Palladium nanoparticle catalysts in ionic liquids: Synthesis, characterisation and selective partial hydrogenation of alkynes to Z-alkenes

Article abstract of DOI:10.1039/c0jm03557b

The simple heating (120 °C) of Pd(OAc)₂ in 1-butyronitrile-3-methylimidazolium-N-bis(trifluoromethane sulfonyl)imide ((BCN)MI·NTf₂) under reduced pressure leads to the formation of stable and small-sized Pd(0)-NPs (diameter: 7.3 ± 2.2 nm). These metal nanoparticles were characterised by means of TEM, HRTEM and XPS analysis techniques. Moreover, the potential for partial hydrogenation of alkynes in multiphase systems was evaluated. The hydrogenation of internal alkynes at 25 °C and under 1 bar of hydrogen yields Z-alkenes (up to 98% selectivity). Application of higher hydrogen pressure (4 bar) in these reactions always led to the formation of alkanes without the detection of any alkenes. TOF values were attained up to 1282 h^-1 with a good recyclability of the system which does not lose its activity for at least 4 runs. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c0jm03557b

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PAPER
Palladium nanoparticle catalysts in ionic liquids: synthesis, characterisation
and selective partial hydrogenation of alkynes to Z-alkenes†
a
Ramprakash Venkatesan, Martin H. G. Prechtl,‡
ab
a
Jackson D. Scholten,a Rafael P. Pezzi,x
*
c
Giovanna Machado and Jairton Dupont
a
*
Received 20th October 2010, Accepted 7th December 2010
DOI: 10.1039/c0jm03557b
The simple heating (120 ꢀC) of Pd(OAc)2 in 1-butyronitrile-3-methylimidazolium-
N-bis(trifluoromethane sulfonyl)imide ((BCN)MI$NTf2) under reduced pressure leads to the
formation of stable and small-sized Pd(0)-NPs (diameter: 7.3 ꢁ 2.2 nm). These metal nanoparticles
were characterised by means of TEM, HRTEM and XPS analysis techniques. Moreover, the potential
for partial hydrogenation of alkynes in multiphase systems was evaluated. The hydrogenation of
internal alkynes at 25 ꢀC and under 1 bar of hydrogen yields Z-alkenes (up to 98% selectivity).
Application of higher hydrogen pressure (4 bar) in these reactions always led to the formation of
alkanes without the detection of any alkenes. TOF values were attained up to 1282 hꢂ1 with a good
recyclability of the system which does not lose its activity for at least 4 runs.
composed of imidazolium aggregates located immediately adja-
cent to the nanoparticle surface, providing both steric and elec-
tronic protection against aggregation and/or agglomeration.23,24
Palladium is among the most popular transition metal nano-
particles used in catalysis and widely used for a significant
number of synthetic transformations.25 Several synthetic
methods have been reported regarding the preparation of stable
Pd-NPs. Some of the methods are sonochemical,26 g-irradia-
tion,27 UV irradiation,28 micro-emulsion technique,29 and polyol
reduction.30 However, the use of ILs as suitable and alternative
reaction media for the preparation of Pd-NPs is relatively
limited.31,32 In fact, the development of a reliable method for the
generation of soluble and stable NPs is intimately bound to the
nature of metal precursor and type of the IL. And the very low
solubility of metal precursors in the IL, such as PdCl2, usually led
to a very broad size distribution and different shapes since the
precursors are heterogeneously distributed in the media. On the
other hand, efficient synthesis of Pd-NPs (diameter: 4.9 nm) from
the reduction of Pd(acac)2 (acac ¼ acetylacetonate) by molecular
hydrogen using non-functionalised ILs as unique stabilisers was
demonstrated earlier.33 However, these nanoparticles are not
stable and the use of phenanthroline as additional capping agent
is essential for the NPs stabilisation.34 The use of external
capping agents, or functionalised ILs such as 2,20-dipyridyl-
amine-functionalised imidazolium salts,35 polymers36 or ionic
carbon nanotubes37 have been used for the stabilisation and
dispersion of Pd-NPs in ILs. Indeed, nitrile functionalised ILs
