Organic carbonates as stabilizing solvents for transition-metal nanoparticles

Article abstract of DOI:10.1039/c2dt30668a

Biodegradable, non-toxic, green and inexpensive propylene carbonate (PC) solvent is shown to function as a stabilizing medium for the synthesis of weakly-coordinated transition-metal nanoparticles. Kinetically stable nanoparticles (M-NPs) with a small and uniform particle size (typically <5 ± 1 nm) have been reproducibly obtained by easy, rapid (3 min) and energy-saving 50 W microwave irradiation under an argon atmosphere from their metal-carbonyl precursors in PC. The M-NP/PC dispersions are stable for up to three weeks according to repeated TEM studies over this time period. The rhodium nanoparticle/PC dispersion is a highly active catalyst for the biphasic liquid-liquid hydrogenation of cyclohexene to cyclohexane with activities of up to and 1875 (mol product) (mol Rh)^-1 h^-1 and near quantitative conversion at 4 to 10 bar H₂ and 90 °C. From the PC dispersion the M-NPs can be coated with organic capping ligands such as 3-mercaptopropionic acid or trioctylphosphine oxide for further stabilization.

Full text of DOI:10.1039/c2dt30668a

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PAPER
Organic carbonates as stabilizing solvents for transition-metal nanoparticles†
Christian Vollmer,a Ralf Thomannb and Christoph Janiak*a
Received 23rd March 2012, Accepted 18th June 2012
DOI: 10.1039/c2dt30668a
Biodegradable, non-toxic, “green” and inexpensive propylene carbonate (PC) solvent is shown to
function as a stabilizing medium for the synthesis of weakly-coordinated transition-metal nanoparticles.
Kinetically stable nanoparticles (M-NPs) with a small and uniform particle size (typically <5 1 nm)
have been reproducibly obtained by easy, rapid (3 min) and energy-saving 50 W microwave irradiation
under an argon atmosphere from their metal–carbonyl precursors in PC. The M-NP/PC dispersions are
stable for up to three weeks according to repeated TEM studies over this time period. The rhodium
nanoparticle/PC dispersion is a highly active catalyst for the biphasic liquid–liquid hydrogenation of
cyclohexene to cyclohexane with activities of up to and 1875 (mol product) (mol Rh)−1 h−1 and near
quantitative conversion at 4 to 10 bar H2 and 90 °C. From the PC dispersion the M-NPs can be coated
with organic capping ligands such as 3-mercaptopropionic acid or trioctylphosphine oxide for further
stabilization.
Introduction
Organic carbonates, such as dimethyl carbonate, diethyl carbon-
ate, ethylene carbonate or propylene carbonate (PC) (Fig. 1) are
polar solvents which are available in large amounts and at low
prices, have a large liquid temperature range (for PC mp.
−49 °C, bp. 243 °C), are of only low (eco)toxicity and are com-
Fig. 1 Selected organic carbonates.
pletely biodegradable.1 Propylene carbonate (PC) is an aprotic,
highly dipolar solvent, which has a low viscosity2,3 and is con-
sidered a green solvent because of its low flammability, volatility
significant interest for technological applications in several areas
and toxicity.4
of science and industry, especially in catalysis due to their high
activity.12–15 The controlled and reproducible synthesis of
Organic carbonates are recognized as solvents in industrial
applications such as cleaning, degreasing, paint stripping, gas
defined and stable M-NPs with a small size distribution is
treating, and textile dyeing.5 Yet, so far organic carbonates are
used primarily for extractive applications and as solvents in
electrochemistry.1 PC is used as a solubilizer and co-solvent in
cosmetics,6 in the FLUOR process for the removal of carbon
dioxide from natural gas streams in the oil industry,7 in lacquer8
and BASF is using PC for waste removal in the copper wire-
coating process.9 PC is also investigated for Li-ion battery
research.10,11
important.16–20 Very small M-NPs tend to aggregate because of
their high surface energy and large surface area. To avoid this
agglomeration, M-NPs need to be stabilized either by organic
donor ligands,21–24 ionic liquids15,25–27 or by deposition on solid
supports.28–30
Reports on metal nanoparticles in organic carbonates are rare
and appear to be of accidental coincidence for Pd-NPs.31–34
Here we demonstrate that PC can generally stabilize small
M-NPs (<5 nm).
