Synthesis and characterization of silica-supported Pd nanoparticles and its application in the Heck reaction

Article abstract of DOI:10.1016/j.cclet.2012.06.016

Well-dispersed palladium nanoparticles immobilized onto modified silica (SiO₂-pr-NH-cyanuric-SH) have been prepared in some facile steps. The catalyst exhibits high catalytic activity in the Heck reaction, and can be easily recovered and reused without significant loss of its activity in several runs.

Full text of DOI:10.1016/j.cclet.2012.06.016

Available online at www.sciencedirect.com
Chinese Chemical Letters 23 (2012) 887–890
www.elsevier.com/locate/cclet
Synthesis and characterization of silica-supported Pd nanoparticles
and its application in the Heck reaction
b
Mehran Ghiaci a, Donya Valikhani a, , Zahra Sadeghi
*
a Department of Chemistry, Isfahan University of Technology, 8415683111, Iran
b Department of Chemistry, Payame Noor University, 19395-4697 Tehran, Iran
Received 21 February 2012
Available online 4 July 2012
Abstract
Well-dispersed palladium nanoparticles immobilized onto modified silica (SiO2-pr-NH-cyanuric-SH) have been prepared in
some facile steps. The catalyst exhibits high catalytic activity in the Heck reaction, and can be easily recovered and reused without
significant loss of its activity in several runs.
# 2012 Donya Valikhani. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Nanoparticles; Palladium; Heck reaction
1. Introduction
Metal nanoparticles exhibit unique and considerably changed physical and chemical properties compared to bulk
ones, because of their large surface area to volume ratio. But the liquid suspensions of metal nanoparticles have some
drawbacks in chemical reactions such as aggregation, recycling and separation. Thus, recent studies focused on
immobilizing metal nanoparticles on suitable materials. Inorganic solids such as silica, carbon, clay, zeolites and TiO2
can be used as appropriate supports to carry metal nanoparticles. Among the inorganic supports, silica is one of the
most utilized supports for metal nanoparticles immobilization. This is mainly because of its special properties, such as
high surface area, inertness, excellent stability (chemical and thermal), its low cost and the fact that organic groups can
be easily anchored to the surface to provide catalytic centers [1].
Among metal nanoparticles, transition metal nanoparticles are in thefocus ofinterest. They have highcatalytic activity in
organic reactions [2–4]. Palladium as catalyst plays an important role in hydrogenation reaction [5,6], oxidation reaction [7]
and carbon-carbon coupling reactions, such as the Heck-, Sonogashira-, Suzuki-, Stille-coupling reactions [8–12].
One of the approaches for heterogenization of Pd nanoparticles is to functionalize the surface of the support with suitable
ligands such as phosphines [13], N-heterocyclic carbenes [14] and thiol [15]. Thiol-modified surfaces have been previously
metalatedwithpalladiumandusedinC–Ccouplingreactions.Actually,therearemanypublicationsintheliteraturesuggested
that palladium can be bound to an immobilized thiol surface and used as catalyst in C–C coupling reactions [16,17].
Herein, we present a new method to immobilize palladium nanoparticles on modified silica [SiO2-pr-NH-cyanuric-
SH] and survey its catalytic activity in the Heck reaction.
* Corresponding author.
E-mail address: d.valikhani@ch.iut.ac.ir (D. Valikhani).
1001-8417/$ – see front matter # 2012 Donya Valikhani. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
http://dx.doi.org/10.1016/j.cclet.2012.06.016

