Ru nanoparticles immobilized on montmorillonite by ionic liquids: A highly efficient heterogeneous catalyst for the hydrogenation of benzene

Article abstract of DOI:10.1002/anie.200502632

(Figure Presented) In three steps to ruthenium-based heterogeneous catalysts: The central element of this novel and simple route is the immobilization of Ru⁰ nanoparticles on montmorillonite (see picture; the arrows indicate a Ru nanoparticle i

Full text of DOI:10.1002/anie.200502632

Communications
Catalytic Hydrogenation
simpler and more efficient method for the preparation of Ru
catalysts is still a challenge in nanocatalysis. As mentioned
above, MMT possesses negatively charged, two-dimensional
silicate sheets with cationic species, and ILs are composed
entirely of ions. The combination of MMT and ILs may
therefore provide a new way to prepare highly efficient
catalysts. Herein, we report the immobilization of Ru nano-
particles onto MMT using the IL 1,1,3,3-tetramethylguanidi-
nium trifluoroacetate ([TMG][TFA]) and their application in
the catalytic hydrogenation of benzene. To the best of our
knowledge, this is the first synthesis of a nanocatalyst that
involves loading catalytic metal nanoparticles onto MMTwith
an IL.
DOI: 10.1002/anie.200502632
Ru Nanoparticles Immobilized on
Montmorillonite by Ionic Liquids: A Highly
Efficient Heterogeneous Catalyst for the
Hydrogenation of Benzene**
Shiding Miao, Zhimin Liu,* Buxing Han, Jun Huang,
Zhenyu Sun, Jianling Zhang, and Tao Jiang
The design of nanocomposites consisting of functional metals
and different matrices is a promising research area for the
fabrication of a variety of catalysts, adsorbents, and optical
and electrical devices. Montmorillonite (MMT), which is a
type of naturally occurring clay, can be structurally defined as
layers of negatively charged two-dimensional silicate sheets
that are separated by interlayer cationic species with a high
The IL used was synthesized by following previously
[
11]
reported procedures. To synthesize the catalyst, Na-MMT
was first treated three times with an aqueous solution of
[TMG][TFA] to exchange the Na cations with those of the IL
(TMG-MMT). After filtration and washing three times with
distilled water, TMG-MMT was dispersed in an aqueous
3
+
solution of RuCl with stirring in order to adsorb Ru and
3
[
1]
exchange ability for other cations. Such layered materials
have been used as host materials for the preparation of
composites and have potential applications in catalysis,
form TMG-MMT/RuCl composites. The solvent was then
3
removed from the solution by evaporation and the Ru/MMT
catalyst (catalyst A) was obtained after hydrogenation at
2208C for 2 h.
[
2]
separation, and the optical and electrical fields. For
example, Pd nanoparticles deposited on MMT with surfac-
tants exhibit a high selectivity for the partial hydrogenation of
The as-prepared catalyst A was used for the hydrogena-
tion of benzene. The results are listed in Table 1. It is clear
that catalyst A is very active for the hydrogenation of benzene
to cyclohexane, with a catalytic activity that is comparable or
superior to Ru-cluster-based catalysts. For comparison, the
TOFs of [(h -C H )(h -C Me ) Ru (m -O)(m -H) ][BF ]at
[
3]
3+
1
-phenyl-1-pentyne to 1-phenyl-cis-1-pentene, and Sc -
exchanged MMT has been demonstrated to function as an
[
4]
efficient catalyst for the Michael reaction. For MMT-based
catalysts, the catalytic nanoparticles are generally intercalated
into the interlayer spaces or deposited on the outer surfaces of
6
6
6
6
6
6
2
3
3
2
3
4
[
12]
1108C and [(h -C H ) Ru (m -H) ]Cl at 908C and a hydro-
gen pressure of 6.0 MPa
6
6
6
4
4
[13]
3
4
2
[
3,5]
MMT with the aid of surfactants or polymers.
in ionic liquids are 3644 and
In recent years, room-temperature ionic liquids (ILs) have
attracted much attention due to their unusual properties,
especially their extremely low vapor pressure, high thermal
and good chemical stability, and excellent solvent power for
organic and inorganic compounds, and they have been widely
utilized as environmentally benign solvents for different
364 molbenzene per molRu per hour, respectively.