seem to offer one of the best media for the generation of cata-
lytically active Pd nanoparticles38 since it binds to the metal
surface providing the stabilisation, but allows the coordination
of substrates.22
Introduction
It is believed that nanoparticles (NPs) which are less than 10 nm
in diameter have properties in between those of bulk and single
particles.1 However, NPs are solely kinetically stable and they
should be stabilised against aggregation into larger particles and
bulk material. The main methods used for the stabilisation of
NPs in solution involve electrostatic or steric protection, for
example the use of water-soluble polymers, quaternary ammo-
nium salts, surfactants or polyoxoanions.2–5 The development of
new approaches for the synthesis and stabilisation of uniformly
sized (or mono-dispersed) NPs constitutes a very important
research area.6–21
In this context, ionic liquids (ILs) have been proven to be
suitable media for the generation and stabilisation of soluble
metal NPs with small sizes, narrow size distribution and different
shapes.7,22 The IL forms a protective layer, which is probably
aLaboratory of Molecular Catalysis, Instituto de Quꢀımica, Universidade
Federal do Rio Grande do Sul (UFRGS), 9500 P.O. Box 15003,
91501-970 Porto Alegre, Brazil. E-mail: jairton.dupont@ufrgs.br; Fax:
+ 55 51 3308 7304; Tel: + 55 51 3308 6321
bInstitute of Chemistry, Humboldt University Berlin, Brook-Taylor-Str. 2,
12489 Berlin, Germany. E-mail: martin.prechtl@hu-berlin.de
cCentro de Tecnologia Estrateꢀgica do Nordeste, Av. Prof. Luiz Freire 01,
50740-540 Recife, Brazil
† Electronic supplementary information (ESI) available: larger image of
Fig. 3. See DOI: 10.1039/c0jm03557b
‡ Present address: Institute of Inorganic Chemistry, University of
€
Cologne, Greinstr. 6, 50939 Koln, Germany. E-mail: martin.prechtl@
uni-koeln.de
ꢀ
x Present address: Instituto de Fısica, Universidade Federal do Rio
Grande do Sul. P.O. Box 15051, 91501-970. Porto Alegre, Brazil.
3030 | J. Mater. Chem., 2011, 21, 3030–3036
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We report herein our preliminary results for the synthesis of
Pd(0)-NPs in ILs and their use as efficient catalysts for the
partial hydrogenation of alkynes to alkenes. Real mono-
dispersed and well-immobilised Pd(0)-NPs were prepared by
simple thermal treatment of the precursor Pd(OAc)2 (OAc ¼
acetate) dissolved in (BCN)MI$NTf2 IL (BCN$MI$NTf2 ¼ 1-
butyronitrile-3-methylimidazolium-N-bis(trifluoromethane sul-
fonyl)imide). Other precursors such as Pd(acac)2, Pd(dba)2
(dba ¼ dibenzylidene acetone) or palladium chloride gave
much less satisfying results in a variety of ILs, i.e. high
agglomeration and precipitation of bulk palladium metal.
Moreover, these Pd-NPs prove to be efficient and highly
selective catalytic systems for the partial hydrogenation of
alkynes to alkenes. Other examples on this topic for Pd-
nanocatalysts are rare.39–42 These are useful intermediates for
the preparation of an enormous number of bioactive molecules
and natural products.14,43–49
Sample preparation for TEM analysis
In order to perform the TEM analysis, a droplet of a suspen-
sion containing Pd(0)-NPs embedded in the ILs was dispersed
in acetone and a slight amount of this dispersion was placed in
a carbon-coated copper grid. Particle size distributions were
determined from the digital images obtained with a CCD
camera. The NPs diameter was estimated from ensembles of
500 particles (1000 counts) chosen in arbitrary areas of the
enlarged micrographs. The diameters of the particles in the
micrographs were measured using the software Sigma Scan
Pro 5.
XPS analysis of Pd precursors in IL
Pd(OAc)2. The precursor Pd(OAc)2 (0.03 g, ꢄ0.13 mmol) was
firstly analysed by XPS.
Pd(OAc)2 in IL. Further, in a Fischer–Porter bottle the same
amount of the precursor was dissolved in a mixture of
(BCN)MI$NTf2 (0.23 g, ꢄ0.53 mmol) and acetone (1 mL). The
acetone was removed under reduced pressure and the resulting
solution was analysed.
