Perfluoro-tagged rhodium and ruthenium nanoparticles immobilized on silica gel as highly active catalysts for hydrogenation of arenes under mild conditions

Article abstract of DOI:10.1039/c2nj40859g

Perfluoro-tagged rhodium and ruthenium nanoparticles have been stabilized on highly fluorinated hybrid organic-inorganic silica gel. The method for preparation of these nanoparticles is highly reproducible and the new materials were robust, recoverable and active catalysts for arene hydrogenation under very mild conditions.

Full text of DOI:10.1039/c2nj40859g

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Perfluoro-tagged rhodium and ruthenium
nanoparticles immobilized on silica gel as highly active
catalysts for hydrogenation of arenes under mild
conditions†
Cite this: NewJ.Chem., 2013,
37, 278
Received (in Montpellier, France)
21st September 2012,
Accepted 7th November 2012
Sandra Niembro,a Silvia Donnici,a Alexandr Shafir,a Adelina Vallribera,*a
Mar´ıa L. Buil,b Miguel A. Esteruelas*b and Carmen Larramonab
DOI: 10.1039/c2nj40859g
www.rsc.org/njc
Perfluoro-tagged rhodium and ruthenium nanoparticles have been
stabilized on highly fluorinated hybrid organic–inorganic silica gel.
The method for preparation of these nanoparticles is highly repro-
ducible and the new materials were robust, recoverable and active
catalysts for arene hydrogenation under very mild conditions.
phosphine-free perfluoro-tagged palladium nanoparticles on a
solid support. If successful, this would extend the family of
materials currently used for particle entrapment, including
polymers, surfactants, dendrimers, cyclodextrins and aerogels.
In this context, we observed that previously formed fluorous-
stabilized Pdnp were successfully immobilized on fluorous
silica gel through fluorous–fluorous interactions or within a
silica gel in the form of a hybrid organic–inorganic framework.
Introduction
The development of nanoparticles has been intensively Both types of supported Pdnp-materials can be successfully
pursued due to their unusual properties that arise from quan-
tum size effects and their large surface areas.1,2 Common
methods by which nanoparticle synthesis is accomplished
comprise electrochemical synthesis, decomposition of organo-
metallic precursors or the reduction of metal salts. In the
reduction of metal salts, the presence of a stabilizer serves to
prevent the nanoclusters from further aggregation into bulk
metal.1 Although at first glance heavily fluorinated com-
pounds3 do not seem to be best suited as stabilizers of
nanoparticles (perfluorinated chains are indeed known to
exhibit very small attractive interactions even among them-
selves), we previously demonstrated that fluorinated
compounds can indeed be used as a protecting shield for
nanoparticles. Thus, palladium nanoparticles were obtained
from the soluble palladate salt Na2Pd2Cl6 using MeOH as a
reducing agent in the presence of several heavily fluorinated
compounds.4 Following this work, we were intrigued by the
possibility of applying this concept to the immobilization of
used as phosphine-free catalysts under air in the alkynylation
of aryl halides (copper-free), in the Suzuki–Miyaura cross-
coupling and in the Heck reaction. At the end of the reaction,
this supported palladium could be recovered by centrifugation
and decantation in air and could be reused in additional
catalytic runs.5a–e Following our methodology of 20085a other
authors have prepared and used the same fluorous-stabilized
Pdnp immobilized on fluorous silica gel successfully in Suzuki
reactions and for C-2 arylation of indoles.5f–h
In order to extend the benefits of the fluorous hybrid silica
to other catalytic processes, we shifted our attention to the
synthesis of Rh and Ru nanoparticles commonly used as
catalysts for hydrogenation reactions.2g Among such processes,
hydrogenation of monocyclic arenes is a process of high
industrial interest.6 As an example, the reduction of benzene
to cyclohexane, followed by the oxidation to adipic acid and
caprolactam, constitutes one of the basic steps for the production
of nylon-6 and nylon-6.6. Arene hydrogenation has also generated
interest from an environment preservation standpoint,7 partly
due to the demand for cleaner-burning low-aromatic-content
diesel fuel, which is otherwise associated with exhaust particles
containing powerful carcinogens. The field is also of interest in
relation to the hydrogen storage.8
a
`
Department of Chemistry, Universitat Autonoma de Barcelona, Campus UAB,
`
08193 Cerdanyola del Valles, Barcelona, Spain.
E-mail: adelina.vallribera@uab.es; Fax: +34 935-812477; Tel: +34 935-813045
b
´
´
´
´
´
Departamento de Quımica Inorganica-Instituto de Sıntesis Quımica y Catalisis
´
Homogenea (ISQCH), Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain.
