Highly dispersed nickel and palladium nanoparticle silica aerogels: Sol-gel processing of tethered metal complexes and application as catalysts in the Mizoroki-Heck reaction

Article abstract of DOI:10.1039/b604544h

Highly dispersed Ni and Pd silica aerogels were prepared through sol-gel processes. They have been synthesized by tethering the metal to the silica matrix with the use of the complexing silanes AEAPTS [N-(aminoethyl)aminopropyl] trimethoxysilane or TRIAMIN [N-((aminoethyl)aminoethyl)aminopropyl] trimethoxysilane. The gels were dried with scCO₂ to afford the aerogels. High metal dispersion and small particle sizes were observed through HRTEM and XRD measurements. The organic groups were removed by oxidation and/or pyrolysis. The Pd reduced aerogels containing 15-17% of palladium are good recoverable catalysts in the Mizoroki-Heck reaction. the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2006.

Full text of DOI:10.1039/b604544h

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Highly dispersed nickel and palladium nanoparticle silica aerogels:
sol–gel processing of tethered metal complexes and application as
catalysts in the Mizoroki–Heck reaction
ab
a
a
Sandra Martı
´
Ulrich Schubert,* Anna Roig and Elies Molins
nez, Marcial Moreno-Man
˜
b
as,w Adelina Vallribera,*
c
c
Received (in Durham, UK) 29th March 2006, Accepted 16th May 2006
First published as an Advance Article on the web 31st May 2006
DOI: 10.1039/b604544h
Highly dispersed Ni and Pd silica aerogels were prepared through sol–gel processes. They
have been synthesized by tethering the metal to the silica matrix with the use of the
complexing silanes AEAPTS [N-(aminoethyl)aminopropyl]trimethoxysilane or TRIAMIN
[
N-((aminoethyl)aminoethyl)aminopropyl]trimethoxysilane. The gels were dried with scCO
2
to
afford the aerogels. High metal dispersion and small particle sizes were observed through
HRTEM and XRD measurements. The organic groups were removed by oxidation and/or
pyrolysis. The Pd reduced aerogels containing 15–17% of palladium are good recoverable
catalysts in the Mizoroki–Heck reaction.
Introduction
of organofunctional alkoxysilanes of the type (CH
3 3
O) Si
(
2 3
CH ) A, in which A is a group capable of coordinating to
Silica aerogels are a group of extremely light and porous solid
materials obtained through sol–gel processes followed by
removing solvent from the wet gels at supercritical conditions
metal ions. The advantage of this method is that a two-
6
component system is transformed in a single precursor. The
solid xerogels were then calcined and reduced. However, it has
never been attempted to prepare aerogels by this approach.
Our interest in the fields of aerogels and catalysis prompted
us to explore this methodology in the synthesis of aerogels.
Therefore, the goal of this work was to obtain highly dispersed
small nanoparticles of Ni and Pd embedded in silica aerogels,
following the organosilane single-source method, which could
be used in the Mizoroki–Heck reaction. The use of palladium
on solid supports as catalysts for this reaction has recently
1
of the solvent. One of the striking properties of aerogels is the
large surface area. In addition, metallic nanoparticles or
metal-oxide nanoparticles can be placed in aerogels and be
used as heterogeneous catalysts. We have previously prepared
silica aerogel–iron oxide nanocomposites which were active as
2
catalysts in conjugate additions and palladium-doped organic
and carbon aerogels that were good recoverable catalysts for
3
Mizoroki–Heck reactions. Moreover europium-containing
organic gels have been proved to be efficient as recoverable
4
catalysts in Michael additions.
7
attracted a great deal of attention. Diverse palladium silica
8
based supports have been used. Recent results show that
In sol–gel processing of metal–silica nanocomposite aero-
gels the control of homogeneity is not trivial. The difference in
chemical properties between the metal salt, used as the metal
precursor, and the most common silica precursors (for exam-
ple TEOS) is evident. There are many reaction parameters that
differently influence hydrolytic polycondensation of each pre-
cursor in a sol–gel process of a multi-precursor system. Two of
the challenges are the control of the metal particle size and the
homogeneity of the metal distribution. A general approach to
control dispersion of the metal compound during sol–gel
processing is to use the approach of tethering the metal to
the Heck coupling can be induced by palladium acetate
immobilized in reversed phase silica gel with the aid of an
9
10
ionic liquid. However, carbon supports gave in general
better results than silica supports, including palladium sup-
10a
ported in carbon nanotubes.
