Rhodium nanoparticles stabilized by ionic copolymers in ionic liquids: Long lifetime nanocluster catalysts for benzene hydrogenation

Article abstract of DOI:10.1021/ja051803v

Novel ionic liquid-soluble ionic copolymers containing imidazolium ionic liquidlike units have been synthesized. Rhodium nanoparticles stabilized by the ionic copolymer in ionic liquids have been successfully obtained. The nanoparticles showed unprecedented lifetime and activity in arene hydrogenation under forcing conditions (a temperature of 75 °C and a hydrogen pressure of 40 bar) with a total turnover (TTO) of 20000 (in five total recycles of 4000 TTOs each) and a turnover frequency of 250 h^-1, demonstrating that the combination of ionic liquids with ionic liquidlike stabilizers is a pathway towards highly stable and active nanoparticle catalysts. Copyright

Full text of DOI:10.1021/ja051803v

Published on Web 06/15/2005
Rhodium Nanoparticles Stabilized by Ionic Copolymers in Ionic Liquids:
Long Lifetime Nanocluster Catalysts for Benzene Hydrogenation
†
‡
‡
,†
Xin-dong Mu, Jian-qiang Meng, Zi-Chen Li, and Yuan Kou*
State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry and Molecular
Engineering, Peking UniVersity, Beijing 100871, China, and Department of Polymer Science and Engineering,
College of Chemistry and Molecular Engineering, Peking UniVersity, Beijing 100871, China
Received March 21, 2005; E-mail: yuankou@pku.edu.cn
Soluble nanoparticle catalysts have attracted increasing attention
in recent years because of their unique properties.1,2 Metallic
Scheme 1. Synthesis of
poly[(N-Vinyl-2-pyrrolidone)-co-(1-vinyl-3-alkylimidazolium halide)]
Copolymers
nanoparticles are kinetically unstable with respect to agglomeration
to the bulk metal and must therefore be stabilized by additives;
examples include quarternary ammonium salts, soluble polymers,
1
and polyoxoanions. Nanoparticles that are both stable and active,
especially under forcing conditions, are highly desired to facilitate
2
actual practical applications. Combining different stabilizing effects
copolymerization of the 1-vinyl-3-alkyl imidazolium halide and
N-vinyl-2-pyrrolidone (NVP) using azobisisobutyronitrile (AIBN)
as the initiator in methanol at 60 °C for 16 h.
The resulting copolymers were highly soluble in 1-butyl-3-
methylimidazolium ILs as well as in other highly polar organic
solvents such as methanol. Their average molecular weights were
determined by gel permeation chromatography (GPC) analysis.
Rhodium nanoparticles were synthesized by the hydrogen
may produce stable and active nanoparticles. For example, rhodium
nanoparticles co-stabilized with polyoxoanions and tetrabutylam-
monium cations can give total turnovers (TTO) up to 193 000 for
olefin hydrogenation (a catalytic lifetime record for olefin hydro-
3
genation by soluble nanoparticles) and a TTO of 2600 for arene
hydrogenation.4
Recently, nanoparticles stabilized by an ionic liquid (IL), 1-butyl-
3
-methylimidazolium hexafluorophosphate ([BMI][PF
6
]), have been
5-8
reduction of RhCl ‚3H O in the presence of the copolymers
reported,
and a TTO of 3509 in 32 h for arene hydrogenation
3
2
dissolved in [BMI][BF
4
] IL. A typical micrograph of the rhodium
by iridium (a catalytic lifetime record for arene hydrogenation by
6
9,10
nanoparticles is shown in Figure 1a. A relatively narrow unimodal
size distribution with a diameter of 2.9 ( 0.6 nm is observed.
Benzene hydrogenation was carried out at 75 °C under 40 bar
hydrogen pressure to evaluate the catalytic performance of the
nanoparticles, with the results shown in Table 1. When a poly-
soluble nanoparticles) has been achieved. Different thiol
and
nitrile11 functionalized imidazolium ILs have also been used to
stabilize metal nanoparticles. More recent studies have shown that
imidazolium ILs may oxidatively add to coordinatively unsaturated
low valent metal centers,1
2,13
implying that the ILs might stabilize
[
(N-vinyl-2-pyrrolidone)-co-(1-vinyl-3-butylimidazolium chloride)]
metal nanoparticles not only electrostatically but also by coordina-
tion involving the cations. The presence of surface-attached
N-heterocyclic carbenes formed by reaction between Ir nanoclusters
and imidazolium-based ILs has been observed very recently by
means of elegant 2H labeling and NMR studies.14 Although
imidazolium ILs are thought to be good stabilizers for nanoparticles,
in some cases it is found that agglomeration still leads to loss of
activity, indicating that IL-stabilization alone does have limitations.6
+
-
copolymer, abbreviated as poly(NVP-co-VBIM Cl ), having an
NVP concentration of 54 mol % was used as co-stabilizer in [BMI]-
[
BF
4
] with a copolymer/metal mole ratio of 5:1, a TTO of 4000
-
1
and a turnover frequency (TOF) of 250 h were obtained in 16 h
(entry 1).
To determine whether the catalyst is heterogeneous or homo-
1
9
geneous, a mercury poisoning experiment was performed; the
result clearly showed the heterogeneous nature of the catalyst (see
Figure S5 in the Supporting Information).
Adding other materials to ILs can combine different types of
