# One-Step Synthesis of Pd-M/ZnO (M=Ag, Cu, and Ni) Catalysts byγ-Irradiation and Their Use in Hydrogenation and Suzuki Reaction

Authors: Mi-Ran Kim; Seong-Ho Choi
Journal: Journal of Nanomaterials
 (2009)

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## Abstract

ZnO-supported Pd, Pd-Ag, Pd-Cu, and Pd-Ni catalysts (Pd-M/ZnO) were prepared inMeOH/H2O mixture (4/1, v/v-%) by γ-irradiation at room temperature. Small mono- and bimetallic nanoparticles were loaded on the surface of ZnO nanopowder as confirmed with TEM, TEM-EDXS, XRD, and ICP-AES data. The catalytic efficiency against Pd-M/ZnO was determined in hydrogenation and Suzuki reaction and compared with the commercial Pd/C catalyst. The Pd-Ag/ZnO showed the highest catalytic efficiency in the Suzuki reaction.


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## Body

## 1. Introduction

Palladium catalysts are used in cross-coupling reaction
(e.g., Suzuki, Heck reaction, Sonogashira, etc.).  While homogeneous palladium catalysts are not
used in industrial application because of the difficulty in separating and recycling
for the catalysts.  Many heterogeneous
palladium catalysts have been made for recycling mainly by immobilizing
palladium onto inorganic supports or polymers [1–5].  Palladium nanoparticles, colloidal palladium
species, and polymer-incarcerated palladiums have been also reported as recycling
catalysts [6, 7].  However, they often
suffer from problems such as low catalytic efficiency, degradation, the
leaching of the metal species, and a difficult synthetic procedure.Palladium catalyzed
Heck or Suzuki C–C forming
reactions using aryl bromides as one of the substrates are among the most versatile
reactions for organic synthesis, since they are compatible with a large variety
of functional groups.  Many palladium complexes
have been investigated as homogeneous catalysts and many supported palladium
complexes as heterogeneous catalysts for these reactions.  An extensive range of reaction conditions have
been explored and literature on this subject has very recently been
reviewed [8].  In a previous paper [9], carbon-supported
monometallic Pd and bimetallic Pd-M (M=Ag, Ni, and Cu) nanoparticles were
synthesized by γ-irradiation at room temperature without reducing
agents.  The prepared Pd/C and Pd-M/C catalysts
were applied as catalyst in C–C coupling reaction (specially, Suzuki and Heck
reactions) in EtOH and in acetonitrile, respectively.  The Pd-Cu/C catalysts showed high catalytic
efficiency in the Suzuki- and Heck-type reactions.  However, the metallic nanoparticle based on Pd
is mainly aggregated on carbon surface because carbon supporters have
hydrophobic properties, while the metallic particles have hydrophilic properties.  In order to increase dispersion of metallic
nanoparticles, we selected hydrophilic property supporters.On the other
hand, the aromatic amines are of significant industrial importance because
they are widely used as the intermediates for synthesis of dyes, pharmaceuticals,
and agrochemicals [10].  Aromatic amines
are generally prepared by the reduction of aromatic nitro compounds. Two
general methods are used for the reduction of aromatic nitro compounds in
industry: catalytic hydrogenation [11, 12] and stoichiometric reduction reaction
[13–17].  The catalytic hydrogenation is
a clean and convenient method for producing amine in high yield.  However, little has been reported about
synthesis of aromatic amine from aromatic nitro compound by using monometallic
or bimetallic nanoparticles prepared by γ-irradiation, to our knowledge.Herein, we
described a simple method for preparing ZnO-supported Pd (Pd/ZnO) and Pd-M (M=Cu,
Ni, and Ag) nanoparticles (Pd-M/ZnO) byγ-irradiation at room
temperature without any reduction agents.  The prepared Pd/ZnO and Pd-M/ZnO catalysts
were characterized by TEM, TEM-EDXS, and ICP-AES spectroscopy.  Furthermore, the catalytic efficiencies of
the Pd/ZnO and Pd-M/ZnO catalysts were evaluated in hydrogenation and Suzuki
reactions.