E-mail: maester@unizar.es; Fax: +34-976-761187; Tel: +34-976-761160
The chemical reduction of common rhodium and ruthenium
chlorides in the presence of an appropriate stabilizer is probably
the most general synthetic route to colloids of these metals. This
† Electronic supplementary information (ESI) available: Experimental
description of the preparation of Rhnp@1 and Runp@1 and TEM images. See
DOI: 10.1039/c2nj40859g
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Table 1 Characteristics of the catalytic materials
methodology was recently used to prepare Runp stabilized by
imidazolium ionic liquids,9 methylated cyclodextrins,10 and
poly(4-vinylpyridine),11 all of which were subsequently employed
as catalysts in arene hydrogenation reactions. As for rhodium-
catalyzed arene hydrogenation, the process has been accom-
plished using nanoparticles supported on TiO2,12 silica,13 in
multiwalled carbon nanotubes14 and nanofibers,15 a PVP-derived
polymer,16 graphene,17 and alumina,18 as well as those stabilized
by polynitrogen ligands in ionic liquids.19
In this paper we report the synthesis of perfluoro-tagged
rhodium and ruthenium nanoparticles immobilized on
functionalized silica gel and the catalytic activity of the newly
prepared composites in the hydrogenation of arenes.
Materiala
Mnp/stabilizer
Mols
MCl3ÁxH2O (mmol)
Entry
Ø (nm)
Mb (%)
1
2
3
4
5
6
7
8
9
10
Rhnp@1
Rhnp@1
Runp@1
Runp@1
Rhnp@1
Rhnp@1
Rhnp@1
Runp@1
Runp@1
Runp@1
45
45
47
47
114
114
114
113
113
113
2.9 0.8
2.8 0.4
1.2 0.1
1.2 0.3
2.4 0.5
2.0 0.3
2.3 0.3
1.0 0.4
1.3 0.4
1.2 0.2
1.51
2.3
2.04
1.9
3.84
3.33
3.71
2.93
2.67
2.69
a
A 1 : 1 stabilizer : metal ratio (mmol of organic component) was used
in all the reactions. RhCl3Á3H2O or RuCl3Á1.5H2O was used as a metal
b
source. Determined by inductively coupled plasma (ICP).
Results and discussion
The stabilizer used in this work is the organic–inorganic hybrid
1 (Scheme 1) which has been previously used successfully in
Pdnp preparation. A fluorinated aromatic thioether combining
several features expected to enhance its stabilizing capacity,
namely the two fluorinated chains; the sterical bulk; as well as
the S and N atoms as potential donors, is incorporated within a
polymeric hybrid silica gel structure. In principle, a combi-
nation of the weak interactions between such a stabilizer and
the particle surface would contribute to the overall stability of
nanoparticles without hindering their catalytic activity, thus
striking a balance between catalytic activity and stability.
Material 1 was prepared by the sol–gel condensation of a
monosilylated fluorous derivative of cyanuric chloride with a
large excess of tetraethoxysilane in formic acid.5a
Thus, Rhnp were prepared by reducing an aqueous solution of
RhCl3Á3H2O with NaBH4 in the presence of stabilizer 1
(Scheme 1). Similarly, Runp were prepared starting from RuCl3Á
1.5H2O. The results are summarized in Scheme 1 and Table 1.
Transmission electron microscopy (TEM) confirmed the
presence of very small (1–3 nm) nanoparticles (see Fig. 1 and 2).
Fig. 1 TEM image and electron diffraction pattern of Rhnp@1.
Fig. 2 TEM image and particle size distribution of Runp@1.
This result was encouraging, since a smaller particle size may lead
to an enhanced catalytic activity. In the case of Rhnp, electron
diffraction showed a pattern characteristic of face-centered cubic
rhodium(0), with clearly observed (111), (200), (220) and (311)
rings. No electron diffraction, however, was obtained for the Runp
In addition, powder X-ray diffractions for Rhnp@1 and Runp@1 as
.
Scheme 1 Preparation of Rh and Ru nanoparticles with highly fluorinated
stabilizing agent 1.
c
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Table 2 Hydrogenation of aromatic substrates with Rhnp@1 and Runp@1a
Entry
1
Catalysts
Substrate
Product
TOFb
111
2 mol% Rhnp@1
2
2 mol% Rhnp@1
37
3
4
2 mol% Rhnp@1
2 mol% Rhnp@1
199
7
Fig. 3 Comparison of powder XRD of 1, Rhnp@1 and Runp@1.
