Amphiphilic ionic liquid stabilizing palladium nanoparticles for highly efficient catalytic hydrogenation

Article abstract of DOI:10.1039/c1cp20255c

The highly water-soluble palladium nanoparticles (NPs) were synthesized by using the amphiphilic poly(ethylene glycol)-functionalized dicationic imidazolium-based ionic liquid (C₁₂Im-PEG IL) as a stabilizing agent. The aqueous dispersed palladium NPs in the range of 1.9 ± 0.3 nm were observed by transmission electron microscopy (TEM). The physicochemical properties of C₁₂Im-PEG IL in aqueous phase have been characterized by electrical conductivity, surface tension and dynamic light scattering (DLS) measurements. It was demonstrated that the amphiphilic ionic liquid can form micelles above its critical micelle concentration (CMC) in aqueous solution and the micelles played a crucial role in stabilizing the palladium NPs and thus promoted catalytic hydrogenation. Furthermore, the dicationic ionic liquid can also act as a gemini surfactant and generated emulsion between hydrophobic substrates and the catalytic aqueous phase during the reaction. The aqueous dispersed palladium NPs showed efficient activity for the catalytic hydrogenation of various substrates under very mild conditions and the stabilizing Pd(0) nanoparticles (NPs) can be reused at least eight times with complete conservation of activity. the Owner Societies 2011.

Full text of DOI:10.1039/c1cp20255c

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PAPER
Amphiphilic ionic liquid stabilizing palladium nanoparticles for highly
efficient catalytic hydrogenationw
a
b
a
a
a
a
Wenwen Zhu, Hanming Yang, Yinyin Yu, Li Hua, Huan Li, Bo Feng and
Zhenshan Hou*a
Received 29th January 2011, Accepted 13th April 2011
DOI: 10.1039/c1cp20255c
The highly water-soluble palladium nanoparticles (NPs) were synthesized by using the amphiphilic
poly(ethylene glycol)-functionalized dicationic imidazolium-based ionic liquid (C Im-PEG IL) as
1
2
a stabilizing agent. The aqueous dispersed palladium NPs in the range of 1.9 Æ 0.3 nm were
observed by transmission electron microscopy (TEM). The physicochemical properties of
C12Im-PEG IL in aqueous phase have been characterized by electrical conductivity, surface tension
and dynamic light scattering (DLS) measurements. It was demonstrated that the amphiphilic ionic
liquid can form micelles above its critical micelle concentration (CMC) in aqueous solution and the
micelles played a crucial role in stabilizing the palladium NPs and thus promoted catalytic
hydrogenation. Furthermore, the dicationic ionic liquid can also act as a gemini surfactant and
generated emulsion between hydrophobic substrates and the catalytic aqueous phase during the
reaction. The aqueous dispersed palladium NPs showed efficient activity for the catalytic
hydrogenation of various substrates under very mild conditions and the stabilizing Pd(0)
nanoparticles (NPs) can be reused at least eight times with complete conservation of activity.
Introduction
particularly to synthesize catalytically active transition-metal
NPs with control of size, near monodispersity, and stabilization.
Dupont’s group is one of the first to recognize the potential of
imidazolium-based ILs in the preparation of NPs. They
demonstrated the utility of imidazolium ionic liquids as a
template, stabilizer, and immobilizing agent for catalytic
The application of transition-metal nanoparticles (NPs) in
catalysis has been an important frontier of research in recent
years, as they have unique electronic, optical and catalytic
properties, due to the quantum size effects and the high
1
,2
surface-to-volume ratio. However, metal NPs are thermo-
dynamically unstable with respect to agglomeration to the
bulk metal and must therefore be stabilized by additives.
Examples include polymers, dendrimers, ionic liquids, surfactants,
10
transition-metal NPs. The ILs form a protective layer, which
is probably composed of imidazolium aggregates located
immediately adjacent to the nanoparticle surface, which
provides both steric and electronic protection against aggregation.
It has been reported that the transition-metal NPs dispersed in
3–8
polyoxoanions or ligands etc. Moreover, the stabilizing agents
also play an important role in the control of the NPs diameter,
imidazolium-based ILs were stable and active catalysts for
11
9
shape, size distribution and surface properties.
some reactions, such as hydrogenation,
12
oxidation of
13
Ionic liquids (ILs), as the conventional solvent alternative
and stabilizer for metal NPs, have attracted much attention
from both the academia and industry. It is now clear that ILs
have many excellent advantages such as negligible volatility,
excellent thermal stability, remarkable solubility, and a variety
of available structures. Recently, ILs have been used
alcohols, and carbon–carbon coupling reactions. However,
in some cases agglomeration still happened in simple imidzaolium
14
ILs after reaction. It suggests that simple imidazolium ILs
have limitations in stabilizing NPs, especially under harsh
reaction conditions.
