Highly stable noble-metal nanoparticles in tetraalkylphosphonium ionic liquids for in situ catalysis

Article abstract of DOI:10.1002/cssc.201100413

Gold and palladium nanoparticles were prepared by lithium borohydride reduction of the metal salt precursors in tetraalkylphosphonium halide ionic liquids in the absence of any organic solvents or external nanoparticle stabilizers. These colloidal suspensions remained stable and showed no nanoparticle agglomeration over many months. A combination of electrostatic interactions between the coordinatively unsaturated metal nanoparticle surface and the ionic-liquid anions, bolstered by steric protection offered by the bulky alkylated phosphonium cations, is likely to be the reason behind such stabilization. The halide anion strongly absorbs to the nanoparticle surface, leading to exceptional nanoparticle stability in halide ionic liquids; other tetraalkylphosphonium ionic liquids with non-coordinating anions, such as tosylate and hexafluorophosphate, show considerably lower affinities towards the stabilization of nanoparticles. Palladium nanoparticles stabilized in the tetraalkylphosphonium halide ionic liquid were stable, efficient, and recyclable catalysts for a variety of hydrogenation reactions at ambient pressures with sustained activity. Aerial oxidation of the metal nanoparticles occurred over time and was readily reversed by re-reduction of oxidized metal salts. Stable morsels of metal: Palladium nanoparticles stabilized in tetraalkylphosphonium halide ionic liquids are stable, efficient, and recyclable catalysts for a variety of hydrogenation reactions at ambient pressures with sustained activity (see picture). Copyright

Full text of DOI:10.1002/cssc.201100413

DOI: 10.1002/cssc.201100413
Highly Stable Noble-Metal Nanoparticles in
Tetraalkylphosphonium Ionic Liquids for in situ Catalysis
Abhinandan Banerjee, Robin Theron, and Robert W. J. Scott*[a]
Gold and palladium nanoparticles were prepared by lithium
borohydride reduction of the metal salt precursors in tetraal-
kylphosphonium halide ionic liquids in the absence of any or-
ganic solvents or external nanoparticle stabilizers. These colloi-
dal suspensions remained stable and showed no nanoparticle
agglomeration over many months. A combination of electro-
static interactions between the coordinatively unsaturated
metal nanoparticle surface and the ionic-liquid anions, bol-
stered by steric protection offered by the bulky alkylated phos-
phonium cations, is likely to be the reason behind such stabili-
zation. The halide anion strongly absorbs to the nanoparticle
surface, leading to exceptional nanoparticle stability in halide
ionic liquids; other tetraalkylphosphonium ionic liquids with
non-coordinating anions, such as tosylate and hexafluorophos-
phate, show considerably lower affinities towards the stabiliza-
tion of nanoparticles. Palladium nanoparticles stabilized in the
tetraalkylphosphonium halide ionic liquid were stable, efficient,
and recyclable catalysts for a variety of hydrogenation reac-
tions at ambient pressures with sustained activity. Aerial oxida-
tion of the metal nanoparticles occurred over time and was
readily reversed by re-reduction of oxidized metal salts.
Introduction
Nanosized materials have attracted extensive interest in recent
years because of their unique structural and chemical proper-
ties, which can often be modulated by changing parameters
such as size, shape, and the surrounding environment of the
nanomaterials.[1] Noble-metal nanoparticles (MNPs), in particu-
lar, have shown impressive catalytic abilities for reactions of
both scientific and industrial importance.[2] Although support-
ed MNPs have traditionally been used as heterogeneous cata-
lysts for enhanced reusability and facile product recovery,
recent research has been focused on their use as “quasi-homo-
geneous” catalysts.[3] Quasi-homogeneous nanocatalysis in-
volves the synthesis and use of stable and catalytically active
nanoparticles (NPs) in solution; this combines the benefits of
catalyst modulation and recyclability without having to hetero-
genize a homogeneous catalyst.[4] Although there is no dearth
of either materials or methodologies for the synthesis of MNPs
in solution, their lack of thermodynamic stability and conse-
quent tendency to agglomerate and precipitate often prevent
their effective application as efficient catalytic systems.[5]
tion. Herein, we show a process that involves the use of tet-
raalkylphosphonium halide ionic-liquid (IL) solvents, which
allow for a robust and reusable catalytic MNP system in the ab-
sence of organic solvents, secondary stabilizers, and excessive
functionalization of the dispersion medium.
