Palladium nanoparticles entrapped in polymeric ionic liquid microgels as recyclable hydrogenation catalysts

Article abstract of DOI:10.1016/j.molcata.2013.07.010

Metallic Pd nanoparticles with narrow size distributions were prepared in cross-linked ionic liquid polymers (microgels). The microgels support Pd nanoparticles and prevent their aggregation. Size control of the Pd nanoparticles in polymers is possible by

Full text of DOI:10.1016/j.molcata.2013.07.010

Accepted Manuscript
Title: Palladium nanoparticles entrapped in polymeric ionic
liquid microgels as recyclable hydrogenation catalysts
Author: Yanmei Zhang Xian-Yang Quek Leilei Wu Yejun
Guan Emiel J. Hensen
PII:
S1381-1169(13)00272-0
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2013.07.010
MOLCAA 8810
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
20-1-2013
13-7-2013
21-7-2013
Please cite this article as: Y. Zhang, X.-Y. Quek, L. Wu, Y. Guan, E.J. Hensen,
Palladium nanoparticles entrapped in polymeric ionic liquid microgels as recyclable
hydrogenation catalysts, Journal of Molecular Catalysis A: Chemical (2013),
http://dx.doi.org/10.1016/j.molcata.2013.07.010
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Palladium Nanoparticles Entrapped in Polymeric Ionic Liquid Microgels
as Recyclable Hydrogenation Catalysts
Yanmei Zhang1,2, Xian-Yang Quek1, Leilei Wu1, Yejun Guan1&, Emiel J Hensen1,*
1 Laboratory of Inorganic Materials Chemistry, Schuit Institute of Catalysis, Technische Universiteit Eindhoven, Den
Dolech 2, 5612 AZ Eindhoven, The Netherlands
2 Life Science College, Dalian Nationalities University, Economical and Technological Development Zone, Dalian,
116600, China
& Present address: Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry,
East China Normal University, North Zhongshan Road 3663, 200032, shanghai, China
Abstract
Metallic Pd nanoparticles with narrow size distributions were prepared in cross-linked ionic liquid
polymers (microgels). The microgels support Pd nanoparticles and prevent their aggregation. Size
control of the Pd nanoparticles in polymers is possible by varying the metal loading and reduction
method. The particle size has a clear effect on the hydrogenation activity of the Pd nanoparticles with 3
nm sized ones being most active in styrene hydrogenation. Also the physicochemical properties of the
polymeric microgels significantly influence catalytic performance. High activity is obtained when
bromide is used as the counterion because of the good swellability of the resulting Pd/microgel
catalysts in polar solvents such as methanol. The use of a hydrophobic counterion results in loss of
catalytic activity. Furthermore, the microgels supported Pd nanoparticles can be recycled at least ten
times without obvious loss of activity.
Keywords: Pd nanoparticles; Polymeric Ionic Liquid Microgels; Hydrogenation; Recycle
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1. Introduction
Metallic nanoparticles have attracted considerable attention in recent years because of their high
catalytic performance in many reactions due to their high surface to volume ratio [1-3]. However,
applications with suspended metallic nanoparticles are limited, because the naked nanoparticles tend to
aggregate resulting in severe decrease of catalytic activity and often also selectivity [4]. An effective
way to resolve this problem is to stabilize the nanoparticles by use of capping agents or to immobilize
them on a support. Many methods have been developed for this purpose [5-8]. Besides particle size, the
nanoparticle-stabilizer interaction is one of the pivotal factors determining catalytic performance.
Generally, catalytic activity is low if the interaction is very strong due to blocking of surface sites,
while a too weak interaction fails to stabilize the nanoparticles resulting in undesired agglomeration.
In recent years, ionic liquids (ILs) have been explored as stabilizers for metallic nanoparticles.