Experimental
Silica aerogel nanocomposites
5
Different nanocomposite silica aerogels were synthesized using
the gel matrix. Some of us have previously described the use
Ni(OAc)
2 2
and Pd(OAc) as metal sources. In the first step, the
21
21
metal complexes [Pd(AEAPTS)
21
[Ni(AEAPTS)
2
]
,
[Pd(TRIAMIN)
21
2
]
,
a
Department of Chemistry, Universitat Auto´noma de Barcelona,
]
and [Ni(TRIAMIN)
2
]
were formed by
2
0
3
8193 Cerdanyola, Spain. E-mail: adelina.vallribera@uab.es; Fax:
4 935811265; Tel: 34 935813045
reaction of nickel or palladium acetate with 2 molar equi-
valents of N-[(aminoethyl)aminopropyl]trimethoxysilane
(AEAPTS) or [N((N-aminoethyl)aminoethyl)aminopropyl]tri-
methoxysilane (TRIAMIN). In the second step, tetraethoxy-
silane (TEOS) was added as the silica source. Sol–gel
processing of the mixtures resulted in different gels that were
b
c
Institute of Materials Chemistry, Vienna University of Technology,
A-1060 Wien, Austria. E-mail: usschubert@mail.zserv.tuwien.ac.at;
Fax: 43 5880115399; Tel: 43 5880115320
Institut de Cie`ncia de Materials de Barcelona (ICMAB-CSIC),
Campus de la UAB, 08193 Cerdanyola, Spain. Fax: 34 935805729;
Tel: 34 935801853
w Deceased on 20th February 2006.
dried with scCO to afford the aerogels.
2
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Typical experimental procedure
2 2
mmol of the metal acetate (Ni(OAc) or Pd(OAc) ) were
iodobenzene appears in the GLC chromatography. The solu-
tion was decanted and the aerogel washed several times with
acetonitrile. The combined solvent extracts were evaporated
and the residue was dissolved in CH Cl and the solution was
4
suspended in 50 ml of water-free ethanol, and 8 mmol of
AEAPTS (1.78 g, series a) or TRIAMIN (2.24 g, series b), and
the solution was stirred until the metal salt was dissolved.
Then 5 g (24 mmol) of TEOS and 16.25 ml of 0.2 M aqueous
2
2
washed with aqueous Na CO , and a saturated solution of
3
2
NaCl. The organic extract was evaporated to afford ethyl
3
1
trans-cinnamate, 5, as an oil (yield = 99%). H NMR
CDCl , 250 MHz) d: 7.69 (d, J = 16 Hz, 1H), 7.52 (m,
H), 7.37 (m, 3H), 6.43 (d, J = 16 Hz, 1H), 4.26 (q, J = 7 Hz,
NH were added. The reaction mixture was stirred and then
3
(
3
poured into plastic moulds that were sealed. The mixture was
allowed to cure for 4 days at room temperature. Next, the gels
were washed with ethanol and dried with supercritical CO2.
Some of the aerogels were first transferred into a ceramic
2
2
1
1
3
H), 1.33 (t, J = 7 Hz, 3H). C NMR (CDCl , 62.5 MHz) d:
3
66.9, 144.5, 134.4, 130.1, 128.8, 127.9, 118.2, 60.4, 14.2. IR
À1
(
ATR, cm ): 2981, 1708, 1635, 1310, 1167, 765.
boat, placed into an oven and heated to 550 1C (heating rate
À1
3
1
1
0 1C min ) during 60 min (calcination or oxidation process).
Then they were reduced by placing the oxidized samples in a
trans-1,2-Diphenylethylene, 6 . Mp 116–120 1C. H NMR
(
CDCl , 250 MHz) d: 7.50 (m, 4H), 7.28–7.40 (m, 6H), 7.14 (s,
3
13
À1
quartz tube, heating to 550 1C (heating rate 10 1C min ) in a
stream of hydrogen and holding for 2 h.