## 2. Experimental

### 2.1. Chemicals

PdNO3, CuNO3, NiNO3,
Iodobenzene, 2-iodothiophene, 4-iodobenzoic acid, phenyl boronic acid, and
4-nitrophenol were analytical reagent grade and supplied by Sigma-Aldrich Korea (Seoul, Korea).
AgNO3 was purchased from
Kojima Chemicals Co., Ltd. (Japan).
The ZnO nanopowder was obtained from
Sunjin Chemical Co., Ltd. (Korea).  The Poly (vinylpyrrolidone), PVP (molecular
weight of 40 000), was purchased from Junsei Co. Ltd. (Japan). All the other chemicals were
of reagent grade and used without further purification.


### 2.2. Radiolytic Synthesis of Pd-M/ZnO (M=Ag, Ni, and Cu) Catalysts

Scheme1
shows the preparation procedure of Pd-M/ZnO catalysts by using γ-irradiation.
The Pd-Ag/ZnO catalyst was prepared by the following procedure. First, Pd(NO3)2, AgNO3,
PVP as anchor agents, and ZnO nanopowder (10.0 g) as supporting material were
dispersed in a mixed solution of MeOH-water (4/1, v/v-%).  Next, nitrogen gas was bubbled through the
solution for 30 minutes to remove the oxygen. The dispersed solution was then
irradiated by the γ-ray
from a Co-60 source under atmospheric pressure and ambient temperature.  The total irradiation dose was 30 kGy (a dose
rate = 1.0×104 Gy/hour).  The Pd/ZnO, Pd-Ni/ZnO, and Pd-Cu/ZnO catalysts were
also prepared by a similar method.  After
γ-irradiation,
the reaction mixture was centrifuged at 1600 rpm for separation of the Pd-M/ZnO
catalysts, which were then dried at 50°C in a
vacuum oven for 6 hours.Scheme 1
Preparation procedure of Pd-M (M=Cu, Ni and Ag)/ZnO
catalyst in MeOH-water mixture by usingγ-irradiation. Reactants: CH3OH (200 mL) and water
(50 mL) as solvent, PVP as a stabilizer, palladium nitrate, silver nitrate, copper nitrate, and nickel
nitrate, ZnO powder (10.0 g).


### 2.3. Catalytic Test

#### 2.3.1. Hydrogenation

For studying the catalytic efficiency for Pd and Pd-M/ZnO catalysts, the
reaction mixtures were prepared with NaBH4 (0.1 M) as hydrogen source and aromatic nitro
compounds (0.01 M).  Pd-M/ZnO catalysts (5 mg) were added in the prepared reaction mixture.   The UV-visible spectra during reduction of
aromatic nitro compounds using Pd-M/ZnO catalysts were recorded
continuously.  In order to compare the
reduction of aromatic nitro compound, the standard aromatic amine compounds
were also measured by UV-visible spectra.


#### 2.3.2. Suzuki Reaction

In a 100 mL
three-necked flask, an aryl halide (3.0 mmol), phenyl boronic acid (6.0 mmol),
Pd-M/ZnO catalyst (43.0 mg, ca. 4.0 mmol), and K3PO4 (12.0 mmol) were added in 50 mL EtOH.  Three different types of aryl halides were
tested.  The mixture solution was reacted
at 78°C
for 3 hours under nitrogen atmosphere.  In
order to remove the Pd-M/ZnO catalyst, the reaction solution was filtered
through a Whatman filter paper (no.2).  The filtered solution was filtered again
through 0.45 μm membrane filter (MFS-25 PVDF).
Finally the solvent was evaporated.
The yield was determined by HPLC with a ODS column.