Imidazolium-based ionic liquids can indeed stabilize nanoparticles electrostatically as well as by
coordination involving bulky imidazolium cations [4,9]. There are already many publications reporting
that IL-stabilized metallic nanoparticles exhibit high activity and selectivity in catalysis [9-12]. It is a
potent field as the ionic liquid can also be tailored by introducing functionalities that interact with
nanoparticles [12]. However, stabilization solely by ILs still has some limitations. For instance, it has
been shown that Pd will leach into solution when suspending palladium-N-heterocyclic carbene
complexes into IL solvents [1]. Moreover, difficulties in separation and reuse also limit their use. An
alternative method to stabilize nanoparticles is their immobilization onto a solid support, which
facilitates recycling. This is the most common method used in heterogeneous catalysis. Examples of
solid supports are carbon [13], metal oxides [14] such as silica [15] or crystalline frameworks such as
zeolite [16], polymer resins [17], and metal-organic frameworks [18]. However, also in this case
immobilization often results in significant loss of catalytic activity due to the strong interaction
between the support and the nanoparticles. In this respect, it is noteworthy that solid materials
containing imidazolium cations may favourably interact with metallic nanoparticles such that good
catalytic performance is obtained, whilst allowing recycling of the supported nanoparticles by
centrifugation or filtration [10,19-26].
Till to date, there are only few examples involving cross-linked polymers having the chemical
structure of ionic liquids. Most of these polymers have been applied as membranes and electrolytes
[27-29], with only a very small number of examples in catalysis [23, 30]. Palladium is widely used for
a significant number of synthetic transformations [31,32]. Herein we report a simple and general
2
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synthesis procedure to obtain Pd nanoparticles stabilized by polymeric ionic liquid microgels and their
evaluation as heterogeneous catalysts for hydrogenation reactions. These new catalysts exhibit
excellent turnover frequencies, selectivity, and reusability for hydrogenation.
Scheme 1 Preparation procedure for palladium nanoparticles entrapped into cross-linked polymeric
ionic liquids.
2. Experimental
2.1 Preparation of materials
The microgels were prepared by a water-in-oil emulsion method with Span 80 as surfactant and
dodecane as co-solvent. The dispersed aqueous phase consists of 1.8 g 1-vinyl-3-ethylimidazolium
bromide or 1-vinyl-3-methanamineimidazolium bromide, 90 mg N, N’-methylene bisacrylamide
(cross-linking agent, corresponds to a cross-linking degree of 5 %), 40 mg ammonium persulfate
(initiator), 2.5 mL phosphate buffer solution (pH = 6). The continuous phase consists of 0.25 g Span 80
and 5 mL dodecane. The emulsion is prepared by dropwise addition with a syringe of the dispersed
phase into the continuous phase under magnetic stirring. A concentrated gel-like emulsion is obtained,
which is then purged with nitrogen to remove residual oxygen. The polymerization is started by
addition of N, N, N’, N’-tetramethylethylenediamine (120 µL). After 6 h of reaction at room
temperature, the obtained microparticles are precipitated with acetone, filtrated and washed with
3
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distilled water. After this, they are freeze-dried to remove residual solvent. The polymers are denoted
as PILs or NH2-PILs.
The process for the introduction of Pd nanoparticles into the ionic liquid polymers is schematically
shown in Scheme 1. The metal precursor of Pd was adsorbed by ion-exchange of Br- for PdCl4 .
2-
Normally, 0.2 g of PILs or NH2-PIL was dispersed in water and mixed with the desired amount of a
2-
PdCl4 solution. The solid was then separated by centrifugation and washed several times by water.
Finally, the Pd cations were reduced by NaBH4 or H2 in water. After separation and washing, the
catalysts were dried and denoted as Pd(x)/PIL-Br or Pd(x)/NH2-PIL-Br, with x being the Pd loading as
determined by ICP-OES analysis.