15
Recently, the functionalized ILs, also frequently referred
to as task specific ILs, have been applied as both ligands and
supports for immobilizing and recycling transition metal
a
Key Laboratory for Advanced Materials, Research Institute of
Industrial Catalysis, East China University of Science and
Technology, Shanghai, 200237, China.
E-mail: houzhenshan@ecust.edu.cn
Key Laboratory of Catalysis and Materials Science of the State
Ethnic Affairs Commission and Ministry of Education, Hubei
Province, South-Central University for Nationalities, Wuhan,
16
homogeneous catalysts or as protective agents and solvents
for the stabilization of metal NPs. Above all, functionalized
ILs can provide stronger stabilization for the metal NPs
without losing their catalytic properties. Dyson’s group has
reported imidazolium and pyridinium-based ILs with nitrile
functionalities attached to the alkyl side. The two kinds of ILs
could serve as both solvent and stabilizer for palladium NPs
b
4
30074, China
w Electronic supplementary information (ESI) available. See DOI:
0.1039/c1cp20255c
1
1
3492 Phys. Chem. Chem. Phys., 2011, 13, 13492–13500
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1
7
reservoirs in certain carbon–carbon coupling reactions. The
nitrile group can stabilize the NPs by coordination principally,
and thus the stabilization effect prevents the formation of
palladium black deposits.
of palladium NPs with the functionalized IL was also
discussed.
Experimental
We have previously reported 2,2-dipyridylamine-functionalized
1
8
imidazolium ionic liquids. Palladium NPs were stabilized
effectively by the bidentate nitrogen ligands attached covalently
to the imidazolium cations, which were highly efficient multi-
phase catalysts for the hydrogenation of various olefins in ionic
liquid media. Subsequently, the ILs were found to effectively
modify the palladium NPs towards the selective hydrogenation
All manipulations involving air-sensitive materials were
carried out using standard Schlenk line techniques under an
atmosphere of nitrogen and all solvents were dried using
standard methods prior to use. Palladium acetate and Pd/C
(10 wt%) were purchased from Aldrich. Poly(ethylene glycol)-
2000, 2-methylimidazole, various substrates and all other
materials were obtained from SCRC (Sinopharm Chemical
Reagent Co, Ltd, Shanghai). The analysis of transmission
electron microscopy (TEM) was performed in a JEOL JEM
2010 transmission electron microscope operating at 200 kV
with nominal resolution of 0.25 nm. The samples for TEM
were prepared by dropping the aqueous solutions containing
the NPs onto the carbon-coated Cu grids. A total of
three surface tension measurements for each solution were
performed on DCA315 (Thermo Cahn, USA) by the Wilhelmy
method. The size distribution of the micelles was determined
by dynamic light scattering (DLS) measurements using a
Malvern NanoZetaSizer spectrometer. The measurement
errors of micellar sizes from DLS spectra are within Æ3%
around the mean value of the three best measurements of each
sample. The conductivity of the IL solution was measured as a
function of concentration with an electrical conductivity meter
(model DDS-307, Shanghai Precision and Scientific Instrument
Co., Ltd., accuracy of Æ1%) at room temperature. All solutions
were prepared in deionized double distilled water having
conductivity in the range of 2–3 mS. The elemental analysis of
C, H and N was performed on an Elementar Vario EI III
1
9
of aldehydes, esters and ketones. Alternatively, transition-
metal NPs stabilized by protective agents in the aqueous phase
have a wider range of application in catalytic reactions. As
we all know, water is cheap, readily available, nontoxic,
non-flammable and safe to the environment. Furthermore,
water can provide several other advantages, such as inertness
against oxidation and reduction, high polarity and high
solution ability. Aqueous/organic biphasic catalysis employing
2
0–23
water-soluble metal nanocatalysts has been reported.
It is
easy to realize catalyst recyclability and product separation,
since the catalyst is dispersed in the aqueous phase and the
products in the organic phase. Recently, the highly dispersed
nickel nano-catalysts stabilized by amino-functionalized
imidzaolium ionic liquids have been reported for highly active
and selective hydrogenation of CQC bonds of various
2
4
functionalized alkenes in the aqueous phase. Obviously,
stable transition-metal NPs immobilized by ILs have proven
to be efficient green catalysts for several reactions in aqueous/
organic biphasic conditions.
It has been known that the amphiphilic surfactants can be
used as stabilizers for aqueous colloidal suspensions of
4,20
transition-metal NPs with an effective electrosteric interaction.
