Palladium nanoparticles supported on ionic liquid modified, magnetic nanobeads-recyclable, high-capacity catalysts for alkene hydrogenation

Article abstract of DOI:10.1039/c3ra46946h

Magnetic hybrid materials have been synthesized as recyclable catalysts for alkene hydrogenation. The materials consist of magnetic nanobeads functionalized with imidazolium-based ionic liquids and optional polymer shells. Palladium nanoparticles (NPs) were synthesized on the surface of these supports by two different methods and evaluated as catalysts for alkene hydrogenation. Deposition of palladium(0) onto the magnetic nanobeads by microwave decomposition of Pd₂(dba)₃·CHCl₃ leads to more efficient catalysts than the reduction of a Pd(ii) precursor. Reactivity, recycling ability and ease of separation of the catalysts are compared. A hybrid material without polymer shells and a quite flexible ionic liquid was identified as the most promising for stabilizing Pd NPs resulting in a catalyst that shows high activity (TOF up to 330 h^-1), good recycling ability, and minor metal leaching into the product. Notably, the activity of this catalyst increases with an enhanced Pd loading, contrasting related systems for which a decrease of activity is observed due to agglomeration. Therefore, this recyclable, high-capacity system is especially attractive for large-scale applications, requiring just a minimal amount of supporting material for the recycling of expensive Pd that is readily achieved by magnetic decantation. The Royal Society of Chemistry.

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DOI: 10.1039/C3RA46946H
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ARTICLE TYPE
Palladium Nanoparticles Supported on Ionic Liquid Modified, Magnetic
Nanobeads – Recyclable, High-Capacity Catalysts for Alkene Hydro-
genation
Roland Linhardt,a Quirin M. Kainz,a Robert N. Grass,b Wendelin J. Starkb and Oliver Reiser*a,c
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Magnetic hybrid materials have been synthesized as recyclable catalysts for alkene hydrogenation. The
materials consist of magnetic nanobeads functionalized with imidazolium-based ionic liquids and optional
polymer shells. Palladium nanoparticles (NPs) were synthesized on the surface of these supports by two
10 different methods and evaluated as catalysts for alkene hydrogenation. Deposition of palladium(0) onto
the magnetic nanobeads by microwave decomposition of Pd2(dba)3ꢀCHCl3 leads to more efficient cata-
lysts than the reduction of a Pd(II) precursor. Reactivity, recycling ability and ease of separation of the
catalysts are compared. A hybrid material without polymer shells and a quite flexible ionic liquid was
identified as the most promising for stabilizing Pd NPs resulting in a catalyst that shows high activity
15 (TOF up to 330 h-1), good recycling ability, and minor metal leaching into the product. Notably, the activ-
ity of this catalyst increases with an enhanced Pd loading, contrasting related systems for which a de-
crease of activity is observed due to agglomeration. Therefore, this recyclable, high-capacity system is
especially attractive for large-scale applications, requiring just a minimal amount of supporting material
for the recycling of expensive Pd that is readily achieved by magnetic decantation.
35 amount of metal needed in organic synthesis. However, aggrega-
tion of the nanomaterial to the bulk phase often causes problems
during catalysis. To overcome these difficulties, nanoparticles
can be stabilized by their immobilization on solid supports5,6 or
their coating with ionic liquids (ILs).7 Additionally a combination
40 of both strategies is also possible using supported ionic liquids
(SILs).8
20 Introduction
Pd-catalyzed reactions are widely applied in today’s organic
synthesis.1 The variety of reactions is enormous ranging from
alkylations, oxidations and carbonylations to coupling reactions
or hydrogenations. Especially palladium promoted hydrogena-
25 tions2 and coupling reactions3 play a very important role in phar-
maceutical, agrochemical and fine chemical industries. Therefore,
the reuse and recycling of palladium and other expensive metals
like platinum or gold is a major concern in organic chemistry and
especially in industry.4
Ionic liquids can stabilize nanoparticles electrostatically or by
coordination of the metal.9 During recent years, palladium NPs
were frequently stabilized by ILs applying either sole ILs,10
45 supported ionic liquid phases (SILPs)11 or ILs covalently sup-
ported on polymers,12 silica,13 and carbon nanotubes.14 Activity
and recycling ability of these Pd NPs is often significantly in-
creased. However, the ease of recycling often suffers from the
need of cost- and time-intensive filtration methods. One concept
50 to solve this problem is the application of magnetic supports
allowing the recovery of Pd NPs by simple magnetic decanta-
tion.15 We recently reported Pd NPs deposited on magnetic car-
bon coated cobalt nanoparticles as catalysts for alkene hydro-
genation. However, these catalysts suffered from agglomeration
55 of the Pd NPs on the carbon surface of the magnetic support over
time, allowing only small loadings of Pd (<1 wt%) to achieve
high activity. Moreover, a significant decrease of activity of these
catalysts was observed upon recycling and reuse.16 Hence, the
overall aim of this study is to generate a high-loading Pd nano-
60 catalyst by the introduction of stabilizing IL groups and, further-
more, to take advantage of a magnetic support allowing simple
magnetic separation.
