Surfactant-stabilized aqueous iridium(0) colloidal suspension: An efficient reusable catalyst for hydrogenation of arenes in biphasic media

Article abstract of DOI:10.1002/adsc.200303157

Aqueous suspensions of iridium nanoparticles produced by the chemical reduction of IrCl₃ assisted by sonication, in the presence of N,N-dimethyl-N-cetyl-N-(2-hydroxyethyl)ammonium chloride salt as surfactant, have shown an efficient activity for the catalytic hydrogenation of various aromatic derivatives in biphasic media under mild conditions. These nanocatalysts can be reused for further runs with a total conservation of activity and provided significant catalytic lifetime for anisole hydrogenation in pure water with 3000 total turnover (TTO).

Full text of DOI:10.1002/adsc.200303157

FULL PAPERS
Surfactant-Stabilized Aqueous Iridium(0) Colloidal Suspension:
An Efficient Reusable Catalyst for Hydrogenation of Arenes in
Biphasic Media
a
a,
b
b
¬
Vincent Mevellec, Alain Roucoux, * Esther Ramirez, Karine Philippot, Bruno
Chaudretb
a
¡
¬
¬
UMR CNRS 6052, ™Syntheses et Activations de Biomolecules∫, Ecole Nationale Superieure de Chimie de Rennes
¬ ¬
Institut de Chimie de Rennes, Avenue du General Leclerc, 35700 Rennes, France
E-mail: alain.Roucoux@ensc-rennes.fr
Laboratoire de Chimie de Coordination du CNRS, UPR 8241, 205 route de Narbonne, 31077 Toulouse Cedex 04, France
b
E-mail: chaudret@lcc-toulouse.fr
Received: October 3, 2003; Accepted: December 20, 2003
Abstract: Aqueous suspensions of iridium nanopar- further runs with a total conservation of activity and
ticles produced by the chemical reduction of IrCl3 provided significant catalytic lifetime for anisole
assisted by sonication, in the presence of hydrogenation in pure water with 3000 total turn-
N,N-dimethyl-N-cetyl-N-(2-hydroxyethyl)ammonium
chloride salt as surfactant, have shown an efficient
activity for the catalytic hydrogenation of various
over (TTO).
aromatic derivatives in biphasic media under mild Keywords: arenes; biphasic catalysis; hydrogenation;
conditions. These nanocatalysts can be reused for iridium; nanoparticles; surfactants
Introduction
this approach, the reaction mixture forms a typical two-
phase system: the lower phase containing the iridium
The total or partial hydrogenation of benzene deriva- particles in 1-n-butyl-3-methylimidazolium hexafluoro-
tives represents an industrial catalytic challenge, in phosphate (BMI ¥ PF6) and the upper one the organic
particular with the increasing industrial demand for low- products. Nevertheless, it seems that the best results
aromatic Diesel fuels[1] due to serious health issues.[2] (conversion, recycling) were obtained with isolated
Moreover the conversion of benzene to cyclohexane, a Ir(0) nanoparticles in the solid state (™solvent-less
key intermediate in adipic acid production, is still the conditions∫) and not in biphasic liquid conditions with
most important industrial hydrogenation reaction of redispersed nanoparticles in BMI ¥ PF6.
monocyclic arenes.[3] The catalytic transformation is
Here, we describe the efficient hydrogenation of
generally carried out under drastic conditions with various benzene compounds in biphasic liquid-liquid
rhodium or ruthenium homogeneous or heterogeneous (water/hydrocarbon) systems by surfactant-protected
catalysts but a few examples also describe the use of Pd, iridium(0) nanoparticles and show the competitive
Pt, Ni and Co as metal catalysts.[4,5] In the past five years, advantage of this catalyst by comparison with previous
the use of nanoparticles in this active research area has catalyst systems (Table 1). The interest of our approach
received increased attention since some homogeneous
catalysts have been shown to be ™microheterogene-
ous∫.[5] In this context, the use of rhodium and ruthe-
nium nanoparticles for the hydrogenation of benzene
derivatives has been widely studied.[5,6] Attention is
presently focused on nanoparticles of an alternative
noble metal such as iridium. For example, iridium
particles supported on g-alumina or silica are known to
catalyze the hydrogenation of benzene and toluene after
important surface treatments.[7] The use of ionic liquids
for the formation and stabilization of iridium(0) nano-
clusters which are active catalysts for the hydrogenation
of olefins and arenes has recently been reported.[8] In Scheme 1.
72
¹ 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/adsc.200303157
Adv. Synth. Catal. 2004, 346, 72 76