2
1
1
H). C NMR (CDCl , 62.5 MHz) d: 137.1, 128.5, 127.4,
3
À1
26.3. IR (ATR, cm ): 3023, 2964, 1946, 1879, 1811, 1752,
Other aerogels were directly reduced without prior calcina-
tion. The materials were characterized by X-ray diffraction
600, 1490, 1447, 958, 758, 686.
3
1
trans-4-Phenyl-3-butene-2-one, 7 . Mp 35–39 1C. H NMR
(CDCl , 250 MHz) d: 7.53 (m, 2H), 7.50 (d, J = 16 Hz, 1H),
7.40 (m, 3H), 6.70 (d, J = 16 Hz, 1H), 2.36 (s, 3H). C NMR
(
XRD) with a D5000 Siemens X-ray powder diffractometer
using Cu-Ka incident radiation. Transmission electron micro-
scopy (TEM) observations and selected area electron diffrac-
tion patterns were performed using a JEOL-JEM-2010
microscope operating at 200 keV. Surface area determinations
were carried out following the BET (Brunauer–Emmett–
Teller) method with a ASAP-2000 surface area analyzer
3
1
3
(
CDCl
3
, 62,5 MHz) d: 198.1, 143.1, 134.1, 130.2, 128.7, 128.7,
À1
1
26.8, 26,2. IR (ATR, cm ): 3059, 2999, 1678, 1600, 1448,
1
359, 1168, 976, 749, 690.
(
Micromeritics Instruments Corp.). Thermal analyses
Results and discussion
were performed on Netzsch TG209c and Shimadzu DSC-50
apparatus.
Different Ni and Pd silica aerogel nanocomposites were pre-
pared following the method previously reported by some of us
5
c
for the preparation of xerogels. The preparation of the
aerogels included the following steps (Scheme 1): (i) in situ
preparation of the metal complexes by reaction of the metal
salt with the amino-substituted organo(alkoxy)silane. (ii) Ad-
dition of the Si(OR) as the network former and aqueous NH
Mizoroki–Heck reaction
Typical experimental procedure. Ethyl acrylate (1.5 ml, 13.5
mmol), iodobenzene (0.5 ml, 4.5 mmol) and triethylamine
(
1.25 ml, 9 mmol) were dissolved in 15 ml of acetonitrile.
4
3
Then, 0.0588 g (0.09 mmol, 2% molar) of Pd(a)R aerogel were
added to the mixture and the reaction was carried out under
reflux and mechanical stirring during 8 h, when no signal of
to start the sol–gel reactions. (iii) Drying of the obtained wet
gels with scCO . The samples are designated as M(x), where M
is the chemical symbol of the metal, and x is the series level
2
Scheme 1 Preparation of different Ni and Pd aerogel nanocomposites (example for series a).
1
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Fig. 2 DSC of the aerogels Pd(a) and Pd(b).
Fig. 1 TGAs of the aerogels Pd(a) and Pd(b).
(Fig. 2) showed the temperature at which the exothermic
pyrolysis and/or thermolysis reactions took place.
Reduction of all these materials resulted in metal/silica
aerogel nanocomposites (Scheme 1). These samples were
designated as M(x)OR. The letters OR were added after
M(x) to indicate that the samples were oxidized and then
reduced.
depending on the complexing organo(alkoxy)silane (AEAPTS
series a; TRIAMIN series b). Moreover, to study the influence
of the complexing organo(alkoxy)silane on the properties of
the composites, a third series of samples (series c) was pre-
pared by using the same metal salts and TEOS but with no
complexing organo(alkoxy)silane added. The materials of the
One of the goals of our previous work on metal oxide or
5c
three series exhibited differences in their gelation behavior.