30
In recent years, the application of palladium nanoparticles
became of strong interest owing to their extremely high surface to
volume ratio compared to the bulk phase. Metal nanoparticles
exceed the catalytic activity of the bulk metal by far, and conse-
quently, the use of nanoparticles as catalysts can reduce the
a Institut für Organische Chemie, Universität Regensburg, Universitätsstr.
31, 93053 Regensburg, Germany. Fax: +49 941 943 4121; Tel: +49 941
943 4631; E-mail: oliver.reiser@chemie.uni-regensburg.de
b Institut für Chemie und Bioingenieurwissenschaften, Department
Chemie und Angewandte Biowissenschaften, ETH Zürich, Wolfgang-
Pauli-Str. 10, 8093 Zürich, Switzerland.
c Department of Chemistry, Tokyo Institute of Technology, 2-12-1 O-oka-
yama, Meguro-ku, Tokyo 152-8551 (Japan).
† Electronic Supplementary Information (ESI) available: Materials and
methods, nomenclature of the nanobeads, ICP-OES analysis, TEM
pictures, IR, GC, XRD and NMR data. See DOI: 10.1039/b000000x/
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Herein, we describe the synthesis of novel hybrid systems
consisting of a magnetic core (carbon-coated Co nanobeads),
covalently attached imidazolium ILs and Pd nanoparticles immo-
flexible spacer which is attached to the nanoparticles (Fig. 1). Our
study includes the synthesis and characterization of the novel
hybrid materials, comparison of different Pd sources for the
bilized on the surface. The attachment of ILs on the particles was 10 nanoparticle synthesis, influence of the different materials on
5
performed in three different ways: (i) directly on the surface of
the nanobeads; (ii) on a polymer coating the nanobeads; (iii) on a
activity/recycling ability, leaching tests, and recycling studies.
Fig. 1 Novel Pd@Co/C hybrid systems: Pd@Co/C 1,22 Pd@PS-IL@Co/C 2, Pd@Bz-IL@Co/C 3, and Pd@Spacer-IL@Co/C 4.
15
Scheme 1 Synthesis of PS-IL@Co/C 6 from poly(benzyl-chloride)styrene functionalized Co/C NPs 5. Reagents and conditions: (i) methyl imidazole,
toluene, microwave heating, 150 °C, 30 min.
for six consecutive runs, however, a loss of activity was observed
from the second run indicating agglomeration of the nanoparticles
Results and discussion
40 deposited on the Co/C support. Furthermore, a quite large amount
Carbon-coated cobalt nanobeads (Co/C) were used as magnetic
of supporting material is needed due to the necessity of low palla-
20 support for our studies. The highly magnetic material (158
emu/g) is synthesized on large scale via reducing flame spray
pyrolysis (> 30 g/h).17 The graphene-like coating of the beads
provides high stability against acids, air/moisture, high tempera-
ture and, furthermore, enables facile functionalization of the
25 surface analogous to carbon nanotubes (CNTs). We successfully
demonstrated covalent as well as non-covalent methods to immo-
bilize catalysts,18 scavengers,19 reagents19,20 or fluorescent dyes21
on the graphene-like surface. While the loading via direct immo-
bilization is limited to ≈ 0.2 mmol/g,18 the introduction of poly-
30 mer-shells can lead to a higher loading of up to 3 mmol/g.22 This
reduces the amount of required support and can even lead to
higher activities due to a dendritic-like effect.22b Most recently,
we reported the deposition of Pd nanoparticles on the carbon
surface of Co/C NPs.16 This novel material (Fig. 1, 1) showed
35 extremely high activity in alkene hydrogenation reactions (TOF
up to 11095 h-1) and exceeded other Pd catalysts, especially
common Pd/C catalysts, by far. These catalysts could be reused
dium loading onto the support in order to achieve high catalytic
activities.
Synthesis of Co/C-supported ionic liquids
45 Since ILs are known to effectively stabilize metal nanoparticles
through electrostatic interactions and coordination of the metal,8
Co/C-supported ILs could serve as a stabilizing material for Pd
nanoparticles and therefore enhance the recycling ability of Pd
catalysts. Hence, we synthesized different Co/C nanobeads with
50 ILs covalently bound to the surface: PS-IL@Co/C 6 having a
polymer shell between the Co/C beads and the IL (Scheme 1),
Bz-IL@Co/C 9 bearing an IL directly on the Co/C surface, and
Spacer-IL@Co/C 12, having a small, flexible spacer between the
Co/C core and the IL (Scheme 2).
