Application of a versatile nanoparticle stabilizer in phase transfer and catalysis

Article abstract of DOI:10.1002/cplu.201200108

Gold and rhodium nanoparticles stabilized by a cationic polymer can be readily modified by anion exchange to afford nanoparticles that may be dispersed in a wide range of organic solvents and ionic liquids. The catalytic activity of the Rh nanoparticle dispersions have been evaluated in the selective hydrogenation of limonene and the nature of the solvent has been shown to play a critical role.

Full text of DOI:10.1002/cplu.201200108

DOI: 10.1002/cplu.201200108
Application of a Versatile Nanoparticle Stabilizer in Phase
Transfer and Catalysis
Ilaria Biondi, Vincent Laporte, and Paul J. Dyson*[a]
Gold and rhodium nanoparticles stabilized by a cationic poly-
mer can be readily modified by anion exchange to afford
nanoparticles that may be dispersed in a wide range of organic
solvents and ionic liquids. The catalytic activity of the Rh nano-
particle dispersions have been evaluated in the selective hy-
drogenation of limonene and the nature of the solvent has
been shown to play a critical role.
Introduction
Metallic nanoparticles, NPs, have applications in catal-
ysis, medicine[1,2] and in optical devices.[3] Although
mostly used in the solid state, NP dispersions are im-
portant in certain applications, and the ability to
transfer NPs into different physicochemical environ-
ments is consequently a fundamental issue of consid-
erable importance. The most common strategy used
to transfer NPs prepared in an aqueous to an organic
phase involves coating the NPs with ligands that di-
rectly coordinate to the metal surface.[4] However, the
strong covalent interaction between the ligands and
the NP surface decreases the number of available sur-
face atoms for catalysis and consequently these
types of stabilizers are not very suitable for catalytic
applications.[5] It would therefore be preferable to
find an alternative method to transfer NPs from
water to other solvents using a noncovalent NP sta-
bilizer. Polymer stabilizers such as poly(vinyl pyrroli-
done), PVP, or poly(vinyl alcohol), PVA, exhibit poor
solubility in most organic solvents and ionic liquids
(ILs).[6] Recently, it has been shown that NPs coated
Scheme 1. P-IL 1 and the synthesis of 2 and 3 by anion exchange.
with a polymer-ionic liquid, P-IL, may be transferred
from aqueous solution into ILs.[7] It has also been
shown that NPs stabilized by PVP can be prepared in
functionalized ILs, but this procedure is limited to very hydro-
philic ILs.[8]
available second-generation Grubbs catalyst as described pre-
viously (Scheme 1).[9] The solubility properties of 1 are readily
modulated by anion exchange, for example exchange of the
methanesulfonate anion, MsOÀ, by bis(trifluoromethylsulfony-
l)imide or hexafluorophosphate anions by reaction with an
excess amount of LiTf2N or NaPF6 affords the hydrophobic P-
ILs 2 and 3, respectively, that precipitate from aqueous solu-
tion. The reactions are fast and quantitative with the immedi-
Herein, we describe the preparation and characterization of
NPs decorated with a P-IL that may be readily switched into
a wide range of different solvents. The catalytic properties of
some of the resulting NPs were also evaluated and show that
the P-IL does not impair the catalytic activity of the NPs.
Results and Discussion
[a] Dr. I. Biondi, Dr. V. Laporte, Prof. P. J. Dyson
Institut des Sciences et Ingꢀnierie Chimiques
Ecole Polytechnique Fꢀdꢀrale de Lausanne (EPFL)
1015 Lausanne (Switzerland)
The water-soluble P-IL, poly-(3-((2,4-divinylcyclopentyl)methyl)-
1,2-dimethyl-1H-imidazolium methanesulfonate) 1, was synthe-
sized in high yield from 3-(bicycle[2.2.1]hept-5-en-2-ylmethyl)-
1,2-dimethyl-1H-imidazolium methanesulfonate through ring-
opening metathesis polymerization, ROMP, using commercially
E-mail: paul.dyson@epfl.ch
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201200108.