Surfactant-Stabilized Aqueous Iridium(0) Colloidal Suspension
FULL PAPERS
Table 1. Comparison of rhodium and iridium nanoparticles for the hydrogenation of anisole and/or benzene in biphasic liq/liq
systems.
Systems
Substrate Conditions
Products (%)
TTO[a] Time TOF[b] Recyclability Ref.
(T[ 8C], PH )
(h)
(hÀ1)
2
Rh0, polyoxyanion and Anisole
tetrabutylammonium
22, 3 bars
Methoxycyclohexane
(91)Methoxycyclohex-
ene (8)
2600
144
54
Not
demonstrated
[5c]
Rh0, PVP
Rh0, tetrabutylammo- Anisole
nium hydrogen sulfate
Benzene 30, 7 bars
Cyclohexane (100)
3600
77
16
62
675
3.6
Yes
[6c]
[6e]
36, 10 bars
Methoxycyclohexane
(77)
Not
demonstrated
Rh0, bmi ¥ PF6
Anisole
75, 4 bars
Methoxycyclohexane
(100)
246
6
123
Yes
[8a]
Benzene 75, 4 bars
Cyclohexane (100)
242
22
37
33
Yes
Yes
[8a]
[9]
Rh0, hydroxyalkyl-am- Anisole
monium bromide salts
25, 1 bar
Methoxycyclohexane
(100)
2000
188
Benzene 25, 1 bar
Cyclohexane (100)
400
180
14
18
91
30
Yes
Yes
[9]
Ir0, solvent-less[c]
Anisole 75, 4 bars
Methoxycyclohexane
(84)Cyclohexane (16)
Cyclohexane (100)
[8a]
Benzene 75, 4 bars
3509
3000
32
45
328
200
Yes
Yes
[8a]
Ir0, hydroxyalkyl-am- Anisole
monium chloride salts
25, 40 bars
Methoxycyclohexane
(100)
Cyclohexane (100)
this paper
d
Benzene 25, 40 bars
100
0.8 375
Yes
this paper
[a]
[b]
[c]
[d]
Total turn-over defined as moles of substrates per moles of metal.
Turn-over frequency (related to H2 consumed) defined as turn-over divided by time.
Not a biphasic liquid/liquid system.
Standard test result, not optimized.
is the use as catalyst of colloidal metallic particles, finely aggregation. For a ratio 1/Ir< 10, we observe a slight
dispersed in water,[9] a solvent that continues to attract agglomeration of metal particles in aqueous solution
attention for economic and ecological reasons since after several days. A ratio of 10 is sufficient to stabilize
biphasic catalysis appears as an undeniable solution to colloidal particles within the aqueous phase if the
easily separate the catalyst from reaction products even preparation is assisted by sonication. In water and under
if trace amounts of aromatic compound are generally these conditions, it is known that surfactant monomers
hard to remove.[8a,10] The solubility of the nanoparticles may associate to form micelles, which favor the stabi-
is assured by their protective agent, a highly water- lization of the particles. Moreover, based on the excess
soluble surfactant which provides an electrosterical of NaBH4 and the low sonication time, it is very unlikely
stabilization to prevent aggregation and to facilitate the that the long chain of 1 contributes significantly to the
recycling process.[9]
reduction of Ir(III) through formation of potential
reducing radical intermediates. To the best of our
knowledge, this is the first example of the preparation
of iridium nanoparticles stabilized by ammonium sur-
factant under sonochemical conditions.[11] Transmission
Results
In this system, the catalytically active aqueous suspen- electron microscopy (TEM) observations show that the
sions consist of metallic iridium(0) nanoparticles pre- particles are well-dispersed on the grid and monodis-
pared by reducing iridium trichloride with sodium persed in size with an average diameter of 1.9 Æ 0.7 nm,
borohydride in a dilute aqueous solution of the pre- as shown on the size histogram which results from the
viously reported N,N-dimethyl-N-cetyl-N-(2-hydroxy- measurement of about 400 particles. Figure 1 shows the
ethyl)ammonium chloride salt 1 (Scheme 1).[9] This obtained particle size distribution, which can be well
reduction of the precursor is assisted by sonication fitted by a Gaussian curve. These Ir(0) aqueous suspen-
during five minutes. The transformation of Ir3‡ ions sions are highly stable and can be stored in air without
dissolved in water in zerovalent nanoparticles, which are special precautions although some ppm O2 can react
known to absorb in the UV region, can be followed by with iridium nanoparticles as previously reported.[12]
UV measurements. A molar ratio 1/Irˆ 10 is necessary
The so-obtained catalytic system (Ir/1/water) is very
to provide very stable suspensions and to prevent efficient for the hydrogenation of various benzene
Adv. Synth. Catal. 2004, 346, 72 76
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73