55
The synthesis of PS-IL@Co/C 6 started from poly(benzylchlo-
ride)styrene functionalized Co/C NPs 5 (3.1 mmol/g chloride)22
via microwave reaction with methyl imidazole (Scheme 1). The
loading of IL on the surface was determined by elemental micro-
2
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analysis as 2.1 mmol/g, indicating 88% displacement of chloride
with the imidazolium moiety.
Scheme 2 Synthesis of Bz-IL@Co/C 9 and Spacer-IL@Co/C 12 from benzyl-alcohol functionalized Co/C NPs 7. Reagents and conditions: (i) SOCl2,
DMF, DCM, 0 °C → rt, 12 h; (ii) methyl imidazole, toluene, microwave heating, 150 °C, 30 min; (iii) KHSO5, MeCN/H2O, reflux, 24 h; (iv) SOCl2,
reflux, 24 h; (v) amino propyl imidazole, 120 °C, 24 h; (vi) bromo butane, 80 °C, 24 h.
5
Table 1 Synthesis of Pd@PS-IL@Co/C 2, Pd@Bz-IL@Co/C 3 and Pd@Spacer-IL@Co/C 4 with varying Pd loadings.
Method of synthe-
sisa
Pd incorporated
[%]c
Pd loading
[mmol/g]c
Pd content
[wt%]c
Entry
Index
Pd source
Mass ratiob
1
2
2a
2b
2c
2d
2e
2f
Pd2(dba)3ꢀCHCl3
Pd2(dba)3ꢀCHCl3
Pd2(dba)3ꢀCHCl3
Pd2(dba)3ꢀCHCl3
Pd2(dba)3ꢀCHCl3
Na2PdCl4
A
A
A
A
A
B
B
B
A
A
A
A
A
A
A
A
A
A
1:1
1:0.6
1:0.2
1:0.1
1:0.01
1:1
56
64
74
69
81
100
90
85
91
83
100
86
96
86
90
81
90
100
2.6
2.3
1.2
0.6
0.08
4.8
3.2
1.3
4.3
2.9
1.6
0.7
0.09
4.0
3.2
1.3
0.8
0.09
28
24
12
6
3
4
5
0.8
50
34
14
45
31
17
8
6
7
2g
2h
3a
3b
3c
3d
3e
4a
4b
4c
4d
4e
Na2PdCl4
1:0.6
1:0.2
1:1
8
Na2PdCl4
9
Pd2(dba)3ꢀCHCl3
Pd2(dba)3ꢀCHCl3
Pd2(dba)3ꢀCHCl3
Pd2(dba)3ꢀCHCl3
Pd2(dba)3ꢀCHCl3
Pd2(dba)3ꢀCHCl3
Pd2(dba)3ꢀCHCl3
Pd2(dba)3ꢀCHCl3
Pd2(dba)3ꢀCHCl3
Pd2(dba)3ꢀCHCl3
10
11
12
13
14
15
16
17
18
1:0.6
1:0.2
1:0.1
1:0.01
1:1
0.9
43
34
14
8
1:0.6
1:0.2
1:0.1
1:0.01
1
a Method A: heating under microwave irradiation; method B: reduction with H2. b Mass ratio of NPs to Pd employed in the synthesis. c Determined by
ICP-OES.
10 Bz-IL@Co/C 9 and Spacer-IL@Co/C 12 were synthesized from
benzyl-alcohol functionalized Co/C NPs 7 (Scheme 2).17a Benzyl-
chloride modified Co/C NPs 8 were generated by substitution
reaction of 7 (0.11 mmol/g benzyl alcohol) with SOCl2. To obtain
Bz-IL@Co/C 9, 8 was subsequently heated under microwave
15 irradiation with methyl imidazole. The loading of IL on the sur-
face was 0.09 mmol/g, reflecting 83% of the maximum loading.
Spacer-IL@Co/C 12 were synthesized by oxidation of 7, fol-
lowed by amide formation of the benzoic acid functionalized
Co/C NPs 8 with 1-(3-aminopropyl)imidazole. After substitution
20 with 1-bromobutane following the lead of Lee et al.14 Spacer-
IL@Co/C 12 were generated with a loading of 0.05 mmol/g,
reflecting 47% of the maximum loading after three steps.
Synthesis of Pd@IL@Co/C with varying Pd loadings
In a second step palladium NPs were deposited on IL modified
25 Co/C NPs (IL@Co/C). Different mass ratios of palladium to
IL@Co/C were used in the synthesis to determine an effect of the
palladium loading on the catalytic activity.