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P. J. Dyson et al.
ate precipitation of 2 and 3 from the aqueous solution ob-
served. These precipitates can be subsequently separated,
washed, and dried to afford white solids.
a small amount of [bmim][PF6].[12] The [TTA]+ cations on the
À
particle surface interact with the PF6 anion of the IL leading
to the formation of a hydrophobic ion pair, which changes the
surface properties of the NPs and allows their transfer into
CH2Cl2.
The solubility of 1–3 was studied in nine imidazolium-based
ILs 4–12 (Scheme 2); 1 is soluble in the hydrophilic ILs 4–6
A similar mechanism can be invoked for the trans-
fer of the NPs described here and to confirm whether
the exchange reaction is complete X-ray photoelec-
tron spectroscopy (XPS) was used to analyze the NP
surfaces. XPS analysis has been successfully em-
ployed to analyze the near-surface electronic struc-
ture of several ILs, leading to a better understanding
of the specific interactions occurring between the
cation and anion in imidazolium-based ILs and hence
to their physical properties.[13–18] Furthermore, this
technique had been widely used to investigate tran-
sition-metal NPs in ILs providing deeper insights in
the stabilization mode.[19–24]
As expected, the high resolution survey spectra
obtained on NP-2 and NP-3 contain the characteristic
Au 4f doublet (Figure 2). Notably, the spectra also
confirm that the anion-exchange process is effective-
ly quantitative, as shown by the appearance of an
F 1s peak, an additional N 1s peak for NP-2, and addi-
tional P 2s and P 2p peaks for NP-3. Moreover, no S
signal is observed in the NP-3 sample. Comparisons
with literature data confirm the presence of the Tf2NÀ
Scheme 2. ILs used in solubility studies of 1–3.
whereas 2 and 3 are soluble in the hydrophobic ILs 7–9 and
10–12, respectively. P-IL 1 was used in the synthesis of gold
NPs, NP-1, using HAuCl4 as a precursor (Au/polymer 1:1) and
NaBH4 as the reducing agent (Au/NaBH4 1:2 w/w). On addition
of the NaBH4 to the HAuCl4 solution the color of the solution
changes from pale yellow to red indicative of the formation of
NPs. Successful NP synthesis was also confirmed by an intense
absorption peak at 517 nm in the UV/VIS spectrum corre-
sponding to the gold surface plasmon band[10] and TEM analy-
sis revealed the particle size of the NPs is in the range 3–
10 nm (see Figure 1 and the Supporting Information).
Two different approaches were explored to transfer NP-
1 into the hydrophobic ILs 7–12. In one approach the aqueous
NP-1 solution and the appropriate IL are vigorously mixed in
the presence of a saturated aqueous solution of LiTf2N or
NaPF6 (see the Supporting Information). Once the methanesul-
fonate counter anion of the P-IL in NP-1 is switched to Tf2NÀ in
NP-2, or PF6À in NP-3, the NPs rapidly transfer into ILs 8–13. Al-
ternatively, NP-1 is directly precipitated from the aqueous solu-
tion through anion exchange with LiTf2N or NaPF6 to afford
NP-2 and NP-3 as solids, respectively. NP-2 and NP-3 may be
separated by filtration and after drying NP-2 may be dispersed
in ILs 7–9 and NP-3 in ILs 10–12. Characterization of the trans-
ferred NPs by TEM reveals that the size of the transferred NPs
is essentially unchanged (see the Supporting Information).
In aqueous NP solutions stabilized by surfactants such as tet-
radecyltrimethylammonium bromide, TTAB, the transfer of the
NPs into ILs is related to the charge stabilization of surface-ad-
sorbed anions.[5,11] For example, gold and silver NPs stabilized
by TTAB can be transferred from water to CH2Cl2 containing
Figure 1. TEM image and size distribution histogram for NP-1 (scale bar:
50 nm).