5
6
2 mol% Rhnp@1
2 mol% Rhnp@1
36
well as the hybrid silica 1 were measured (Fig. 3). Material Rhnp@1
did show an intense new peak at 2y = 40.851, corresponding to the
spacing of d = 2.210 Å and attributable to the Rhfcc [111] reflection
(theoretical d = 2.196 Å). Still, only the matrix pattern was observed
35
7
8
9
2 mol% Runp@1
2 mol% Runp@1
29
in the XRD pattern of the corresponding Runp
.
One of the general problems in the preparation of metallic
nanoparticles is the reproducibility in terms of nanoparticle
size and metal content. As a test, three different batches of Rhnp
were prepared under the same experimental conditions
(see entries 5–7 of Table 1) obtaining in all the cases nano-
particles of 2 nm diameter and a metal content between 3.3
and 3.8%. In the case of Runp the size was around 1 nm and
the metal content between 2.7 and 2.9% demonstrating once
more the reproducibility of the method (see entries 8–10 of
Table 1).
195
2 mol% Runp@1
2 mol% Runp@1
5
10
32
a
Reaction conditions: 60 1C, 0.024 mmol of catalyst, 1.2 mmol of arene,
b
5 mL of 2-propanol, 1 atm of H2. Turnover frequency defined as mol
of H2/mol of metal  h.
The catalytic properties of these new materials were tested in
the hydrogenation of arenes. We found that Rhnp@1 catalyzes
the quantitative hydrogenation of aromatic hydrocarbons such
as benzene, toluene, and styrene; as well as phenol and measured by H2 uptake (Fig. 4 and Table 2). Acetone was not
pyridine at a constant atmospheric pressure of hydrogen, at detected during the reaction, indicating that the reduction of
60 1C, in the presence of 2% mol of Rh(0) and using 2-propanol the substrates by hydrogen transfer does not take place.
as solvent. The catalysis proceeds at considerable rates, as
Benzene is rapidly reduced to cyclohexane in quantitative yield at
a turnover frequency (TOF) of 111 mol H2 (mol Rh(0))À1 hÀ1
(Table 2, entry 1), and the nanoparticles maintained their catalytic
activity for the following five cycles (Fig. 5). Less turnover frequency
Fig. 4 Initial reaction profiles (H2 uptake vs. time) in the catalytic hydrogenation
of aromatic compounds with Rhnp@1 in 2-propanol (5 mL) at 60 1C and
1 atm of H2.
Fig. 5 Initial reaction profiles (mL of H2 uptake) for 6 consecutive cycles in the
hydrogenation of benzene with Rhnp@1.
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was observed for Runp (Table 2, entry 7). Substituents on the Acknowledgements
aromatic ring slow down the hydrogenation, possibly as a conse-
This research was supported by the Ministerio de Ciencia e
quence of the substituents ‘‘bumping’’ into the nanoparticle
stabilizer. Thus, even for toluene the TOF is reduced to 37 mol
H2 (mol Rh(0))À1 hÀ1 (entry 2). Although styrene (entry 3) is quickly
reduced to ethylbenzene (TOF about 199 mol H2 (mol Rh(0))À1 hÀ1),
the hydrogenation of the latter to ethylcyclohexane takes place at a
rate of 6 mol H2 (mol Rh(0))À1 hÀ1) much slower than the reduction
of toluene (entries 3 and 4). Phenol was reduced to cyclohexanol and
pyridine to piperidine at rates similar to the reduction of toluene
(entries 5 and 6).
´
Innovacion (Projects CTQ2008-05409-C02-01, CTQ2011-22649,
CTQ2011-23459 and Consolider Ingenio 2010 CSD2007-00006)
and the DURSI-Generalitat de Catalunya (2009SGR-1441).
´
´
The Diputacion General de Aragon (E35) and FEDER are
´
acknowledged. A.S. was funded through a Ramon y Cajal
´
fellowship from the Ministerio de Ciencia e Innovacion.
Notes and references
Runp@1 also hydrogenates benzene (entry 7), styrene
(entries 8 and 9), and phenol (entry 10) to the corresponding
saturated compounds, under the same conditions. Neverthe-
less, the catalytic activity of Runp@1 was found to be lower
than that of the rhodium catalyst. Moreover, Rhnp@1 can be
easily recovered and reused in 6 consecutive cycles in the hydro-
genation of benzene with only a marginal loss of activity
(Fig. 5).
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organic–inorganic silica gel. The method used to prepare these
nanoparticles is extremely reproducible giving rise to small
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30 m  250 mm HP-5MS 5% phenyl methyl siloxane column
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