The deposition of palladium NPs on IL@Co/C was carried out
using two different methods. Urriolabeitia et al.6 reported a pro-
30 cedure to immobilize palladium NPs on CNTs using
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Pd2(dba)3ꢀCHCl3 as Pd(0) source in dry toluene under microwave
4a-e). The palladium incorporation into the polystyrene coated
heating in just two minutes. Alternatively Lee et al.14 reported a 35 platforms PS-IL@Co/C 6 is not as effective as the deposition on
procedure reducing Na2PdCl4 under hydrogen atmosphere to
deposit palladium NPs on ionic liquid functionalized CNTs.
The tendency of Pd2(dba)3ꢀCHCl3 to rapidly form Pd NPs upon
decomposition was recently intensely discussed by the group of
Bz-IL@Co/C 9 and Spacer-IL@Co/C 12 (Fig. 2A). Especially
with an increasing amount of palladium used in the synthesis the
gap of Pd incorporation between polymeric and non-polymeric
NPs is increasing. Furthermore, the results with PS-IL@Co/C 6
5
Ananikov.23 The nanoparticles are not just formed under heating 40 show that the palladium incorporation starting from a Pd(II)
conditions, but also at room temperature upon storing for longer
time. The amount of decomposed complex furthermore depends
source is much more effective than starting from a Pd(0) source
(Fig. 2B). This is probably due to the higher polarity of the sol-
vent used in the synthesis, being consistent with the synthesis of 2
starting from Pd2(dba)3ꢀCHCl3, where a solvent mixture of dry
10 on the commercial source or the applied method of synthesis,
respectively. The nanoparticles formed at room temperature were
found to have a size of 60-200 nm with minor amounts of smaller 45 toluene/MeOH leads to a better swelling and dispersibility than in
ones (10-20 nm). The complex we used for our studies was com-
mercially available (see supporting information), which was
15 determined by the method of Ananikov to contain 68% of
Pd2(dba)3ꢀCHCl3 along with Pd-NPs, being in good agreement
with the results reported for commercial sources.23
pure toluene.
The novel hybrid materials were characterized using transmis-
sion electron microscopy (TEM) and x-ray powder diffraction
(XRD) techniques. TEM pictures clearly show the existence of
50 Pd nanoparticles (5-15 nm) on the surface of the Co/C nanobeads
(see supporting information), while XRD proves the presence of
Pd and Co both in oxidation state 0: For Co(0) the characteristic
peaks at 2θ of 44.25°, 51.5°, and 75.81° and for Pd the character-
istic peaks at 2θ of 40.06°, 46.49°, 67.94°, and 81.85° were ob-
55 served (see supporting information). However, at Pd contents
lower than 10 wt% the characteristic Pd(0) peaks could not be
distinguished from the background any longer.
As the nanoparticles we observed on the surface of the sup-
ports are between 5-15 nm, we can conclude that Pd NPs which
60 were already present in the Pd2(dba)3ꢀCHCl3 complex (10-200
nm)23 were crushed under the high temperature microwave condi-
tions applied.
Hydrogenation of trans-stilbene
To evaluate the activity of the different catalysts and the effects
65 of varying Pd loading the hydrogenation of trans-stilbene with
molecular hydrogen was chosen as test reaction. The amount of
catalyst was adjusted to ensure either 0.1 mol% or 1 mol% of Pd
in each reaction.
Table 2 shows the results of the catalytic tests. Starting with
70 catalyst 2, we observed a higher activity of those stemming from
the Pd(0) source.24 For example, comparing catalyst 2c (12 wt%
Pd, Table 2, entry 4) with 2h (14 wt% Pd, Table 2, entry 9) the
activity is almost four times higher in case of the material pre-
pared from the Pd(0) precursor. In both cases the activity of the
75 materials increases with a higher Pd content on the surface which
is opposite to what was observed in case of carbon stabilized Pd
NPs (Pd@Co/C).22 The maximum turn over frequency for the
polymeric material 2 was 100 h-1.
Fig. 2 Comparison of Pd incorporation depending on A) the support
20 (Method A) and B) the deposition method.
Applying the two described methods we synthesized catalysts
with varying palladium loadings for PS-IL@Co/C 6, Bz-
IL@Co/C 9, and Spacer-IL@Co/C 12 (Table 1). For PS-
IL@Co/C 6 dry toluene was replaced by a mixture of dry tolu-
25 ene/MeOH in order to increase the swelling of the polystyrene
coating. The amount of palladium used in the synthesis is given
as the mass ratio of ionic-liquid functionalized Co/C NPs to Pd.