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Heterogeneous Catalysis
Transfer of the aqueous NP-4 solution into ILs 7 and 10 (to
afford NP-5 and NP-6) was achieved using the same anion-ex-
change reaction applied to the gold NPs (see above and the
Supporting Information for characterization). XPS analysis con-
firmed that the anion-exchange reaction went to completion
(see the Supporting Information).[17,18,20]
The NPs were evaluated in the hydrogenation of limonene,
a cyclic terpene occurring in many essential oils,[25] and a re-
newable material obtained in large quantities as a by-product
in orange juice manufacture. Limonene contains a terminal
and an internal C=C bond, and partial hydrogenation (of the
terminal C=C bond) provides carvomenthene, which can be
converted into para-menthane after hydrogenation of the in-
ternal C=C bond (Scheme 3). The hydrogenation of related
substrates by ruthenium and platinum NPs has been reported
previously,[26,27] and reviews describing the application of rhodi-
um NPs in catalysis have been published.[28,29]
Figure 2. XPS survey spectra for NP-2 and NP-3 (the indium 3d doublet orig-
inates from the indium foil used to support the samples).
The catalytic reactions were performed using a catalyst/sub-
strate ratio of 1:2000 at 808C under 40 bar of hydrogen. After
30 minutes with the NP-4 solution carvomenthene is selective-
ly obtained in 35%. A longer reaction time increased the con-
version to 74%, and provided carvomenthene in 71% (Table 1,
entries 1 and 2). The selective hydrogenation of limonene with
rhodium complexes in aqueous solution has been reported
before,[30] but this appears the first report of aqueous rhodium
À
anion in NP-2[18] and the PF6 anion in NP-3.[17,20] The atomic
ratio calculated from the high-resolution spectra is very close
to the expected stoichiometric atomic ratio (see the Support-
ing Information).
The stabilization of NPs by 1 is presumably due to a combi-
nation of steric and electrostatic effects. Because of the incom-
plete coordination of the surface atoms and the elec-
trophilic character of the NP surface, the MsOÀ
anions presumably form a layer immediately adjacent
to the NP surface, thus providing electrostatic stabili-
zation.[5] Replacement of 1 with the corresponding
monomer in the preparation of NPs resulted in the
immediate precipitation of the NPs from the aqueous
solution, strongly suggesting that a steric effect is
also involved in the stabilization process. After addi-
tion of LiTf2N or NaPF6 to NP-1, the cationic chain
À
moieties of 1 rapidly interact with the Tf2NÀ or PF6
anions forming the corresponding hydrophobic poly-
mers 2 and 3 and consequently modifying the sur-
face properties of the NPs such that they become hy-
drophobic. The hydrophilic polymer 1 is not only
soluble in water, but it is also soluble in methanol
and ethanol, and the hydrophobic polymers 2 and 3
are soluble in acetone, acetonitrile, dimethyl sulfox-
ide (DMSO), and N,N-dimethylformamide (DMF). Con-
sequently, NP-2 and NP-3 were also redispersed in
acetone, acetonitrile, DMF, and DMSO. Analysis by
UV/VIS spectroscopy shows an absorption peak at
517 nm for all the samples, indicating the NPs are es-
sentially unchanged and verified by TEM images (see
the Supporting Information).
Rhodium NPs stabilized by 1 were synthesized
from the reduction of RhCl3 with NaBH4 (Rh/poly-
mer 1:1). On addition of the reducing agent the solu-
tion immediately turned from orange to black, indica-
tive of NP formation, NP-4. TEM analysis of NP-4 con-
firmed the presence of rhodium NPs with a diameter
<3 nm (Figure 3).
Figure 3. TEM image and size distribution histogram for NP-4 (scale bar: 20 nm).
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P. J. Dyson et al.
Conclusion
The solubility properties of the hydrophilic ionic polymer 1 de-
scribed here are readily modulated by exchanging the counter
ion allowing NPs to be dispersed in a wide range of solvents.
XPS analysis demonstrates that efficient and quantitative anion
exchange takes place. Because the polymer stabilizer interacts
with the NP surface mostly through noncovalent interactions
(note that the oxygen atoms can potentially coordinate) the
catalytic activity of the NPs is not suppressed and excellent ac-
tivities can be obtained. Moreover, when dispersed in ILs the
selectivity of catalytic reactions can be modified, and thus, the
appropriate combination of NP stabilizer and dispersion sol-
vent is vital to afford nanocatalysts with optimum properties.