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Vincent Mevellec et al.
FULL PAPERS
Figure 2. Recycling and catalytic lifetime.
corresponds to an average turnover frequency (TOF) of
66 mol anisole converted/(mol of Ir¥ h), the highest
known TOF for arene hydrogenation in a two-phase
system with a catalytic aqueous suspension of metallic
nanoparticles. TOF are calculated according to the
amount of introduced iridium without taking into
account the true number of active metal sites, conse-
quently TOF may be underestimated.[5b,5c] In addition to
ˆ
the hydrogenation of the aromatic ring, an exo C C
double bond may also be reduced (Table 2, entry 8). A
rapid hydrogenation is also observed with disubstituted
compounds such as cresol and xylene derivatives (Ta-
ble 2, entries 9 14) leading to the major or exclusive
production of the cis-diastereomers. This demonstrates
the continuous coordination of the substrate to iridium
nanoparticles during hydrogenation leading to the mi-
gration of hydrogen to only one face of the compound.[4,5]
The catalytic lifetime was studied by reusing the
aqueous phase containing iridium nanoparticles after
simple decantation in a separating funnel for three
successive hydrogenation runs of various substrates:
monoalkyl, monofunctionalized and disubstituted ben-
zenes such as toluene, anisole and cresol, respectively.
The results are given in Figure 2. In the first cycle, the
Figure 1. TEM micrograph and size distribution of Ir(0)
nanoparticles stabilized by 1.
derivatives such as mono- or disubstituted arenes and turnover frequencies were found between 300 and
provides the corresponding saturated compounds with- 400 hÀ1 (400 hÀ1 for anisole, 375 hÀ1 for toluene,
out the hydrogenolysis products as usually observed 300 hÀ1 for p-cresol). The aqueous suspensions are still
with soluble transition metal nanoparticles.[5,8] In all active and present similar turnover activities during the
cases, the conversion is complete after a few hours successive runs (2nd run ˆ 397 hÀ1, 3rd run ˆ 395 hÀ1 for
(Table 2) and no intermediates such as cyclohexene anisole hydrogenation; 2nd run ˆ 370 hÀ1, 3rd run ˆ
derivatives are observed during the catalysis. Moreover, 366 hÀ1 for toluene; 2nd run ˆ 296 hÀ1, 3rd run ˆ 291 hÀ1
no aggregates are visually observed at the end of these for p-cresol). These preliminary results demonstrate the
experiments. No induction period is observed but steric efficient stability of the catalytically active iridium
and electronic effects of substituents affect the reaction suspensions as previously observed with rhodium nano-
rate. Indeed, arenes substituted by electron-withdraw- particles.[9]
ing groups react slowly (Table 2, entry 7) whereas the
reaction is favored with electron-donating substituents
(Table 2, entries 1, 4, 6). A total conversion is also Conclusion
observed for significant anisole/Ir molar ratios (entries
2, 3). 3000 TTO are demonstrated for anisole hydro- New aqueous colloidal solutions of N,N-dimethyl-N-
genation in 45 h at 208C and 40 bars of H2, which cetyl-N-(2-hydroxyethyl)ammonium chloride salt-pro-
74
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Surfactant-Stabilized Aqueous Iridium(0) Colloidal Suspension
FULL PAPERS
Table 2. Hydrogenation of aromatic compounds under biphasic conditions.[a]
Entry
Substrate
Substrate/Ir0
(mol/mol)
Product (yield %)[b]
Time
(h)
TOF[c]
(hÀ1)
1
2
3
4
5
6
7
8
9
Anisole
Anisole
Anisole
Phenol
benzene
Toluene
Ethyl benzoate
Styrene
100
1000
3000
100
100
100
100
100
100
Methoxycyclohexane (100)
Methoxycyclohexane (100)
Methoxycyclohexane (100)
Cyclohexanol (100)
0.75
12
45
400
250
200
300
375
375
157
344
200
1
Cyclohexane
0.8
0.8
1.9
1.16
1.5
Methylcyclohexane(100)
Ethyl cyclohexanoate (100)
Ethylcyclohexane (100)
2-Methylcyclohexanol
cis (90), trans (10)
o-Cresol
10
11
12
13
14
m-Cresol
p-Cresol
o-Xylene
m-Xylene
p-Xylene
100
100
100
100
100
3-Methylcyclohexanol
cis (80), trans (20)
4-Methylcyclohexanol
cis (78), trans (22)
1,2-Dimethylcyclohexane
cis (95), trans (5)
1,3-Dimethylcyclohexane
cis (85), trans (15)
1.2
1
250
300
181
214
250
1.65
1.4
1.2
1,4-Dimethylcyclohexane
cis (80), trans (20)
[a]
Conditions : catalyst (1.85 Â 10À5 mol), surfactant (1.85 Â 10À4 mol), water (20 mL), hydrogen pressure (40 bars),
temperature (208C), stirred at 1000 minÀ1.
[b]
Determined by GC analysis.
Turn-over frequency defined as moles of H2 per moles of iridium per hour.
[c]
Analytical Procedures
tected Ir(0) nanoparticles could be easily prepared by
reduction of IrCl3. These solutions display a quantitative A multiwave ultrasonic generator (Bioblock Scientific VCX
600) and a Ti-6Al-4V oscillator of 13 mm diameter were used
for the ultrasonic irradiation and were operated at 20 kHz with
an input power of 600 W. Sonication was carried out in a
temperature-controlled water bath (about 258C). UV-Vis
spectral measurements were obtained with a spectrophotom-
eter UV-vis Cary 4000 (Varian) at 258C.
Specimens for TEM analysis were prepared by slow
evaporation of a drop of the colloidal solution deposited
onto a holey carbon grid. TEM studies were conducted using a
JEOL 200CX-Telectron microscope operating at 200 kV with
activity for the hydrogenation of various arene derivatives
under mild conditions. In addition, an efficient recycling
process without a significant loss of activity has been
obtained. Finally, the study of the selectivity in the
hydrogenation of disubstituted monocyclic reveals that
without exception, the cis diastereomer is the major
product. The surfactant-stabilized aqueous colloidal sus-
pensions described here constitute the first efficient
catalysts using iridium as noble metal for the hydro-
genation of various monocyclic arenes in a pure biphasic a point-to-point resolution of 4.5 ä. The size distribution was
built through a manual analysis of enlarged micrographs by
measuring at least 400 particles on a given grid.
system.
Gas chromatography was performed on a Carlo Erba GC
6000 with FID detector equipped with an Altech AT1 column
(30 m, 0.25 mm inner diameter).
Experimental Section
StartingMaterials
Synthesis of Aqueous Ir(0) Suspensions
Iridium chloride hydrate was obtained from Strem chemicals.
Sodium borohydride, N,N-dimethylethanolamine, chlorohexa-
decane and all substrates were purchased from Aldrich or
Acros and were used without further purification. Water was
distilled twice before use by conventional methods. N,N-
The suspensions were prepared at 258C. Sodium borohydride
(25 mg, 6.48 Â 10À4 mol) was added to an aqueous solution of
surfactant (90 mL, 1.44 Â 10À2 mol LÀ1). This solution was
added all at once to an aqueous solution (50 mL) of the
precursor IrCl3 ¥ 3 H2O (38.8 mg, 1.3 Â 10À4 mol) to give an
aqueous Ir(0) colloidal suspension (140 mL). This reduction
was assisted by sonication during five minutes and the trans-
formation of Ir3‡ ions dissolved in water into zerovalent
Dimethyl-N-cetyl-N-(2-hydroxyethyl)ammonium
chloride
salt 1 was prepared and fully characterized as previously
reported.[9]
Adv. Synth. Catal. 2004, 346, 72 76
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¹ 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
75