E.g. a mass ratio of 1:0.1 means 100 mg NPs to 10 mg Pd. The
Pd content for the resulting hybrid materials was determined by
30 ICP-OES analysis.
80 Using the non-polymer coated catalyst Pd@Bz-IL@Co/C 3 we
observed an increase in activity by a factor of nine (Table 2, entry
14) and could even reach a turn over frequency of 4000 h-1 apply-
ing a hydrogen pressure of 10 bar (Table 2, entry 15). The load-
ing/activity relationship in this case is reversed compared to the
85 polystyrene coated catalyst 2. The material with the lowest load-
ing shows the highest activity in the hydrogenation (Table 2,
entry 14). The activity of Pd@Spacer-IL@Co/C 4 turned out to
be in between 2 and 3, the maximum TOF was observed for 4a
(333 h-1, Table 2, entry 16). Activity increases again with an
90 increase in loading as previously seen for the polystyrene coated
particles 2.
Thus, hybrid materials were obtained with Pd contents ranging
from 0.8-50 wt% (Pd@PS-IL@Co/C, 2a-h), 0.9-45 wt%
(Pd@Bz-IL@Co/C, 3a-e) and 1-43 wt% (Pd@Spacer-IL@Co/C,
4
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Generally, we observed also differences during the catalytic
tests regarding the ease of separation of the materials (Fig. 3).
Magnetic separation in case of polystyrene coated particles 2,
especially the ones with high Pd content, took several minutes
applying a 1.2 T neodymium based magnet (Fig. 3A). However,
hybrid materials 3 (Fig. 3B) and 4 (Fig. 3C) were separated from
the reaction mixture within seconds.
5
Table 2 Hydrogenation of trans-stilbene using Pd@PS-IL@Co/C 2,
Pd@Bz-IL@Co/C 3 and Pd@Spacer-IL@Co/C 4.a
10
Catalyst
[mol%] [min]
t
Conversion
[%]b
Entry Catalyst Pd [wt%]
TOF [h-1]c
1
2
2a
2b
2b
2c
2d
2e
2f
28
24
24
12
6
1.0
1.0
0.1
1.0
1.0
1.0
1.0
1.0
1.0
0.1
0.1
0.1
0.1
0.1
0.1
60
60
60
60
60
60
60
60
60
60
60
60
60
60
15
87
100
5
87
100
50
3
4
91
52
1
91
5
52
6
0.8
50
34
14
45
31
17
8
1
7
48
22
23
15
19
33
61
88
100
48
8
2g
2h
3a
3b
3c
3d
3e
3e
22
9
23
10
11
12
13
14
15d
150
190
330
610
880
4000
0.9
0.9
16
17
18
19d
20
21
22
23
4a
4a
4b
4b
4b
4c
4d
4e
43
43
34
34
34
14
8
0.1
1.0
0.1
0.1
1.0
0.1
0.1
0.1
90
75
90
90
120
90
90
90
50
100
34
58
100
16
10
1
333
80
Fig. 3 Collecting the nanocatalysts from the reaction mixture by an exter-
nal magnet. A) Pd@PS-IL@Co/C 2b, B) Pd@Bz-IL@Co/C 3b, C)
Pd@Spacer-IL@Co/C 4b.
200
387
50
20
107
93
1
7
Recycling studies
a Stilbene (2 mmol) in iPrOH (20 mL) was hydrogenated by 2 ꢁmol (0.1
mol%) or 20 ꢁmol (1 mol%) of catalyst using dodecane as internal GC
standard. b Determined by GC analysis using internal standard. c Mol of
We next investigated the recycling ability of the three different
hybrid materials (Fig. 4). Recycling experiments in all cases were
15 substrate transformed per mol catalyst per hour. d 10 bar of H2 pressure.
25 performed with the two high-loading catalysts of each material,
i.e. with Pd@PS-IL@Co/C 2a and 2b, Pd@Bz-IL@Co/C 3a and
3b, and Pd@Spacer-IL@Co/C 4a and 4b. Reactions were carried
out with an initial palladium amount of 1 mol% and stopped in
each case after reaching full conversion in the first run. For fur-
30 ther runs the reaction time was held constant.
In cases of 2
and 3 a drop of activity is observed after the first or the second
run, respectively. This indicates that these materials are not effec-
tively stabilizing the Pd NPs on the surface resulting in aggrega-
tion of the Pd NPs and/or leaching of Pd. However, the material
35 with the more flexible ionic liquid on the surface shows much
better results. In case of 4a we also see a decrease in yield after
the first run, whereas for 4b recycling is possible for at least five
runs without loss of activity demonstrating the efficient stabiliza-
tion of Pd NPs on the surface of the hybrid material. The observa-
40 tion of a slight decrease for catalyst 4a is probably caused by
agglomeration due to higher Pd contents. From now on 4b was
used as catalyst for further leaching/recycling studies.
