Supported ionic liquid phase rhodium nanoparticle hydrogenation catalysts

Article abstract of DOI:10.1039/b708111a

Rh(0) nanoparticles (ca. 4 nm) dispersed in an ionic liquid (1-n-butyl-3-methylimidazolium tetrafluoroborate) were immobilized within a silica network, prepared by the sol-gel method. The effect of the sol-gel catalyst (acid or base) on the encapsulated ionic liquid and Rh(0) content, on the silica morphology and texture, and on the catalyst alkene hydrogenation activity was investigated. The Rh(0) content in the resulting xerogels (ca. 0.1 wt% Rh/SiO₂) was shown to be independent of the sol-gel process. However, acidic conditions afforded higher contents of encapsulated ionic liquid and xerogels with larger pore diameters, which in turn might be responsible for the higher catalyst activity in hydrogenation of the alkenes. The Royal Society of Chemistry.

Full text of DOI:10.1039/b708111a

View Article Online / Journal Homepage / Table of Contents for this issue
This paper is published as part of a Dalton Transactions themed issue on:
Emerging Strategies in Catalysis
Guest Edited by Carmen Claver
Universitat Rovira i Virgili, Tarragona
Published in issue 47, 2007 of Dalton Transactions
Images reproduced by permission of Christian Muller (outside) and Challa Kumar (inside)
Other papers published in this issue include:
Access to well-defined isolated Fe(II) centers on silica and their use in oxidation
Charbel Roukoss, Steven Fiddy, Aimery de Mallmann, Nuria Rendón, Jean-Marie Basset,
Emile Kuntz and Christophe Copéret, Dalton Trans., 2007, DOI: 10.1039/b711015d
Phosphinines as ligands in homogeneous catalysis: recent developments, concepts
and perspectives
Christian Müller and Dieter Vogt, Dalton Trans., 2007, DOI: 10.1039/b712456m
Solventless continuous flow homogeneous hydroformylation of 1-octene
Anja C. Frisch, Paul B. Webb, Guoying Zhao, Mark J. Muldoon, Peter J. Pogorzelec and
David J. Cole-Hamilton, Dalton Trans., 2007, DOI: 10.1039/b712683b
Palladium catalyzed Suzuki C–C couplings in an ionic liquid: nanoparticles responsible
for the catalytic activity
Fernando Fernández, Beatriz Cordero, Jérôme Durand, Guillermo Muller, François Malbosc,
Yolande Kihn, Emmanuelle Teuma and Montserrat Gómez, Dalton Trans., 2007, DOI:
10.1039/b713449e
Visit the Dalton Transactions website for cutting-edge inorganic research
www.rsc.org/dalton

PAPER
www.rsc.org/dalton | Dalton Transactions
Supported ionic liquid phase rhodium nanoparticle hydrogenation catalysts
Marcos A. Gelesky,a Sandra S. X. Chiaro,b Fla´vio A. Pavan,a Joa˜o H. Z. dos Santos*a and Jairton Dupont*a
Received 29th May 2007, Accepted 25th July 2007
First published as an Advance Article on the web 9th August 2007
DOI: 10.1039/b708111a
Rh(0) nanoparticles (ca. 4 nm) dispersed in an ionic liquid (1-n-butyl-3-methylimidazolium
tetrafluoroborate) were immobilized within a silica network, prepared by the sol–gel method. The effect
of the sol–gel catalyst (acid or base) on the encapsulated ionic liquid and Rh(0) content, on the silica
morphology and texture, and on the catalyst alkene hydrogenation activity was investigated. The Rh(0)
content in the resulting xerogels (ca. 0.1 wt% Rh/SiO2) was shown to be independent of the sol–gel
process. However, acidic conditions afforded higher contents of encapsulated ionic liquid and xerogels
with larger pore diameters, which in turn might be responsible for the higher catalyst activity in
hydrogenation of the alkenes.