In the last decade, many groups have shown that ILs can be
used as intriguing media for the stabilization of MNPs.[7] Immo-
bilization of NPs by supporting them in an IL rather than on a
surface provides catalytic centers in the liquid phase; there
have been many reports in the literature exploring the catalytic
properties of MNPs in imidazolium-based ILs since the pioneer-
ing work of Dupont and co-workers.[8] Previously, well-defined
Pd NPs have been prepared in situ and used for CÀC coupling
reactions, such as Heck and Suzuki processes,[9] while we have
shown that bimetallic NPs can be transferred from water to
imidazolium ILs for efficient nanocatalysis.[10] The reasons
behind such efforts can be understood, if we consider the nu-
merous attractive chemical properties of ILs, such as limited
volatility and flammability, recyclability, wide ranges of solubili-
ty and miscibility, and structural tunability (through both the
cation and anion), although their green credentials are a
matter of debate.[11] Despite these developments, optimal strat-
egies for MNP stability in ILs are still being determined; there
is some evidence that MNP dispersions are inherently more
stable in ILs because of the IL microstructure and/or weak in-
teractions of the anions and/or cations of ILs with the NP sur-
face, although we have previously shown that inherent impuri-
To date, the electrostatic and/or steric protection that stabi-
lizes such NPs is introduced through the addition of an exter-
nal stabilizer; synthetic strategies, in which such stabilization is
inherent in the reaction system, are few and far between.
Stabilizers, such as poly(vinylpyrrolidone) and poly-N-donor li-
gands, and molecular stabilizers, such as alkanethiolates, can
stabilize MNPs; however, they can suffer from numerous disad-
vantages: very efficient capping ligands often passivate the
catalytic activity of MNPs, and the protective ligand may not
be soluble in the medium of dispersion of MNPs.[6] Moreover,
the concept of involving a secondary stabilizer in a catalyst
system just for the sake of protection may have undesirable ef-
fects on the selectivity and the reactivity of the catalyzed reac-
[a] A. Banerjee, R. Theron, Prof. R. W. J. Scott
Department of Chemistry, University of Saskatchewan
110 Science Place, Saskatoon SK S7N 5C9 (Canada)
Fax: (+1)306 966 4730
E-mail: robert.scott@usask.ca
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R. W. J. Scott et al.
ties in imidazolium ILs (such as unreacted 1-methylimidazole)
and/or secondary stabilizers can dramatically affect the stabili-
ty.[12] In addition, “task-specific” functionalized imidazolium and
pyridinium ILs with thiol, alcohol, and nitrile moieties have
been used for the synthesis of MNPs by many groups.[13]
generated by re-reduction of the metal salts to reform MNPs,
thus enhancing the recyclability of such MNP/IL composites.
Results and Discussion
To date, the majority of research concerned with MNP cata-
lysts in ILs has involved imidazolium-based ILs; this can be par-
tially attributed to the moderate viscosities of room-tempera-
ture imidazolium ILs.[14] The room-temperature imidazolium ILs
Synthesis of MNPs in tetraalkylphosphonium ILs
Inherently stabilized Au and Pd MNPs were synthesized in
tetraalkylphosphonium ILs through in situ LiBH4 reduction,
without the involvement of any organic solvents or secondary
stabilizers. As these ILs are considerably viscous, cooling of the
product led to highly viscous systems; however, the viscosity
reduced drastically upon heating.
