Highly efficient and stable palladium nanocatalysts supported on an ionic liquid-modified xerogel

Article abstract of DOI:10.1039/b814559h

Incorporation of a phosphorylated ionic liquid as both a complexing and reducing agent for Pd(II) in a xerogel results in the formation of highly dispersed, uniformly sized Pd nanocatalysts, tightly supported on the surface of the silica and not embedded

Full text of DOI:10.1039/b814559h

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Highly efficient and stable palladium nanocatalysts supported on an ionic
liquid-modified xerogelw
Afsaneh Safavi,*ab Norouz Maleki, Nasser Iranpoor, Habib Firouzabadi,
ab
a
a
ab
a
ab
Ali Reza Banazadeh, Roya Azadi and Fatemeh Sedaghati
Received (in Cambridge, UK) 20th August 2008, Accepted 12th September 2008
First published as an Advance Article on the web 15th October 2008
DOI: 10.1039/b814559h
9
Incorporation of a phosphorylated ionic liquid as both a com
plexing and reducing agent for Pd(II) in a xerogel results in the
formation of highly dispersed, uniformly sized Pd nanocatalysts,
tightly supported on the surface of the silica and not embedded in
the bulk of the xerogel.
bone. However, most of these supports are fragile and cannot
withstand thermal activation or regeneration or their prepara-
tion is quite tedious. Also, the encapsulation of palladium
nanoparticles in silicates or other solid substrates causes the
active surface area of the nanoparticle to be very small which
results in a decrease in their catalytic activity. This is the main
disadvantage of the above procedures. Previous works on
encapsulation of metal nanodispersions in sol–gel-derived
silicates necessitate avoiding aggregation and precipitation of
the metal salt or metallic particles in the organic sol–gel
precursors and during the various stages of sol–gel polymer-
ization, drying, and aging. Several general schemes were
reported for the encapsulation of metallic nanoparticles in
sol–gel matrices. Sequential reduction of metal salt and sol–
The application of metal nanoparticles in catalysis has been an
important frontier of research in recent years with numerous
review articles published during the last decade in both
1
2
heterogeneous and homogeneous catalysis. Heterogeneous
catalysts are attractive for both laboratory and large scale
processing. In fact, one of the challenging areas of research is
heterogeneous catalysis with nanocatalysts supported on a
solid surface. The possibility of the use of supported nano-
catalysts in many areas of research, particularly in flow
systems, offers not only the advantages of ease of automation,
1
0
gel processing in inverse micelle solutions were used for
the preparation of metal–silicate nanodispersions. How-
ever, the presence of gel precursors destabilizes the inverse
micelles resulting in bigger particles and wider particle size
3
reproducibility, speed and safety, but also the fact that no
purification steps are usually needed and flow reactors can be
used repeatedly, thus lowering the production costs.
1
0
distribution.
Among different metal nanocatalysts, palladium nano-
particles have gained much reputation. This is because
palladium is a versatile catalyst in modern organic synthesis
and is widely used for a significant number of synthetic
In the search for an ideal support for Pd nanoparticles
without such complications, we now report on the use of a
phosphorylated ionic liquid (IL) modified xerogel which could
be evenly coated on glass slides. As has been reported for other
4
2b,5
11
transformations such as Mizoraki–Heck,
5
Suzuki-cross
Stille and Sonogashira coupling reactions.
immidazolium ILs, incorporation of this IL (I) in sol–gel
b,c,6
7
7c
coupling,
films imparts considerable transparency, elasticity and reduced
gel shrinkage to the silica matrix leading to the ease of evenly
coating the sol–gel material on glass slides. It has been
However, its use is often complicated by issues surrounding
the separation of the palladium and ligands from the desired
product while reuse, ideally without loss of catalytic activity,
becomes a major practical and economic issue with respect to
large-scale applications. For this reason there has been much
effort to immobilize palladium nanoparticles on suitable
support surfaces. The most generally utilized methods for
immobilization are physical adsorption in which palladium
nanoparticles are supported on solid surfaces (e.g. silica,
1
2
reported that immidazolium based ILs form worm-like self
assebmblies in sol–gel matrices through p–p stacking inter-
actions of the neighboring imidazolium rings.