Moreover, metal nanoparticles (MNPs) with small diameter and
Introduction
narrow size distribution can be prepared by simple H2 reduction
The combination of an ionic liquid with a solid support material is
of metal compounds or decomposition of organometallic species
dissolved in ILs.20–22 Although these MNPs dispersed in ILs are
catalysts for reactions under multiphase conditions, in several
cases the MNPs are not stable and tend to aggregate.23 Alterna-
tively, these nanoparticles can be used in conjunction with other
stabilizers or be easily transferred to other organic and inorganic
supports to generate more stable and active catalysts.24–27 The
nanoparticle/ionic liquid/stabilizer combination usually exhibits
an excellent synergistic effect that enhances activity and robustness
of the catalyst. Therefore, it is quite plausible to assume that more
efficient and stable catalytic systems could be prepared by the
generation of metal nanoparticles associated with silica using ILs
as templates for both catalytic partners i.e. the metal nanoparticles
and the silica support.28,29
emerging as a new alternative for the immobilization of transition-
metal catalyst precursors.1,2 This process named Supported Ionic
Liquid Phase (SILP) catalysis is a concept which combines the
advantages of ionic liquids with those of heterogeneous support
materials. These materials are prepared by the covalent attachment
of ionic liquids to the support surface or by simply depositing the
catalytically active species-containing ionic liquid phases on the
surface of a support, usually silica- or polymeric-based materials.
In several cases, the procedure involves the simple dissolution of a
sulfonated phosphine-modified rhodium catalyst into a supported
ionic liquid, while the substrates constitute the organic phase.
This method reduces the amount of ionic liquid and affords
easy and efficient separation of products from the catalyst. In
comparison to the traditional biphasic systems, higher catalytic
activity and lower metal leaching can be obtained by the suitable
tuning of the experimental conditions.1,3–7 The SILP systems have
been used in various catalytic reactions such as Friedel–Crafts
reactions,8 Rh-catalyzed hydroformylations6 and hydrogenations,9
Pd-catalyzed Heck reactions,10 and Rh-, Pd-, and Zn-catalyzed
hydroaminations.11
We present herein our results which show that imidazolium ionic
liquids can be used for the generation of silica supported Rh(0)
nanoparticles via sol–gel processes.
Experimental
Imidazolium ionic liquids (ILs) possess pre-organized struc-
tures mainly through hydrogen bonds which induce structural
directionality.12 These IL structures can adapt or be adaptable
to many species, as they provide hydrophobic or hydrophilic
regions, and a high directional polarizability.13,14 This structural
organization of ILs can be used as “entropic drivers” for sponta-
neous, well-defined and extended ordering of nanoscale structures.
Indeed, the unique combination of adaptability towards other
molecules and phases associated to the strong hydrogen-bond-
driven structure makes ionic liquids potential key tools in the
preparation of a new generation of chemical nanostructures15,16
such as templated porous silica prepared in a sol–gel process.17–19
General
Except for the preparation of the Rh(0) nanoparticles all ex-
periments were performed in air. RhCl3·3H2O and Rh/C (5%)
were provided by Degussa (Brazil). The halide free 1-n-butyl-
3-methylimidazolium tetrafluoroborate (BMI.BF4) ionic liquid30
and the Rh(0) nanoparticles31 were prepared according to known
procedures. Solvents and alkenes were dried with the appropri-
ate drying agents and distilled under argon prior to use. All
other chemicals were purchased from commercial sources and
used without further purification. Gas chromatography analysis
was performed with a Hewlett-Packard-5890 gas chromatograph
with an FID detector and a 30 m capillary column with a
dimethylpolysiloxane stationary phase.
aLaboratory of Molecular Catalysis, Institute of Chemistry-UFRGS,
P.O.Box 15003, 91501-970, Porto Alegre, RS, Brazil. E-mail: dupont@
iq.ufrgs.br; Fax: +55 51 3308 7304; Tel: +55 51 3308 6321
The nanoparticle formation and hydrogenation reactions were
carried out in a modified Fischer–Porter bottle immersed in
a silicone oil bath and connected to a hydrogen tank. The
temperature was maintained at 75 ◦C by a hot-stirring plate
bPETROBRAS S.A. -R&D Center Quadra 7-Cidade Universitaria-Ilha do
Funda˜o, 21949-900, Rio de Janeiro, RJ, Brazil
This journal is
The Royal Society of Chemistry 2007
Dalton Trans., 2007, 5549–5553 | 5549
©

connected to a digital controller (ETS-D4 IKA) with stirring
at 1200 rpm. The fall in the hydrogen pressure in the tank was
monitored with a pressure transducer interfaced through a Novus
Scanning Electron Microscopy (SEM) and Electron Dispersive
Spectroscopy (EDS) elemental analysis
The materials were analyzed by SEM using a JEOL model JSM
5800 with 20 kV and 5000 magnification. The same instrument
with was used for the EDS with a Noran detector (20 kV and
acquisition time of 100 s and 5000 magnification).
converter to a PC and the data workup via Microcal Origin 5.0TM
.