À
have weakly coordinating anions, such as BF4 and PF6À, with
limited MNP stabilization capabilities. Moreover, the imidazoli-
um cation can undergo deprotonation in the presence of
strong bases to form carbenes,[15] and hexafluorophosphate
groups can undergo anion hydrolysis to produce strongly toxic
HF at moderate temperatures.[16] Another class of ILs is the tet-
raalkylphosphonium family of ILs, which are considerably more
stable in strongly basic media,[17] cost less, and are fairly chemi-
cally inert (although Wittig substitution reactions on the phos-
phonium center have been reported).[18] Several other groups
have previously shown that tetraalkylphosphonium ILs can be
used successfully for MNP stabilization and catalysis. In particu-
lar, Kalviri and Kerton reported a Suzuki coupling reaction cata-
lyzed by Pd MNPs, which were formed in phosphonium ILs in
the absence of an external reductant, and it was postulated
that the IL itself was acting as a reducing agent.[19] Yinghuai
et al. used the same IL to prepare Ru MNPs from an organome-
tallic precursor with ethylene glycol as a reductant.[20] However,
to the best of our knowledge, all of the tetraalkylphosphonium
IL systems studied to date have anions that either do not coor-
dinate to metal surfaces or coordinate weakly to them. We
were particularly intrigued by the P[6,6,6,14]X {P[6,6,6,14]=tri-
hexyl(tetradecyl)phosphonium cation; X=Cl, Br} family of
room-temperature ILs, because of their striking structural re-
semblance to the quaternary ammonium surfactants which
were already successfully used for electrostatic MNP stabiliza-
tion.[21] We postulated that such P[6,6,6,14]X (X=Cl, Br) ILs
would be particularly interesting to examine as efficient dual
“solvent/stabilizers” for MNP synthesis.
TEM images of Au MNPs synthesized in P[6,6,6,14]Cl and
P[6,6,6,14]Br are shown in Figure 1a and d, respectively. The
average sizes of the free Au MNPs synthesized in P[6,6,6,14]Cl
Herein, we outline the synthesis of a catalytic system of
MNPs dispersed in several tetraalkylphosphonium ILs in the ab-
sence of organic solvents and external stabilizers conventional-
ly used for protection against MNP agglomeration. Au and Pd
MNPs synthesized directly in P[6,6,6,14]X (X=Cl, Br) ILs showed
prolonged stability over several months, whereas MNPs
formed in systems with weakly coordinating tosylate (OTs) and
hexafluorophosphate anions showed poor stability towards ag-
gregation. Pd MNPs dispersed in P[6,6,6,14]X (X=Cl, Br) ILs
were recyclable catalysts for the hydrogenation of olefinic alco-
hols, aromatic nitro compounds, 1,2-unsaturated carbonyls,
and alkynes. The dependence of the catalytic activities on the
nature of the IL used for dispersion of the MNPs further con-
firms the important role played by the ILs in MNP stabilization,
and sheds light on the exact mechanism of protection against
aggregation. In addition, we show that, although both the Au
and Pd MNPs dispersed in P[6,6,6,14]X (X=Cl, Br) ILs are prone
to oxidation in the presence of air, the MNPs can be easily re-
Figure 1. a) TEM image of Au MNPs prepared in P[6,6,6,14]Cl. b) TEM image
of Au MNPs prepared in P[4,4,4,1]OTs. c) Size distribution of Au MNPs in
P[6,6,6,14]Cl. d) TEM image of Au MNPs prepared in P[6,6,6,14]Br. e) UV/Vis
spectra of 1.4 mm solutions of HAuCl4 (c), Au MNPs in P[6,6,6,14]Cl (b),
and Au MNPs in P[6,6,6,14]Br (g) in cyclohexane.
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Nanoparticles in Tetraalkylphosphonium Ionic Liquids
and P[6,6,6,14]Br determined from the TEM images were (4.1Æ
0.7) and (5.4Æ0.9) nm, respectively. This is in agreement with
the UV/Vis spectra of the MNPs (Figure 1e). The spectra of the
Au MNPs in P[6,6,6,14]Cl show a broad plasmon band at l=
520 nm after reduction. For Au MNPs in P[6,6,6,14]Br, the plas-
mon band is significantly shifted to higher wavelengths be-
cause of the larger average particle size. Au MNPs synthesized
in tributyl(methyl)phosphonium tosylate (P[4,4,4,1]OTs) aggre-
gated quickly (TEM micrograph shown in Figure 1b) and pre-
cipitated within a day.
very slow aerial oxidation, as the red Au0 system becomes col-
orless and then pale yellow after several weeks at 908C; this is
likely to be caused by oxidative etching of Au assisted by
halide absorption on the surface, leading to the formation of
AuI, AuIII, or a mixture of both.[22] The Pd MNPs in the
P[6,6,6,14]Cl IL are completely oxidized over 48 h upon heating
in air at 908C. Oxidative etching of Pd in the presence of O2
and ClÀ is well known: Kalviri and Kerton attribute their failure
to grow Pd NPs in P[6,6,6,14]Cl in the absence of reductants to
this effect.[19] In both cases, the oxidized metal species could
be readily re-reduced by dropwise addition of excess LiBH4.