Recently, the monodispersed pure nickel NPs were synthesized
Elementa. All NMR spectra were recorded on a Bruker Avance
and TMS as solvent
1
500 instrument (500 MHz H) using CDCl
3
2
5
in water by CTAB and a mixture of TEAB and TBAB.
and reference. Chemical shifts (d) were given in parts per million
and coupling constants (J) in hertz. X-Ray photoelectron
spectroscopy (XPS) measurements were performed on a Thermo
ESCALAB 250 spectrometer. The UV/Vis measurements were
performed using a Varian Cary 500 spectrophotometer. The GC
analyses were performed in a GC112A with FID detector
equipped with an HP-5 column (30 m, 0.25 mm i.d.).
The NPs were stable in air up to 325 1C and catalyzed
the reduction of p-nitrophenol in the presence of hydrazine
hydrate as a reducing agent. Moreover, surfactants can break
a kinetic barrier due to mass-transport limitations between
2
6
the aqueous and organic phase. In this work, we synthesized
a new amphiphilic IL to stabilize the aqueous colloidal
suspension of palladium NPs, which is composed of poly-
General preparation procedure for palladium nano-catalysts
(
and chlorine as anion. On the other hand, the present
ethylene glycol)-functionalized alkylimidazolium as cation
À6
A mixture of 2.25 Â 10 mol Pd(OAc)
2
dissolved in acetone
2
7
À5
amphiphilic IL is also a gemini surfactant, which bears
two alky chains of 12 carbon atoms each and is joined by a
PEG spacer of about 43 ethylene glycol units in the
cationic section. Generally, gemini surfactants possess several
characteristics which make them superior to similar conventional
surfactants, including a lower critical micelle concentration
(0.1 mL) and 1.77 Â 10
mol dicationic ionic liquid
C
C
12Im-PEG (Scheme 1S)w or nonionic surfactants C12-PEG-
12 (Scheme 2S)w dissolved in water (1.5 mL) was stirred at
room temperature in a stainless steel autoclave (50 mL) for
12 h, and then the acetone was evaporated under vacuum,
followed by reducing with molecular hydrogen (0.1 MPa) at
60 1C for 20 min, which afforded a black aqueous colloidal
palladium.
(
CMC) and a higher efficiency at reducing the aqueous/
organic interfacial tension. To our knowledge, it is the first
time that the amphiphilic dicationic ionic liquid has been
utilized to stabilize colloidal suspension of palladium NPs.
The present catalytic system (Pd/C12Im-PEG IL) was
employed for aqueous hydrogenation under very mild
conditions and the stability of the catalyst was evaluated
through an aqueous recycling process in eight successive
hydrogenation runs. Additionally, the stabilizing mechanism
General procedure for olefin hydrogenation under
hydrogen pressure
All hydrogenation reactions were carried out under standard
conditions (25 1C, 1.0 MPa of H ), unless otherwise indicated.
2
The stainless steel autoclave containing previously prepared
palladium(0) nanoparticles dispersed in aqueous phase
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(
1.5 mL) was charged with the appropriate substrate, then
absorption spectra of Pd(0)/C12Im-PEG IL were almost
identical after first and third runs (Fig. 1b and c). The
above results indicated that the reduced Pd species served as
catalytically active species in the present system.
hydrogen was admitted to the system at a constant pressure up
to 1.0 MPa. The mixture was stirred for appropriate time at
room temperature. After the reaction, the products were
extracted by liquid–liquid extraction with diethyl ether three
times. For the recycling procedure, diethyl ether remaining in
the autoclave was evaporated under vacuum, and then the
substrate was charged into the autoclave for the next recycling.
The XRD pattern (Fig. 2) of the isolated material confirmed
the presence of crystalline Pd(0). The most representative
reflections of Pd(0) were indexed as face-centered cubic (fcc)
structure. The Bragg reflections at 40.13, 46.59, 68.21, 81.96,
and 86.901 corresponded to the indexed planes of the crystals
of Pd(0) (111), (200), (220), (311) and (222).
The XPS signal of Pd 3d regions was shown in Fig. 2S.w The
main peak at 334.6 eV of Pd 3d5/2, was virtually the same as
the specimens of the giant clusters of Pd(0) or the bulk metal
Pd (334.8 eV), corresponding to palladium in a zero oxidation
Results and discussion
Characterization of palladium NPs
In this work, poly(ethylene glycol)-functionalized dicationic
ionic liquid C12Im-PEG is intentionally protected in the C2
position of imidazolium cation by a methyl group, because
N-heterocyclic carbene might form by C2 deprotonation or
oxidative addition of the C2–H bond of imidazolium
cation, leading to poisoning the palladium catalyst and then
3
0
state. Moreover, there is no peak of Pd(II) species in the
spectra. In a word, the results of XPS, XRD and UV/Vis
indicated that the palladium(II) species were completely
reduced to palladium NPs by hydrogen reduction, and the
palladium NPs could be protected by IL without any change
of valence.