### 2.4. Characterization

UV-visible spectra were measured by using a Shimadzu
UV-160 digital spectrophotometer (Kyoto, Japan) in a 1 cm
quartz cuvette.  Field emission transmission
electron microscopy (FE-TEM) images were collected at 200 kV using a Hitachi HF-2000
TEM equipped with a field emission source.  An energy dispersive X-ray spectrometer (EDXS)
attached to the Hitachi HF-2000 TEM was used to analyze the chemical composition
of the samples.  FE-TEM specimens were
prepared by placing microdrops of colloid solution on a carbon film supported on
copper grids. The elemental analysis of the catalysts was performed by an Ultima-C
Inductively Coupled Plasma-Atomic Emission Spectrometer (ICP-AES, Jobin-Yvon
Co. USA).


## 2.1. Chemicals

PdNO3, CuNO3, NiNO3,
Iodobenzene, 2-iodothiophene, 4-iodobenzoic acid, phenyl boronic acid, and
4-nitrophenol were analytical reagent grade and supplied by Sigma-Aldrich Korea (Seoul, Korea).
AgNO3 was purchased from
Kojima Chemicals Co., Ltd. (Japan).
The ZnO nanopowder was obtained from
Sunjin Chemical Co., Ltd. (Korea).  The Poly (vinylpyrrolidone), PVP (molecular
weight of 40 000), was purchased from Junsei Co. Ltd. (Japan). All the other chemicals were
of reagent grade and used without further purification.


## 2.2. Radiolytic Synthesis of Pd-M/ZnO (M=Ag, Ni, and Cu) Catalysts

Scheme1
shows the preparation procedure of Pd-M/ZnO catalysts by using γ-irradiation.
The Pd-Ag/ZnO catalyst was prepared by the following procedure. First, Pd(NO3)2, AgNO3,
PVP as anchor agents, and ZnO nanopowder (10.0 g) as supporting material were
dispersed in a mixed solution of MeOH-water (4/1, v/v-%).  Next, nitrogen gas was bubbled through the
solution for 30 minutes to remove the oxygen. The dispersed solution was then
irradiated by the γ-ray
from a Co-60 source under atmospheric pressure and ambient temperature.  The total irradiation dose was 30 kGy (a dose
rate = 1.0×104 Gy/hour).  The Pd/ZnO, Pd-Ni/ZnO, and Pd-Cu/ZnO catalysts were
also prepared by a similar method.  After
γ-irradiation,
the reaction mixture was centrifuged at 1600 rpm for separation of the Pd-M/ZnO
catalysts, which were then dried at 50°C in a
vacuum oven for 6 hours.Scheme 1
Preparation procedure of Pd-M (M=Cu, Ni and Ag)/ZnO
catalyst in MeOH-water mixture by usingγ-irradiation. Reactants: CH3OH (200 mL) and water
(50 mL) as solvent, PVP as a stabilizer, palladium nitrate, silver nitrate, copper nitrate, and nickel
nitrate, ZnO powder (10.0 g).


## 2.3. Catalytic Test

### 2.3.1. Hydrogenation

For studying the catalytic efficiency for Pd and Pd-M/ZnO catalysts, the
reaction mixtures were prepared with NaBH4 (0.1 M) as hydrogen source and aromatic nitro
compounds (0.01 M).  Pd-M/ZnO catalysts (5 mg) were added in the prepared reaction mixture.   The UV-visible spectra during reduction of
aromatic nitro compounds using Pd-M/ZnO catalysts were recorded
continuously.  In order to compare the
reduction of aromatic nitro compound, the standard aromatic amine compounds
were also measured by UV-visible spectra.


### 2.3.2. Suzuki Reaction

In a 100 mL
three-necked flask, an aryl halide (3.0 mmol), phenyl boronic acid (6.0 mmol),
Pd-M/ZnO catalyst (43.0 mg, ca. 4.0 mmol), and K3PO4 (12.0 mmol) were added in 50 mL EtOH.  Three different types of aryl halides were
tested.  The mixture solution was reacted
at 78°C
for 3 hours under nitrogen atmosphere.  In
order to remove the Pd-M/ZnO catalyst, the reaction solution was filtered
through a Whatman filter paper (no.2).  The filtered solution was filtered again
through 0.45 μm membrane filter (MFS-25 PVDF).
Finally the solvent was evaporated.
The yield was determined by HPLC with a ODS column.