The UV/Vis spectra before and after regeneration and the TEM
images of the particles after re-reduction are shown in
Figure 3. After oxidation and re-formation of the Pd and Au
MNPs in P[6,6,6,14]Cl, TEM analysis revealed average particle
sizes of (3.9Æ1.1) and (3.5Æ1.1) nm, respectively. The reasons
for the moderate increase in Pd MNP size are still unknown.
Pd MNPs synthesized in P[6,6,6,14]Cl had a deep brown
color. A TEM image of Pd MNPs synthesized in P[6,6,6,14]Cl is
shown in Figure 2a; the average size of these particles was
(2.3Æ0.6) nm. The UV/Vis spectrum, shown in Figure 2c, shows
an exponential rise in absorbance with decreasing wavelength
typical of Pd MNPs in solution.
Figure 3. TEM images of a) Pd MNPs and b) Au MNPs in P[6,6,6,14]Cl regen-
erated after aerial oxidation followed by LiBH4 reduction. UV/Vis spectra of
the re-reduction of c) Pd MNPs (c: Pd NPs, a: re-reduced Pd NPs, g:
post-oxidation Pd) and d) Au MNPs in P[6,6,6,14]Cl (c: Au NPs, a: re-
reduced Au NPs, g: post-oxidation Au).
Figure 2. a) TEM image of Pd MNPs prepared in P[6,6,6,14]Cl. b) Size distribu-
tion of Pd MNPs prepared in P[6,6,6,14]Cl. c) UV/Vis spectra of 1.4 mm solu-
tions of K2PdCl4 (c) and Pd0 in P[6,6,6,14]Cl (a) in cyclohexane.
Borohydride reduction is a well-documented method for the
synthesis of MNPs in conventional solvents, and its mechanism
has been thoroughly studied.[23] Reduction of dissolved metal
salts give metal atoms in the embryonic stage of nucleation,
followed by irreversible ion/atom or atom/atom collisions that
produce metal cluster “seeds”. It has been pointed out by
Dupont et al.[7b] that there are electrostatic forces at play be-
tween the charged MNP surfaces and the three-dimensional
network of cations and anions that constitute the ILs.
Dupont[7b] and others[24] proposed a two-phase system to ex-
plain imidazolium IL stabilization of MNPs: a crystalline MNP
and a semi-ordered IL phase. A similar model may also be ap-
plied to the tetraalkylammonium phosphonium IL systems
NP stability and regeneration
The P[6,6,6,14]Cl-stabilized MNPs remained unagglomerated
for months under N2. This stability is exceptional compared to
that of MNPs stabilized in tetraalkylphosphonium ILs bearing
poorly coordinating anions, such as N(CN)2À, OTs, and methyl-
sulfate (SO4MeÀ), in which precipitation of the particles is ob-
served within 12–24 h, or MNPs stabilized directly in imidazoli-
um ILs, which tend to precipitate over several days or almost
immediately under hydrogenation conditions.[12] Under air at
elevated temperatures, Au MNPs in the P[6,6,6,14]Cl IL show
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111

R. W. J. Scott et al.
greater the stabilization provided by it to the MNP. Similarly,
one should expect a similar trend in catalytic activities. This is
confirmed by Figure 4, which shows the hydrogenation of 2-
methyl-3-butene-2-ol at 758C by using 14.0 mm Pd MNPs in
Scheme 1. Schematic representation of MNP stabilization by P[6,6,6,14]Cl IL.
(Scheme 1), in which the halide anions bind strongly to the
metal surface and the associated bulky tetraalkylphosphonium
cations offer steric protection. Significant work using tetraal-
kylammonium halide systems, such as cetyltrimethylammoni-
um bromide (CTAB) and tetraoctylammonium bromide (TOAB),
have shown that halide absorption onto metal surfaces can
lead to significant stability of the resulting particles.[25] Another
significant validation for this model is the direct dependence
of MNP stability on the anion composition of the IL: the stron-
ger the coordinating ability of the anion (e.g., ClÀ >OTsÀ), the
greater the protection of the MNP surface, which leads to in-
creased stability of the MNP in the IL.
Application of Pd MNPs in P[6,6,6,14]Cl IL for catalytic
hydrogenations
The catalytic behavior of Pd MNPs in P[6,6,6,14]Cl IL was evalu-
ated by performing the hydrogenation of several organic com-
pounds with unsaturated moieties present in their structures.