Experimental
General methods
All manipulations involving the palladium complexes were
carried out under an argon atmosphere using Schlenk tech-
niques. Palladium acetate, palladium acetylacetonate and palla-
dium dibenzylidene acetone were prepared according to the
known literature procedures.50 Potassium tetrachloropalladate
was purchased from Strem Chemicals, phenanthroline was
purchased from Aldrich and triphenylphosphine was purchased
from Acros Organics. The ILs BMI$BF4 (BMI: 1-n-butyl-3-
methylimidazolium),51 BM2I$NTf2 (1-n-butyl-2,3-dimethylimi-
Pd(0)-NPs in IL. In a typical experiment, the Fischer–Porter
bottle was loaded with Pd(OAc)2 (0.03 g, ꢄ0.13 mmol),
(BCN)MI$NTf2 (0.23 g, ꢄ0.53 mmol) and acetone (1 mL). The
acetone was removed under reduced pressurꢀe. The susꢀpension
was heated for 24 hours (or 48 hours) at 120 C or 150 C. The
obtained black suspension was analysed by XPS.
Isolated Pd(0)-NPs. The Pd(0)-NPs could be isolated from the
IL phase by the addition of acetone followed by centrifugation.
The black powder was dried under reduced pressure and then
analysed.
52
dazolium),51 BMI$NTf2,51 and (BCN)MI$NTf2 were prepared
according to known procedures. Tetrabutylammonium bromide
(TBAB) and tributylamine were purchased from Acros Organics.
All other chemicals were purchased from commercial sources
and used without further purification. NMR spectra were
recorded on a Varian VNMRs spectrometer (300 MHz). Mass
spectra were obtained using a GC-MS Shimadzu QP-5050
(EI, 70 eV). Gas chromatography analyses were performed with
a Hewlett-Packard-5890 gas chromatograph with a flame ion-
isation detector and a 30 m capillary column with a dime-
thylpolysiloxane stationary phase. Transmission electron
microscopy (TEM) was performed on a JEOL JEM 1200Ex
operating at 100 kV (CME-UFRGS, Brazil) and HRTEM
was performed on an FEI Tecnai20 equipment at 200 kV
(CETENE, Brazil). X-Ray photoelectron spectroscopy (XPS)
measurements were obtained in ultra-high vacuum (base pres-
sure of 1 ꢃ 10ꢂ10 mbar) using an omicron station equipped with
an Mg source (ka radiation of 1253.6 eV) and an EAC2000-
SPHERA analyser at 53ꢀ detection angle.
Synthesis of Pd(0)-NPs in ILs
In a typical experiment, a Fischer–Porter bottle was loaded
with palladium acetate (2.3 mg, 0.01 mmol). Then
(BCN)MI$NTf2 (0.60 g, ꢄ1.39 mmol) in an organic solvent
(acetone 1 mL) was added via syringe under an argon flow. The
mixture was stirred at room temperature for five minutes to
yield a dark yellow suspension. Afterwards, the organic solvent
was removed under reduced pressure. The suspension was then
heated for 1 hour at 120 ꢀC under reduced pressure. This
resulted in the formation of a black suspension. The Pd(0)-NPs
embedded in the IL were analysed by TEM and used for
catalytic experiments (see below).
The NP formation and catalytic hydrogenation reactions were
carried out in a modified Fischer–Porter bottle immersed in
a silicone oil bath and connected to a hydrogen reservoir. The
decrease of the hydrogen pressure in the reservoir was monitored
with a pressure transducer interfaced through a Novus converter
to a PC, and the data were worked up via Microsoft Excel. The
substrate/catalyst ratio was calculated from the initial quantity of
palladium precursor used.
Catalytic hydrogenation with Pd(0)-NPs in ILs
In a typical experiment, a Fischer–Porter bottle loaded with
freshly prepared Pd(0)-NPs in IL (see above) was mixed with an
alkyne as the substrate (substrate/catalyst ratio see Table 3). The
mixture was stirred under a constant pressure of hydrogen gas
(1 or 4 bar) at room temperature for the indicated times and
analysed by GC, GC-MS and NMR (Table 3).
