Transition-metal nanoparticles in imidazolium ionic liquids: Recycable catalysts for biphasic hydrogenation reactions

Article abstract of DOI:10.1021/ja025818u

1-n-Butyl-3-methylimidazolium hexafluorophosphate room-temperature ionic liquid is not only suitable as a medium for the preparation and stabilization of iridium nanoparticles but also ideal for the generation of recyclable biphasic catalytic systems for hydrogenation reactions. Thus, Ir(0) nanoparticles with a mean diameter of 2 nm have been prepared by reduction of Ir(I) dissolved in the ionic liquid with H₂. This catalytic solution can be reused several times for the biphasic hydrogenation of olefins under mild reaction conditions. Copyright

Full text of DOI:10.1021/ja025818u

Published on Web 03/29/2002
Transition-Metal Nanoparticles in Imidazolium Ionic Liquids: Recycable
Catalysts for Biphasic Hydrogenation Reactions
Jairton Dupont,*,† Gledison S. Fonseca, Alexandre P. Umpierre, Paulo F. P. Fichtner, and
†
†
‡
§
Sergio R. Teixeira
Laboratory of Molecular Catalysis, Institute of Chemistry, Department of Metallurgy and Institute of Physics,
UFRGS, AV. Bento Gon c¸ alVes, 9500 Porto Alegre 91501-970 RS Brazil
Received February 5, 2002
The search for more efficient catalytic systems that might
combine the advantages of both homogeneous (catalyst modulation)
and heterogeneous (catalyst recycling) catalysis is one of the most
Table 1. Hydrogenation of Olefins by Ir Nanoparticles in BMI‚PF6
Pa
conv.b
T
(°C)
time
(h)
entry
olefin
1-decene
1-decene
1-decene
1-decene
1-decene
styrene
cyclohexene
methyl methacrylate
4-vinylcyclohexene
4-vinylcyclohexene
olefin/Ir
(atm)
(%)
1
exciting challenges of modern chemistry. In most of the cases the
1
2
3
4
1 200
1 200
12 000
1 200
1 200
1 200
1 200
1 200
1 200
1 200
4
4
4
2
4
4
4
4
4
4
75
30
75
75
75
75
75
75
75
75
0.5
3.0
2.0
0.7
0.5
1.0
3.2
17
100
92
100
100
56
63
100
100
concepts behind these methods involve, in simplistic terms, the
heterogenization of homogeneous catalysts or inversely the ho-
mogenization of heterogeneous catalysts. Indeed, various efficient
homogeneous catalyst immobilization methods have been developed
in the last years through the use, for example, of liquid-liquid1
organometallic biphasic catalysis, heterogenization of molecular
c
5
6
7
8
9
,2
d
3
1.0
4.0
91
100
catalysts in solid supports. More recently, with the advances of
e
10
nanochemistry it has been possible to prepare “soluble” analogues
4
of heterogeneous catalysts. It is believed that nanoparticles (near
a
Constant pressure. b Conversion (GC). c Reaction performed with [Ir-
d
monodispersed particles that are usually less than 100 Å in diameter)
will have properties intermediate between those of bulk and single
Cl(cod)]2 in CH Cl2 (in the absence of BMI‚PF ). Conversion and
selectivity in 4-ethylcyclohexene. Conversion in ethylcyclohexane.
2
6
e
5
particles. However, nanoparticles are solely kinetically stable and
they should be stabilized against aggregation into larger particles
and bulk material. The main methods used for the stabilization of
nanoparticles in solution involves electrostatic or steric protection
by, for example, the use of water-soluble polymers, quaternary
ammonium salts, surfactants, or polyoxoanions.5
Herein we report our preliminary results on the use of room-
6
temperature imidazolium ionic liquids (RTILs) for the formation
and stabilization of iridium nanoparticles that are recyclable catalysts
for biphasic hydrogenation reactions.7
Among the various methods available for the generation of
nanoparticles we employed ligand reduction and displacement of
4
__
Figure 1. Curve-fit of 1-decene (ooo) and cyclohexene ( ) loss (reaction
conditions as described in Table 1, entries 1 and 7).
[
IrCl(cod)]
dazoilum hexafluorophosphate ([BMI][PF
of an orange solution of [IrCl(cod)] (16 mg, 0.05 mmol) in 3 mL
of [BMI][PF ] with hydrogen (4 atm) at 75 °C for 10 min affords
2
(cod ) 1,5-cyclooctadiene) in 1-n-butyl-3-methylimi-
6
]) IL. Thus, treatment
2
6
a black “solution”. This ionic solution promotes the biphasic
hydrogenation of various olefins under mild reaction conditions,
and the products were isolated almost quantitatively by simple
decantation (Table 1). The catalytic activity of this system (TOF
-
1
)
6000 h at 1200 rpm and 75 °C) is significantly superior to
those obtained in biphasic conditions by classical transition-metal
6
catalyst precursors in RTILs under similar reaction conditions. Of
Figure 2. Comparison of catalyst recycling in the hydrogenation of
1-decene by Crabtree’s catalyst (gray) and iridium nanoparticles (black) in
BMI‚PF6 at 75 °C and 4 atm of H2 for 40 min. (olefin/Ir ) 1200).
note is that the hydrogenation can be conducted at room temperature
with a slightly positive pressure of hydrogen (entry 2). Figure 1
shows the 1-decene and cyclohexene loss with time under a constant
pressure of H (4 atm) at 75 °C with an olefin/Ir of 1200. Moreover,
2
the cyclohexene curve is sigmoidal and can be fitted to the A f
B, A + B f 2B autocatalytic mechanism that is characteristic of
nanocluster formation and growth (analogous to those observed with
Ir nanoparticles stabilized by polyoxoanions).5g
The catalytic behavior and the recycling of these ionic liquid-
8
stabilized iridium particles were compared with Crabtree’s catalyst
*
Author for correspondence. E-mail: dupont@iq.ufrgs.br.
Laboratory of Molecular Catalysis.
Department of Metallurgy.
3 6 6
precursor [Ir(cod)py(PCy )][PF ] in [BMI][PF ] under the same
reaction conditions (Figure 2). The catalytic activity of Crabtree’s
catalyst decreases significantly upon recycling, whereas the Ir
†
‡
§
Insitute of Physics.
4228
9
J. AM. CHEM. SOC. 2002, 124, 4228-4229
10.1021/ja025818u CCC: $22.00 © 2002 American Chemical Society

