4-CN
4-CF3
2-CF3
8
10
5
8
8
10
4
7
7
5
95
95
100
90
95
90
92
98
85
I
Br
Br
Br
Cl
Cl
Cl
10
11
12
100
100
5
a
Reaction conditions: arylhalide (1 mmol), phenylacetylene (1 mmol), K2CO3 (1 mmol), Pd/ZnO (0.009 g), and H2O (1.0 mL) at 90 ◦C.
Isolated yield.
b
Table 9
Comparison of activity of different catalysts in the Heck and Sonogashira cross-coupling reactions.
Catalyst
Reaction Conditions
CH3CN, NEt3, 82 ◦
Time (h)
Yield (%)
Ref.
Hecka
nano Pd/AT-Mont (0.07 mol%)
nano Pd/SiO2 (0.3 mol%)
nano PdO/PS (1.0 mol%)
nano Pd/NH2-SiO2 (0.05 mol%)
nano Pd–Fe3O4 (1 mol%)
C
5
5
87
54a
14
95
91
98
90
99
91
90
98
[9f]
[9h]
[9g]
[9l]
[9j]
This study
[9f]
[9i]
[9j]
NMP, NaOAc, 150 ◦
C
H2O, KOH, 90 ◦
C
20
2
24
17
3
DMF, K2CO3, 110 ◦
DMF, NaOAc, 110 ◦
C
C
nano Pd/ZnO (8.32 × 10−6 mol% of Pd)
nano Pd/AT-Mont (0.07 mol%)
Pd-2QC-MCM-41 (0.02 g)
H2O, K2CO3, 90 ◦
C
Sonogashirab
CH3CN, NEt3, 82 ◦
NMP, piperidine, 80 ◦
DMF, piperidine, 110 ◦
DMF, NEt3, 80 ◦
H2O, K2CO3,CuI, 90 ◦C
C
C
C
3
nano Pd-Fe3O4 (1 mol%)
24
10
7
Pd/MgLa (1.5 mol%)
C
[9k]
This study
nano Pd/ZnO (8.32 × 10−6 mol% of Pd)
a
Phenyl iodide and styrene as starting materials;
Phenyl iodide and phenyl acetylene as starting materials.
b
and finally dried at 80 ◦C. A new reaction was then performed with
fresh solvent and reactants under the same conditions. The results
are shown in Table 10 and Figure 7.
The amount of palladium leaching of the catalyst was further
investigated by ICP and AAS. The result indicated that only 0.01%
wt of the Pd metal (the initial 9.84 w% Pd goes to 9.83 ww% after
5th cycle) is leached out from the catalyst surface after five cycles
(for both Heck and Sonogashira reactions) (Table 10), which was
determined by ICP and ASS in both liquid phase and solid mate-
rial. Furthermore, the filtered solution exhibited no reactivity to
Table 10
Reusabilities of the Pd/ZnO catalyst.
Run
Yield (%) 3a (Heck reaction)a
Yield (%) 3b (Sonogashira reaction)b
First
98
98
97
95
95
98
98
97
97
97
Second
Third
Fourth
Fifth
a
The reaction was terminated after 17 h.
The reaction was terminated after 7 h.
b

M. Hosseini-Sarvari et al. / Applied Catalysis A: General 475 (2014) 477–486
485
Figure 8. XRD pattern of the nano Pd/ZnO used for the reaction of 1-(4-
bromophenyl)ethanone with styrene in the Heck reaction, after the 5th run recycling
of the catalyst.
(3 h), respectively. The filtrate was further heated, no extra forma-
tion of coupling product was observed via GC analysis even after
20 h and 10 h for each reaction, respectively. These observations
confirm that the reactions catalyzed by the nano Pd/ZnO are het-
erogeneous in nature.
4. Conclusions
In conclusion, we have successfully prepared Pd supported on
ZnO nanoparticles as heterogeneous catalyst for Heck and Sono-
gashiro cross coupling reactions, without using any ligands and Ar
or N2 flow. The results reported indicate that the behavior of palla-
dium catalyst, in the C–C coupling reactions, can be modified using
an appropriate preparation method (coprecipitation technique).
Moreover, the easy extractive recovery of the final product, and the
solid residue can be reused for several times, can be considered as
strong practical advantages of this method. This methodology could
provide a facile, efficient, and environmentally friendly process for
the Heck and Sonogashira reactions because of its wide applicabil-
ity to various substrates, the use of less toxic reagents, and mild
reaction conditions. We believe that this type of nano crystalline
metal oxide finds excellent applications as active catalysts not only
for laboratory scale research but also for industrial applications.
Figure 7. Plot of the yields versus the reused cylces as a function of reaction time
for the a) Heck and b) Sonogashira reaction.
Acknowledgments
We gratefully acknowledge the support of this work by the Shi-
raz University and the Iran National Science Foundation (Grant
No.90003779) for financial support.
both Heck and Sonogashira reactions. The absence of any obvious
decrease leached metal in the filtrate suggests that high stabil-
ity of the heterogeneous catalyst and confirming that the catalytic
process occurs on solid surface. The XRD pattern (Figure 8) of the
recovered nano catalyst for Heck reaction after the 5th run showed
that the aggregation of the particles has not occurred significantly,
and the size of the particles were not disturbed notably in compar-
ison with the XRD picture of the catalyst before its application for
the reaction.
This observation confirms that nano Pd/ZnO are stable under
the reaction conditions and are not affected by the reactants.
This could be attributed to the high stability of the nano Pd/ZnO,
which behaves truly as a heterogeneous solid catalyst. Also,
in another experiment when the catalyst separated from the
reaction mixture in the middle of the Heck reaction between 1-(4-
bromophenyl)ethanone and styrene (8 h) or Sonogashira reaction
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