1
2
8
alumina or charcoal). Over the past few years, a number of
groups have immobilized a variety of palladium catalysts on a
range of other insoluble supports, e.g. by coordination
through resin-bound ligands or encapsulation in which
palladium nanoparticles are trapped within a polymer back-
Herein we report the development of a novel method for the
deposition of palladium nanoparticles on silica substrates.
In this regard, we elucidated the effect of addition of a newly
synthesized ionic liquid containing a phosphinite group
a
Department of Chemistry, College of Sciences, Shiraz University,
Shiraz, 71454, Iran
Nanotechnology Research Institute, Shiraz University, Shiraz, Iran.
E-mail: safavi@chem.susc.ac.ir; Fax: +98-711-2286008;
Tel: +98-711-613751
b
13
(I) attached to the silica matrix.
In contrast to the previous methods for immobilization of
nanoparticles in silica matrices in which palladium nano-
particles were either added to the sol mixture during sol–gel
preparation, or prepared during sol formation by calcinations
w Electronic supplementary information (ESI) available: Experimental
details; absorbance spectra and AMF images of the Pd nanoparticles.
See DOI: 10.1039/b814559h
This journal is ꢀc The Royal Society of Chemistry 2008
Chem. Commun., 2008, 6155–6157 | 6155

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and reduction processes, which result in dispersed nano-
particles inside a composite glass, here the xerogel containing
self assemblies of the ionic liquid with a phosphinite group is
first prepared and is then contacted with a solution of
palladium acetate. The phosphinite group present in the IL
acts both as a complexing and reducing agent for Pd(II). It has
been established that a Pd(0) complex is spontaneously formed
from reaction of Pd(II) and phosphine ligands by an irre-
versible intramolecular reduction of a Pd(II) complex by the
Table 1 Suzuki and Heck reactions of aryl halides catalyzed by
palladium nanoparticles
1
4
phosphine.
The important role of the presence of the ionic liquid
containing a phosphinite group in a xerogel was further
elucidated by substitution of the IL with 4-aminophenyl
diphenyl phosphinite. In the latter case the formed xerogel
did not have the ideal physical properties of a xerogel prepared
with an IL and was not suitable for the formation of Pd
nanoparticles.
a
Entry Ar–X
1
Product
t/h Yield
%)
(
9
95
2
3
10 89
The synthesis of phosphine stabilized palladium nano-
particles supported on silica was carried out using the
following procedure. The xerogel was prepared by mixing
tetraethoxysilane (TEOS) (1 ml), HCl (0.5 ml of 0.001M)
and 0.1 g of IL powder. 1.5 ml of ethanol solution was added
to this solution and the mixture was stirred for 30 min at room
temperature. The solution was then allowed to gelate at room
temperature for 24 h. The formed xerogel was deposited on a
glass slide (activated with concentrated nitric acid) by immer-
sion of the glass into the gel for 15 s and drying at room
temperature under an ambient atmosphere for several days.
Palladium nanoparticles were then supported on the xerogel
by contacting (for 1 h), the glass slide with a solution of
palladium acetate. Fig. 1 shows the occurrence of highly
dispersed, uniformly sized Pd nanoparticles with an average
size of 70 nm. The amount of Pd nanoparticles on the glass
5
8
88
84
4
5
15 95
6
7
15 88
slide was determined by the ICP method and was found to be
À2
2
86 Æ 4 ng cm .
Further support for the association of Pd nanoparticles with
10 85
the surface of the glass slide was obtained by UV-Vis spectro-
scopy (Fig. 1S, ESIw) and AFM experiments (Fig. 2S, ESIw).