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Table 1 Reaction conditions for the Pd(0)-NP synthesisa
Entry
Precursor
IL
T/ꢀC
t/min
Immobilisation
1
2
3
4
5
6
Pd(acac)2
Pd(cod)2
Pd2(dba)3
PdCl2
Pd(OAc)2
K2PdCl4
Pd(acac)2
PdCl2 + Ph3P
Pd(OAc)2 + Phen
Pd(OAc)2
Pd(OAc)2
BMI$BF4
BM2I$NTf2
BM2I$NTf2
(BCN)MI$NTf2
(BCN)MI$NTf2
(BCN)MI$NTf2
(BCN)MI$NTf2
BMI$NTf2
75
40
75
75
75
75
75
80
15
15
15
15
15
15
15
20
15
360
60
Metal precipitation
Metal precipitation
Metal precipitation
Metal precipitation
Metal precipitation
Metal precipitation
Metal precipitation
Metal precipitation
Dispersed NPs
7
8b
9
(BCN)MI$NTf2
TBAB + Bu3N
(BCN)MI$NTf2
75
120
120
10c
11c
Dispersed NPs
Dispersed NPs
a
Reactions carried out under 4 bar of H2 (constant pressure). b System without H2 (under argon). c System under vacuum. Phen ¼ Phenanthroline.
NP solution was stable for a long time without precipitation.
Results and discussion
Additionally, these metal NPs were successfully employed for
typical catalytic C–C coupling reactions.53 In the present case,
Synthesis of mono-dispersed Pd(0)-NPs in ILs and
characterisation by TEM and XPS analysis
the TEM micrographs of the ‘‘soluble’’ NPs in the IL
(BCN)MI$NTf2 showed mono-dispersed Pd(0)-NPs with
a mono-modal distribution with a mean diameter of 7.3 ꢁ 2.2 nm
The NP catalysts were prepared with metal precursors dispersed
in ILs by using different reduction methods (I) using hydrogen as
(Fig. 1a). Here, the obtained size distribution histogram was
a reducing agent (II), thermal reduction of Pd(OAc)2 in IL
reasonably fitted to a Gaussian curve (Fig. 1b). Furthermore,
without any additional reducing agent, where the acetate acts as
high magnification and high-resolution TEM images of these
a reductant and (III) hydrogen as the reducing agent with the
Pd(0)-NPs showed the planar defects and interplanar distances.
addition of a ligand (Table 1). The reactions employed show that
These NPs could be isolated from the IL by the addition of
the simple thermal treatment of palladium acetate provides the
acetone and centrifugation.
best results towards the formation of well-dispersed and immo-
Although the use of additional N-donor ligands such as phe-
bilised NPs. Instead, other reducing agents provoke that NP
nanthroline (entry 9) or tributylamine (entry 10) as stabiliser
formation is followed by metal precipitation (Table 1). In most
provides the formation of immobilised NPs, the ligands are not
cases it can be assumed that the reduction with hydrogen as the
necessary in the presence of an anchored nitrile group like in IL
reductant is too fast, therefore the local concentration of Pd(0) is
(BCN)MI$NTf2. The conditions are summarised in Table 1.
too high which provokes fast agglomeration and metal precipi-
tation. In contrast, the heating (120 ꢀC) of Pd(OAc)2 in
In order to further investigate the elemental chemical state of
the precursors, the IL and the NPs isolated and dispersed in
(BCN)MI$NTf2 under reduced pressure leads to the formation
(BCN)MI$NTf2, samples corresponding to different steps of the
of stable and small-sized metal Pd(0)-NPs (7.3 ꢁ 2.2 nm, Fig. 1
processing were subjected to XPS analyses (Fig. 3). XPS results
and 2, Table 1: entry 11). Pointing out that the NP dispersion
show the presence of palladium, fluorine, carbon, nitrogen,
remains stable for several months without any metal precipita-
oxygen and a small contribution of silicon (from the glassware).
tion. Noteworthy, Dyson and co-workers reported in a previous
No significant changes are observed on the fluorine 1s, nitrogen
work the synthesis of metal Pd-NPs in the same nitrile-func-
tionalised IL using an ionic polymer as additional stabiliser.25,53
1s and sulfur 2p regions (Fig. 3b,c and f). In addition, the
nitrogen 1s signal represents two components with roughly the
In that case, the metal NPs are around 5 nm in diameter and the
same intensity. The component centered near 399.7 eV corre-
sponds to nitrogen in the nitrile group and the NTf2 anion, while
the one near 402 eV corresponds to nitrogen located in the imi-
dazolium ring.