2
8,29
decreasing the catalytic activity.
poly(ethylene glycol)-functionalized ionic liquid C Im-PEG is
The synthetic route of the
Transmission electron microscopy observations showed
that the average particle size of system Pd/C12Im-PEG IL
was 1.9 nm (Fig. 3). Moreover, the palladium NPs were still
well-dispersed with an average diameter of 2.1 nm after the
third run. The size distribution histogram was obtained on the
basis of a measurement of about 300 particles. The catalytic
suspensions had a similar average diameter (1.9 nm versus
1
2
illustrated in Scheme 1S.w The palladium NPs were prepared
by reducing Pd(II) species with hydrogen at 60 1C for 20 min in
the presence of C12Im-PEG IL. The as-synthesized Pd NPs
have been characterized by UV/vis, XRD, XPS and TEM
methods.
The UV/Vis spectra of C12Im-PEG IL, Pd(II)/C12Im-PEG
IL and Pd(0)/C12Im-PEG IL complex in deionized water
showed remarkable differences in intensity (Fig. 1). There is
a weak absorption peak in the region about 263 nm (inset) for
higher concentration of C Im-PEG IL due to the presence of
2
.1 nm) by the comparative TEM, which confirmed the
conservation of the size distribution after the third run. These
‘nearly monodispersed’’ aqueous suspensions are highly
‘
stable. Thus, it is reasonable that the catalyst can still maintain
the initial activity without any decrease in consecutively
catalytic recycles.
1
2
an imidazole ring. The strong absorption peak of Pd(II)
complex appeared at about 264 nm and was obviously different
from that of the reduced Pd species at low concentration IL
conditions (Fig. 1b and d). In addition, it was found that the
Effect of emulsification on the catalytic activity
Styrene was firstly chosen as a model substrate to explore
hydrogenation by using the present palladium colloidal
suspension and Pd/C as catalysts. The hydrogenation of
Fig. 1 UV/Vis spectrum of (a) C12Im-PEG IL, (b) Pd(0)/C12Im-PEG
IL species after hydrogenation of styrene for 75 min, (c) Pd(0)/C12Im-
PEG IL species after the third run, (d) Pd(II)/C12Im-PEG IL complex
in the aqueous phase. Concentration of the C12Im-PEG IL was all
À3
À1
1
.2 Â 10 mol L for a, b, c and d. The high concentration of C12Im-
Fig. 2 X-Ray diffraction pattern of isolated palladium NPs after
three consecutive recycles of styrene hydrogenation. The peaks are
labelled with the hkl of the planes for the corresponding Bragg angles.
À2
À1
PEG in aqueous solution (1.2 Â 10 mol L ) showed a weak
adsorption peak at 263 nm (inset).
1
3494 Phys. Chem. Chem. Phys., 2011, 13, 13492–13500
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Fig. 3 Comparison of colloidal Pd stabilized by C12Im-PEG IL: (a) before and (b) after the third run (scale bar = 50 nm).
styrene produced only ethylbenzene in this system. The plot of
the product’s yield as a function of the reaction time was shown
in Fig. 4. It was found that the yield increased almost linearly
along with time and no induction period was observed, which
indicated a quasi-zero-order kinetics to styrene concentration.
Obviously, the colloidal palladium system showed a higher
reaction rate than that of a standard Pd/C heterogeneous
catalyst in the aqueous phase. It was worth noting that the
C Im-PEG IL was actually an amphiphilic protective agent,
which can act as a gemini surfactant. It was observed that the
mixture of aqueous suspension and substrates can form an
emulsion under stirring. We believed that the lipophilic alkyl
chain and hydrophilic poly(ethylene oxide) (PEO) chains in
the ionic liquid played a vital role in the formation of the
emulsion system. The amphiphilic properties of the IL in
aqueous/organic biphasic system are favorable for increasing
the interfacial area of the aqueous and organic phase and
breaking the phase barrier. Therefore, the emulsification
promoted a substrate transfer to the interface of the two
phases where the substrate contacted the active species of
palladium nano-catalysts. As a result, the catalytic hydrogenation
process could be promoted by the emulsification and thus the
present colloidal palladium exhibited better catalytic activity.
Moreover, the emulsion was very stable and could be stored for
several weeks without obvious phase separation. However,
because the C12Im-PEG IL was completely immiscible with
diethyl ether, after the reaction diethyl ether was added to the
emulsion, which can break the emulsion easily and facilitate phase
separation. Then, the product was extracted with diethyl ether
and the palladium NPs dispersed in water were separated
conveniently.
12
Role of micelles formed by the amphiphilic ionic liquid in
hydrogenation
Generally, the surfactant molecules can form micelles by self-
association of their monomers. In an emulsion system, the
drops of the dispersed phase (hydrophobic oil phase) are
separated by liquid films or lamellae. The interaction between
Fig. 4 Comparison of standard catalysts versus the Pd/C12Im-PEG
IL catalyst for hydrogenation of styrene. Reaction conditions: styrene/
À3
Pd = 10 000/1, styrene (4.5 Â 10 mol); water (1.5 mL); room
2
temperature (25 1C); H (0.2 MPa).