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.
Fig. 4 Recycling studies with Pd@PS-IL@Co/C (2a, 2b), Pd@Bz-IL@Co/C (3a, 3b) and Pd@Spacer-IL@Co/C (4a, 4b). Reaction times are given below
the columns; yields in % are given above every single column.
times from 180 to 300 mins. In this case, we observed Pd con-
5
Table 3 Recycling studies of Pd@Spacer-IL@Co/C (34 wt%, 4b) in the
hydrogenation of trans-stilbene.a
tents of < 10 ppm and Co contents of < 8 ppm with two excep-
30 tions (17 ppm Pd in run 7 and 14 ppm Co in run 8). These results
are also promising for industrial uses as the acceptable palladium
limits are reported to be between 10 and 20 ppm.25
Reaction time to full
Run
Pd leaching
[ppm]c
Co leaching
[ppm]c
conversion [min]b
1
2
120
120
120
120
120
120
120
120
120
120
120
150
9
17
9
Catalyst 4b is additionally attractive due to its very high capac-
ity in palladium, requiring only little amounts of supporting
35 material, which is also important for the recycling of expensive
metals like Pd. For experiments using a Pd content of 0.1 mol%
only 0.7 mg of catalyst 4b were needed in order to generate 20
mmol of product within 10 runs. This would in scale up lead to
30 mol of product (or 5 kg in case of trans-stilbene) applying 1 g
40 of catalyst 4b.
11
20
11
16
28
19
8
3
13
8
4
5
17
14
17
21
14
13
22
15
6
7
8
9
11
28
64
48
10
11
12
Table 4 Recycling studies of Pd@Spacer-IL@Co/C (34 wt%, 4b) in the
hydrogenation of trans-stilbene under 10 bar H2-pressure.a
Reaction time to full
conversion [min]b
Pd leaching
[ppm]c
Co leaching
[ppm]c
a
i
Run
Stilbene (2 mmol) in PrOH (20 mL) was hydrogenated by 20 ꢁmol (1
mol%) of catalyst 4b. Reaction was monitored by GC analysis and
b
c
stopped at full conversion. In ꢁg per g of product. Determined by ICP-
10 OES.
1
2
180
180
180
210
210
210
210
240
240
300
10
2
2
1
3
4
2
Next, another reaction batch with 4b was run without internal
4
6
3
standard in order to determine the contents of Pd and Co in the
product by ICP-OES analysis (Table 3). Contamination of the
product is possible either by leaching of Pd or Co from the cata-
15 lyst or by particles that are not separated from the reaction mix-
ture effectively. The material was recycled for 11 runs without
any need of increasing the reaction time (120 min). In run 12 the
reaction time had to be increased to 150 min to reach full conver-
sion. After these 12 runs the Pd retained on the hybrid material
20 was determined as 87% of the starting value. TEM pictures do
not show any significant change of the nanocatalysts (see sup-
porting information). The Pd and Co contents in the products
were in the first 10 runs between 8 and 28 ppm, whereas the Pd
content increases in run 11 and 12 to 64 and 48 ppm.
5
6
4
6
5
3
7
17
8
7
8
14
4
9
3
10
6
7
a
i
Stilbene (2 mmol) in PrOH (20 mL) was hydrogenated by 2 ꢁmol (0.1
45 mol%) of catalyst 4b under 10 bar of H2-pressure. Reaction was moni-
b
c
tored by GC analysis and stopped at full conversion. In ꢁg per g of
product. Determined by ICP-OES.
Recycling experiments varying the substrate
We also investigated the scope of the reaction with catalyst 4b by
50 reusing the particles and changing the substrate after each run
(Table 5). Various styrene derivatives were hydrogenated in short
reaction times of 15-60 minutes (Table 5, run 2-6) bearing elec-
25
We also carried out leaching studies with 4b using 0.1 mol%
Pd under additional 10 bar of hydrogen pressure (Table 4). The
particles could be recycled for 10 consecutive runs with reactions
6
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Table 5 Recycling of catalyst Pd@Spacer-IL@Co/C (34 wt%, 4b) in the
hydrogenation of olefins and nitro compounds.a
tron withdrawing (Table 5, run 5) as well as electron donating
functional groups (Table 5, run 6). Additionally, chalcones and
cinnamic esters were hydrogenated selectively (Table 5, run 7+8),
t
[min]
Conversion
[%]b
Run
1
Substrate
Product
1
which was additionally proved by H-NMR and 13C-NMR. Fur-
5
thermore, strained compounds like norbornene were hydrogenat-
ed in very short reaction times (Table 5, run 9) as well as nitro
compounds bearing electron donating (Table 5, run 10) or elec-
tron withdrawing groups (Table 5, run 11).