The absence of the peak at 420 nm due to Pd(II) reveals
complete conversion of Pd(II) to Pd(0). Also the AFM images
show that the sol–IL functionalized surface is covered with
tightly bound Pd nanoparticles.
a
1
Isolated yield. Purity was confirmed by H NMR.
The Pd nanoparticles thus obtained were found to be highly
active, recyclable, and stable catalysts for wide applications.
As some examples, the high catalytic effects of these nano-
catalysts are presented in Suzuki and Heck coupling reactions
in both organic solvents such as ethanol or DMF and also in
water. Some representative reactions in ethanol are shown in
Table 1. More interestingly, the Pd nanoparticles were found to
be remarkably stable even under the high temperatures of reflux
processes. The reusability of the Pd nanoparticles was tested
for the Suzuki reaction between boromobenzene and phenyl
boronic acid. A clear stability in activity was observed with the
number of cycles up to 5 runs (RSD = 2.9%), (Table 1S, ESIw).
The SEM images before and after the reactions show no
considerable change in the morphology of the Pd nanoparticles.
Thus, there is virtually no leaching of any Pd species.
The high catalytic effect of the Pd nanoparticles was also
demonstrated on the chemiluminescent reaction of lumigen
PS-atto and H
2 2
O which was chosen as a model system
(Fig. 2). In this case the nanocatalyst can again be used several
times without deterioration of its catalytic activity.
Fig. 1 SEM images of glass slides contacted with Pd(II) solution: (a) a
glass slide covered with xerogel with no IL and (b) a glass slide covered
with IL-modified xerogel.
The important feature of these highly dispersed supported
Pd nanoparticles is that the nanoparticles are tightly
6
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G. Machado, S. I. Wolke and J. Dupont, J. Am. Chem. Soc., 2005,
127(10), 3298–3299; (c) V. P. Ananikov, N. V. Orlov,
I. P. Beletskaya, V. N. Khrustalev, M. Y. Antipin and
T. V. Timofeeva, J. Am. Chem. Soc., 2007, 129(23), 7252–7253;
(
d) F. J. Ibanez and F. P. Zamborini, J. Am. Chem. Soc., 2008,
30(2), 622–633.
1
3
4
G. Jas and A. Kirschning, Chem.–Eur. J., 2003, 9, 5708–5723.
M. Moreno-manas and R. Pleixats, Acc. Chem. Res., 2003, 36,
6
38–643.
5
(a) N. Nikbin, M. Ladlow and S. V. Ley, Org. Process Res. Dev.,
2007, 11, 458–462; (b) Z. Zhang and Z. Wang, J. Org. Chem., 2006,
2 2
Fig. 2 Chemiluminescent intensity for the lumigen PS-atto–H O
7
1(19), 7485–7487; (c) B. Baruwati, D. Guin and S. V. Manorama,
system in the presence (a) and absence (b) of Pd nanoparticles.
Org. Lett., 2007, 9(26), 5377–5380; (d) M. T. Reetz and
E. Westermann, Angew. Chem., Int. Ed., 2000, 39, 165–168.
X. Chen, Y. Hou, H. Wang, Y. Cao and J. He, J. Phys. Chem. C,
supported on the surface of the silica and not embedded in the
bulk of the xerogel. Thus, a larger active surface area of the
catalyst is exposed on the xerogel. Due to this feature,
mesoporous silica with nanoparticles can be successfully used
as a catalyst in reactions where nanoparticles embedded in
polymeric systems cannot be employed. The possibility of
coating IL-modified xerogels, with high elasticity and low gel
shrinkage, on a glass slide greatly facilitates handling and later
separation of the catalyst from the reaction medium.
Moreover, stabilization of nanoparticles in silica-based ma-
trices provides a promising way to benefit from the advantages
of nanocrystalline dispersions for optical and catalytic appli-
cations and still enjoy the convenience of solid handling.