Here we show that PC can function as a solvent for metal
nanoparticle synthesis and stabilization without the need for
additional capping ligands. Metal nanoparticles (M-NPs) are of
aInstitut für Anorganische Chemie und Strukturchemie, Heinrich-Heine-
Universität Düsseldorf, D-40204 Düsseldorf, Germany.
E-mail: janiak@uni-duesseldorf.de; Fax: +49 211 81 12287;
Tel: +49 211 81 12286
Results and discussion
M-NP synthesis and characterization
bFreiburger Material Forschungszentrum (FMF) and Institut für
Makromolekulare Chemie, Universität Freiburg, Stefan-Meier-Str.
21-31, 79104, Germany
The metal carbonyl Mo(CO)6, W(CO)6, Re2(CO)10, Fe2(CO)9,
Ru3(CO)12, Os3(CO)12, Co2(CO)8, Rh6(CO)16 or Ir4(CO)12 was
dissolved/suspended under an argon atmosphere in dried and
deoxygenated propylene carbonate (PC). Complete decompo-
sition by microwave irradiation of the metal carbonyl in PC was
†Electronic supplementary information (ESI) available: Additional
TEMs, DLS and SAED parameters, catalytic activity with comparative
activity data from literature. See DOI: 10.1039/c2dt30668a
9722 | Dalton Trans., 2012, 41, 9722–9727
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achieved after only 3 min using a low power of 50 W under
argon (Fig. 2).
size distribution (Table 1). SAED patterns do not show reflec-
tions indicative of a very crystalline material. We therefore con-
clude that the particles obtained from the synthesis are
amorphous M-NPs stabilized by PC. Still the diffraction rings
match the known d-spacing of the respective metal diffraction
pattern (see Tables S3–S5 in ESI†).
The resulting orange-red Os-, yellow Mn- and W-, and dark-
brown to black Mo-, Re-, Ru-, Co-, Rh- and Ir-NP dispersions
were reproducibly obtained through the microwave decompo-
sition route. Complete Mx(CO)y decomposition from short,
5 min microwave irradiation was verified by infrared spec-
troscopy with no (metal–)carbonyl bands between 1800 and
2000 cm−1 being observed any more after the microwave treat-
ment (Fig. 3).
The resulting M-NPs were analyzed by transmission electron
microscopy (TEM; Fig. 4–6), selected area electron diffraction
(SAED), and dynamic light scattering (DLS) for their size and
Fig. 5 TEM (left) and SAED (right) of Rh-NPs in PC from Rh6(CO)16
(entry 8 in Table 1). The black bar is the beam stopper. Diffraction rings
at (Å) 2.23 (very strong), 1.94 (strong), 1.39, 1.17 and 0.89 (weak)
match the d-spacing of the Rh-metal diffraction pattern (see Table S4†).
Fig. 2 Microwave decomposition of metal carbonyls to M-NPs in PC.
Fig. 6 TEM (left) and SAED (right) of Ir-NPs in PC from Ir4(CO)12
(entry 10 in Table 1). The black bar is the beam stopper. The diffraction
rings at (Å) 2.25 (strong), 1.38 and 1.20 (weak) match with the
d-spacing of the Ir-metal diffraction pattern (see Table S5†).
Fig. 3 IR-spectra of Rh6(CO)16/PC (light curve) and the resulting Rh-
NP (0.5 wt%)/PC-dispersion (bold curve) after microwave treatment
(50 W, 3 min).
Table 1 M-NP size and size distribution in PCa
Entry
Metal carbonyl
TEM Ø (σ)b [nm]
DLS Ø (σ)b [nm]
1
2
3
4
5
6
7
8
Mo(CO)6
W(CO)6
2.2 ( 0.5)
2.9 ( 0.6)
<1
2.4 ( 0.9)
2.7 ( 0.5)
3.0 ( 1.5)
6.1 ( 7.4)
2.1 ( 0.6)
3.2 ( 1)
3.5 ( 1.1)
3.7 ( 1.4)
1.4 ( 0.5)
3.2 ( 0.8)
2.6 ( 0.8)
4.0 ( 1.2)
6.7 ( 2.2)
2.4 ( 0.8)
—
Re2(CO)10
Fe2(CO)9
Ru3(CO)12
Os3(CO)12
Co2(CO)8
Rh6(CO)16
c
9
Rh6(CO)16
10
11
Ir4(CO)12
Ir4(CO)12
1.3 ( 0.5)
1.5 ( 0.4)
6.0 ( 1.6)
—
c
a 0.5 wt% M-NP/PC dispersions obtained by MWI with 50 W for 3 min
unless noted otherwise. b Median diameter (Ø) and standard deviation
(σ). See experimental section for TEM and DLS measurement
conditions. c 1 wt% M-NP/PC dispersion by MWI with 50 W for 5 min;
see Fig. S1 and S2 in ESI for TEM pictures.