Slightly elevated temperatures were necessary to reduce the
viscosities of the ILs for attenuation of mass-transfer effects.
Although the solubility of H2 in ILs is moderate, it has been ob-
served that this solubility actually increases at higher tempera-
tures, which is in contrast to other gases, such as CO2.[26] Semi-
nal work by Brennecke et al. showed the dependence of gas
solubility in an IL on the nature of the anion present.[27a] Earlier,
Dyson et al. determined the Henry constant for H2 in
P[6,6,6,14][PF3(C2F5)3] to be 70 MPa at 293 K, which indicated
that H2 was more soluble (by an order of magnitude) in many
phosphonium ILs than the imidazolium or pyridinium ILs.[27b]
For the Pd MNP/P[6,6,6,14]Cl system, most of the hydroge-
nations reached completion within a few hours. The progress
of the reaction was followed by differential H2 pressure meas-
urements; the details of which are given in the Experimental
Section. The monometallic Pd MNPs in the P[6,6,6,14]Cl IL
showed high catalytic activity and considerable stability. It was
noted earlier that the mechanism of stabilization of MNPs in-
volves protective anchoring of the IL anions onto MNP sur-
faces; the better the coordinating ability of the anion, the
Figure 4. a) Turnover number (TON) versus time plot for the hydrogenation
of 2-methyl-3-butene-2-ol by using 14.0 mm Pd MNPs in various phosphoni-
~
&
^
um ILs { : P[6,6,6,14]Br, : P[4,4,4,1]OTs, : P[6,6,6,14]PF6, ꢁ: P[6,6,6,14]Cl,
*
: P[4,4,4,1]OSO3Me, –: P[6,6,6,14]N(CN)2}. b) Comparison of the turnover fre-
quency (TOF) per min of the reaction in different ILs.
different ILs. The relative turnover frequencies (TOFs) of this re-
action over the first half hour can be seen in Figure 4b; the
largest TOF of 10.2 minÀ1 was found for P[6,6,6,14]Cl compared
to only 1.8 minÀ1 for P[6,6,6,14]N(CN)2. Traces of precipitate
were observed in the reaction flask after completion of the re-
action for MNPs in P[6,6,6,14]N(CN)2, P[4,4,4,1]OTs, and
P[4,4,4,1]OSO3Me.
According to Figure 4b, the TOFs increase in the following
order:
P[6,6,6,14]N(CN)2 <P[4,4,4,1]OTs<P[4,4,4,1]OSO3Me<
P[6,6,6,14]PF6 <P[6,6,6,14]Br !P[6,6,6,14]Cl. The activities here
are substantial: we previously observed significantly lower
TOFs (approximately 4.5 minÀ1) for hydrogenations with polyvi-
nylpyrrolidone (PVP)-stabilized Pd MNPs in imidazolium ILs.[10]
Trends can be deduced from this order, namely, shorter
alkyl-chain-bearing phosphonium ILs are less efficient at pro-
tecting against agglomeration than those with longer chains,
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Nanoparticles in Tetraalkylphosphonium Ionic Liquids
and the halides in general provide maximum stability to MNPs,
leading to high catalytic activities. The first observation could
easily be justified if, we consider the representation in
Scheme 1, in which a surrounding layer of tetraalkylphosphoni-
um ions is shown to offer steric protection to the MNPs. Clear-
ly, shorter alkyl chains would be less efficient at offering resist-
ance to aggregation than longer ones. A number of other fac-
tors make P[6,6,6,14]Cl the best IL for Pd MNP stabilization and
catalysis. Ferguson and Scovazzo found that the diffusivity of
many gases at 308C was maximum in P[6,6,6,14]Cl[28] and ob-
served that gas diffusivity in phosphonium-based ILs was sig-
nificantly higher than diffusivity in imidazolium-based ILs of
equal viscosity. P[6,6,6,14][NTf2] with a viscosity of 200 cP
(1 P=0.1 Pas) has the same gas diffusivity as an imidazolium-
based IL with a viscosity of 20 cP. They attributed this to the
small size of ClÀ, which prevents it from competing with diffus-
ing gas molecules to fill the same microvoids within the IL.