120
15
100
100
100
100
100
100
100
100
100
2
Conclusion
10 In this report, we successfully developed novel hybrid materials
consisting of magnetic ionic liquids and stabilized Pd nanoparti-
cles. The catalyst with a quite flexible ionic liquid on the surface
turned out to be the most promising regarding the stabilization of
Pd NPs. It showed high activity in the hydrogenation of trans-
15 stilbene, is easily separable, and furthermore was recycled for at
least 11 runs without significant loss of activity. Furthermore, we
could show that the leaching of Pd and Co into the product was
not noticeably high if a Pd amount of 0.1 mol% was used. Recy-
cling of the catalyst was also possible varying the substrate after
20 each run.
3
50
4
60
5
15
6
15
This novel catalyst is especially interesting regarding the point
of sustainability as one can recycle expensive Pd with a very
small amount of supporting material. This makes it especially
attractive for industry where not only costs of catalyzing metal
25 but also of supporting material play an important role.
7
420
40
8
9
10
10
11
20
20
100
100
a Substrate (2 mmol) in iPrOH (20 mL) was hydrogenated by 20 ꢁmol (1
30 mol%) of catalyst 4b using dodecane as internal GC standard. Deter-
mined by GC analysis using internal standard.
b
Experimental
Benzyl-chloride functionalized carbon coated cobalt nanopar-
ticles (8, Bz-Cl@Co/C)
35 450 mg (0.12 mmol/g) Bz-OH@Co/C (7) and 1.4 ꢁL (18 ꢁmol)
dry DMF were predispersed in 4.5 mL dry dichloromethane for
10 min under N2-atmosphere using an ultrasonic bath. The dis-
persion was cooled to 0 °C and 65.3 ꢁL (0.9 mmol) SOCl2 was
added slowly. The reaction mixture was allowed to warm to rt
40 and stirred over night. After magnetic decantation the particles
were washed with dichloromethane (5 x 5 mL) and dried in vacuo
to obtain Bz-Cl@Co/C (440.8 mg, 0.11 mmol/g).
IR (ν/cm-1): 1676, 1596, 1503, 1263, 1015, 778; elemental micro-
analysis (%): C, 8.54; H, 0.14; N, 0.07; Cl, 0.40.
45
Benzyl-methyl imidazolium chloride functionalized carbon
coated cobalt nanoparticles (9, Bz-IL@Co/C)
400 mg (0.12 mmol/g) Bz-Cl@Co/C (8) and 4 mL dry toluene
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were introduced to a microwave vial and sonicated for 10 min 60 washed with dry THF (5 x 10 mL) and dried under vacuum to
under N2-atmosphere using an ultrasonic bath. After adding 24
ꢁL (0.3 mmol) N-methyl-imidazole the reaction mixture was
heated in a focused microwave oven to 150 °C for 30 min. After
removing the supernatant by magnetic decantation the particles
were washed with toluene (5 x 5 mL), and dichloromethane (3 x
5 mL) and dried under vacuum at 80 °C yielding 9 (397.6 mg,
0.09 mmol/g).
obtain 12 (909.2 mg, 0.05 mmol/g).
IR (ν/cm-1): 2913, 2850, 1738, 1455, 1366, 1222, 1217, 1160,
753; elemental microanalysis (%): C, 9.40; H, 0.24; N, 0.51.
5
65 Representative procedure for the microwave deposition of Pd
nanoparticles on PS-IL@Co/C nanobeads (Method A)
100 mg PS-IL@Co/C (6) and 1 mL of a mixture dry tolu-
IR (ν/cm-1): 2920, 2852, 2363, 1214, 1161, 1014, 814, 654; ele-
10 mental microanalysis (%): C, 8.98; H, 0.16; N, 0.23; Cl, 0.27.
ene/MeOH were introduced to a microwave vial and sonicated in
an ultrasonic bath for 10 min under N2-atmosphere. 4.9 mg (4.7
70 ꢁmol) Pd2(dba)3ꢀCHCl3 was added and the reaction mixture
heated in a focused microwave oven to 110 °C for 2 min. After
magnetic decantation the particles were washed with dichloro-
methane (5 x 5 mL) and dried under vacuum to obtain Pd@PS-
IL@Co/C (2e) (96.9 mg). The loading of Pd was determined by
Poly(benzyl-methyl imidazolium chloride)styrene functional-
ized carbon coated cobalt nanoparticles (6, PS-IL@Co/C)
500 mg (3.1 mmol/g) PS-Cl@Co/C (5) and 5 mL dry toluene
15 were introduced to a microwave vial and sonicated for 10 min
under N2-atmosphere using ultrasonic bath. 309 ꢁL (3.88 mmol) 75 ICP-OES (0.08 mmol/g, 81%).