Fig.
1 (a) Selected TEM image of Pd(0)-NPs dispersed in
(BCN)MI$NTf2 and (b) the histogram showing the particle size distri-
bution of Pd(0)-NPs dispersed in (BCN)MI$NTf2. The particles
display an irregular shape with a monomodal particle size distribution of
7.3 ꢁ 2.2 nm.
Fig. 2 Experimental HRTEM images of Pd(0)-NPs in (BCN)MI$NTf2,
showing the planar defects and interplanar distance indicated by arrows.
3032 | J. Mater. Chem., 2011, 21, 3030–3036
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Fig. 3 XPS analyses of palladium in IL. (A larger image of Fig. 3 is provided in the ESI†).
Fig. 3e shows the XPS signal corresponding to the palladium
3d transition from the Pd(OAc)2 salt (dashed dotted line),
Pd(OAc)2 in IL (dashed line), metal Pd-NPs in IL after thermal
treatment of the palladium salt (black line) and isolated Pd(0)-
NPs (red line). It was observed that two peaks for each palladium
3d signal were 5.25 eV apart, corresponding to the spin orbit
splitting. Thus these peaks do not represent different chemical
states. The binding energies for the palladium 3d5/2 transition
corresponding to the analysed samples are shown in Table 2.
The Pd 3d5/2 transition in the palladium acetate was observed
to occur with a binding energy of 337.7 eV. Moreover,
dissolving this salt in IL, a small shift of 0.7 eV to a higher
binding energy in the transition was observed. One would
expect to observe palladium shifting towardsꢀ a metallic state
after Pd(OAc)2 is heated for 24 hours at 120 C as this would
explain the catalytic activity. Surprisingly, the sample contain-
ing Pd(0)-NPs in IL showed a similar binding energy (338.7 eV)
as the palladium precursor Pd(OAc)2 dispersed in the IL
(338.4 eV). Presumably, the surface of the IL layer only
contains small quantities of Pd(0)-NPs and the majority is
located in the inner regions of the IL aliquot, and the low
concentration is not accessible by XPS.
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Table 2 Palladium 3d5/2 binding energies as determined by XPS using
Mg ka radiation of 1253.6 eV
Palladium 3d5/2
binding energy/eV
Sample
Pd(OAc)2
337.7
338.4
338.7
338.7
—
Pd(OAc)2 in (BCN)MI$NTf2
Pd(0)-NPs in IL (24 h @ 120 ꢀC)
Pd(0)-NPs in IL (48 h @ 120 ꢀC)
Pd(0)-NPs in IL (24 h @ 150 ꢀC)
Pd(0)-NPs in IL (48 h @ 150 ꢀC)
Isolated Pd(0)-NPs
—
335.1
Scheme 1 Hydrogenation of alkynes with Pd(0)-NPs in IL.
In order to investigate the possibility of incomplete reduction
of the palladium salt when dissolved in IL and explain the peaks
at 338.7 eV, the thermal treatment of Pd(OAc)2 in IL was con-
ducted for 48 hours at 120 ꢀC and 24 or 48 hours at 150 ꢀC prior
to XPS analysis. No changes were observed in the energy of the
Pd 3d5/2 transition after 48 hours at 120 ꢀC and at 150 ꢀC no
signal corresponding to the Pd 3d signal was observed by XPS
(not shown). This indicates that (I) the reduction of Pd(OAc)2 in
(BCN)MI$NTf2 was not completed at 120 ꢀC, even after
48 hours. And (II) the resulting Pd(0)-NPs are repelled from the
first few nanometres near the IL surface, preventing photoelec-
trons generated on the Pd(0)-NPs from reaching the electrostatic
energy analyser. This effect may be attributed to the same
mechanism that prevents the formation of metal Pd aggregates,
resulting in the observed stable NP dispersion. However, metallic
palladium could only be located by XPS after isolating the NPs
from the IL phase. The signal centred at 335.1 eV corresponds to
the metallic Pd indicating that isolated Pd(0)-NPs are indeed in
the metallic state. Although no sensible modifications on C 1s
binding energies (Fig. 3a) were observed, it cannot be discarded
that classical and abnormal N-heterocyclic carbenes (NHCs) are
formed in situ and these species might act as ligand.54,55 There-
fore, Pd(II)carbene complexes may be formed, most likely by
reaction of the imidazolium cation with Pd(0) species.25,56–59
Fig. 3a shows the carbon 1s transition corresponding to the
pure (BCN)MI$NTf2 (dotted line) Pd(OAc)2 in (BCN)MI$NTf2
(dashed line) and Pd(0)-NPs in the same IL (solid line). The
intensity of this peak was found to vary between samples, as
adventitious carbon contaminations are known to represent this
binding energy.