metal/SiO nanocomposites by the single-source method had
2
No gelation was achieved in the case of the Ni(c) (sol–gel
been to obtain carbon-free materials. For this reason, the
xerogels were calcined prior to reduction to remove the
organic groups. For an application as catalysts in organic
reactions, the presence of organic groups may not be detri-
mental. We therefore also prepared aerogels, designed with the
letters R after M(x), where the oxidation step was omitted. In
oxidized/reduced aerogels M(x)OR the organic groups were
removed by pyrolysis reactions. In contrast, in the only
reduced aerogels M(x)R the organic groups are partially
retained.
processing of Ni(OAc) /TEOS mixtures). For Pd(c), a hetero-
2
geneous gel was formed and an accumulation of the metal salt
on the surface of the gel was clearly observed. On the contrary,
when AEAPTS and TRIAMIN were employed, which contain
the strongly coordinating ethylenediamine or diethylenetria-
mine groups, homogeneous gels were obtained.
The M(a) and M(b) aerogels were calcined in air to remove
organic groups and to obtain metal oxide/silica aerogel nano-
composites Ni(a)O and Pd(b)O (Scheme 1). The letter O after
M(x) indicates that the samples were oxidized. The TGAs of
the aerogels denoted the temperature of the thermal degrada-
tion of the organic groups. The TGAs of Pd(a) and Pd(b) are
shown in Fig. 1. The weight loss in Pd(b) was higher due to the
larger proportion of organic groups. The TGAs of Ni xerogels
The nitrogen absorptions measurements showed that in all
the aerogels the shape of the adsorption isotherm was of type
II, characteristic of macroporous solids. In general, when the
aerogels were calcined and then reduced, (M(x)OR), a large
percentage of micropores were created and this resulted in an
increase in porosity and specific surface area (Table 1). We
5c
materials were previously documented and similarly we
decided to calcine the aerogels in air at T = 550 1C. The
corresponding DSC curves of the aerogels Pd(a) and Pd(b)
5c
have previously observed this behaviour in xerogels. More-
over, the aerogels directly reduced also presented an increase
Table 1 Synthesis features and characteristics of aerogels
Metal
source
Surface
area/m g
Pore
volume/cm g
Density/
g cm
Metal
(%)
Mean particle
size /nm
2
À1
3
À1
À3
a
Material
XRD phase
Ni(a)
Ni(OAc)
Ni(OAc)
Ni(OAc)
Ni(OAc)
Ni(OAc)
Ni(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
2
2
2
2
2
2
441
575
393
439
756
515
516
690
1020
475
530
642
1.8
3.7
1.9
1.3
2.8
1.6
1.7
2.0
2.4
1.1
0.9
0.9
0.09
0.1
0.13
0.27
0.7
0.5
0.4
0.4
0.3
—
—
—
—
—
—
11.4
16.3
15.8
—
Amorphous
fcc-Ni
fcc-Ni
Amorphous
fcc-Ni
fcc-Ni
Amorphous
fcc-Pd
fcc-Pd
Amorphous
fcc-Pd
fcc-Pd
—
Ni(a)R
Ni(a)OR
Ni(b)
Ni(b)R
Ni(b)OR
Pd(a)
Pd(a)R
Pd(a)OR
Pd(b)
Pd(b)R
Pd(b)OR
a
1.7
12.3
—
2.5
2.3
b
2
2
2
2
2
2
1.3
1.4
8.3
—
1.6
5.6
0.4
0.74
0.9
—
—
b
Determined from XRD. Determined from HRTEM.
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Fig. 3 Powder X-ray diffractograms of Ni(a), Ni(a)R and Ni(a)OR.
in porosity and surface area. So, the temperature used to
2
reduce the aerogels in the presence of H promoted the partial
removal of the organic groups with the concomitant creation
of micropores.
The powder X-ray diffraction (XRD) patterns for the
calcined–reduced aerogels showed the expected crystallinity
of metallic fcc-Ni (Fig. 3) and fcc-Pd (Fig. 4) nanoparticles.
We have determined the particle size by the XRD line broad-
ening in case of sharp reflections (Table 1). As expected, small
metallic nanoparticles were observed due to the high disper-
sion of the metal. For Ni(x)R aerogels, Ni(0) nanoparticles
between 1.7 and 2.5 nm were observed. In the case of Pd(x)R
the sizes of Pd(0) nanoparticles were even smaller (1.4–1.6
nm). Average particles size was larger for M(a)OR materials,
the reason could be the longer thermal treatment which could
result in an aggregation of the small particles, therefore
favouring the particle growth. The influence of the various
synthesis parameters on the particle size and the size distribu-
tion such as the nature of the complexing silane, the counter
ion of the employed metal salt and the thermal process has
Fig. 5 Particle size distribution and HRTEM image of Pd(a).