Synthesis of Rh(0) nanoparticles immobilized in silica
Silica immobilized Rh(0) nanoparticles were prepared by the sol–
gel method under acidic and basic conditions. Typical procedure
for acid catalysis: 10 mL of tetraethoxy orthosilicate (9.34 g,
45 mmol) was introduced in a Becker under vigorous stirring at
60 ◦C. The Rh(0) nanoparticles (10 mg, 0.1 mmol) were dissolved
in BMI.BF4 (1 mL, 5.1 mmol) and ethanol (5 mL). This solution
was submitted to stirring and sonication for 2 min and then added
to the solution containing TEOS. Consecutively, an acid solution
(HF or HCl) was added as acid catalyst. The temperature was kept
at 60 ◦C for 18 h. The resulting material was washed several times
with acetone and dried under vacuum.
Typical procedure for base catalysis: 10 mL of TEOS (9.34 g,
45 mmol) was added to ethanol (5 mL), containing the ionic
liquid BMI.BF4 (1 mL, 5.1 mmol) and previously isolated Rh(0)
nanoparticles (10 mg, 0.1 mmol). Then ethanol (95 mL) and
ammonium hydroxide (20 mL) were added. The mixture was kept
under stirring for 3 h at room temperature and left to stand for a
further 18 h. The resulting xerogel was filtered and washed with
acetone and dried under vacuum for 1 h.
Transmission Electron Microscopy (TEM) analysis
The morphologies and the electron diffraction (ED) patterns of
the obtained particles were determined on a JEOL JEM-2010
equipped with an EDS system and a JEOL JEM-120 EXII electron
microscope, operating at accelerating voltages of 200 and 120 kV,
respectively. The TEM samples were prepared by deposition of the
Rh(0) nanoparticles or Rh(0)/SiO2 isopropanol dispersions on a
carbon-coated copper grid at room temperature. The histograms
of the nanoparticle size distributions were obtained from the
measurement of around 300 diameters and reproduced in different
regions of the Cu grid assuming spherical shapes.
Hydrogenations
The catalysts (100 mg) were placed in a Fischer–Porter bottle and
the alkene (12.5 mmol) was added. The reactor was placed in an oil
bath at 75 ◦C and hydrogen was admitted to the system at constant
pressure (4 atm) under stirring until the consumption of hydrogen
stopped. The organic products were recovered by decantation and
analyzed by GC. The re-use of the catalysts were performed by
simple extraction of the organic phase (upper phase) followed by
the addition of the alkene. After ten recycles the solid was isolated,
re-dispersed in isopropanol and placed in a cooper carbon grid.
Elemental analysis (CHN)
The organic phases present in the xerogels were analyzed using
CHNelemental PerkinElmer elementalCHNS/Oanalyzer, model
400. Triplicate analysis of the samples, previously heated at 100 ◦C
under vacuum for 1 h, was carried out.
Results and discussion
The sol–gel process allows us to obtain solid products by creating
an oxide network via progressive polycondensation reactions of
molecular precursors in a liquid medium. This process is consid-
ered as a soft chemical (chimie douce) approach for the synthesis of
metastable (amorphous) oxide materials.33 The process essentially
comprehends two steps: hydrolysis and condensation. Both reac-
tions are affected by the nature of the catalyst. Therefore, in the
present study, two main routes were evaluated: (i) an acid-catalyzed
one using either HF or HCl, and (ii) a base-catalyzed approach,
using NH4OH as catalyst. In both routes, the hydrolysis and
condensation of tetraethoxy orthosilicate (TEOS) were performed
in the presence of Rh(0) nanoparticles, which were prepared by
hydrogen reduction (4 atm) of RhCl3·nH2O dissolved in the hy-
drophilic 1-n-butyl-3-methylimidazolium tetrafluoroborate ionic
liquid at 75 ◦C. The hydrophilic 1-n-butyl-3-methylimidazolium
tetrafluoroborate ionic liquid was selected for this study in order
to guarantee the complete solubilization of the aqueous base and
acid catalysts. These nanoparticles of 4.8 nm were isolated and
re-dispersed in the ionic liquid. Table 1 presents the elemental
analysis of the resulting silicas.