C O MMU N I C A T I O N S
In summary we have demonstrated for the first time that the
6
room-temperature [BMI][PF ] IL is not only a suitable medium
1
1
for the preparation and stabilization of transition-metal nanopar-
ticles but also ideal for the generation of recyclable biphasic
hydrogenation systems. Inasmuch, a plethora of imidazolium ILs
(with different physical-chemical properties) can be easily prepared
by varying the anion and the alkyl chain on the aromatic ring, and
this thereby opens the possibility for the preparation of distinct
nanoparticles, for biphasic catalysis.
Acknowledgment. We are grateful to FAPERGS and CTPE-
TRO for financial support and to CAPES for scholarships.
Supporting Information Available: Experimental procedures
(preparation and characterization of the nanoparticles) (PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
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Figure 3. (a) TEM micrograph showing the Ir nanoparticles observed at
2
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Histogram illustrating the particle size distribution. The approximate number
of Ir atoms estimated for spherical particles of the corresponding diameter
is indicated in each bar. (Ir particles isolated after catalysis).
6
nanoparticles in [BMI][PF ] maintain their efficiency for up to at
least seven recycles (total turnovers > 8 400).
An Hg test5b clearly indicated the presence of Ir particles (see
Supporting Information) in the system formed by the reduction of
6
the Ir(I) precursor in [BMI][PF ]. Moreover, these particles can be
isolated by centrifugation (0.5 h at 3000 rpm) from the catalytic
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(
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9
3
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are similar to those obtained by Finke from the H
Bu N]Na [1,5-cod)Ir‚P
62] in acetone.5g The isolated Ir
nanoparticles can be used directly for heterogeneous processes or
re-immobilized in [BMI][PF ] and reused for the hydrogenation
2
reduction of
[
4
3
2
W
15Nd
3
O
(10) The presence of water in the system causes the decomposition of the [BMI]-
[
PF
6
] IL, however, without affecting the catalytic performance of the ionic
catalytic “solution”.
6
(11) The H
2
reduction of Ru(III) and Rh(III) compounds in [BMI][PF
6
] also
produces nanoparticles that are active for biphasic catalysis.
reactions showing the same catalytic performance to those that have
been freshly prepared.10
JA025818U
J. AM. CHEM. SOC.
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