more labile and isomerisation prone 3-hexyne was partially
hydrogenated to the corresponding olefin without any trace of
isomerised by-products such as 1-hexene and 2-hexene (entry 5).
This result is in agreement with those reported for the hydroge-
nation of 1,3-butadiene catalysed by metal Pd-NPs in IL.33 In
that case, the difference in solubility between the substrate
(1,3-butadiene) and the major product 1-butene plays the role for
the observed selectivity (up to 72% in 1-butene). Moreover, no
isomerisation process was detected at the different stages of the
diene consumption, indicating that the NPs dispersed in IL act as
heterogeneous catalysts. In the present case, the same conclusion
can be adopted, i.e. the absence of isomerisation during the
3-hexyne hydrogenation process suggests that the Pd(0)-NPs in
(BCN)MI$NTf2 possess a typical behaviour of surface-like
(multi-site) catalytic properties. Noteworthy, no hydrogenation
of the nitrile group of the IL was observed under the applied mild
conditions, this shows the suitability of the functional IL–solvent
for such hydrogenation reactions.
Notably, the pressure of hydrogen has a great influence on
these reactions. The application of higher pressure (4 bar) in
these reactions always led to the formation of alkanes without
the detection of any alkenes. Interestingly, when the reactions are
carried out under mild conditions of 1 bar of hydrogen it was
possible to stop the reaction at the alkene stage with a good
Z-selectivity. In the case of diphenylacetylene, exclusively
Z-stilbene was formed without further hydrogenation to the
corresponding alkane, even when higher pressure (4 bar) was
applied. In most cases, the catalyst remained highly immobilised
in the IL phase even after hydrogenation. The high selectivity
observed is most likely related to the surface electronic modifi-
cation induced by the coordination of functionalised nitrile ionic
liquid. Although we have not observed sensible modifications on
C 1s binding energy by XPS, it cannot be discarded that
a modification of the near-surface region of palladium, in which
carbon (from fragmented feed molecules) occupies interstitial
lattice sites could be also responsible for the high partial alkyne
hydrogenation as observed in more classical Pd heterogeneous
catalysts.61
Partial hydrogenation of alkynes in liquid–liquid biphasic
systems with Pd(0)-NPs in ILs
The catalyst system Pd(0)-NPs in (BCN)MI$NTf2 was applied
for partial hydrogenation of a substrate scope of internal and
terminal alkynes with a high selectivity and under very mild
reaction conditions (Scheme 1).
Concerning the semi-hydrogenation of internal alkynes (Table
3), symmetrical diaryl-substituted alkynes (diphenyl acetylene,
entry 1) as well as dialkyl-substituted alkynes (3-hexyne, entry 5)
were reduced to the corresponding alkenes with good conver-
sions (75–100%) and with exclusive Z-stereochemistry. Also, the
benzylated terminal alkyne was hydrogenated to the terminal
olefin without debenzylation (entry 4, Table 3) which is usually
observed using Pd/C as a catalyst.60 It is noteworthy that even the
Moreover, the partial hydrogenation of the selected alkynes
provides TOFs up to 1282 hꢂ1. The TOF values were calculated
considering (I) all the metal amounts employed in the reaction
and (II) only the metallic atoms exposed on the NP surface.