Transmission electron microscopy (TEM) of Pd(a) aerogels
was carried out. It showed well defined spherical particles
fairly homogeneously dispersed in the silica matrix (Fig. 5).
The mean diameter of the nanoparticles determined by TEM
was about 1.3 Æ 0.3 nm. The electron diffraction patterns of
this sample were obtained and the diffraction rings can be
ascribed to the (111), (200), (220) and (311) crystallographic
planes of the fcc-Pd. Therefore, the Pd is reduced in the
5a,b,f,g,h
already been investigated in detail.
2
supercritical conditions of the CO and without the presence
of any reduction step. This reduction process in supercritical
conditions was observed previously for other Pd-doped aero-
3
gels dried in supercritical conditions of EtOH.
The materials were tested as catalysts in Mizoroki–Heck
reactions (Scheme 2, Table 2). For all the Mizoroki–Heck
Fig. 4 Powder X-ray diffractograms of Pd(a), Pd(a)R and Pd(a)OR.
Scheme 2 Mizoroki–Heck reactions tested.
1
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Table 2 Results for Mizoroki–Heck reactions shown in Scheme 2
vantages of using aerogels are the easy reaction work-up, and
the easy recovery and reuse of the catalyst.
a
Conversion (%)
Catalyst
Olefin
Time/h
Product
Pd(a)
2
2
2
3
3
3
4
4
4
24/35/45/40
8/11/13/20
7/8/9/12
3 days
10/25/23/48
2 days
3 days
5
5
5
6
6
6
7
7
7
97/90/85/98
99/93/90/83
94/93/90/93
82
74/65/56/79
5
98
99
99
Pd(a)R
Pd(a)OR
Pd(a)
Pd(a)R
Pd(a)OR
Pd(a)
Pd(a)R
Pd(a)OR
a
Acknowledgements
Financial support from ‘‘Ministerio de Educacio
of Spain (Projects CTQ2005-04968/BQU, BQU2002-04002,
MAT2003-01052 and Accion Integrada HU2003-0041) and
‘DURSI-Generalitat de Catalunya’’ (Project SGR2005-00305,
´
n y Ciencia’’
´
2 days
2 days
‘
SGR2005-00452) is gratefully acknowledged.
Quantitative conversion by GLC chromatography.
References
reactions we used 2% molar palladium with respect to the
limiting reagent and triethylamine as a base in refluxing
1
¨
For reviews on aerogels, see: (a) N. Husing and U. Schubert,
Angew. Chem., Int. Ed., 1998, 37, 22; (b) A. C. Pierre and G. M.
Panjonk, Chem. Rev., 2002, 102, 4243; (c) H. D. Gesser and P. C.
Goswami, Chem. Rev., 1989, 89, 765.
CH CN. In general the best results were obtained for the
3
Pd(a)R aerogel as catalyst. This material is very active for the
reactions of iodobenzene with ethyl acrylate and styrene. In
both cases we carried out the reaction three consecutive times
with the same batch of catalyst without loss of activity. The
analysis of the crude reaction mixture for the first reaction
indicates a leaching of 3% for the Pd(a)R. In comparison with
our previous results obtained with organic and carbon aero-
gels doped with Pd, we have improved the catalyst which is
now more active and can be recovered and reused. As an
example, the reaction of iodobenzene with ethyl acrylate is
complete after 8 h using Pd(a)R as a promoter which can be
reused three times, whereas our best results using a recoverable
catalyst were obtained with Pd-carbon aerogel and a reaction
2 S. Martınez, M. Meseguer, L. Casas, E. Rodriguez, E. Molins, M.
´
Moreno-Man
Tetrahedron, 2003, 59, 1553.