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Vincent Mevellec et al.
FULL PAPERS
nanoparticles was followed by UV measurements (lmax
240 nm) and characterized by a color change from yellow to
black.
ˆ
[4] A. Roucoux, J. Schulz, H. Patin, Chem. Rev. 2002, 102,
3757.
[5] a) P. J. Dyson, J. Chem. Soc. Dalton Trans. 2003, 2964;
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Chem. 2002, 41, 1558 and 1625.
General Procedure for Hydrogenation
[6] a) K. Pelzer, O. Vidoni, K. Philippot, B. Chaudret, V.
The stainless steel autoclave was charged with the previously
prepared aqueous suspension of Ir(0) (20 mL) and a magnetic
stirrer. The appropriate substrate (1.85 Â 10À3 mol) was added
into the autoclave and dihydrogen was admitted to the system
at constant pressure (40 atm). The mixture was stirred until the
reaction was finished. Samples for gas chromatographic
analysis were removed from time to time. After complete
hydrogenation the two phases were separated by extraction
and decantation. The aqueous phase was reused in a second
hydrogenation in the cleaned autoclave. The TOF were
determined for 100% conversion.
¡
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H. Ohde, C. M. Wai, Chem. Commun. 2002, 2388; c) J.-L.
Pellegatta, C. Blandy, V. Colliere, R. Choukroun, B.
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2002, 178, 55; d) R. J. Bonilla, P. G. Jessop, B. R. James,
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III, R. G. Finke, J. Am. Chem. Soc. 1998, 120, 5653;
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We thank the Region Bretagne (France) for financial support
¬
and the Service de Microscopie de l×Universite Paul Sabatier
¡
(TEMSCAN) and V. Colliere for TEM analysis.
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¹ 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
Adv. Synth. Catal. 2004, 346, 72 76