Rutherford Backscattering Spectrometry (RBS)
Rhodium loadings in catalysts were determined by RBS using
He+ beams of 2.0 MeV incident on homogeneous tablets of the
compressed (12 MPa) catalyst powder. The method is based on the
determination of the number and energy of the detected particles
which are elastically scattered in the Coulombic field of the atomic
nuclei in the target. In this study, the Rh/Si atomic ratio was
determined by the heights of the signals corresponding to each of
the elements in the spectra and converted to wt% Rh(0)/SiO2. For
an introduction to the method and applications of this technique
the reader is referred elsewhere.32
Nitrogen adsorption–desorption isotherms
The adsorption–desorption isotherms of previous degassed solids
(150 ◦C) were determined at liquid nitrogen boiling point in a
volumetric apparatus, using nitrogen as probe. The specific surface
areas of xerogels were determined from the t-plot analysis and
pore size distribution was obtained using the BJH method. A
homemade equipment with a vacuum line system employing a
turbo-molecular Edwards vacuum pump was used. The pressure
measurements were made using a capillary Hg barometer and a
Pirani gauge.
Carbon and nitrogen content was taken as a sign for the presence
of ionic liquid BMI.BF4 once encapsulated within the silica
network. According to Table 1, higher ionic liquid contents were
incorporated with the acid catalysts. It is worth noting that in the
case of acidic conditions, hydrolysis is faster than condensation.
5550 | Dalton Trans., 2007, 5549–5553
This journal is
The Royal Society of Chemistry 2007
©

Table 1 Elemental analysis of Rh(0)/SiO2
The textural properties were further characterized by nitrogen
adsorption. Specific area was calculated by the BET method, while
pore diameter, by the BJH one (Table 2). According to Table 2,
silicas prepared in the absence of Rh(0) present higher specific area
(ca. 100 m2 g−1). The introduction of metal particles during the
synthesis led to a reduction in the specific area, independently of
the synthetic route. The pore volume was shown to be independent
of the presence or not of Rh(0) and of acidic or basic conditions.
Nevertheless, the pore diameter was shown to be smaller for the
materials prepared in the presence of NH4OH as catalyst.
Sample
C/mmol g−1
N/mmol g−1
Rh(0) (%)a
Rh(0)/SiO2/HF
Rh(0)/SiO2/HCl
Rh(0)/SiO2/NH4OH
16.3
13.9
6.7
4.2
3.5
1.7
0.11
—
0.14
a Determined by RBS.
The supported catalysts were evaluated in hydrogenation reac-
tions of aliphatic compounds. Table 3 presents data regarding 1-
decene and cyclohexene hydrogenation reactions. For comparative
purposes we also included the data concerning the catalytic activity
of commercial Rh/C and of isolated Rh(0) nanoparticles.
The rate of condensation slows down with increasing number
of siloxane linkages around a central silicon atom. This leads to
weakly branched polymeric networks. On the other hand, in the
case of basic conditions, condensation is accelerated relative to
hydrolysis. The rate of condensation increases with increasing
number of siloxane bridges. Thus, highly branched networks
result.34 In the present case, based on the carbon and nitrogen
contents, it seems that the resulting weakly branched structure
generated in the presence of acid catalyst (either HF or HCl)
guarantees the constraint of the ionic liquid.
Metal contents were determined by Rutherford backscattering
spectrometry (RBS). According to Table 1, the immobilized Rh
content is roughly the same for silica prepared by both routes,
corresponding to ca. 60–75% of the initial Rh content employed
in the synthesis.