Although it is not possible to exclude that in heterogeneous
systems some partial aggregation and topology changes can
occur on the metallic surface,62 the second case is more accurate
3034 | J. Mater. Chem., 2011, 21, 3030–3036
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Table 3 Pd(0)-NP-catalysed partial hydrogenation (1 bar) of alkynes at 25 ꢀC with Pd(OAc)2 as a catalyst precursor in (BCN)MI$NTf2
e
Entry
Alkynea (substrate)
Alkene (product)
t/h
Conv. (%)
Selectivity (%)
TOF (hꢂ1
)
1
2
6.5
2.8
87
97
98 : 2b
95 : 5c
13 (74)
35 (193)
3
4
2.8
3.5
1.3
75
96
97 : 3c
80 (446)
55 (305)
100
5
100
92 : 4 : 4d
231 (1282)
a
Substrate/catalyst ratio (mol/mol) based on Pd(OAc)2: entries 1 and 2 (S/C ¼ 100), entry 3 (S/C ¼ 300), entry 4 (S/C ¼ 200), entry 5 (S/C ¼ 300).
b
c
Selectivity determined by analytical GC, cis-alkene : trans-alkene. Selectivity determined by analytical GC, alkene : alkane. Selectivity
d
e
determined by analytical GC, cis-alkene : trans-alkene : alkane. TOF ¼ [product (mmol)]/[catalyst (mmol) ꢃ time (h)]; the TOFs in parenthesis
were calculated considering only the exposed metal atoms on the NP surface. The TOF values are considered to be under-estimated, because the
initial conversions were already high (>20%). In the case of entry 2, pentane (1 mL) was added as co-solvent; in the case of entries 1, 3 and 4,
cyclohexane (1 mL) was added as co-solvent.
in catalysis, once merely the surface atoms are able to catalyse the
reaction. In order to determine the percentage of surface atoms
on the Pd(0)-NPs, firstly we calculated the volume of one particle
considering spherical NPs with a mean diameter of 7.3 nm. The
volume obtained was around 203.7 nm3 for each metal NP
leading to around 14 048 atoms per particle. Using the method
based on ‘‘magic numbers’’63 which is an established tool for the
evaluation of cluster formation, the estimation of the number of
layers and the percentage of surface atoms per particle. The
number of layers per particle was estimated as 16, which means
that ꢄ18% of Pd atoms are on the NP’s surface. These results
were used to determine the corrected TOFs for the hydrogena-
tion reactions promoted by the metal Pd-NPs in ILs. Note that in
all cases the TOF values had to be estimated once the reduction
of Pd(OAc)2 was not completed as observed by XPS analysis.
These conditions are summarised in Table 3.
In order to test the recyclability of the system, 3-hexyne was
chosen as the standard substrate for the hydrogenation reactions.
The Pd(0)-NPs in IL showed efficient catalytic activity during
recycles providing a selective formation of the Z-alkene (selec-
tivity up to 96%) with total 3-hexyne conversions (Fig. 4). These
results evidenced the robustness of the catalyst which did not lose
its activity for at least 4 runs.
Conclusions
In summary, a new mild and simple methodology for the efficient
selective partial hydrogenation of alkynes has been developed
based on the generation of metal Pd-NPs in ILs. The thermal
treatment of Pd(OAc)2 in (BCN)MI$NTf2 results in the forma-
tion of stable and highly mono-dispersed Pd(0)-NPs which are
catalytically active for hydrogenation of alkynes. Remarkably,
depending on the hydrogen pressure it is possible to influence the
selectivity towards alkenes or alkanes as final products. The
catalytic system showed TOFs up to 1282 hꢂ1 and there is no
significant loss in its activity for at least 4 runs. Further research
on the application of these Pd(0)-NPs for the selective reduction
of other multiple bonds in ILs is currently underway.
Fig. 4 Catalyst recycles for the hydrogenation of 3-hexyne (ꢄ0.25 g;
3.0 mmol) promoted by Pd(0)-NPs (S/C ¼ 300) in (BCN)MI$NTf2
(0.60 g; ꢄ1.39 mmol) at 25 ꢀC and 1 bar of hydrogen gas (constant
pressure). In all catalytic runs the conversion was 100% and the
cis-hexene (grey) was detected as the major product, but the presence of
trans-hexene (white) and n-hexane (black) was also observed in small
amounts.
Acknowledgements
We gratefully acknowledge for financial support the CNPq,
TWAS (R. Venkatesan), Petrobras and the Alexander-von-
Humboldt Foundation (Dr M. H. G. Prechtl).
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 3030–3036 | 3035

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