Fig. 4 TEM (left) and SAED (right) of Ru-NPs in PC from Ru3(CO)12
(entry 5 in Table 1). The black bar is the beam stopper. The diffraction
rings at (Å) 2.10 (very strong), 1.61, 1.26, 1.15 (all weak) match with
the d-spacing of the Ru-metal diffraction pattern (see Table S3†).
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The hydrodynamic radius from DLS is roughly two to three
times the size of the pure kernel cluster. For very small M-NPs
(<1 nm) the size of the hydrodynamic radius was measured to be
more than three times the radius found from TEM. The median
M-NP diameter for the microwave-synthesized Mo-, W-, Re-,
Ru-, Os-, Rh- and Ir-NPs at 0.5 wt% of M-NPs in PC was
between <1 and 3.0 nm, with a narrow size distribution (TEM
data in Table 1). No extra stabilizers or capping molecules are
needed to achieve this small particle size. It is, at present, not
trivial to routinely and easily prepare uniform nanoparticles in
the size range between <1 and 3 nm without strong capping
ligands. For the magnetic Fe-NPs and Co-NPs the median diam-
eter was somewhat larger with 2.4 and 6.1 nm, respectively. For
Rh- and Ir-NPs also 1 wt% M-NP dispersions were prepared and
by comparison the higher metal concentrations give larger par-
ticles. The M-NP/PC dispersions are stable for up to three weeks
according to repeated TEM measurements over this time period.
Less-noble M-NPs in PC will be oxidized when exposed to
air whereas noble M-NPs (Rh, Ir) can be air stable, as seen
before for M-NPs in ionic liquids.25c–e
its low solubility in PC and is an intermediate in the hydrogen-
ation of benzene to cyclohexane.39,40
For activity comparison Tables S6 and S7 in ESI† summarize
related hydrogenation activities for cyclohexene and 1-hexyne
with M-NPs in ILs and on supports from the literature. Activities
for cyclohexene hydrogenations with Rh-NPs/PC were twice as
good than for Ru- or Rh-NP catalysts in ILs,26,41,42 but not as
good as for Rh-NPs on supports30,43,44 (Table S5 in ESI†). The
lower hydrogenation activities of Ru- and Rh-NPs in ILs are
traced to the IL diffusion barrier for H2 and the substrate.
Ligand-free NPs on supports have a lower diffusion barrier,
hence, higher activity.30
Rh- and Ru-NPs surface capping
The addition of an organic ligand to the bare M-NP surface is
generally described as a surface functionalization. However deri-
vatization, coating or capping are better terms.45 The post syn-
thetic introduction of an organic capping ligand on the dispersed
M-NPs in PC is possible. Surface capping of Rh- or Ru-nanopar-
ticles dispersed in the propylene carbonate was carried out here
with 3-mercaptopropionic acid, HS–(CH2)2–COOH or trioctyl-
phosphine oxide (TOPO). Both 3-mercaptopropionic acid45 and
TOPO46,47 are well-known stabilizing reagents and both are
soluble in propylene carbonate. The transformation of M-NP/PC
to M-NP/HS–(CH2)2–COOH or M-NP/TOPO was done by treat-
ing the M-NP/PC dispersion with an excess of HS–(CH2)2–
COOH or TOPO at room temperature over night. The strong
affinity between the thiol (–SH) or phospine oxide (–PvO)
group and the rhodium or ruthenium nanoparticles replaces the
PC protective layer. The ligand-capped nanoparticles are signifi-
cantly larger (Table 3, Fig. 8 and 9, see Fig. S3–S6 in ESI† for
additional TEM pictures).
Hydrogenation of cyclohexene and 1-hexyne
Organic carbonates have recently been used as solvents in cataly-
sis, e.g., for platinum-catalyzed hydrosilylation of unsaturated
fatty acid esters35 in Pd-catalyzed substitution reactions,36 regio-
selective rhodium-catalyzed hydroformylation37 and in the asym-
metric iridium-catalyzed hydrogenation of olefins.2,38 Palladium
colloids in PC were used to hydrogenate dienes and alkynes31,32
and for Heck-reactions.33 The weak interactions between a nano-
particle and organic carbonates could be of interest to develop
more efficient catalyst processes.