Table 2 Textural properties of the xerogelsa
Entry Sample
S
BET/m2 g−1 Vp/cm3 g−1 dp/nm
1
2
3
4
SiO2/BMI.BF4/HF
SiO2/BMI.BF4/NH4OH
Rh(0)/SiO2/HF
106
96
65
0.13
0.11
0.11
0.10
8.4
4.2
7.8
3.4
Rh(0)/SiO2/NH4OH
64
a SBET = specific area determined by BET method, Vp = pore volume, dp =
pore diameter.
The metal distribution was determined by SEM-EDX analyses.
Mapping showed a homogeneous Rh distribution in the silica
grains, independently of the preparative route.
Table 3 Hydrogenation of alkenes by encapsulated Rh(0) nanoparticles,
Rh(0) nanoparticles and Rh/Ca
Fig. 1 illustrates the micrography of Rh(0)/SiO2 prepared by
both routes. According to Fig. 1, particle morphologies are in
accordance to that usually observed for pure silica synthesized
by these routes. In the case of acid-catalyzed conditions, a less
organized, plate-like structure was observed, while in the case of
basic conditions, spherical particles were obtained. It is worth
noting that smaller particles were produced in the latter case.
TEM was also employed for the characterization of the supported
catalyst. Fig. 2a shows the micrograph of the isolated Rh(0)
particles, the mean size of which was shown to be ca. 4.8 nm.
In the case of Rh(0)/SiO2 prepared by acid catalysis (HF), both
the morphology and size (ca. 4.1 nm) were maintained within the
silica framework. It is very likely that the presence of ionic liquid
affords stability, avoiding sintering of the metallic particles. The
same behavior was observed for the material prepared by the base
catalysis.
Entry Catalyst
Alkene
t/min TOF/min−1b
1
2
3
4
5
6
7
8
Rh(0)/SiO2/HFc
Rh(0)/SiO2/HFc
1-Decene
Cyclohexene
22
50
60
54
24
16
8
Rh(0)/SiO2/NH4OHd 1-Decene
Rh(0)/SiO2/NH4OHd Cyclohexene 120
Rh(0)e
1-Decene
Cyclohexene
1-Decene
1-Decene
40
70
21
5
6
31
Rh(0)f
Rh/Cg
Rh(0)/SiO2/HF in
BMI.BF4
740
0.8
a Reaction conditions: temperature 75 ◦C; PH2 (4 atm, constant pressure);
conversion (100%); b Turnover frequency defined as mol hydrogenated
product per mol of Rh(0) catalyst per minute; c (1) Rh(0)/SiO2/HF
(100 mg, 0.11% of Rh(0)) 12.5 mmol of alkene; d (2) Rh(0)/SiO2/NH4OH
(100 mg, 0.14% of Rh(0)) 12.5 mmol of alkene; e Rh(0) nanoparticles (3 mg,
0.03 mmol), 6.25 mmol of alkene; f Rh(0) nanoparticles (3 mg, 0.03 mmol),
12.5 mmol of alkene; g 20 mg of Rh/C (5%), 6.25 mmol of alkene.
Fig. 1 Micrographs obtained by SEM of the resulting xerogels: (A) Rh(0)/SiO2/HF and (B) Rh(0)/SiO2/NH4OH.
The Royal Society of Chemistry 2007 Dalton Trans., 2007, 5549–5553 | 5551
This journal is
©

Fig. 2 Micrographs obtained by TEM of: (A) Rh(0) nanoparticles isolated from the ionic liquid and re-dispersed in isopropanol and (B) Rh(0)/SiO2/HF.
As shown in Table 3, all the supported systems were more active
than those constituted of isolated Rh(0) nanoparticles. Among
the silica-based systems, those prepared under acidic conditions
are the most active, exhibiting higher TOF in comparison to those
of commercial Rh/C. The denser and bulkier structure generated
under basic conditions might have afforded less active systems
as shown by some clues. First, the ionic liquid content, which
seems to be important in order to guarantee stability for the
nanoparticles, was lower for these systems. Besides, according to
porosimetric measurements, the pore diameter was much smaller
for the Rh(0)/SiO2/NH4OH system. Rh(0) encapsulated particles,
in spite of a slightly higher content in comparison to that afforded
with an acid catalyst (see Table 1), might be not accessible in the
supported systems prepared under basic conditions.