However, of greater importance is the idea of MNP stabilization
by anionic species of the IL. Among all of the ILs studied, the
anion with the largest surface charge density is ClÀ. In other
words, the catalytic activities of MNPs dispersed in ILs depend
overwhelmingly on the stability of the MNPs in those ILs. As
ClÀ is capable of offering excellent electrostatic protection to
the MNP surface, it enhances the catalytic abilities of the MNPs
dispersed in Cl-bearing ILs. A similar effect is observed for BrÀ,
but to a smaller extent, which may be explained by the larger
size and smaller surface charge density of BrÀ. All other anions
show comparatively smaller catalytic efficiencies and for OTsÀ
and OSO3MeÀ ILs, precipitation of Pd MNPs was observed
within several days. The only IL that shows a slight deviation
from this trend is P[6,6,6,14]PF6, the large, poorly coordinating
anion of which should lead to poor MNP stabilization; howev-
er, the comparatively higher stability of Pd MNPs in this IL can
be explained by the moderate ClÀ content of the IL of
200 ppm, even after repeated washings (P[6,6,6,14]PF6 was syn-
thesized from P[6,6,6,14]Cl by using ion exchange).
Table 1. Data for the hydrogenation of unsaturated species using H2 in
the presence of 14mm Pd NP/P[6,6,6,14]Cl.[a]
Reactant
Product(s)
Yield[b]
[%]
T
t
[8C] [h]
75
70 2.5
>99
2
90
52[c]
100
2
>99 (1: 61, 2: 38) 110
8
90 (1: 78, 2: 22)
80 4.5
PhNO2
PhNH2
95
115
60
6
6
>99 (3: 18, 4: 82)
92 (3: 41, 4: 59)
55 2.5
>99 (5: 8, 6: 92)
85 (5: 21, 6: 79)
120
90
8
5
[a] The substrate (1–2 mL) was added to the 14 mM Pd NP/P[6,6,6,6,14]Cl
catalyst system (10 mL) and heated at the given temperature for the
given length of time under an atmosphere of H2 at ambient pressures.
[b] Yields calculated from the areas of the signals in the 1H NMR spectra
of the pure products and/or residual reactants recovered from the reac-
tion mixture after the completion of the reaction by using vacuum strip-
ping at high temperatures. [c] Extensive formation of polystyrene gran-
ules that precipitated can explain the low yield.
duced the extent of conversion to approximately 90%, but
also gave more cyclooctene (78%, as opposed to 61% for a re-
action for 8 h at 1108C). Similar observations were recorded for
trans-cinnamaldehyde and 3-hexyn-1-ol. For the former, reac-
tion for 5 h at 908C produced 79% 3-phenylpropanal and 21%
of the phenyl alcohol; the extent of conversion was also found
to drop to approximately 85%. Another possible hydrogena-
tion product, cinnamyl alcohol, was not detected at all. This is
in accordance with previous results in other systems.[10] In the
hydrogenation of 3-hexyn-1-ol, hexenols and hexanol were
formed as the only products; a reaction for 6 h at 608C gave
respective yields of 18 and 82%. When the reaction time was
reduced to 2.5 h and the temperature to 558C, the hexenol
yield increased to 41%, whereas 59% of hexanol was pro-
duced. Combined evidence, therefore, indicates that the hy-
drogenations proceed by the accepted Horiuti–Polanyi mecha-
nism, with one unsaturated moiety reduced at a time.[1] Expo-
sure to H2 for smaller amounts of time and/or at a lower tem-
perature favors the partially hydrogenated product.