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Fig. 6 The catalytic activity changes with the concentration of IL.
Fig. 5 Surface tension of aqueous solutions of C12Im-PEG IL as a
Reaction condition: styrene/Pd = 10 000 (molar ratio), water (1.5 mL),
À6
function of the IL concentration in the absence or presence of Pd NPs.
À2
palladium NPs (2.25 Â 10 mol); styrene (2.25 Â 10 mol); reac-
tion time (30 min); room temperature (25 1C); hydrogen pressure
(
1.0 MPa). Turnover frequency was defined as the number of moles of
the droplets in the presence of common emulsifiers (surfactant
micelles) in the continuous phase (aqueous phase) plays an
consumed substrate per mole of palladium per hour.
3
1
important role in controlling the stability of emulsion. To
confirm the formation of the amphiphilic IL micelles in the
aqueous phase, Fig. 5 illustrated a surface tension plot of
example (Fig. 6). Here, the turnover frequency (TOF) was
defined as the number of moles of consumed substrate per
moles of palladium per hour, and the concentration of
palladium in the suspension was kept constant. When the con-
C
12Im-PEG IL in water versus the concentration. As depicted
in the plot, the surface tension of the aqueous solution
decreased with incremental additions of the IL. At a finite
concentration, referred to as the critical micelle concentration
À3
À1
centration of IL was below the CMC (ca. 3.9 Â 10 mol L ),
it was found that the palladium particles were almost
precipitated and the aqueous phase was colorless and clear
after reduction of palladium(II) species, and the corresponding
catalytic activity for hydrogenation of styrene was very low.
Once the concentration of IL reached CMC, the homogeneous
‘‘solution’’ was formed and palladium NPs were well dispersed
in the aqueous phase. This revealed that the formation of
micelles played an important role in the stabilization of the
aqueous colloidal suspension of palladium NPs. When the IL
(
CMC), the surface tension was observed to level off and
remained constant at higher concentrations. In a word, surface
tension measurements demonstrated that C Im-PEG IL can self-
1
2
À3
À1
aggregate into micelles above the CMC of 3.9 Â 10 mol L
.
The CMC value of C Im-PEG IL was also determined by
1
2
conductance measurements as 3.8 Â 10À3 mol L (Fig. 1S),w
which was well in agreement with the results of the surface
tension measurements.
À1
À3
À1
It is well known that the technique of dynamic light
scattering (DLS) is one of the most popular methods used to
concentration increased further to 6.5 Â 10 mol L , the
palladium colloid became highly stable, and the hydrogenation
activity increased to the maximum. We believed that the
number of micelles was adequate to stabilizing the whole
palladium NPs in the aqueous phase at the concentration of
3
2
determine the size of the micelles. Therefore, DLS was used
to examine the size and distribution of the micelles in the
À3
À1
À2
aqueous solution containing 4.73 Â 10 mol L , 7.0 Â
À3
À1
À2
À1
À1
À3
À1
1
0
mol L , 1.03 Â 10 mol L and 1.31 Â 10 mol L
IL (6.5 Â 10 mol L ). A decrease in the catalytic activity of
C Im-PEG IL, respectively (Fig. 3S).w Going up in
palladium colloid was observed above the concentration of IL
1
2
À3
À1
concentration of the IL surfactant, the average hydrodynamic
diameters (D ) of micelles slightly increased from 152 to 179 nm.
These results clearly indicated that the growth of micelle size was
6.5 Â 10 mol L . In this case, the quantity of IL presented
around the palladium NPs was excessive and therefore might
prevent the access of substrates to the active sites.
h
À3
rather limited as the concentration changed from 4.73 Â 10 to
It should be worth noting that if C Im-PEG IL was
1
2
À2
À1
.
1
.31 Â 10 mol L
12 12
replaced by C -PEG-C as the stabilizer for preparation of
The IL aqueous solution containing palladium NPs was also
a similar catalyst under the same conditions, the palladium
particles can also be stabilized well in the colloidal suspension,
which was similar to the case of Pd/C Im-PEG IL. In terms of
analyzed by surface tension measurements. As shown in Fig. 5,
the C12Im-PEG IL aqueous solution with Pd NPs still
possessed a critical micelle concentration although the value
1
2
the configuration of the two gemini surfactants, C -PEG-C
12
1
2
À3
À1
of CMC (3.2 Â 10 mol L ) was lower than that without Pd
has identical PEG spacer and alkyl chains except for the ionic
group. It can be expected that the hydrophilic PEO chain
formed the micelle corona, while the hydrophobic alkyl chains
form the core in aqueous solution for both of surfactants.