The authors wish to express their gratitude to the Third World
Academy of Sciences, Iran Chapter (TWASIC) and Shiraz Uni-
versity Research Council for the support of this work and to Prof.
H. Akhavan Tafti for the kind donation of Lumigen PS-atto.
6
7
2
008, 112(22), 8172–8176.
(a) J. C. Garcia-Martinez, R. Lezutekong and R. M. Crooks,
J. Am. Chem. Soc., 2005, 127(14), 5097–5103; (b) V. Calo,
A. Nacci, A. Monopoli and F. Montingelli, J. Org. Chem., 2005,
7
0(15), 6040–6044; (c) B. M. Choudary, S. Madhi, N. S. Chowdari,
M. L. Kantam and B. Sreedhar, J. Am. Chem. Soc., 2002, 124,
4127–14136.
1
8 (a) S. S. Lee, B.-K. Park, S.-H. Byeon, F. Chang and H. Kim,
Chem. Mater., 2006, 18(24), 5631–5633; (b) V. Sanchez-Escribano,
L. Arrighi, P. Riani, R. Marazza and G. Busca, Langmuir, 2006,
2
2(22), 9214–9219.
9
(a) C. Y. K. Lee, A. B. Holmes, S. V. Ley, I. F. McConvey, B. Al-
Duri, G. A. Leeke, R. C. D. Santos and J. P. K. Seville, Chem.
Commun., 2005, 2175–2177; (b) K. Okamoto, R. Akiyama,
H. Yoshida, T. Yoshida and S. Kobayashi, J. Am. Chem. Soc.,
2005, 127, 2125–2135; (c) R. Akiyama and S. Kobayashi, Angew.
Chem., Int. Ed., 2001, 40, 3469–3471.
10 A. Martino, S. A. Yamanaka, J. S. Kawola and D. L. Loy, Chem.
Mater., 1997, 9, 423–429.
1
1 A. Safavi and M. Bagheri, J. Mater. Chem., 2007, 17,
674–1681.
2 Y. Zhou, J. H. Schattka and M. Antonietti, Nano Lett., 2004, 4(3),
1
Notes and references
1
1
(a) M. Zhang and W. Zhang, J. Phys. Chem. C, 2008, 112(16),
245–6252; (b) C. R. Henry, Appl. Surf. Sci., 2000, 164, 252–259;
c) B. Brandt, T. Schalow, M. Laurin, S. Schauermann, J. Libuda and
H.-J. Freund, J. Phys. Chem. C, 2007, 111, 938–949; (d) S. Mandal,
D. Roy, R. V. Chaudhari and M. Sastry, Chem. Mater., 2004, 16,
477–481.
6
(
13 (a) N. Iranpoor, H. Firouzabadi and R. Azadi, Tetrahedron Lett.,
2006, 14, 5531–5534; (b) N. Iranpoor, H. Firouzabadi and
R. Azadi, Eur. J. Org. Chem., 2007, 13, 2197–2201;
(c) N. Iranpoor, H. Firouzabadi and R. Azadi, J. Organomet.
Chem., 2008, 693, 2469–2472.
14 (a) C. Amatore, A. Jutand and M.-A. M’Barki, Organometallics,
1992, 11, 3009–3013; (b) C. Amatore, E. Carre, A. Jutand and
M.-A. M’Barkii, Organometallics, 1995, 14, 1818–1826.
3
714–3724; (e) J. M. Thomas, B. F. G. Johnson, R. Raja, G. Sankar
and P. A. Midgley, Acc. Chem. Res., 2003, 36, 20–30.
(a) H. Bonnemann and R. Richards, Synth. Methods Organomet.
Inorg. Chem., 2002, 10, 209–224; (b) C. C. Cassol, A. P. Umpierre,
2
¨
This journal is ꢀc The Royal Society of Chemistry 2008
Chem. Commun., 2008, 6155–6157 | 6157