2.2 Catalytic activity measurements
The reaction was performed in a 10 mL stainless-steel reactor equipped with a magnetic stirrer. In
a typical hydrogenation experiment, catalyst and 0.2 g of styrene together with 5 mL of methanol were
mixed and air was replaced by H2. The reactor was filled with H2 until the pressure was 10 bar. The
reaction was started by stirring at room temperature. After the reaction, the filtrate was analyzed by gas
chromatography (QP5050, Shimadzu) equipped with a Rxi-5ms capillary column (30 m × 0.25 mm ×
0.5 μm) and a flame ionization detector (FID).
2.3 Characterization
X-ray powder diffraction (XRD) patterns were recorded on a Bruker D4 Endeavour using Cu Kα
(λ = 0.15405 nm) radiation at 40 kV and a current of 40 mA. Infrared spectra were recorded on a
Thermo Nicolet Nexus 470 Fourier Transform infrared spectrometer. Scanning electronic microscopy
(SEM) images were recorded on Quanta 3D FEG instrument. The size and morphology of the Pd
nanoparticles were determined by a Tecnai G2 F20 transmission electron microscopy (TEM) operated
at an acceleration voltage of 200 kV with a LaB6 filament. Samples were obtained by placing a drop of
a colloidal solution onto a carbon-coated copper grid and evaporation of the solvent in air at room
temperature.
3. Results and discussion
Supported ionic liquids are promising stabilizers for metallic nanoparticles and Pd nanocatalysts
were studied extensively in hydrogenation reactions [33-38]. Therefore, Pd nanoparticles entrapped
4
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into polymeric ionic liquids were prepared by following the synthetic approach outlined in Scheme 1.
Briefly, a vinyl functionalized ionic liquids precursor was prepared and used as starting material to
prepare cross-linked polymers by radical polymerization. Then, PdCl42- was loaded by ion-exchange
with Br-. The metal salt was further reduced to obtain nanoparticles using hydrogen or sodium
borohydride. The final Pd loadings and the average particle size were determined by ICP and TEM,
respectively. The results are summarized in Table 1.
The chemical structure of the PILs was characterized by FT-IR (Fig. 1). Bands in the range of
2800-3000 cm-1 originate from the linear CH2-CH2 groups attached to the imidazolium rings. Features
at 1550 cm-1 and 1650 cm-1 as well as bands above 3000 cm-1 are from the C-H and C=C vibrational
modes of the imidazolium rings [4]. After ion-exchange and reduction, a shift of the band at 1650 cm-1
is observed. This is most likely caused by the interaction of the metal nanoparticles with the -C=C-
groups in the ionic liquid. Overall, the spectra are very similar before and after reduction indicating that
the structure of PILs remains intact.
Fig. 1: FT-IR spectra of materials.
Fig. 2. XRD patterns for materials and catalysts.
The Pd/PIL-Br catalysts were also characterized by XRD. Fig. 2 shows that before and after ion-
exchange a broad amorphous feature is present between 20 and 30 º. After reduction by NaBH4 or H2,
the solid exhibits characteristic peaks at angles (2θ) of 40.2, 46.7, 68.3, 82.2 and 86.7 º, corresponding
to (111), (200), (220), (311) and (307) planes, respectively, of fcc metals [22]. It is also seen that the
peak intensities increase with Pd loading. As expected, the diffraction peaks become broader at low
loading due to decreasing size of the nanoparticles. The effect of the reduction method was also
investigation. When instead of NaBH4 the Pd precursor anions were reduced in H2 with methanol as the
solvent, much larger Pd particles were obtained.
5
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The morphology of the PILs was further studied with SEM. Fig. 3 shows representative SEM
micrographs, which indicate that the particle size is around 25 μm and the particles are not damaged
during the following Pd loading process. In general, the materials also contain a small fraction of
smaller particles. The EDX data (not shown here) give the elemental composition before and after
introduction of the Pd. It follows that part of Br- was replaced by Pd and the Pd loading corresponds
well to the results by ICP element analysis (Table 1). Moreover, the presence of P and F elements
-
indicate the presence of the PF6 after exchange with this ion.