## 2.3.1. Hydrogenation

For studying the catalytic efficiency for Pd and Pd-M/ZnO catalysts, the
reaction mixtures were prepared with NaBH4 (0.1 M) as hydrogen source and aromatic nitro
compounds (0.01 M).  Pd-M/ZnO catalysts (5 mg) were added in the prepared reaction mixture.   The UV-visible spectra during reduction of
aromatic nitro compounds using Pd-M/ZnO catalysts were recorded
continuously.  In order to compare the
reduction of aromatic nitro compound, the standard aromatic amine compounds
were also measured by UV-visible spectra.


## 2.3.2. Suzuki Reaction

In a 100 mL
three-necked flask, an aryl halide (3.0 mmol), phenyl boronic acid (6.0 mmol),
Pd-M/ZnO catalyst (43.0 mg, ca. 4.0 mmol), and K3PO4 (12.0 mmol) were added in 50 mL EtOH.  Three different types of aryl halides were
tested.  The mixture solution was reacted
at 78°C
for 3 hours under nitrogen atmosphere.  In
order to remove the Pd-M/ZnO catalyst, the reaction solution was filtered
through a Whatman filter paper (no.2).  The filtered solution was filtered again
through 0.45 μm membrane filter (MFS-25 PVDF).
Finally the solvent was evaporated.
The yield was determined by HPLC with a ODS column.


## 2.4. Characterization

UV-visible spectra were measured by using a Shimadzu
UV-160 digital spectrophotometer (Kyoto, Japan) in a 1 cm
quartz cuvette.  Field emission transmission
electron microscopy (FE-TEM) images were collected at 200 kV using a Hitachi HF-2000
TEM equipped with a field emission source.  An energy dispersive X-ray spectrometer (EDXS)
attached to the Hitachi HF-2000 TEM was used to analyze the chemical composition
of the samples.  FE-TEM specimens were
prepared by placing microdrops of colloid solution on a carbon film supported on
copper grids. The elemental analysis of the catalysts was performed by an Ultima-C
Inductively Coupled Plasma-Atomic Emission Spectrometer (ICP-AES, Jobin-Yvon
Co. USA).