The selectivity of a catalyzed reaction is also profoundly in-
fluenced by the surrounding environment of the catalyst. The
Pd MNPs in P[6,6,6,14]Cl catalyzed the hydrogenation of vari-
ous organic molecules with unsaturated moieties (such as ole-
finic and acetylinic multiple bonds) in their structures. Accord-
ing to Table 1, yields were almost quantitative, except for sty-
rene, for which polystyrene was also obtained. The aromatic
moiety remained unaffected in all relevant reactions. The Pd
MNPs in P[6,6,6,14]Cl showed very high thermal stability to rel-
atively elevated temperatures (up to approximately 1408C);
thus, a relatively high reaction temperature was chosen for
several of the hydrogenations. Of the various organic mole-
cules hydrogenated, the catalytic activity was found to be the
lowest for the hydrogenation of trans-cinnamaldehyde; this
could be because of the presence of highly conjugated unsa-
turation, along with steric effects of the bulky benzene ring at-
tached to the double bond. The selectivity towards partial hy-
drogenation of 1,3-cyclooctadiene to generate cyclooctene
(rather than the completely saturated cyclooctane) was investi-
gated; this was found to be a function of time and tempera-
ture of the reaction. Reaction for 4.5 h at 808C not only re-
The catalytic efficiency of the regenerated MNPs in an IL is
shown in Figure 5, which shows the recyclability of this system
for the hydrogenation of 2-methyl-3-butene-2-ol. For the recy-
clability evaluation, the same system (Pd MNPs in P[6,6,6,14]Cl)
was used repeatedly for the reduction of 2-methyl-3-butene-2-
ol at 758C. For each cycle, the catalytic activity was obtained
by performing differential pressure measurements of H2 inside
the sealed reaction flask; the products and unreacted substrate
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R. W. J. Scott et al.
Figure 6. a) TEM image of Pd MNPs in P[6,6,6,14]Cl regenerated after aerial
oxidation followed by LiBH4 reduction; size distributions are given in c).
b) TEM image of Pd MNPs in P[6,6,6,14]Cl after five cycles of catalytic hydro-
genations; size distributions are given in d).
organic solvents. These MNPs were found to have stabilities
ranging from days to months, depending upon the composi-
tion of the IL. The Pd NPs showed excellent activities for the
hydrogenation of a range of substrates (cyclohexene, 2-
methyl-3-butene-2-ol, nitrobenzene, 1,3-cyclooctadiene, trans-
cinnamaldehyde, and 3-hexyn-1-ol) when using H2 at ambient
pressures. Unreacted substrates (if any) and products were
easily removed from the catalytic mixture by applying a
vacuum treatment. The catalytic dispersions of Pd MNPs in
P[6,6,6,14]Cl were stable for months, highly recyclable, and
could be readily regenerated after aerial oxidation.
Figure 5. a) Recyclability of Pd MNPs prepared in P[6,6,6,14]Cl for hydroge-
nation of 2-methyl-3-butene-2-ol. The final column shows the TOF per min
after complete Pd oxidation at 908C in air over two days followed by re-re-
duction of Pd with LiBH4. b) From left to right: 14 mm Pd NPs in P[6,6,6,14]Cl
after 5 catalytic cycles, after oxidation, and finally, after re-reduction with
LiBH4.
were removed by vacuum stripping, and the catalyst system
was used for the next cycle. After removal of volatile com-
pounds from the catalytic mixture, there was very little loss in
activity for the hydrogenation of the substituted allyl alcohol
(<5% after 5 cycles), which confirmed the re-usability/recycla-
bility of the catalysts. The physical appearance of the system
can be seen in Figure 5b; there are no significant differences
after five catalytic cycles or after the oxidation/reduction
procedure.
Experimental Section
Materials
All chemicals, except for the ones listed below, were purchased
from Sigma Aldrich and used as received. HAuCl4 and K2PdCl4
(both 99.9%, metals basis) were obtained from Alfa Aesar and
were stored under vacuum and flushed with N2 after every use.
Trihexylphosphine (98%), as well as commercial samples of the
ionic liquids (ILs), were generously donated by Cytec Industries.
Commercial samples of ILs were dried under vacuum at 708C for
10–12 h with stirring before use. Deuterated solvents were pur-
chased from Cambridge Isotope Laboratories. 18 MWcm Milli-Q
water (Millipore, Bedford, MA) was used.
However, moderate particle-size growth was observed upon
use of the Pd MNPs as hydrogenation catalysts, as shown in
Figure 6b; after five cycles, the average size of the particles
grew to (5.5Æ1.9) nm. We are uncertain as to why this particle
size growth occurs, but no MNP precipitation was observed in
the system after multiple cycles. This growth could be reversed
by Pd oxidation in air at elevated temperatures followed by re-
reduction of the Pd salts (Figure 5b).