Fig. 3. SEM images of PIL-Br, Pd(10)/PIL-Br(b) and Pd(10)/ PIL-PF6(c).
TEM was used to determine the size and size distribution of Pd nanoparticles entrapped in the
microgels. As can been seen in Fig. 4, at a loading of 2.8 wt%, the average particle size is 1.8 nm and
the nanoparticles are well dispersed. When the loading is increased to 5.5 wt%, the average particle
size is around 2.1 nm. Pd loadings of 10 wt% and 28 wt% result in average particles sizes of 3.3 nm
and 3.6 nm, respectively. Furthermore, when hydrogen was used for reduction instead of NaBH4, the
average particle size is around 7.3 nm for the sample with a loading of 28 wt%. Amine groups have
strong affinity to Pd nanoparticles [39,40]. Therefore, amine-functionalized PILs were also used to
entrap Pd nanoparticles. The Pd loadings were 5.1 and 10 wt% with average nanoparticle sizes of 3.5
and 5.0 nm, respectively. To investigate the effect of the physical and chemical properties of polymeric
-
ionic liquids on catalysis, the catalysts were further exchanged with PF6 to render the materials
hydrophobic. As shown in Table 1, the Pd particle remains unchanged during the exchange process.
6
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Fig. 4. TEM images and mean particle size of Pd(x)/PIL-Br and Pd(x)/NH2-PIL-Br.
Pd-catalyzed hydrogenation is of significant importance in modern synthetic chemistry [4,40]. Styrene
hydrogenation in methanol was used as a model reaction to evaluate the catalytic activity of the
entrapped Pd nanoparticles. This solvent was chosen because the bromide-containing PILs swell well
in methanol. The results are summarized in Table 1. A blank experiment did not show any styrene
conversion. There was also no styrene conversion without Pd species in the microgel (not shown),
indicating the hydrogenation activity is from the Pd nanoparticles and not from the polymer
components. The influence of particle size on the hydrogenation of olefinic bonds is of great industrial
interest. As can be seen in Table 1, our results show a significant size effect on the catalytic activity for
styrene hydrogenation. The TOF is highest for Pd(10%)/PIL-Br(NaBH4) with an intermediate particle
size of 3.3 nm. A catalyst with a much larger particle size produced by reduction of the precursor in H2
in methanol (average diameter 7.3 nm) showed a much lower TOF of only 1656 h-1. From these
findings, we conclude that the styrene hydrogenation reaction is structure sensitive with optimal
particle size being around 3 nm. In general, hydrogenation of double bonds is not considered to be
structure sensitive. We can only speculate that the interaction of the Pd nanoparticles with the ionic
liquid environment brings about this peculiar effect. Earlier, we have shown that suspending metal
nanoparticles in an ionic liquid solvent may totally suppress their hydrogenation activity [42].
Finally, to verify that the supported Pd nanoparticles and not leached Pd catalyze styrene
hydrogenation, we filtered Pd(10%)/PIL-Br(NaBH4) after 5 min of reaction and added fresh styrene to
7
Page 7 of 14

the filtrate. After 5 min of reaction, we determined that styrene had not been further converted. This
result is in line with the finding that the Pd concentration in the filtrate is below the ICP detection limit
for this particular measurement of 1 μg Pd/g solution.
Table 1. Hydrogenation of styrene using Pd entrapped in polymeric ionic microgels.