## 3. Results and Discussion

### 3.1. Characterization of Pd-M/ZnO Catalysts Prepared byγ-Irradiation

Figure1 shows the TEM image and the EDXS data of the Pd/ZnO catalysts.  The TEM image clearly shows the Pd particles
are successfully loaded on the surface of the ZnO nanopowder.  The presence of Pd is also seen in the
TEM-EDXS spectrum. The formation of Pd nanoparticles from the metal ions can be
explained by the following equation.  In
an aqueous solution, hydrated electrons were generated by γ-irradiation (see (1): (1)H2O→eaq−,H3O+,H·,H2,OH·,H2O2. The Pd ions are reduced
by the hydrated electrons to form metallic Pd (Pd0) (see (2),
which are aggregated to form more stable Pd particles (Pdagg) (see (3): (2)eaq−+Pd2+→Pd0,(3)nPd0→Pd2→Pdn→Pdagg. In (3), n is the number of aggregated Pd0 and Pdagg is the aggregate in
the final stable state.TEM images (a) and EDXS data (b) of the Pd/ZnO catalyst prepared
byγ-irradiation.
(a)(b)Figure2 shows a typical TEM image and the EDXS data of the Pd-Ag/ZnO catalysts.  The TEM image and TEM-EDXS data inform that the
Pd-Ag alloy particles were formed on the surface of ZnO by γ-irradiation.  In Figures 3 and 4, the TEM and TEM-EDXS data
were shown for Pd-Ni/ZnO and Pd-Cu/ZnO catalysts, respectively.  It is seen in both figures that the Pd-M bimetallic
nanoparticles are again successfully loaded on the surface of the ZnO
nanopowder.  It is noted that the loaded
amount of the Pd-Ni/ZnO catalysts (shown in Figure 3) was smaller than that of
the other particles. In order
to determine the amounts of the metallic nanoparticles, an elemental analysis
was performed on the nanoparticles using ICP-AES, and the results are shown in
Table 1.  The Pd content in Pd-Ni/ZnO (5.06%)
is much lower than that in Pd/ZnO (19.4%), Pd-Ag/ZnO (9.73%), and Pd-Cu/ZnO (6.36%).
Also, the Ni content in Pd-Ni/ZnO (0.65%)
is much lower than the Ag content in Pd-Ag/ZnO (8.53%) and the Cu content in
Pd-Cu/ZnO (11.3%).  The results shown in
Table 1 confirm that a lesser amount of Pd-Ni/ZnO nanoparticles was formed than
other nanoparticles. Belloni et al. [18–20] published that the control of
the conditions for the synthesis of the metallic nanoparticle and alloy
clusters.  He concluded that the
reduction of the respective metal ions should be related to the radiation dose,
time, and concentration of metal ions.Table 1
ICP data of Pd-M (M=Ag, Cu, Ni)/ZnO
catalysts prepared byγ-irradiation.
Pd (%)Ag (%)Cu (%)Ni (%)Pd/ZnO19.4Pd-Ag/ZnO9.738.53Pd-Cu/ZnO6.3611.3Pd-Ni/ZnO5.060.650TEM images (a) and EDXS data (b) of the Pd-Ag/ZnO catalyst prepared
byγ-irradiation.
(a)(b)TEM images (a) and EDXS data (b) of the Pd-Ni/ZnO catalyst prepared
byγ-irradiation.
(a)(b)TEM images (a) and EDXS data (b) of the Pd-Cu/ZnO catalyst prepared
byγ-irradiation.
(a)(b)