Although the stabilizing effect of polyether surfactant is well
À3
À1
NPs (3.9 Â 10 mol L ). The result unambiguously proved
the existence of micelles in the presence of palladium NPs. To
investigate the relationship between the stability and activity
of colloidal Pd as a function of the concentration of
3
3
C
12Im-PEG IL, the hydrogenation of styrene with hydrogen
documented for metal NPs in the solution phase, when we
used the equal mole of pure PEG-2000 to stabilize palladium
(
1.0 MPa) at room temperature was evaluated as a typical
1
3496 Phys. Chem. Chem. Phys., 2011, 13, 13492–13500
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Table 1 Comparison of Pd/C12-PEG-C12 versus Pd/C12Im-PEG IL
for hydrogenation of styrene
micelle structure, resulting in easy contact between substrate
molecules and Pd NPs and then enhancing the reaction rate.
À1
Catalysts
Yield [%]
TOF [h
]
Recyclability of the catalyst and scope of the substrates
Pd/C12Im-PEG IL
Pd/C12-PEG-C12
97.8
37.8
12 300
4360
To investigate the recyclability of the palladium nanocatalysts,
À1
À2
Reaction conditions: styrene/Pd = 10 000 (molar ratio), protective agent
À2
we chose the concentration of IL c = 1.2 Â 10 mol L
À1
À6
(
1.2 Â 10 mol L ), water (1.5 mL), palladium (2.25 Â 10 mol);
which was sufficient to maintain stable NPs within the aqueous
phase after recycles, although the corresponding activity is
slightly lower than the highest one (Fig. 6). In the next step,
the effect of Pd(0) concentration on the catalytic activity was
investigated using the hydrogenation of styrene as a typical
example (Fig. 8). Interestingly, as the mole ratio of the
substrate to Pd was kept unchanged, the activity of the catalyst
decreased dramatically as palladium concentration increased
À2
styrene (2.25 Â 10 mol); reaction time (1 h); room temperature (25 1C);
hydrogen pressure (1.0 MPa).
NPs, the bulk palladium particles were formed rapidly and
deposited completely from the solution after reduction with
hydrogen under the present conditions, and the aqueous phase
became colorless and clear. It suggests that palladium NPs can
not be stabilized only by PEG-2000 molecules, but in C -
À1
1
2
from 0.5 to 6 mmol L at room temperature.
The TEM images demonstrated that a higher concentration
PEG-C12 or C12Im-PEG IL micelles. As the PEO chains
constitute the hydrophilic surface of a micelle by self-
aggregation, the local concentration of PEO chains in the
micelle corona is enough to stabilize palladium NPs. In other
words, palladium NPs were mainly located in the micelle
structure, which then resulted in the formation of an
À1
of palladium (3 mmol L ) induced the growing of palladium
NPs (1.9 nm versus 3.4 nm) (Fig. 3 and 9), leading to a
decrease of activity. Generally, the growing in the size of
metal NPs resulted in decreasing of catalytic activity,
3
7
which is usually attributed to the low surface area. This
phenomenon indicated that the colloidal palladium nano-
catalysts were highly effective even if the concentration of
palladium decreased to a very low level since the number of
catalytic sites of Pd NPs were enough for the complete
conversion of styrene under the present conditions.
3
4
analogous ‘‘nanoreactor’’. For the C12Im-PEG IL micelles,
the surface of the micelle cores is charged positively with
imidazolium cations. The palladium NPs might be inclined
3
5
to close with the surface of the micelle cores. In this case, the
catalytic reaction took place in the framework of the micelle.
The comparison activity study displayed that the catalytic
activity of Pd/C12-PEG-C12 was much lower than that of Pd/
The reusability of the colloidal suspension of palladium NPs
was also examined. After the hydrogenation was completed,
the aqueous phase and organic phase formed a stable
emulsion, which was broken by diethyl ether. Then, the diethyl
ether phase was removed. The catalyst was washed with
diethyl ether three times and was further treated under
vacuum for the next run. It was found that the amounts of
leaching C Im-PEG IL and Pd were negligible by elemental
C12Im-PEG IL (Table 1). We believed that the imidazolium
cation in C12Im-PEG IL could change the surface properties of
palladium NPs, such as the surrounding electronic characteristics,
which enhanced the hydrogenation activity of palladium nano-
3
6
catalysts for hydrogenation.
Because the amphiphilic IL concentration was much higher
than its CMC during the reaction, the aqueous continuum, the
oil droplets (substrates) and the micelles coexisted in the
1
2
and ICP-AES analysis in the consecutive catalytic recycles.