Scheme 3. Hydrogenation of limonene.
Table 1. Hydrogenation reactions using NP-4–NP-6.[a]
Entry Catalyst Solvent Substr. Conv. [%][b] Product Selec. [%] Yield [%][c]
1
NP-4
NP-4
water
water
35
74
100
96
4
35
71
3
Experimental Section
Reagents were purchased from Aldrich or Acros and used as re-
ceived. Water was double distilled using a Water purification Milli-
Q Gradient, Elix 3 with 30 L reservoir. Membrane dialysis tubes
(MWCO 1000 and 2000) were purchased from Spectrum Laborato-
ries. P-IL 1[9] and ILs 4–6,[45,46] 7–9[47–49], and 10–12[48,50,51] were syn-
thesized according to literature procedures. The UV/VIS spectra
were recorded on a Jasco V-550 spectrometer. The NMR spectra
were measured on a Bruker DMX 400 using SiMe4 as an external
standard at 208C. The TEM images were obtained on a PHILIPS/FEI
CM20 transmission electron microscope (200 KeV) using Quantifoil
Carbon film Cu grids on 200 mesh as specimen supports. The size
distribution was estimated from 200 particles. The XPS data were
obtained on an Axis Ultra instrument (Kratos Analytical) under
ultra-high vacuum (<10À8 Torr), using a monochromatic AlKa X-ray
source (1486.6 eV). The source power was maintained at 150 W
and the emitted photoelectrons were sampled from a 700ꢁ
350 mm2 area with a 908 photoelectron take-off angle. The analyzer
pass energy was 80 eV for survey spectra and 40 eV for high-reso-
lution spectra. The adventitious carbon 1s peak was calibrated at
285 eV and used as an internal standard to compensate for any
charging effects. Both curve fitting of the spectra and quantifica-
tion were performed with the CasaXPS software, using relative sen-
sitivity factors given by Kratos.
2[d]
3
4
NP-5
NP-6
IL 7
76
100
75
25
76
74
25
IL 10
>99
[a] Hydrogenation reactions performed at 40 bar of hydrogen, 808C for
30 min. [b] Determined by NMR spectroscopy. [c] Yield of isolated pro-
duct.[d] 60 min reaction time.
NPs catalyzing this reaction. In ILs the rate of reaction and the
selectivity both increase—NP-5 and NP-6 are extremely active
and the selectivity toward the partially reduced product is very
high (Table 1, entries 3 and 4).
General procedure for the synthesis of 2 and 3
To 1 (10 mg, 0.033 mmol of monomer units) dissolved in double-
distilled water (0.5 mL), LiTf2N (29 mg, 0.1 mmol) or NaPF6 (17 mg,
0.1 mmol) was added with the immediate precipitation of an off-
white solid observed. The solution was stirred at RT for 1 min, cen-
trifuged (30 s, 16000 rpm), and the supernatant water removed.
Double-distilled water (1 mL) was added, the solution stirred at RT
for 1 min, centrifuged (30 s, 16000 rpm), and the water removed.
This procedure was repeated four times and poly-(3-((2,4-divinylcy-
clopentyl)methyl)-1,2-dimethyl-1H-imidazolium bis-(trifluoromethyl-
sulfonyl)-imide) 2 or poly-(3-((2,4-divinylcyclopentyl)methyl)-1,2-di-
methyl-1H-imidazolium hexafluorophosphate 3 were dried under
Indeed, in recent years ionic liquids (ILs) have received in-
creasing attention as reaction media because of their favorable
properties such as high polarity, negligible vapor pressure,
high ionic conductivity, high thermal stability, and low interfa-
cial surface tension.[31–38] The latter feature makes ILs good
media for NP synthesis because a low interfacial surface ten-
sion results in high nucleation rates, allowing the formation of
very small particles which undergo weak Ostwald ripen-
ing.[19,39–42] Moreover, ILs have also been shown to influence
the selectivity of numerous reactions.[43,44]
1
vacuum for 12 h. H NMR (258C, 400.1 MHz, [D6]acetone) for 2: d=
7.56, 5.64, 5.52, 5.43, 5.25, 4.31, 4.22, 4.08, 3.91, 3.43, 3.24, 2.96,
2.86, 2.72, 1.85, 1.75, 1.30 ppm; for 3: d=7.46, 5.61, 5.50, 5.42,
5.24, 4.22, 4.16, 4.02, 3.84, 3.35, 3.20, 3.06, 2.83, 2.67, 2.29, 1.79,
1.53, 1.28 ppm.