Our Ru/MMT nanocatalyst is a heterogeneous catalyst,
which means that it is superior to homogeneous catalysts in
that it can be easily separated from the products and can be
used in a fixed-bed process. Catalyst A was reused for a
further four runs under the same reaction conditions without
any significant loss of activity (entries 6–9). This confirms that
it is very stable. We also performed benzene hydrogenation
with the traditional Ru/C and Ru/Al O catalysts under the
[
6]
[7]
processes, including chemical reactions and separations.
ILs are also attractive media to stabilize metal nanoparti-
[
8]
cles. Recently, Pd nanoparticles immobilized on the molec-
ular sieve SBA-15 by 1,1,3,3-tetramethylguanidinium lactate
have been found to exhibit high catalytic activity for hydro-
2
3
same conditions; these results are also given in Table 1
(entries 15–18). Comparing entries 2 and 17, it can be seen
that our Ru/MMT catalyst has a much higher activity than
Ru/Al O . The Ru/MMT catalyst is also more active than Ru/
[
9]
genation reactions.
Ruthenium catalysts are important catalysts for the
hydrogenation of aromatic compounds, and have been
2
3
C (see entries 3 and 16).
[
10]
extensively investigated. However, the development of a
We carried out additional experiments that allowed us to
obtain useful information about the very high activity and
stability of catalyst A. First, the IR spectrum of the catalyst
was recorded (Bruker Tensor 27). The bands at 1614 and 1419,
[*] S. Miao, Dr. Z. Liu, Prof. B. Han, Dr. J. Huang, Z. Sun, Dr. J. Zhang,
Dr. T. Jiang
Center for Molecular Science
Institute of Chemistry
Chinese Academy of Sciences
Beijing 100080 (P.R. China)
Fax: (+86)10-6256-2821
À1
1456, and 2945 cm can be attributed to the C=N and CH3
groups in the TMG cation, thus indicating that this cation is
present in the catalyst.
The X-ray photoelectron spectrum of the as-prepared
catalyst A was collected on an ESCALab220i-XL spectrom-
E-mail: liuzm@iccas.ac.cn
eter operating at 15 kV and 20 mA at a pressure of about 3 ”
[
**] This work was financially supported by the National Natural Science
Foundation of China (20374057, 50472096) and the Opening
Research Fund of the State Key Laboratory of Heavy Oil Processing
À9
1
1
0
mbar with AlKa radiation as the excitation source (hn =
486.6 eV). In the survey XPS analysis for catalyst A no Cl
(
2004-10).
could be detected, which suggests that RuCl is completely
3
2
66
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Angew. Chem. Int. Ed. 2006, 45, 266 –269

Angewandte
Chemie
Table 1: Hydrogenation of benzene with different catalysts.
The N sorption isotherms of
2
[
a]
[b]
pristine MMT, TMG-exchanged
Entry
Catalyst
Benzene/Ru [mol/mol]
T [8C]
P(H ) [MPa]
t [h]
Yield [%]
TOF
2
MMT, and catalyst A were deter-
mined with an ASAP-2405N
instrument at liquid-nitrogen tem-
perature after degassing the sam-
1
2
3
4
5
A
A
A
A
A
A
A
A
A
B
B
C
C
D
1000
1000
1000
40
40
40
2.0
4.0
6.0
8.0
8.0
8.0
8.0
8.0
8.0
4.0
8.0
4.0
6.0
4.0
4.0
6.0
4.0
4.0
2.5
2.5
1.8
1.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
5.0
2.5
3.5
4.5
84.5
100
100
100
100
100
100
100
100
100
100
54.0
94.2
44.5
99.5
98.5
35.6
79.5
–
400
555
667
4000
4000
4000
4000
4000
400
4000
–
376
–
200
394
102
177
1000
40
À4
10000
10000
10000
10000
10000
1000
10000
1000
1000
1000
1000
1000
1000
110
110
110
110
110
40
110
40
40
40
40
40
40
ples at 2008C and 10 Torr for
[
[
[
[
c]
c]
c]
c]
6
7
8
9
0
1
2
3
4
5
6
7
8
12 h. They show type-IV character-
istics, thereby indicating the meso-
porous structures of these samples.
According to the nitrogen-desorp-
tion data, the specific surface area
1
1
1
1
1
1
1
1
1
2
À1
increases from 37 m g for pris-
2
À1
tine MMT to 43 m g for TMG-
2
À1
exchanged MMTand 57 cm g for
catalyst A due to the loading of Ru
nanoparticles.