Catalyst
Loading
(wt%)
~
daverage
(nm)
time
Yield
(%)
0
TOF
(h-1)
0
(min)
10
5
Blank
~
Pd(2.8%)/PIL-Br(NaBH4)
Pd(5.5%)/PIL-Br (NaBH4)
Pd(10%)/PIL-Br (NaBH4)
Pd(28%)/PIL-Br (NaBH4) (2.5‰)
Pd(28%)/PIL-Br (Reduced by H2) (5‰)
Pd(10%)/PIL-Br (NaBH4)
Pd(9.1%)/PIL-PF6 (NaBH4)
Pd(10%)/NH2-PIL-Br (NaBH4)
Pd(9.2%)/NH2-PIL-PF6 (NaBH4)
Pd(5.1%)/NH2-PIL-Br (NaBH4)
2.8
5.5
10
1.8 ± 0.3
2.1 ± 0.8
3.3 ± 0.8
3.6 ± 0.8
7.3 ± 1.3
3.3 ± 0.8
3.3 ± 0.8
3.5 ± 0.8
3.5 ± 0.8
5.0 ± 0.8
19
912
5
63
3024
4800
3744
1656
4800
1176
3600
1848
1344
5
100
78
28
5
28
5
69
10
5
100
49
9.1
10
10
6
90
9.2
5.1
10
5
77
28
Reaction conditions: The mixture of 2.5 ‰ of Pd and 2 μmol of styrene in 5 ml of methanol was stirred under 10 bar
of H2 at room temperature for 5 or 10 min.
It is not only the size of the nanoparticle that has significant effect on catalysis, interaction
between the nanoparticle and the support is also important. Strong interaction with the support may
lower the activity of the Pd nanoparticles. When the Pd nanoparticles are entrapped in an amine-
functionalized support, the activity (TOF = 3600 h-1) is only slightly affected compared to the non-
amine-containing PILs supported catalysts (TOF = 4800 h-1) under the otherwise similar reaction
conditions.
Besides optimum particle size, accessibility of the Pd nanoparticles entrapped in the polymeric gel
particle is a key prerequisite for good catalytic performance. Therefore, the control over the
morphology and physical properties of the polymers is important. The physical and chemical properties
8
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of ionic liquids can be varied over a wide range by proper selection of cations and anions. The presence
of strongly coordinating anions such as chloride or bromide results in hydrophilicity of the PILs. When
the anion is replaced by hexafluorophosphate, hydrophobicity is conferred to the PILs. Microscopy was
used to evidence the significant swelling ability of the Br--containing PILs in polar solvents, such as
water and methanol (not shown here). This swelling behaviour did not occur in THF nor in
cyclohexane. In line with this difference, Pd(10%)/PIL-Br shows high hydrogenation activity in
methanol, whereas almost no activity was observed in THF and cyclohexane (Table 2). It is speculated
that the lower accessibility of the substrate to the Pd nanoparticles in the two latter cases causes the low
activity. Also, the PILs became hydrophobic when Br- was replaced by PF6 and, accordingly, could not
-
swell in methanol. Also in this case, no catalytic activity was detected. Whereas microscopy confirmed
-
the immediate swelling of Br-containing PILs, those exchanged with PF6 did not change their
morphology and size. Accordingly, it is inferred that direct accessibility of substrates due to swelling of
the PILs is essential for the catalytic performance of these PILs-supported Pd nanoparticles. These
findings also evidence that the hydrogenation reaction occurs inside the microgel particles where Pd
nanoparticles are stabilized by the ionic liquids groups. In summary, the Pd nanoparticles are entrapped
inside the polymers and their activities are tunable by changing solvents or the counter ions of the PILs.
Table 2. Styrene hydrogenations in various organic solvents.
time
(min)
10
Yield
(%)
0
TOF
(h-1)
0
Catalyst
Blank
Solvent
methanol
methanol
5
100
4800
Pd(10%)/PIL-Br
Pd(10%)/PIL-Br
THF
Cyclohexane
methanol
methanol
methanol
THF
10
10
10
6
6
0
144
0
Pd(10%)/PIL-Br
Pd(9.1%)/PIL-PF6
Pd(10%)/NH2-PIL-Br
Pd(9.2%)/NH2-PIL-PF6
Pd(9.2%)/NH2-PIL-PF6
49
90
77
0
1176
3600
1848
0
10
10
9
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Reaction conditions: A mixture of 2.5‰ of Pd and 2 μmol of styrene in 5 ml of organic solvent was stirred under 10
bar of H2 at room temperature.