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Heterogeneous Catalysis
Synthesis of NP-1
Acknowledgements
An aqueous solution of HAuCl4 (1 mL, 1 mgmLÀ1, 2.5 mmol of Au),
an aqueous solution of 1 (13 mL, 0.1905 m, 2.5 mmol), and water
(3 mL) were added together and stirred for 2 min at RT. After this
time an aqueous solution of NaBH4 (1 mL, 0.5 mgmLÀ1, 12.5 mmol)
was rapidly added and the color of the reaction mixture immedi-
ately turned from pale yellow to red. The solution was stirred for
additional 30 min at RT and then dialyzed for 24 h (MWCO: 1000).
The solution was stored at 48C.
We thank the EPFL and Swiss National Science Foundation for fi-
nancial support. We also thank Prof. Philippe Andrꢀ Buffat, Dr.
Marco Cantoni and the staff at the Interdisciplinary Centre for
Electron Microscopy (CIME) at the EPFL for assistance and useful
discussions regarding TEM analysis.
Keywords: heterogeneous catalysis · hydrogenation · ionic
liquids · gold · nanoparticles · rhodium
General procedure for the transfer of NP-1 from aqueous
solution to ILs 7–12
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Method 1: The hydrophobic IL 7–12 (1 mL), NP-1 solution
(0.5 mmol of Au, 2.6 mL of 1, 1 mL) and an aqueous solution of
LiTf2N (0.1 mmol, 287 mg, 1 mL) or NaPF6 (1 mmol, 168 mg, 1 mL)
were stirred for 1 min. The NPs immediately transferred from the
aqueous phase to the IL phase. The solution was stirred for addi-
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was washed with water (5ꢁ2 mL) and dried under vacuum at 808C
for 10 h.
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Method 2: The NP-1 solution (0.5 mmol of Au, 2.6 mL of 1, 1 mL)
and an aqueous solution of LiTf2N (30 mmol, 9 mg, 0.1 mL) or
NaPF6 (30 mmol, 5 mg, 0.1 mL) were stirred for 1 min; the immedi-
ate precipitation of the NPs was observed, affording NP-2 and NP-
3, respectively. The solution was filtered and the precipitate was
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24 h and then re-dispersed in the appropriate IL, 7–9 for NP-2 and
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An aqueous solution of RhCl3 (1 mL, 1 mgmLÀ1, 5.06 mmol of Rh),
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Catalytic experiments were performed using a home-built multicell
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autoclave and sealed. After flushing with H2 (5ꢁ10 bar), the auto-
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1
pressure released. Conversions were determined by H NMR spec-
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Received: May 7, 2012
Published online on && &&, 0000
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPlusChem 2012, 00, 1 – 7
ÝÝ
These are not the final page numbers!

FULL PAPERS
I. Biondi, V. Laporte, P. J. Dyson*
&& – &&
Application of a Versatile Nanoparticle
Stabilizer in Phase Transfer and
Catalysis
Polymer coated: Metal nanoparticles
coated with an ionic polymer may be
readily dispersed in a wide range of sol-
vents including ionic liquids (see figure).
In catalysis the nature of the solvent
strongly influences the outcome of the
reaction and consequently the solvent
can be used to fine-tune the surface
features of the nanoparticles and their
corresponding catalytic properties.
ChemPlusChem 2012, 00, 1 – 7
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chempluschem.org
&
7
&
ÞÞ
These are not the final page numbers!