Here we tested the Rh-NP/PC dispersions for their catalytic
activity in the biphasic liquid–liquid hydrogenation of cyclohex-
ene to cyclohexane (Fig. 7, Table 2). The low miscibility of sub-
strates and products with the PC phase allows for easy separation
by simple decantation of the hydrophobic phase. Cyclohexene
was chosen as a substrate since it presents a challenge because of
The use of a protic organic thiol ligand and the unpolar TOPO
ligand more than doubles the size of the resulting capped metal
nanoparticles (Table 3). The aggregation is a result of the introduc-
tion of the capping ligands into the polar PC network. Sub-
sequently the stabilizing property of propylene carbonate towards
the M-NPs is weakened and results in further M-NP agglomera-
tion which is driven by the surface–surface interactions.
Conclusion
We describe here a simple, reproducible, and broadly applicable
microwave-induced metal carbonyl decomposition for the
Fig. 7 Hydrogenation of cyclohexene to cyclohexane with Rh-NPs.
Table 2 Hydrogenation of Rh-NPs/PC with different substratesa
Entry
Substrate
t [min]
pH2 [bar]
T [°C]
Conversion [%]
Activity [mol product × (mol Rh)−1 × h−1
]
1
2
3
4
5
Cyclohexene
Cyclohexene
Cyclohexene
1-Hexyneb
108
34
61
104
4
4
10
10
4
75
90
25
25
90
95c
95c
95c
88
0
590
1875
1045
51
Cyclohexened
1440 (= 24 h)
0
a 10 mL (0.1 mol) cyclohexene or 1.0 mL (1.8 mmol) 1-hexyne; 0.75 mL of the Rh-NP/PC dispersion with 1 wt% Rh (9 mg, 8.8 × 10−5 mol Rh).
b 1 mL n-decane was added to provide a biphasic liquid–liquid catalytic system. No n-hexene detected by GC analysis see experimental section. c The
reaction was intentionally stopped at 95 or 88% conversion as thereafter the decrease in cyclohexene concentration lowered the reaction rate (see
Fig. S7 in ESI). d Hydrogenation was carried out with Rh/SR-NPs (see Table 3, entry 3).
9724 | Dalton Trans., 2012, 41, 9722–9727
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Table 3 Ligand capped M-NP size and size distribution
M-NP/ligand in PC
Entry
Metal
Ligand
M-NP/PC original size from TEM [nm]
TEM Ø (σ)a [nm]
DLS Ø (σ)a,b [nm]
1
2
3
4
Rh
Rh
Ru
Ru
HS–(CH2)2–COOH
TOPO
HS–(CH2)2–COOH
TOPO
2.1 ( 0.6)
2.7 ( 0.5)
9 ( 5)
24 ( 17)c
30 ( 14)d
6.0 ( 1.5)c
37 ( 12)d
10 ( 5)
13 ( 4)
13 ( 5)
a Median diameter (Ø) and standard deviation (σ). See experimental section for TEM and DLS measurement conditions. b Hydrodynamic radius,
median diameter from the measurements at 633 nm. The resolution of the DLS instrument is 0.6 nm. c Measurement performed in ethanol.
d Measurement performed in chloroform.
carbonates are established industrial and low-priced solvents.4
PC appears as an attractive alternative for weakly coordinated,
albeit sufficiently stabilized metal nanoparticles.
Experimental section
Materials and instrumentation for M-NP synthesis: Mo(CO)6,
W(CO)6, Re2(CO)10, Fe2(CO)9, Ru3(CO)12, Os3(CO)12,
Co2(CO)8, Rh6(CO)16, and Ir4(CO)12 were obtained from Strem
and Aldrich, racemic propylene carbonate (PC) from Sigma-
Aldrich (purity 99.7%, H2O free). Cyclohexene (>99%),
1-hexyne (>97%), 3-mercaptopropionic acid (>99%), n-decane
Fig. 8 TEM of Rh-NP/HS–(CH2)2–COOH from propylene carbonate
after thiolation with 3-mercaptopropionic acid, centrifugation and re-dis-
persion in ethanol.