### 3.2. Catalytic Efficiency of Pd-M/ZnO Catalysts

The ZnO-supported Pd and Pd-M nanoparticles prepared in this study can be
applied as a catalyst in hydrogenation. Figure5 shows the UV-Vis spectra (a)
and absorbance versus time plot (b) of the reduction of 4-nitrophenol by Pd/ZnO
catalysts.  It has been observed that the
intensity at 400 nm due to nitro group peak was decreased, while the intensity at
290 nm due to amine group peak was increased.  This means the formation of 4-aminophenol. Pradhan
et al. [21] published about the reduction of nitro group on 4-nitrophenol by
using sodium borohydride as a hydrogen source in the presence of the Pt-Ni
nanoparticle as catalyst.   Figure 6 exhibited that the absorbance versus time
plot during reduction of 4-nitrophenol using Pd and Pd-M/ZnO (M=Ag, Ni, and Cu)
catalysts.  This data shows reduction
process of 4-nitrophenol on Pd-M/ZnO monitored by UV-Visible spectroscopy.  The order of catalytic efficiency for Pd-M/ZnO
catalysts was as follows: Pd/ZnO > Pd-Ni/ZnO > Pd-Cu/ZnO > Pd-Ag/ZnO.  In a previous paper [22], we also prepared the
PVP stabilized Pd colloids by γ-irradiation in aqueous solution without additional reducing agent.  The Pd colloid was used as catalysts for the
reduction of 2-, 3-,  4-nitrophenol, 2,4,6-trinitrotoluene, and
4-nitrobenzo-15-crown in aqueous solution at room temperature. However, Pd
colloid could not be recycled, while these Pd-M/ZnO catalysts could be recycled.UV-Vis spectra (a) of the reduction of 4-NP and absorbance versus time plot (b) of the reduction of 4-nitrophenol by Pd/ZnO catalyst.
(a)(b)Figure 6
Absorbance versus time plot of reduction of
4-nitrophenol by Pd-M/ZnO catalysts.The ZnO-supported Pd and Pd-M nanoparticles prepared in this study can also
be applied in Suzuki reactions as catalyst.
Table2
shows the catalytic efficiencies (measured by the reaction
yield) of Pd/ZnO and Pd-M/ZnO catalysts in three different Suzuki
reactions.  The reaction yield was
determined by an HPLC with an ODS column, with MeOH as the mobile phase, and a
UV/VIS detector set at 254 nm.  As shown
in Table 2, in the first reaction, the catalytic efficiency (measured by the
yield of product, 2-phenylthiophene) decreases in the order of Pd-Ag/ZnO >
Pd-Cu/ZnO > Pd-Ni/ZnO > Pd/ZnO.  However,
C-C coupling formation was not formed at no. 11–15 in Table 2, whereas the
biboronic compound was mainly obtained.
Figure 7 shows HPLC and GC/Mass data obtained for the reactants and the
products of biphenyl compound in Suzuki reactions.  There are
two important factors, such as catalytic efficiency and recycling, in catalytic
reaction.  Figure 8
show the recycling results of Pd-Ag/ZnO catalysts in the Suzuki reaction.  The product yield of 2-phenylthiophene was
98.9% in the first reaction. After the fifth recycling, the yield was 81.5%.
These results indicate that the ZnO supported Pd and Pd-M nanoparticles prepared
by γ-irradiation were good catalysts in the Suzuki reaction.Table 2
Catalytic efficiency of Pd-M/ZnO
catalysts in Suzuki reaction. Reaction conditions: Pd-M/ZnO catalysts (0.4 mmol), aryl halide (3 mmol), boronic acid (6 mmol), K3PO4 (12 mmol), in EtOH (50 mL), at 78C°, for 3 hours, yield
obtained from HPLC data.
EntryCatalystsYield (%)Aryl haledeBoronic acidProduct1Pd/C99.52Pd/ZnO99.03Pd-Ni/ZnO99.24Pd-Cu/ZnO99.15Pd-Ag/ZnO1006Pd/C95.67Pd/ZnO96.98Pd-Ni/ZnO97.19Pd-Cu/ZnO98.010Pd-Ag/ZnO98.311Pd/C37.012Pd/ZnO38.013Pd-Ni/ZnO42.814Pd-Cu/ZnO44.715Pd-Ag/ZnO40.4HPLC chromatograms of standard (a) and GC/Mass spectra (b) of  biphenyl compound.Recycling results of Pd-Ag/ZnO catalyst.