S. Martınez, A. Vallribera, C. L. Cotet, M. Popovici, L. Martı
Roig, M. Moreno-Manas and E. Molins, New J. Chem., 2005, 29,
342.
S. Martı
and A. Vallribera, Monatsh. Chem., 2006, 137, 627.
˜
as, A. Roig, R. M. Sebastian and A. Vallribera,
´
3
4
´
´
n, A.
˜
1
´
˜
nez, L. Martın, E. Molins, M. Moreno-Manas, A. Roig
´
5 (a) B. Breitscheidel, J. Zieder and U. Schubert, Chem. Mater.,
1991, 3, 559; (b) C. Lembacher and U. Schubert, New J. Chem.,
1
2
998, 721; (c) G. Trimmel and U. Schubert, J. Non-Cryst. Solids,
001, 296, 188; (d) W. Rupp, N. Husing and U. Schubert, J. Mater.
¨
Chem., 2002, 12, 2594; (e) G. Trimmel, C. Lembacher, G. Kick-
elbick and U. Schubert, New J. Chem., 2002, 26, 759; (f) U.
Schubert, S. Amberg-Schwab and B. Breitscheidel, Chem. Mater.,
1989, 1, 576; (g) U. Shubert, B. Breitscheidel, H. Buhler, C. Egger
and W. Urbaniak, Mater. Res. Soc. Symp. Proc., 1992, 271, 621;
3
time of 24 h. A possible explanation could be the difference in
the size of the metal particles. With the use of the ligand
AEAPTS the metal is tethered to the silica matrix during
sol–gel processing. This creates a good dispersion of the metal
through the matrix and renders the aggregation of the Pd
particles more difficult. This control over the particle size
could be of importance in the case of using expensive metals.
(
Chem. Mater., 1996, 6, 1659.
U. Schubert, Adv. Eng. Mater., 2004, 6, 173.
For a review on Mizoroki–Heck reaction catalyzed by supported
palladium, see: A. Biffis, M. Zecca and M. Bassato, J. Mol. Catal.
A: Chem., 2001, 173, 279.
¨
h) W. Morke, R. Lamber, U. Schubert and B. Breitscheidel,
6
7
8 (a) A. Papp, K. Miklos, P. Forgo and A. Molnar, J. Mol. Catal. A:
´
´
Chem., 2005, 229, 107; (b) L. Li, L.-X. Zhang, J.-L. Shi, J.-N. Yan
and J. Liang, Appl. Catal., A, 2005, 283, 85.
Conclusions
9 H. Hagiwara, Y. Sugawara, T. Hoshi and T. Suzuki, Chem.
Commun., 2005, 2942.
Different Ni or Pd/silica aerogel nanocomposites were pre-
pared. The use of the complexing silanes AEAPTS or TRIA-
MIN allows the metal to be tethered to the silica matrix during
the preparation of the materials. Through TEM and XRD we
can observe high metal dispersion and small particle sizes. The
Pd aerogels were studied as catalysts in the Mizoroki–Heck
reactions and they gave good results in the reaction between
the iodobenzene and the ethyl acrylate or styrene. The ad-
10 For recent references, see: (a) G. Garcia and A. Leyva, J. Mol.
Catal. A: Chem., 2005, 230, 97; (b) Q. Hu, J. Pang, N. Jiang, J. E.
Hamsey and Y. Lu, Microporous Mesoporous Mater., 2005, 81,
1
49; (c) M. Gruber, S. Chouzier, K. Koehler and L. Djakovitch,
Appl. Catal., A, 2004, 265, 161; (d) K. Shimizu, S. Koizumi, T.
Hatamachi, H. Yoshida, S. Komai and Y. Kitiyama, J. Catal.,
2
004, 202, 141; (e) A. Perosa, P. Tundo, M. Selva, S. Zinovyev and
A. Testa, Org. Biomol. Chem., 2004, 2, 2249; (f) F. Zhao and M.
Arai, React. Kinet. Catal. Lett., 2004, 81, 281; (g) N. Kim, M. S.
Known, C. M. Park and J. Park, Tetrahedron Lett., 2004, 45, 7057.
This journal is ꢀc the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2006
New J. Chem., 2006, 30, 1093–1097 | 1097