The palladium dispersion after every cycle was found to be the
same as in the fresh catalyst, indicating the NPs’ resistance to
agglomeration due to the function of micelles. The recycling of
the catalyst for the hydrogenation of styrene is shown in
3
1
colloidal system. As shown in Fig. 7, substrate molecules
can diffuse into surfactant micelles near the oil/water interface
for micelles to take up the oil molecules. The micelles near
the oil/water interface can prevent the oil droplets from
approaching each other and thus enhance the stability of
emulsion, since palladium NPs were mainly located in the
Fig. 8 Influence of the concentration of Pd(0) in colloidal suspension
on the catalytic activity. Reaction conditions: styrene/Pd = 10 000, 1.5 mL
water; room temperature (25 1C); hydrogen pressure (1.0 MPa).
Fig. 7 Pd NPs catalyzing the hydrogenation reaction in a multiphase
system.
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À1
Fig. 9 Transmission electron micrograph (scale bar = 50 nm) and size distribution histogram of Pd/C12Im-PEG IL suspension ([Pd] = 3 mmol L ).
was only identified after the reaction. We further chose
benzylidene acetone as a model substrate to investigate the
catalytic activity and selectivity of hydrogenation conveniently
over Pd/C12Im-PEG IL catalyst, because both of the CQC
and CQO double bonds are present in the molecule (entry 9,
Table 1S).w Under the present conditions, the product was
only 4-phenylbutan-2-one, and no 4-phenyl-2-butanol was
identified. For the hydrogenation of olefinic alcohol, propylene
alcohol and cinnamyl alcohol were totally converted to
propanol and 3-phenylpropan-1-ol respectively (entries 6 and
7
, Table 1S).w However, the aggregates of Pd particles
were visually observed at the end of the hydrogenation of
propylene alcohol. It was observed that when highly water-
soluble propylene alcohol was added to the colloidal Pd
aqueous solution, the palladium particles were deposited
completely from the solution after stirring for several
minutes. This was the reason why a lower value of TOF was
obtained for the hydrogenation of propylene alcohol. To
investigate the effect of water-soluble propylene alcohol on
the micelles, the fresh IL aqueous solution containing
propylene alcohol was determined by DLS measurements.
The results showed that no aggregates with sizes in the
range of 10 to 500 nm were detected. In other words,
propylene alcohol can destroy the C Im-PEG IL micelles in
Fig. 10 Recyclability of the palladium NPs for hydrogenation of
styrene. Reaction conditions: styrene/Pd = 10 000 (molar ratio),
À2
À1
C
12Im-PEG IL (1.2 Â 10 mol L ), water (1.5 mL), palladium
À6 À2
catalyst (2.25 Â 10 mol); styrene (2.25 Â 10 mol); room temperature
25 1C); hydrogen pressure (1.0 MPa).
(
Fig. 10, and the results show that the catalyst can be used at
least eight times without any decrease in activity.
The scope of the substrates was established for a wide range
of functionalized alkenes and in all instances the reaction
was completed under 1.0 MPa hydrogen pressure at room
temperature with excellent conversion for the hydrogenation
of CQC double bonds (Table 1S).w The reaction was
monitored by a decrease of hydrogen pressure and gas
chromatographic analysis. Hydrogenation of several typical
aliphatic, aromatic, and alicyclic olefins proceeded efficiently
and almost quantitive yields were achieved. For aliphatic
olefins, n-hexene was totally converted to hexane in 4 h, and
the isomerization of n-hexene did not take place during the
reaction. Because competitive hydrogenation between the
aromatic ring and the carbonyl groups of aromatic ketone
1
2
aqueous solution. As a result, it can be concluded that the
palladium NPs are stabilized by IL micelles in the present
system.
Selective hydrogenation of nitrobenzene (NB) and substituted
nitrobenzene, being a major challenge for chemists, is
commonly used to manufacture aniline and its derivatives,
which are significant intermediates for dyes, polyurethanes,
3
9–42
pharmaceuticals, explosives, and agrochemicals.
The NB
and methyl-substituted NB could be efficiently converted into
the corresponding amino aromatic compounds under mild
conditions in the aqueous phase (entries
9 and 10,
Table 1S).w It is worth noting that although cinnamyl alcohol,
benzylidene acetone and p-nitrotoluene are solid and water-
insoluble at room temperature (entries 7, 9 and 11, Table 1S),w
the reaction took place smoothly in an aqueous/solid biphasic
system. In other words, the conversion rate was not influenced
by the state of substrate.