## 3.1. Characterization of Pd-M/ZnO Catalysts Prepared byγ-Irradiation

Figure1 shows the TEM image and the EDXS data of the Pd/ZnO catalysts.  The TEM image clearly shows the Pd particles
are successfully loaded on the surface of the ZnO nanopowder.  The presence of Pd is also seen in the
TEM-EDXS spectrum. The formation of Pd nanoparticles from the metal ions can be
explained by the following equation.  In
an aqueous solution, hydrated electrons were generated by γ-irradiation (see (1): (1)H2O→eaq−,H3O+,H·,H2,OH·,H2O2. The Pd ions are reduced
by the hydrated electrons to form metallic Pd (Pd0) (see (2),
which are aggregated to form more stable Pd particles (Pdagg) (see (3): (2)eaq−+Pd2+→Pd0,(3)nPd0→Pd2→Pdn→Pdagg. In (3), n is the number of aggregated Pd0 and Pdagg is the aggregate in
the final stable state.TEM images (a) and EDXS data (b) of the Pd/ZnO catalyst prepared
byγ-irradiation.
(a)(b)Figure2 shows a typical TEM image and the EDXS data of the Pd-Ag/ZnO catalysts.  The TEM image and TEM-EDXS data inform that the
Pd-Ag alloy particles were formed on the surface of ZnO by γ-irradiation.  In Figures 3 and 4, the TEM and TEM-EDXS data
were shown for Pd-Ni/ZnO and Pd-Cu/ZnO catalysts, respectively.  It is seen in both figures that the Pd-M bimetallic
nanoparticles are again successfully loaded on the surface of the ZnO
nanopowder.  It is noted that the loaded
amount of the Pd-Ni/ZnO catalysts (shown in Figure 3) was smaller than that of
the other particles. In order
to determine the amounts of the metallic nanoparticles, an elemental analysis
was performed on the nanoparticles using ICP-AES, and the results are shown in
Table 1.  The Pd content in Pd-Ni/ZnO (5.06%)
is much lower than that in Pd/ZnO (19.4%), Pd-Ag/ZnO (9.73%), and Pd-Cu/ZnO (6.36%).
Also, the Ni content in Pd-Ni/ZnO (0.65%)
is much lower than the Ag content in Pd-Ag/ZnO (8.53%) and the Cu content in
Pd-Cu/ZnO (11.3%).  The results shown in
Table 1 confirm that a lesser amount of Pd-Ni/ZnO nanoparticles was formed than
other nanoparticles. Belloni et al. [18–20] published that the control of
the conditions for the synthesis of the metallic nanoparticle and alloy
clusters.  He concluded that the
reduction of the respective metal ions should be related to the radiation dose,
time, and concentration of metal ions.Table 1
ICP data of Pd-M (M=Ag, Cu, Ni)/ZnO
catalysts prepared byγ-irradiation.
Pd (%)Ag (%)Cu (%)Ni (%)Pd/ZnO19.4Pd-Ag/ZnO9.738.53Pd-Cu/ZnO6.3611.3Pd-Ni/ZnO5.060.650TEM images (a) and EDXS data (b) of the Pd-Ag/ZnO catalyst prepared
byγ-irradiation.
(a)(b)TEM images (a) and EDXS data (b) of the Pd-Ni/ZnO catalyst prepared
byγ-irradiation.
(a)(b)TEM images (a) and EDXS data (b) of the Pd-Cu/ZnO catalyst prepared
byγ-irradiation.
(a)(b)