[
[
d]
d]
Ru/C
Ru/C
Ru/Al O3
[
[
e]
e]
2
We also characterized the cata-
lysts by means of X-ray diffraction
Ru/Al O3
1000
60
2
[
a] Yield of cyclohexane. [b] TOF (turnover frequency) was calculated as conversion of mols of benzene
per mol of Ru per hour. [c] Catalyst A was reused for a further four runs. [d] Ru/C, 5% Ru on carbon
powder, from ICI Co., Japan. [e] Ru/Al O , 5% Ru on Al O powder, provided by XiAA Catal. Chem. Tech.
(XRD) with an XꢀPERT SW dif-
fractometer (40 kV, 40 mA, CuKa
radiation) in the small-angle range
2
3
2
3
Co. Ltd., China.
of 2q = 1–128 and the wide-angle
range of 2q = 4–708. On the basis of
converted into the metal under the conditions used. Figure 1
shows the Ru 3d spectrum of the composite. It can be seen
that although the Ru 3d signal is obscured by the C 1s signal of
the XRD analysis in the small-angle range (Figure 2), the
basal spacing of pristine MMT is 0.966 nm, while the spacing
of TMG-exchanged MMT is enlarged to d001 = 1.42 nm due to
Figure 1. XPS spectra at the Ru 3d edge of catalyst A. The three vertical
lines indicate the peak positions of the binding energies of C 1S, RuÀ
O, and Ru, respectively.
Figure 2. XRD patterns of A) pristine MMT, B) TMG-exchanged MMT,
C) TMG-MMT/Ru (catalyst A).
a carbon contaminant at 284.6 eV, the deconvoluted spectrum
shows a doublet for two chemically different Ru entities with
peak binding energies of 280.3 (Ru 3d ) and 284.5 eV (Ru
the incorporation of the organic cations of the IL into the
galleries of MMT upon ion exchange. The characteristic peak
of catalyst A is shifted slightly to wider angle relative to the
peak of TMG-MMT, which suggests the intercalation of Ru
nanoparticles in the interlayers of MMT. However, there is no
diffraction peak for Ru nanoparticles in the XRD pattern in
the wide-angle range (not shown), which implies that the Ru
particles are either very small or amorphous.
The morphology of catalyst A was investigated with a
transmission electron microscope (TEM, JEOL, JEM-2010)
equipped with an energy-dispersive X-ray spectrometer
(EDS) at an accelerating voltage of 200 kV. It can be seen
that irregular nanoparticles with a size of less than 3 nm are
uniformly distributed on the MMT surfaces (Figure 3a).
These were confirmed as Ru particles by EDS analysis during
5
/2
0
[14]
3
d ), which confirms the presence of Ru in catalyst A.
3/2
Moreover, the peak at 281.1 eV suggests the presence of a
Ru–O component, which probably results from oxidation of
the ruthenium nanoparticles upon exposure to air.
Similar phenomena have also been reported by other
[
10c,d]
groups.
The XPS results indicate that the RuCl loaded on
3
0
MMT is reduced to Ru by hydrogen at 2208C within 2 h. This
reaction temperature is lower than those commonly used.
[
10e]
The reduction in the reaction temperature probably results
from the interaction of RuCl with the IL and/or the substrate.
3
A similar phenomenon has also been reported by Zhuang and
co-workers, who reduced RuCl and H PtCl dispersed on
3
2
6
[
15]
activated carbon with hydrogen at 1208C.
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www.angewandte.org
267

Communications
the TMG-MMT/RuCl composite with water to remove the
3
3
+
soluble Ru before hydrogen reduction. In other words, the
only difference in the procedures for preparing catalyst A and
3
+
catalyst B is that the water-soluble Ru , which could be
precipitated and mixed with the MMT/Ru composite during
the vaporization process, is removed before hydrogen reduc-
3
+
tion when preparing catalyst B, while all of the Ru is
reduced when preparing catalyst A. Catalyst B displays a
similar catalytic activity for benzene hydrogenation to
3
+
catalyst A, which suggests that the Ru in the RuCl aqueous
3
solution is firmly fixed onto the TMG-exchanged MMT
during the soaking process.
We also treated pristine MMT directly with the aqueous
solution of RuCl in the absence of the IL (catalyst C), and we
3
synthesized catalyst D by treating MMT with 1-n-butyl-3-
methylimidazolium tetrafluoroborate ([bmim][BF ]), another
4
IL, in a similar manner to catalyst A. Both catalysts C and D
exhibit much lower activity for the hydrogenation of benzene
than catalysts A and B, as shown in Table 1. Therefore, it can
be deduced that the TMG cations present in catalysts A and B
play an important role in immobilizing the Ru nanoparticles
on MMT.