The hydrogenation of simple alkenes by Rh(0)/SiO2/HF de-
=
pends on steric hindrance at the C C double bond and follows
Fig.
3
Hydrogenation of 1-hexene (–᭹–) and cyclohexene (–ꢀ–)
the same trend as observed with classical rhodium complexes
in homogeneous conditions, that is, the reactivity follows the
order: terminal–internal. This is exemplified in Fig. 3 which
shows the conversion rate of 1-hexene and cyclohexene hydro-
genation. Finally, the catalytic material can be recovered by
simple decantation and re-used at least ten times without any
significant loss in catalytic activity. The recovered material shows
no sign of agglomeration as observed from the TEM analysis of
Rh(0)/SiO2/HF catalyst isolated after ten recharges (Fig. 4).
by Rh(0)/SiO2/HF under 4 atm (constant pressure) at 75 ◦C, and
[Alkene]/[Rh(0)] = 1179.
or base catalysis). The Rh(0) content in the resulting xerogels
was shown to be independent of the preparative route, but acidic
conditions afforded higher encapsulated ionic liquid content and
xerogels with larger pore diameter, which in turn might have
guaranteed higher catalyst activity in the hydrogenation of alkenes.
The use of ionic liquid for the preparation of both nanoparticles
and silica affords encapsulated ionic liquid/Rh(0)/silica materials
with different morphology, texture, and catalytic activity. This
combination exhibits an excellent synergistic effect that enhances
the activity and robustness of the Rh hydrogenation catalysts.
All the supported systems were more active than that constituted
of isolated Rh(0) nanoparticles for the hydrogenation of alkenes.
Conclusions
The Rh(0) nanoparticles dispersed in ionic liquid (1-n-butyl-3-
methylimidazolium tetrafluoroborate) can be easily immobilized
within a silica network when prepared by the sol–gel method (acid
5552 | Dalton Trans., 2007, 5549–5553
This journal is
The Royal Society of Chemistry 2007
©

8 C. deCastro, E. Sauvage, M. H. Valkenberg and W. F. Holderich,
J. Catal., 2000, 196, 86–94.
9 C. P. Mehnert, E. J. Mozeleski and R. A. Cook, Chem. Commun., 2002,
3010–3011.
10 H. Hagiwara, Y. Sugawara, K. Isobe, T. Hoshi and T. Suzuki, Org.
Lett., 2004, 6, 2325–2328.
11 S. Breitenlechner, M. Fleck, T. E. Muller and A. Suppan, J. Mol. Catal.
A: Chem., 2004, 214, 175–179.
12 J. Dupont and P. A. Z. Suarez, Phys. Chem. Chem. Phys., 2006, 8,
2441–2452.
13 C. S. Consorti, P. A. Z. Suarez, R. F. de Souza, R. A. Burrow, D. H.
Farrar, A. J. Lough, W. Loh, L. H. M. da Silva and J. Dupont, J. Phys.
Chem. B, 2005, 109, 4341–4349.
14 J. Dupont, J. Braz. Chem. Soc., 2004, 15, 341–350.
15 M. Antonietti, D. B. Kuang, B. Smarsly and Z. Yong, Angew. Chem.,
Int. Ed., 2004, 43, 4988–4992.
16 Y. Zhou and M. Antonietti, J. Am. Chem. Soc., 2003, 125, 14960–14961.
17 Y. Zhou, J. H. Schattka and M. Antonietti, Nano Lett., 2004, 4, 477–
481.
Fig. 4 Micrographs obtained by TEM of Rh(0)/SiO2/HF isolated after
ten re-cycles (1-decene hydrogenation).
In particular the silica-based systems prepared under acidic
conditions were shown to be the most active, exhibiting higher
TOF in comparison to that of commercial Rh/C (5%). The denser
and bulkier silica structure generated under basic conditions (less
active catalytic system) incorporated less ionic liquid. A high level
of ionic liquid incorporation seems to be important in order to
guarantee stability for the nanoparticles.