## 3.2. Catalytic Efficiency of Pd-M/ZnO Catalysts

The ZnO-supported Pd and Pd-M nanoparticles prepared in this study can be
applied as a catalyst in hydrogenation. Figure5 shows the UV-Vis spectra (a)
and absorbance versus time plot (b) of the reduction of 4-nitrophenol by Pd/ZnO
catalysts.  It has been observed that the
intensity at 400 nm due to nitro group peak was decreased, while the intensity at
290 nm due to amine group peak was increased.  This means the formation of 4-aminophenol. Pradhan
et al. [21] published about the reduction of nitro group on 4-nitrophenol by
using sodium borohydride as a hydrogen source in the presence of the Pt-Ni
nanoparticle as catalyst.   Figure 6 exhibited that the absorbance versus time
plot during reduction of 4-nitrophenol using Pd and Pd-M/ZnO (M=Ag, Ni, and Cu)
catalysts.  This data shows reduction
process of 4-nitrophenol on Pd-M/ZnO monitored by UV-Visible spectroscopy.  The order of catalytic efficiency for Pd-M/ZnO
catalysts was as follows: Pd/ZnO > Pd-Ni/ZnO > Pd-Cu/ZnO > Pd-Ag/ZnO.  In a previous paper [22], we also prepared the
PVP stabilized Pd colloids by γ-irradiation in aqueous solution without additional reducing agent.  The Pd colloid was used as catalysts for the
reduction of 2-, 3-,  4-nitrophenol, 2,4,6-trinitrotoluene, and
4-nitrobenzo-15-crown in aqueous solution at room temperature. However, Pd
colloid could not be recycled, while these Pd-M/ZnO catalysts could be recycled.UV-Vis spectra (a) of the reduction of 4-NP and absorbance versus time plot (b) of the reduction of 4-nitrophenol by Pd/ZnO catalyst.
(a)(b)Figure 6
Absorbance versus time plot of reduction of
4-nitrophenol by Pd-M/ZnO catalysts.The ZnO-supported Pd and Pd-M nanoparticles prepared in this study can also
be applied in Suzuki reactions as catalyst.
Table2
shows the catalytic efficiencies (measured by the reaction
yield) of Pd/ZnO and Pd-M/ZnO catalysts in three different Suzuki
reactions.  The reaction yield was
determined by an HPLC with an ODS column, with MeOH as the mobile phase, and a
UV/VIS detector set at 254 nm.  As shown
in Table 2, in the first reaction, the catalytic efficiency (measured by the
yield of product, 2-phenylthiophene) decreases in the order of Pd-Ag/ZnO >
Pd-Cu/ZnO > Pd-Ni/ZnO > Pd/ZnO.  However,
C-C coupling formation was not formed at no. 11–15 in Table 2, whereas the
biboronic compound was mainly obtained.
Figure 7 shows HPLC and GC/Mass data obtained for the reactants and the
products of biphenyl compound in Suzuki reactions.  There are
two important factors, such as catalytic efficiency and recycling, in catalytic
reaction.  Figure 8
show the recycling results of Pd-Ag/ZnO catalysts in the Suzuki reaction.  The product yield of 2-phenylthiophene was
98.9% in the first reaction. After the fifth recycling, the yield was 81.5%.
These results indicate that the ZnO supported Pd and Pd-M nanoparticles prepared
by γ-irradiation were good catalysts in the Suzuki reaction.Table 2
Catalytic efficiency of Pd-M/ZnO
catalysts in Suzuki reaction. Reaction conditions: Pd-M/ZnO catalysts (0.4 mmol), aryl halide (3 mmol), boronic acid (6 mmol), K3PO4 (12 mmol), in EtOH (50 mL), at 78C°, for 3 hours, yield
obtained from HPLC data.
EntryCatalystsYield (%)Aryl haledeBoronic acidProduct1Pd/C99.52Pd/ZnO99.03Pd-Ni/ZnO99.24Pd-Cu/ZnO99.15Pd-Ag/ZnO1006Pd/C95.67Pd/ZnO96.98Pd-Ni/ZnO97.19Pd-Cu/ZnO98.010Pd-Ag/ZnO98.311Pd/C37.012Pd/ZnO38.013Pd-Ni/ZnO42.814Pd-Cu/ZnO44.715Pd-Ag/ZnO40.4HPLC chromatograms of standard (a) and GC/Mass spectra (b) of  biphenyl compound.Recycling results of Pd-Ag/ZnO catalyst.


## 4. Conclusion

The Pd/ZnO and Pd-M/ZnO catalysts
(M=Ag, Cu, and Ni) were one-pot synthesized byγ-irradiation
in an MeOH-water mixture without a reducing agent.  TEM, TEM-EDXS, and ICP-AES data showed that the
Pd and Pd-M (M=Ag, Cu, and Ni) nanoparticles were successfully loaded on the
surface of the ZnO nanopowder.   The
catalytic efficiencies of the Pd-M/ZnO catalysts were examined in hydrogenation
and Suzuki reactions.  In hydrogenation
of 4-nitrophenol, the catalytic efficiency of Pd and Pd-M/ZnO catalysts was similar.  In the Suzuki reactions, the catalytic
efficiency (measured by the yield of the product) decreases in the order of Pd-Ag/ZnO
> Pd-Cu/ZnO > Pd-Ni/ZnO > Pd/ZnO.
In comparison of the catalytic efficiency between Pd-M/ZnO and the commercial
Pd/C catalyst, the catalytic efficiency of Pd and Pd-M/ZnO catalysts prepared
by γ-irradiation was higher than that of the commercial
Pd/C.   The heterogeneous Pd-M/ZnO catalysts prepared
by γ-irradiation can be used for carbon-carbon coupling
reactions such as Still-, Heck-, and Sonogashira-reactions.



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*Source: 302919-2009-05-31.xml*