stabilizing effects, leading to more stable nanoparticles. It is
important to note, however, that commonly used organic stabilizers,
After the reaction, the cyclohexane product can be easily
separated from the system by simple decantation and the IL phase
containing the nanoparticles can be reused. When hydrogenation
of benzene was repeated five times under the conditions shown in
entry 1, there was no loss in activity and the TTO exceeded
15
for example, 1,10-phenanthroline (phen) and poly(N-vinyl-2-
pyrrolidone) (PVP),16 generally have low solubility in ILs. Our new
approach, described in this communication, is to design and
synthesize ionic copolymers containing imidazolium IL-like units,
which are potentially capable of acting as soluble bifunctional co-
stabilizers when dissolved in ILs. This has enabled us to synthesize
highly stable and active metal nanoparticles.
20 000 mol/mol Rh, which is 5.7 times higher than the previous
record of 3509 (Ir nanoparticles stabilized by [BMI][PF ]) . The
6
6
size distributions of rhodium nanoparticles before (Figure 1a) and
after (Figure 1b) reaction also remained unchanged. ICP analysis
of the benzene layer separated from the first run indicated absence
of any rhodium within the detection limits (i.e., less than
The synthesis of the copolymers is illustrated in Scheme 1. Unlike
previously reported syntheses of vinyl-substituted imidazolium
salts,17,18
our 1-vinyl-3-alkyl imidazolium salts were prepared from
0.1 µg/mL).
the N-alkylation of 1-vinylimidazole with the corresponding alkyl
halides. The polymers were then synthesized by the free radical
In contrast, when phen or PVP was used as an additional
stabilizer, the catalysts deactivated rapidly and a black precipitate
was observed (entries 2 and 3); the limited solubility of these
additives in ILs may limit their stabilizing abilities.
†
State Key Laboratory for Structural Chemistry of Unstable and Stable Species.
Department of Polymer Science and Engineering.
‡
9694
9
J. AM. CHEM. SOC. 2005, 127, 9694-9695
10.1021/ja051803v CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
nanoparticles were stable, their activity was reduced compared with
that observed when a moderate amount of copolymer was used
(entry 1).
It is important to stress that the outstanding stability and activity
of our ionic copolymer-stabilized rhodium nanoparticles in ILs result
from a highly synergistic effect between the polymer and IL. When
4
hydrogenation was carried out in [BMI][BF ] IL in the absence of
the copolymer (entry 4) or in methanol in the presence of the
copolymer (entry 5), the catalysts showed poor activities and the
formation of black precipitates was observed. This indicates that
neither the copolymer nor the IL alone can effectively stabilize the
rhodium nanoparticles under these reaction conditions. Clarification
of the mechanism of nanoparticle stabilization and the origin of
the synergy between the two components requires further detailed
study, which is currently underway in our laboratory.
In conclusion, novel ionic liquid-soluble ionic copolymers
containing imidazolium ionic liquidlike units have been synthesized.
Rhodium nanoparticles stabilized by the ionic copolymer in ionic
liquids have been successfully obtained. The nanoparticles showed
unprecedented lifetime and activity in arene hydrogenation under
forcing conditions (a temperature of 75 °C and a hydrogen pressure
of 40 bar) with a TTO of 20 000 (in five total recycles of 4000
TTOs each) and a TOF of 250 h , demonstrating that the
combination of ionic liquids with ionic liquidlike stabilizers is a
pathway towards highly stable and active nanoparticle catalysts.
Figure 1. TEM micrographs and particle size histograms of copolymer
stabilized rhodium nanoparticles in [BMI][BF4] (a) before and (b) after
recycling four times (20 000 TTOs) (200 particles counted for each sample,
scale bar ) 50 nm).
-
1
Table 1. Hydrogenation of Benzene Catalyzed by
+
-
Poly(NVP-co-VBIM Cl ) Polymers and IL Co-Stabilized Rhodium
Nanoparticlesa
Acknowledgment. This work was financially supported by the
National Science Foundation of China (Project No. 20473002.).
We are grateful for the helpful discussion with Dr. David G. Evans.
concn of NVP
stabilizer/
metal
mole ratioc
(
mol %) in the
conv
(%)d
TOF
(h-1)e
catalyst
stability
entry
stabilizer
copolymerb
1
2
3
4
5
6
7
8
9
polymerf
phen
PVP
54
-
5:1
1:1
1:1
-
5:1
5:1
5:1
1:1
10:1
100
<1
<1
<1
<1
79
93
2
64
250
-
Supporting Information Available: Synthesis and characterization
of copolymers, synthesis of the rhodium nanoparticles, and procedures
and experimental data for hydrogenation reactions. This material is
available free of charge via the Internet at http://pubs.acs.org.
poorg
poorg
-
-
g
-
-
-
-
197
212
5
poor
f,h
poorg
polymer
polymer
polymer
polymer
polymer
54
39
42
54
54
f
f
f
f
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poorg
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(
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J. AM. CHEM. SOC.
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