Synthesis of PEG Supported Palladium Nanoparticles: Simple and

recyclable palladium catalytic system for Heck reaction

P. Ahmadian Namini, A. A. Babaluo* , M. Tahmasebpour

Nanostructure Materials Research Center (NMRC), Sahand University of Technology,

P.O. Box 51335/1996, Tabriz, I.R.Iran

Research Center of Polymeric Materials, Sahand University of Technology,

P.O. Box 51335/1996, Tabriz, I.R. Iran.

Abstract

Palladium nanoparticles with narrow size distribution as a catalytic system for Heck

reaction were synthesized by applying poly(ethyleneglycol) (PEG) and Pd(OAc)2. PEG

acts as a reducing agent for reduction of Pd2+ to Pd and also as a stabilizer for synthesized

palladium nanoparticles. Both XRD and TEM results exhibit production of nanoparticles

with about 7nm in diameter. This polymeric palladium catalyst is relatively inexpensive,

environmentally benign, and simply functionalizable and can be easily recovered from

the products.

Keywords: Palladium; nanoparticle; PEG; Heck reaction; catalyst

Introduction

Heck reaction is one of the most versatile and useful tools in organic synthesis.

From the corresponding alkene and aryl compounds, the Heck reaction provides a direct

route to synthesize the important olefins [1]. The Heck coupling, one of the most

important palladium catalyzed CC bond formation reactions, is a process described as

olefin arylation (Scheme 1) [15].

+ R RRX R

Scheme 1.

* E-mail: a.babaluo@sut.ac.ir

[Pd]

-HX

Arylated olefins, the final products of Heck reaction, have very broad application

in synthesis of pharmaceuticals, agrochemicals, and natural products [4,5]. Therefore,

very intensive research is addressed to elaboration of new, simple and efficient catalytic

systems for Heck coupling reaction [5].

In recent years, there are growing interests on the catalytic properties of transition

metal nanoparticles because of their large surface area and a great ratio of atoms

remaining at the surface. Although many researches have been done, there is still the

paramount challenge for the wide application of transition metal nanoparticle as catalysts

in the industry, i.e. how to separate and recycle them completely from the products. One

approach to separate and recycle metal nanoparticles is to immobilize them onto a

polymeric or inorganic support, making it really easy and simple to separate the catalyst

from the reaction products mixture [6].

In this context, using polymers that are soluble in some solvents but insoluble in

others is gaining increasing importance as scaffolds on which to attach reagents and

catalysts. This methodology allows the reaction to be conducted in a homogeneous phase

using a solvent that dissolves the polymer as a medium, and after completion of the

reaction, the catalytic system can be recovered by precipitation, adding a second solvent

in which the polymer becomes insoluble [7].

Polyethylene glycol (PEG) has been emerging as a very convenient support for

the synthesis of a variety of catalysts, ligands and organic compounds. It is relatively

inexpensive, environmentally benign and can readily be functionalized. It is soluble in

many organic solvents but insoluble in some others such as diethyl ether. These

properties render it very attractive as a mobile support wherein a reaction catalyzed by

the PEG supported catalyst. This reaction can be carried out under homogeneous

condition while the PEG-catalyst system can be easily recovered after extracting the

reaction mixture with diethyl ether [8]. Herein, we report a new and facile route for

preparation of nano-Pd by exploiting PEG, which was found to act as both reducing agent

and stabilizer [1].

2. Experimental

2.1. Materials

The characteristics of the materials used in this work are given in Table 1.

Table 1. Characteristics of materials

Materials Function Molecular formula Characteristics Supplier

Palladium acetate Palladium Precursor C4H6O4Pd

Solid Brown Powder

Mw: 224.49 Merck1

Poly(ethyleneglycole) Reducing agent HO(C2H4O)nH

Ave. Molecular Weight:

2000, 6000 and 35000 Merck1

Methane Dichloride Solvent CH2Cl2 Transparent Liquid Merck1

1 E.Merck, D 6100 Darmstadt, Germany.

2.2. Preparation of nano-Pd

0.05 gr palladium acetate was added into 4 gr PEG 6000 [9] at 80 oC by magnetic

stirring. The resulting light yellow homogeneous solution was further stirred for 2 h at the

same temperature. During this process the color of the solution changed from light yellow

to brown and finally turned black, indicating the formation of nano-Pd. The process then

followed by ultrasonic irradiation of solution for 1 h at the same temperature. Then the

mixture of PEG and palladium nanoparticles was cooled immediately to ambient

temperature to form a solidified mixture.

2.3. Characterization

The X-ray Diffraction (XRD) patterns of Pd nanoparticles were determined on a

TW3710 Philips XPert diffractometer using CuK as radiation and CuK as a filter (=1.54 ). Data were collected in the range 5 2 80 with a 0.02 2-step and 2 sec per step (40 kV and 30 mA). The average size of crystallites was calculated from the

peak broad (111) by using the DebyeScherrer equation [10]. The transmission electron

micrographs (TEM) were obtained by employing CM-200 FEG Philips microscope. TEM

samples were prepared by solving Pd/PEG in CH2Cl2 and then drops of prepared

colloidal solution were placed onto a carbon-coated copper grid.

40 38 44 36 42

2

3. Results and discussion

3.2. Crystal structure

The crystalline properties of the prepared samples were investigated by XRD. The

result of XRD pattern was shown in Fig. 3(a). The two prominent peaks of PEG (at 2 =

19.2 and 23.4) were presented in this pattern, indicating the presence of pure polymer,

also the characteristic peak of Pd (about 2 =40) was presented [11]. In addition, the

standard XRD pattern of the palladium nanoparticles was shown in Fig. 3(b). The

obtained cluster size of palladium nanoparticles using the Scherrer formula showed that

the synthesized nanoparticles size is less than 10 nm.

a

0 10 20 30 40 50 60 70 80 90

2

PEG

38 39 40 41 42 43 44 45 46

2

Inte

nsity

b

Fig. 3. (a) XRD pattern of the synthesized Pd nanoparticles by 0.05 gr Pd(OAc)2 into 4 gr

PEG 6000, (b) standard XRD pattern for palladium [12].

3.3. Particle size and morphology

Fig. 4 presents the TEM image of palladium nanoparticles prepared by adding

0.05 gr Pd(OAc)2 into 4 gr PEG 6000. TEM image of this sample exhibits palladium

nanoparticles with narrow size distribution and average dimensions about 7nm.

Fig. 4. TEM images of Pd nanoparticles prepared at 0.05 gr of Pd(OAc)2

Conclusion

The obtained results are presented as follow:

Polymer supported nano-Pd catalyst was synthesized by applying poly(ethyleneglycol) (PEG) and Pd(OAc)2 as a catalyst system for Heck reaction.

The obtained results of XRD confirmed the formation of palladium nanoparticles with cluster size less than 10nm.

TEM image showed palladium nanoparticles with narrow particle size distribution and average dimension 7nm.

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