3
8
has been previously observed, therefore aromatic ketone
such as acetophenone was employed as a substrate for
hydrogenation by the present catalyst system. It was found
that acetophenone was hardly hydrogenated under the present
conditions (entry 8, Table 1S).w A trace of 1-phenylethanol
1
3498 Phys. Chem. Chem. Phys., 2011, 13, 13492–13500
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Finally, we have observed the formation of 2-phenylethanol
by reduction of styrene oxide (entry 12, Table 1S).w Catalytic
hydrogenation of epoxides, a useful transformation in organic
synthesis and several homogeneous as well as heterogeneous
11 (a) G. S. Fonseca, A. P. Umpierre, P. F. P. Fichtner, S. R. Teixeira
and J. Dupont, Chem.–Eur. J., 2003, 9, 3263–3269; (b) X. Mu,
J. Meng, Z. Li and Y. Kou, J. Am. Chem. Soc., 2005, 127,
9
694–9695; (c) G. S. Fonseca, J. B. Domingos, F. Nome and
J. Dupont, J. Mol. Catal. A: Chem., 2006, 248, 10–16.
12 X. Xue, T. Lu, C. Liu, W. Xu, Y. Su, Y. Lv and W. Xing,
Electrochim. Acta, 2005, 50, 3470–3478.
4
3
catalysts, have been reported in many literature sources. The
hydrogenation of styrene oxide is usually accompanied with
formation of several side products such as phenylacetaldehyde,
ethylbenzene and styrene. However, in our system, it showed
total conversion and afforded 2-phenylethanol in high yields
1
3 (a) C. C. Cassol, A. P. Umpierre, G. Machado, S. I. Wolke and
J. Dupont, J. Am. Chem. Soc., 2005, 127, 3298–3299; (b) V. Calo,
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0, 6040–6044; (c) C. Chiappe, D. Pieraccini, D. Zhao, Z. Fei and
P. J. Dyson, Adv. Synth. Catal., 2006, 348, 68–74.
(
90.6%). The minor side product was 1-phenylethane-1,
-diol, which could be resulted from the hydrolyzation of
styrene oxide.
1
4 (a) A. P. Umpierre, G. Machado, G. H. Fecher, J. Morais and
J. Dupont, Adv. Synth. Catal., 2005, 347, 1404–1412;
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2
13011–13020.
5 A. E. Visser, R. P. Swatloski, W. M. Reichert, R. Mayton, S. Sheff,
A. Wierzbicki, J. H. Davis Jr. and R. D. Rogers, Chem. Commun.,
1
1
Conclusions
2
001, 135–136.
6 D. M. Chisholm and J. S. McIndoe, Dalton Trans., 2008,
933–3945.
We have shown that well-dispersed palladium colloids could be
stabilized with poly(ethylene glycol)-functionalized dicationic
ionic liquid (C12Im-PEG IL) via reducing palladium acetate
by hydrogen. The ionic liquid micelles in aqueous solution
play an important role in stabilizing and dispersing the
palladium NPs. The novel palladium colloids showed highly
catalytic activity as well as strong stability in the hydro-
genation reaction under very mild conditions. The high
dispersion of 1.9 Æ 0.3 nm Pd NPs and the emulsification of
colloidal suspension are the most important factors that
contribute to the high reactivity of the present system.
3
17 (a) D. Zhao, Z. Fei, T. J. Geldbach, R. Scopelliti and P. J. Dyson,
J. Am. Chem. Soc., 2004, 126, 15876–15882; (b) D. Zhao, Z. Fei,
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(
c) S. R. Dubbaka, D. Zhao, Z. Fei, C. M. R. Volla, P. J. Dyson
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4
7, 3292–3297.
1
8 Y. Hu, Y. Yu, Z. Hou, H. Li, X. Zhao and B. Feng, Adv. Synth.
Catal., 2008, 350, 2077–2085.
19 Y. Hu, H. Yang, Y. Zhang, Z. Hou, X. Wang, Y. Qiao, H. Li,
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0 J. Schulz, A. Roucoux and H. Patin, Chem.–Eur. J., 2000, 6,
18–624.
2
6
2
2
1 F. Lu, J. Liu and J. Xu, Adv. Synth. Catal., 2006, 348, 857–861.
2 R. Zhang, W. Ding, B. Tu and D. Zhao, Chem. Mater., 2007, 19,
Acknowledgements
4
379–4381.
3 D. E. Bergbreiter and S. D. Sung, Adv. Synth. Catal., 2006, 348,
352–1366.
The authors are grateful for support from the National
Natural Science Foundation of China (No. 20773037, No.
2
1
2
1073058), the Research Fund for the Doctoral Program of
24 Y. Hu, Y. Yu, Z. Hou, H. Yang, B. Feng, H. Li, Y. Qiao,
X. Wang, L. Hua, Z. Pan and X. Zhao, Chem.–Asian J., 2010, 5,
Higher Education of China (20100074110014), China, and
the Key Laboratory of Catalysis and Materials Science of
the State Ethnic Affairs Commission and the Ministry of
Education, Hubei Province, South-Central University for
Nationalities (No.CHCL08001), P. R. China. The authors
would also like to express their thanks to Prof Walter Leitner’s
helpful suggestion and Dr Nils Theyssen’s assistance.
1178–1184.
5 M. L. Singla, N. Negi, V. Mahajan, K. C. Singh and D. V. S. Jain,
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