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M. Ghiaci et al. / Chinese Chemical Letters 23 (2012) 887–890
2. Experimental
Immobilization of palladium nanoparticles on silica was followed as shown in Scheme 1. Amino-functionalized
silica [18] (1.0 g) was refluxed with 0.29 g (1.57 mmol) cyanuric chloride in dry toluene (30 mL) for 24 h. The solid
was filtered and washed off with dry toluene and dried at room temperature under vacuum [19]. After that, 0.25 mL
(3.46 mmol) 1,2-ethanedithiol and 1.0 g SiO2-pr-NH-Cyanuric-Cl in 30 mL of dry toluene were refluxed for 24 h. The
solution was filtered and washed off with ethanol and dried at room temperature. The modified silica was characterized
by elemental analysis (CHNS) (C: 9.19%, H: 1.79%, N: 5.03%, S: 1.23%). At the end, in a 100 mL round-bottom flask
with a magnetic stirring bar, 0.02 g Pd (OAc)2 was dispersed in the 50 ml of dry THF for 30 min, then 1.0 g
functionalized silica (SiO2-pr-NH-cyanuric-SH) was added to the mixture and stirred for 6 h at room temperature.
After filtration, the solid was washed with dry THF several times, and then dried at room temperature. The palladium
content of the catalyst based on inductively coupled plasma (ICP) is estimated to be 0.153 mmol/g.
The activity of the SiO2-pr-NH-cyanuric-SH-Pd catalyst for the Heck reaction under a variety of conditions is reported
in Table 1. All reactions were carried out as follows: 1 mmol iodobenzene, 1.5 mmol of terminal alkenes, 2 mmol of
sodiumacetate, 5 mLofDMFandthe preparedcatalystweremixedinasealedreactor. Thereactionmixturewaspouredin
a Teflon lined stainless steel hydrothermal bomb, and heated at the desired temperature in an oil bath and stirred for 24 h.
At last, the bomb was allowed to cool to room temperature and the solution was separated from the catalyst by
centrifugation. The reaction was analyzed by GC and the products were analyzed using GC–MS. For recycling of the
catalyst, after the first run, it was washed by water and CH2Cl2, and dried at room temperature. To show the catalytic
activity of the present Pd nanoparticles, Pd complex in bulky form was synthesized via this method; 0.2 g Pd(OAc)2 was
added tothe mixture of modified silica (1.0 g)and methanol, the mixturewas refluxed for 24 h. Then, the solid was filtered
and washed off with methanol and dried at room temperature under vacuum. Then it was used in Heck reaction. During
reflux, the Pd particles were aggregated and became bulk on modified silica, so we must use more amount of palladium
than we used for our nanocatalyst to obtain high conversion, and this was not beneficial from the economic view.
3. Results and discussion
It is not desirable to prepare palladium catalyst by impregnation silica with a palladium salt aqueous solution,
because the palladium particles are randomly dispersed on the surface of silica with broad size distribution, therefore
the catalytic activities decrease. So we used thiol-modified silica (SiO2-pr-NH-Cyanuric-SH) for preparation of
supported palladium catalyst. The necessary ligand was first synthesized from amino-functionalized silica, cyanuric
]
Cl
N
Cl
MeO
MeO
MeO
N
N
Si
NH2
OH
O
Cl
O
Si
NH2
OH
toluene, reflux
O
toluene, reflux
OH
SH
SH
Cl
S
N
S
N
N
SH
N
O
O
O
SH
O
O
O
Si
NH
N
Si
NH
toluene, reflux
N
Cl
SH
S
Pd(OAc)2
THF
N
O
O
O
Pd(OAc)2
Si
NH
N
N
S
SH
Scheme 1. The preparation of the catalyst.