(p.a., purity >99.5%) and trioctylphosphine oxide (99%) were
obtained from Sigma-Aldrich and used without further purifi-
cation. All synthesis experiments were carried out with Schlenk
techniques under argon since the metal carbonyls are hygro-
scopic and air sensitive. The PC was dried under high vacuum
(10−3 mbar) for several days.
FT-IR (Fourier transform infrared) measurements were
carried out on a Bruker TENSOR 37 IR spectrometer in a range
from 4000 to 500 cm−1 in a KBr cuvette (thickness 0.05 mm).
Transmission electron microscopy (TEM) photographs were
taken at room temperature on a Zeiss LEO 912 TEM operating at
an accelerating voltage of 120 kV. Samples were deposited on
200 μm carbon-coated copper grids. TEM was operated in com-
bination with selected area electron diffraction, SAED (also
known as transmission electron diffraction, TED) which gives
information concerning compound phase(s) and crystallinity,
similar to X-ray powder diffraction.
A Malvern Zetasizer Nano-ZS was used for the dynamic light
scattering (DLS) measurements working at 633 nm wavelength.
Care was taken for choosing the right parameters, such as the
index of refraction of the transition metals at their wavelength
(Table S2†). Samples were prepared by dissolution of 0.05 or
0.1 mL of the 0.5 wt% metal/PC dispersion in acetone
(99% p.a.; particle free) in a glass cuvette before measurement.
Acetone is also capable of stabilizing nanostructured metal
clusters.48,49
Fig. 9 TEM of Rh-NP/TOPO from propylene carbonate after stabiliz-
ation with TOPO, centrifugation and re-dispersion in chloroform.
synthesis of common transition-metal nanoparticles in propylene
carbonate, PC. The M-NP sizes of about 1 to 3 nm for most of
these transition-metal nanoparticles are very small and uniform
with no extra stabilizers or capping molecules needed to achieve
this small particle size in a stable M-NP/PC dispersion. Polar
organic carbonates are susceptible for microwave irradiation
which, thus, provides a very simple and reproducible way for the
rapid (3 min) and energy-saving (50 W power) synthesis of
defined and very small M-NPs from their binary metal–carbonyl
complexes in PC. The obtained Rh-NP/PC dispersions can be
used – without further treatment – as highly active hydrogenation
catalysts. In comparison to ionic liquids, PC and other organic
Metal nanoparticle (M-NP) synthesis
Decomposition by means of microwave irradiation was carried
out under argon. In a typical reaction, the fine metal carbonyl
powder Mx(CO)y (M = Mo, W, Re, Fe, Ru, Os, Co, Rh, Ir; 19.8
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to 8.8 mg, respectively; see Table S1 in ESI†) was dissolved/sus-
pended (≈1 h) under an argon atmosphere at room temperature
in dried and deoxygenated PC (density: 1.19 g mL−1, 1 mL,
1.19 g) for a 0.5 wt% M-NP/PC dispersion. For the synthesis of
a 1 wt% M/PC dispersion the metal carbonyl Mx(CO)y (M =
Rh 15.5 mg; M = Ir 12.9 mg) was suspended/dissolved (≈1 h)
under an argon atmosphere at room temperature in dried and
deoxygenated PC (0.75 mL, 0.9 g). For the synthesis, the
mixture was placed in a microwave (CEM, Discover) under an
inert argon atmosphere and the conversion was finished within
3 min at a power of 50 W. For the 1 wt% dispersions a time of
5 min and a power of 50 W were chosen. Each decomposition
reaction was carried out at least twice. Decomposition reactions
to produce the Rh-NPs that were used in the catalysis in this
work were carried out ten or more times.
M-NPs were dried for several days under high vacuum to
remove the PC solvent.
Acknowledgements
Financial support through DFG grant Ja466/17-1 is gratefully
acknowledged.
Notes and references
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Rh) and 10 mL of cyclohexene (0.1 mol, density 0.811 g mL−1
,
M = 82.14 g mol−1) or 1.0 mL of 1-hexyne (1.8 mmol, density
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column (60 m × 0.32 mm film thickness 25 μm) oven tempera-
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Preparation of ligand-capped M-NP in PC
The obtained Rh- and Ru-NPs/PC (0.4 mL, 0.5 wt%) were
stirred with 3-mercaptopropionic acid (2 mL, 3.5 × 10−5 mol) or
TOPO (14 mg, 3.5 × 10−5 mol) over night. The ligand-capped
M-NPs were collected by centrifugation (2000 rpm, 15 min) and
decantation of the clear propylene carbonate phase. The capped
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