It has been reported that guanidine and guanidinium
Figure 3. a) TEM image of fresh catalyst A, b) EDS analysis of cata-
[
16]
readily form coordination complexes.
On the basis of
lyst A, c) HRTEM image of a cross-section of catalyst A. The white
arrows indicate the interlaminar space of montmorillonite intercalated
with an Ru nanoparticle, d) TEM image of catalyst A after five runs.
results reported in the literature and our experimental results
we can propose a possible mechanism for the formation of
catalyst A. During the treatment of MMT with the aqueous
solution of [TMG][TFA] the cations of the IL exchange with
+
the TEM observation (Figure 3b). No large particles were
found on MMT. The nanoparticles are firmly immobilized on
MMT and could not be removed from the substrate even
when the catalyst was treated ultrasonically for a long time.
To detect whether Ru nanoparticles are present in the
interlayers of MMT, we embedded catalyst A in epoxy resin
to form a solid specimen, and cut the specimen with an
ultramicrotome to get an ultrathin section of the catalyst,
which was examined by high-resolution TEM (HRTEM).
Figure 3c shows an HRTEM image of the section of
catalyst A. The layered structure of MMT in catalyst A can
be clearly seen from this image, and the interlaminar spacing
is about 1.2 nm (as indicated by the white arrows in
Figure 3c), which is consistent with the XRD results. More-
over, there are many nanoparticles of less than 1.2 nm in the
interlaminar spaces of MMT, thus indicating that Ru nano-
particles are intercalated into the MMT interlayers. This
means that the Ru nanoparticles exist in the catalyst in two
forms: some are present on the outer surfaces and others are
embedded in the interlayers. We believe that both these kinds
of Ru nanoparticles catalyze the hydrogenation of benzene.
We also obtained the morphology of catalyst A after five
catalytic cycles by TEM (Figure 3d). It can be seen that the
nanoparticles on the MMT surface tend to aggregate. How-
ever, as discussed above, the activity of catalyst A does not
decrease significantly after five runs. This may imply that the
Ru particles in the interlayers play the main role in the high
activity of the catalyst, and that most of the Ru particles are in
the interlayers.
the Na ions present in the interlayers of pristine MMT to
form the TMG-exchanged MMT, which has a larger interlayer
spacing than MMT, as confirmed by XRD analysis. In the
3
+
subsequent process to load Ru onto the TMG-exchanged
3
+
MMT, some Ru ions could diffuse into the enlarged MMT
interlayers , whilst others are distributed on the outer surface,
3
+
due to the coordination between TMG and Ru . This
3
+
coordination between TMG and Ru is favorable to the
3
+
0
loading of Ru . Finally, Ru nanoparticles are produced by
3
+
reduction of Ru with H . Thus, for catalyst A, the TMG
2
cations interact strongly with the surface of MMT by electro-
static forces and at the same time they stabilize Ru nano-
particles by coordination due to their electron-donor abil-
[
16]
ity.
0
In this way, Ru nanoparticles could be prepared on an
MMT support by a combination of electrostatic forces and
coordination. Both the electrostatic and coordination forces
0
are very strong, which results in a very stable catalyst. The Ru
nanoparticles in the interlayers of MMTare very small, which
may be a reason for the very high activity for benzene
hydrogenation. The TEM study clearly showed that some Ru
nanoparticles are distributed on the outer MMT surfaces, and
that these could not be separated from the MMT even after
lengthy ultrasonic treatment. Therefore, the nanoparticles on
the outer surface may also be stabilized by the IL.
In summary, we presented a method to prepare Ru
nanocatalysts using [TMG][TFA] and MMT. In this method
the IL is first exchanged with the ions in the clay, and then
3
+
Ru ions are loaded onto the IL-exchanged MMT. The final
Ru/MMT nanocatalyst is obtained after hydrogen treatment.
This catalyst shows very high activity for the hydrogenation of
Using a similar procedure to that used to prepare
catalyst A, we prepared catalyst B by thoroughly washing
2
68
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Angewandte
Chemie
benzene and is very stable, due to the excellent synergistic
effects of TMG, MMT, and Ru nanoparticles. We believe that
other transition metals, such as Rh, Pd, and Ir, could also be
supported on MMT using this method and the resulting
materials could be used as efficient nanocatalysts.
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À1
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ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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269