M. Ghiaci et al. / Chinese Chemical Letters 23 (2012) 887–890
889
Table 1
Heck olefination of iodobenzene by the SiO2-pr-NH-cyanuric-SH-Pd.a
R
I
SiO2-pr-NH-cyanuric-SH-Pd
DMF, NaOAc, heating
[TD$INLINE]
R
Entry
R
Time (h)
Temperature (8C)
Amount of Pd (mol%)
Conversion (%)b
Selectivity (%)c
1
2
COOMe
COOMe
COOMe
COOMe
COOMe
COOMe
COOMe
Phenyl
24
24
24
24
24
24
18
24
24
24
70
100
120
130
130
130
130
130
130
130
5 Â 10À4
5 Â 10À4
5 Â 10À4
5 Â 10À4
2 Â 10À4
3 Â 10À4
5 Â 10À4
5 Â 10À4
5 Â 10À4
5 Â 10À4
0
0
0
0
3
40
100
100
100
100
100
88
4
100
43
5
6
63
7
36
8
100
100
91
9
COOt-Bu
COMe
100
100
10
a
Reaction conditions: 1 mmol iodobenzene, 1.5 mmol terminal alkenes, 2 mmol sodium acetate, 5 mL DMF.
The conversion of reactant was determined by gas chromatography measurement.
The selectivity is the ratio of trans-isomer/(cis-isomer + trans-isomer).
b
c
mentioned above. Parallel to these syntheses, the sample were studied by TEM that shows (Fig. 1) the distribution of
Pd nanoparticles in the range of 20–30 nm size on the surface of modified silica. A FT-IR spectrum was recorded for
confirmation after each step of functional group addition on the surface of silica (Fig. 2). The band at around
1100 cmÀ1 is due to the stretching vibration of Si–O–Si units of the silica. The bands at 2921–2940 and 2840–
2870 cmÀ1 are assigned to the stretching modes of –CH2 groups. From the presence of these bands, and the N–H
deformation peak at 1540–1560 cmÀ1, it can be deduced that the silica surface is modified by amine spacer groups
successfully. Peaks that appeared in the 1500–1700 cmÀ1 range, belong to the skeletal vibration of the cyanuric ring.
The S–H vibration band does not appear clearly because of its low content.
Different conditions were tested for the Heck reaction (Table 1). Full conversion and selectivity was obtained using
only 0.03 g catalyst (5 Â 10À4 mol% Pd) at 130 8C (entries 4 and 9). Also the full conversion and selectivity of 88%
for the reaction of styrene with iodobenzene were achieved in the same condition (entry 8).
It is believed that, during the reaction, the Pd (II) complexes present in the catalyst are reduced to Pd (0) which can
interact with each other under the condition of the reaction to form Pd clusters [20]. Regard to the result of our study, we
suggest that the immobilization of Pd on modified silica (SiO2-pr-NH-cyanuric-SH) restricts the moving of reactive
forms of the Pd (II) and Pd (0) complexes, so the intermolecular interaction of unstable Pd intermediates can be retarded.
The Heck reaction of methyl acrylate and iodobenzene with 0.03 g catalyst at 130 8C for 24 h was performed for 3
times without any change in the results (100% conversion). At this stage of work, we tried to determine the Pd in
[(Fig._1)TD$FIG]
Fig. 1. TEM micrographs of the Pd nanoparticles on modified silica.

[(Fig._2)TD$FIG]
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M. Ghiaci et al. / Chinese Chemical Letters 23 (2012) 887–890
Fig. 2. FT-IR of (a) SiO2, (b) SiO2-pr-NH2, (c) SiO2-pr-NH-cyanuric-Cl, and (d) SiO2-pr-NH-cyanuric-SH.
solution after removal of the solid catalyst by centrifugation. We used inductively coupled plasma optical emission
spectroscopy for measuring palladium in solution. The palladium content in solution was not appreciable, to think of
homogeneous reaction. Therefore, loss of activity might be due to aggregation of nanoparticles or leaching of Pd from
the active sites of the catalyst, and reprecipitation on the surface of silica.
In conclusion, we have presented a new method for preparing silica-supported palladium nanocatalyst. Although,
the loading amount of palladium is less than other works, the results demonstrate clearly the applicability of this new
heterogeneous SiO2-pr-NH-Cyanuric-SH-Pd catalyst in the Heck reaction. Also, the high reusability shows that thiol
ligand is one of the best ligands to anchore palladium and prevents the agglomeration of palladium particles during the
reactions. Further efforts are currently underway in our laboratory to test this catalyst in other coupling reactions such
as the Sonogashira and Suzuki reactions.
Acknowledgments
Thanks are due to the Research Council of Isfahan University of Technology and Iran nanotechnology Initiative
Council for supporting of this work.
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