18 Y. Zhou, Cur. Nanosc., 2005, 1, 35–42.
19 S. Dai, Y. H. Ju, H. J. Gao, J. S. Lin, S. J. Pennycook and C. E. Barnes,
Chem. Commun., 2000, 243–244.
20 C. W. Scheeren, G. Machado, S. R. Teixeira, J. Morais, J. B. Domingos
and J. Dupont, J. Phys. Chem. B, 2006, 110, 13011–13020.
21 E. T. Silveira, A. P. Umpierre, L. M. Rossi, G. Machado, J. Morais,
G. V. Soares, I. L. R. Baumvol, S. R. Teixeira, P. F. P. Fichtner and J.
Dupont, Chem.–Eur. J., 2004, 10, 3734–3740.
22 J. Dupont, G. S. Fonseca, A. P. Umpierre, P. F. P. Fichtner and S. R.
Teixeira, J. Am. Chem. Soc., 2002, 124, 4228–4229.
23 J. Dupont and P. Migowski, Chem.–Eur. J., 2007, 13, 32–39.
24 X. D. Mu, D. G. Evans and Y. A. Kou, Catal. Lett., 2004, 97, 151–154.
25 X. D. Mu, J. Q. Meng, Z. C. Li and Y. Kou, J. Am. Chem. Soc., 2005,
127, 9694–9695.
Acknowledgements
Thanks are due to the following agencies: CNPq, PETROBRAS
and CT-PETRO for financial support and CME-UFRGS for
the TEM and SEM microscopy analyses. We also thank Prof.
H. S. Schrekker for proof reading of the manuscript.
26 S. D. Miao, Z. M. Liu, B. X. Han, J. Huang, Z. Y. Sun, J. L. Zhang and
T. Jiang, Angew. Chem., Int. Ed., 2006, 45, 266–269.
27 J. Huang, T. Jiang, B. X. Han, W. Z. Wu, Z. M. Liu, Z. L. Xie and J. L.
Zhang, Catal. Lett., 2005, 103, 59–62.
28 V. Mevellec, A. Nowicki, A. Roucoux, C. Dujardin, P. Granger, E.
Payen and K. Philippot, New J. Chem., 2006, 30, 1214–1219.
29 K. Zhu, F. Pozgan, L. D’Souza and R. M. Richards, Microporous
Mesoporouos Mater., 2006, 91, 40–46.
30 C. C. Cassol, G. Ebeling, B. Ferrera and J. Dupont, Adv. Synth. Catal.,
2006, 348, 243–248.
31 G. S. Fonseca, A. P. Umpierre, P. F. P. Fichtner, S. R. Teixeira and J.
Dupont, Chem.–Eur. J., 2003, 9, 3263–3269.
32 F. C. Stedile and J. H. Z. dos Santos, Phys. Status Solidi A, 1999, 173,
123–134.
Notes and references
1 C. P. Mehnert, Chem.–Eur. J., 2004, 11, 50–56.
2 A. Riisager, R. Fehrmann, M. Haumann and P. Wasserscheid, Top.
Catal., 2006, 40, 91–102.
3 C. Bianchini and G. Giambastiani, Chemtracts, 2003, 16, 301–309.
4 A. Riisager, R. Fehrmann, M. Haumann and P. Wasserscheid, Eur. J. In-
org. Chem., 2006, 695–706.
5 C. L. Feng, Y. H. Wang and Z. L. Jin, Prog. Chem., 2005, 17, 209–216.
6 C. P. Mehnert, R. A. Cook, N. C. Dispenziere and M. Afeworki, J. Am.
Chem. Soc., 2002, 124, 12932–12933.
7 P. B. Webb, T. E. Kunene and D. J. Cole-Hamilton, Green Chem., 2005,
7, 373–379.
33 U. Schubert, N. Husing, Inorganic Materials: A Chemical Approach,
1st edn, Wiley, Weinheim, 2000.
34 Characterization and Chemical Modification of the Silica Surface.
(Studies in Surface Science and Catalysis, vol. 93), ed. E. F. Vansant,
P. Van Der Voort and K. C. Vrancken, Elsevier, Amsterdam, 1995.
This journal is
The Royal Society of Chemistry 2007
Dalton Trans., 2007, 5549–5553 | 5553
©