Synthesis of the tetraalkylphosponium ILs
The synthesis of P[6,6,6,14]Cl and the generation of nanosized Au
and Pd clusters in the IL were performed by using standard
Schlenk techniques. For a typical synthesis of P[6,6,6,14]Cl, trihexyl-
phosphine was added to a slight excess of 1-chlorotetradecane
under N2 at 1458C, stirred overnight, and vacuum stripped to
Conclusions
We have synthesized Au and Pd NPs in a variety of tetraalkyl-
phosphonium ILs by in situ reduction without the use of any
114
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ChemSusChem 2012, 5, 109 – 116

Nanoparticles in Tetraalkylphosphonium Ionic Liquids
remove volatile impurities.[29] A transparent, pale-yellow viscous IL
was generated in 90% yield. Chloride contents of the ILs prepared
by metathesis were checked by using an AgNO3 test. 1H and
31P NMR spectra of the as-prepared samples matched with those
reported in the literature.[27] The commercial sample of P[6,6,6,14]Cl
supplied by Cytec was of comparable purity. P[6,6,6,14]Br was syn-
thesized by using an identical procedure and was a deep yellow
viscous liquid obtained in 93% yield; it was also indistinguishable
from the dried Cytec samples in terms of purity. P[6,6,6,14]PF6 was
synthesized from P[6,6,6,14]Cl by using an additional ion-exchange
step, in which HPF6 was added dropwise to ice-cold P[6,6,6,14]Cl
under N2 and stirred overnight under ambient temperatures. The
waxy white solid generated was washed repeatedly with deionized
water until the washings failed to generate a precipitate upon re-
action with a large excess of a 0.05m AgNO3 solution. It was then
within 5% of values obtained by using differential pressure meas-
urements. Yields and product distributions were studied by using
NMR spectroscopy.
Characterization
UV/Vis spectra were obtained by using a Varian Cary 50 Bio UV/
Visible spectrophotometer with a scan range of l=200–800 nm
1
and an optical path length of 1.0 cm. H and 31P NMR spectra were
obtained by using a Bruker 500 MHz Avance NMR spectrometer;
chemical shifts were referenced to the residual protons of the deu-
terated solvent. TEM analyses of the Au and Pd MNPs in different
ILs, both before and after catalytic cycles, were conducted by
using a Philips 410 microscope operating at 100 kV. The samples
were prepared by ultrasonication of a 1% solution of the MNP/IL
solution in CHCl3 followed by dropwise addition onto a carbon-
coated copper TEM grid (Electron Microscopy Sciences, Hatfield,
PA). To determine particle diameters, a minimum of 100 particles
from each sample from several TEM images were manually mea-
sured by using the ImageJ program.[30]
dried under vacuum, giving
a yield of 95%. P[4,4,4,1]OTs,
P[4,4,4,1]OSO3Me, and P[6,6,6,14]N(CN)2 ILs were donated by Cytec
and were moderately heated under vacuum to remove volatile im-
purities prior to use.
Synthesis of Pd and Au MNPs in ILs
For the synthesis of Pd MNPs, K2PdCl4 (45 mg; 0.14 mmol on the
basis of Pd content) was added under N2 to a sample of the IL
(10 mL) at 808C (all of the ILs studied were liquids at this tempera-
ture) and vigorously stirred to give a 14 mm solution. The solution
was cooled to 608C, and a stoichiometric excess of LiBH4 reagent
(1.5 mL, 2.0m in THF) was injected dropwise over a period of
5 min. Rapid effervescence followed, and the entire solution turned
brown, indicating NP formation. After the addition of LiBH4, volatile
impurities were removed by vacuum stripping the system at 808C.
The Pd MNP solution thus obtained was stored under N2 in
capped vials until use. For the synthesis of Au MNPs, an identical
solvent-free procedure was followed, except the reactions were
performed in air. HAuCl4 (20 mg, equivalent to 0.05 mmol of Au)
was dissolved in P[6,6,6,14]X (10 mL; X=Cl, Br) at 808C to give a
golden yellow solution, which turned violet and then wine-red
upon the dropwise addition of 2.0m LiBH4 reagent (1.5 mL) after
cooling the solution to 608C.
Acknowledgements
We would like to thank the NSERC and the University of Sas-
katchewan for funding, Dr. Pia Wennek for her help and support,
and Tesfalidet Balcha for help with the TEM imaging and pictures
of the samples. A.B. would like to acknowledge scholarships from
the University of Saskatchewan and VWR. We would also like to
express our gratitude towards Al Robertson of Cytec for donation
of the phosphonium ILs as well as the phosphine precursors.
Keywords: catalysis · green chemistry · hydrogenation · ionic
liquids · nanoparticles
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