Pd is known to selectively reduce double and triple C-C bonds in the presence of C=O bonds
[43,44]. By comparing different substrates (Table 3), we found that this also holds for the PIL-
supported Pd catalysts. For instance, when cyclohexenone is used as the substrate, only the C=C bond
is hydrogenated, leaving the C=O bond unaffected. Also, a substrate only containing a C-O double
bond was not reduced. Therefore, it is concluded that these catalysts can efficiently catalyze the
selective reduction of C=C in the presence of C=O bonds. The selective hydrogenation of
phenylacetylene is important from the industrial perspective because a small amount of
phenylacetylene in the feed stream can deactivate styrene polymerization processes [45]. Polymer-
supported Pd is selective in the semi-hydrogenation of phenylacetylene to styrene.
To establish whether these catalysts can indeed be recycled, it was attempted to separate the PILs-
supported catalysts from the reaction mixture by centrifugation. The recycling process was
straightforward and involved separation by centrifugation, twice washing in methanol, and after
redispersing the catalyst in the same solvent a new catalytic test.. The recycling experiments for styrene
hydrogenation for ten cycles showed no significant loss in activity or selectivity as shown in Table 3.
The small loss after 10 recycles might be due to a small loss in dispersion as indicated by the increased
particle size following from an XRD measurement (not shown).
Table 3. Hydrogenation of various substrates by Pd intrapped in microgels.
Time
(min)
Yield
(%)
TOF
(h-1)
4320
Substrate
Products
5
5
5
(1st) 90
(6th) 100
(10th) 85
4800
4080
1176
O
O
20
25
98
100
10
Page 10 of 14

2112
88
11
10
30
O
OH
0
0
Reaction conditions: The mixture of 2.5‰ of Pd and 2 μmol of substrate in 5 ml of methanol was stirred under 10 bar
of H2 at room temperature. For the recycle experiment, the scale is 10 μmol of substrate in 25 ml of methanol.
4. Conclusions
Metallic Pd nanoparticles were successfully prepared in a cross-linked ionic liquid (imidazolium)
polymer microgel network. Pd is introduced by ion-exchange of the counter ions of the ionic liquid
2-
fragments with PdCl4 . Reduction is preferably done by NaBH4. When these microgel particles are
swollen in an appropriate solvent (methanol), the embedded Pd nanoparticles can be used for
hydrogenation of C-C double and triple bonds. The cross-linked ionic liquid polymers serve both as
support and stabilizer for the Pd nanoparticles, preventing extensive aggregation. Size control of the Pd
nanoparticles in polymers is possible by varying the loadings and the reduction method. The optimum
Pd nanoparticle size is ~3 nm. In solvents that do not swell the microgel particles such as THF, the
-
activity is lost. Also, when the hydrophilic Br anion is replaced by the hydrophobic PF6 one, the
microgel particles cannot swell with concomitant loss of activity. The selectivity of Pd catalysts in
selective C=C over C=O hydrogenation was retained for the microgel-embedded Pd nanoparticles. The
Pd nanoparticles stabilized in a polymeric ionic liquid gel matrix can be recycled for at least 10 times
without obvious loss of activity.
Acknowledgements
This work was financially supported by Program for Strategic Scientific Alliances between China and
Netherlands funded by the Royal Netherlands Academy of Arts and Science and the Chinese Ministry
of Science and Technology. The authors thank mrs. Adelheid Elemans-Mehring for ICP measurements.
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Highlights
Metallic Pd nanoparticles prepared in cross-linked ionic liquid polymer gel
Particle size dependence on styrene hydrogenation activity
Hydropholicity influence swellability in solvent and determines catalytic activity
Catalysts can be recycled ten times without obvious loss of activity
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