N-methyl-imidazole was added and the reaction mixture heated in
Representative procedure for the microwave deposition of Pd
nanoparticles on Bz-IL@Co/C nanobeads (Method A)
a focused microwave oven to 150 °C for 30 min. After magnetic
decantation the particles were washed with toluene (5 x 5 mL),
20 dichloromethane (3 x 5 mL) and dried under vacuum at 80 °C
yielding 6 (637.2 mg, 2.1 mmol/g).
IR (ν/cm-1): 3373, 2927, 2851, 1562, 1511, 1449, 1422, 1158,
1018; elemental microanalysis (%): C, 43.95; H, 4.62; N, 5.75;
100 mg Bz-IL@Co/C (9) and 1 mL dry toluene were introduced
80 to a microwave vial and sonicated in an ultrasonic bath for 10
min under N2-atmosphere. 4.9 mg (4.7 ꢁmol) Pd2(dba)3ꢀCHCl3
was added and the reaction mixture heated in a focused micro-
wave oven to 110 °C for 2 min. After magnetic decantation the
particles were washed with dichloromethane (5 x 5 mL) and dried
85 under vacuum to obtain Pd@Bz-IL@Co/C (3e) (99.8 mg). The
loading of Pd was determined by ICP-OES (0.089 mmol/g, 96%).
Cl, 7.41.
25
Benzoic acid functionalized carbon coated cobalt nanoparti-
cles (10, Ph-COOH@Co/C)
1.3 g (0.26 mmol of benzylalcohol units) Bz-OH@Co/C (7) were
Representative procedure for the microwave deposition of Pd
nanoparticles on Spacer-IL@Co/C nanobeads (Method A)
90 50 mg Spacer-IL@Co/C (12) and 1 mL dry toluene were intro-
sonicated in 30 mL MeCN/H2O (1:1) for 10 min using an ultra-
30 sonic bath. After adding 479.5 mg (0.78 mmol) oxone the mix-
ture was refluxed for 24 h. Another portion of oxone (479.5 mg,
0.78 mmol) was added after 12 h. After magnetic decantation the
particles were washed with MeCN/H2O (3 x 20 mL), MeCN (3 x
20 mL) and dried under vacuum to obtain Ph-COOH@Co/C
35 (1.18 g).
duced to a microwave vial and sonicated in an ultrasonic bath for
10 min under N2-atmosphere. 2.5 mg (2.4 ꢁmol) Pd2(dba)3ꢀCHCl3
was added and the reaction mixture heated in a focused micro-
wave oven to 110 °C for 2 min. After magnetic decantation the
95 particles were washed with dichloromethane (5 x 5 mL) and dried
under vacuum to obtain Pd@Spacer-IL@Co/C (4e) (50.1 mg).
The loading of Pd was determined by ICP-OES (0.093 mmol/g,
100%).
IR (ν/cm-1): 2358, 2330, 1731, 1360, 1219; elemental microanal-
ysis (%): C, 8.71; H, 0.28; N, 0.30.
(N-Imidazole)propyl-benzamide functionalized carbon coated
40 cobalt nanoparticles (11)
100 Representative procedure for the deposition of Pd nanoparti-
cles on PS-IL@Co/C nanobeads by H2-reduction (Method B)
To a schlenk tube 20 mg PS-IL@Co/C (6) and 4 mL of a freshly
1.0 g Ph-COOH@Co/C (10) were stirred in 5.0 mL SOCl2 under
N2-atmosphere for 24 h. After magnetic decantation the particles
where washed with dry THF (5 x 5 mL) and dried under vacuum.
Subsequently 5.0 mL 1-(3-aminopropyl)imidazole were intro-
45 duced under N2-atmosphere and the mixture heated to 120 °C for
another 24 h. After magnetic decantation the particles were
washed with THF (3 x 5 mL), 1M HCl (1 x 5 mL), saturated
NaHCO3 solution (1 x 5 mL), H2O (1 x 5 mL) and EtOH (3 x 5
mL). After drying the particles in vacuo 11 (934.7 mg, 0.07
50 mmol/g) was obtained.
prepared Na2PdCl4 solution (Pd conc. 1 mg/mL) were introduced.
The reaction mixture was stirred vigorously for 30 min followed
105 by 30 min under 1 atm H2 pressure (balloon). After magnetic
decantation the particles were washed with H2O (5 x 3 mL),
MeOH (3 x 3 mL), and acetone (2 x 3 mL) and dried under vacu-
um to obtain Pd@PS-IL@Co/C (2h) (15.8 mg). The loading of
Pd was determined by ICP-OES (1.3 mmol/g, 85%).
110
IR (ν/cm-1): 2893, 2840, 1564, 973, 867, 848, 835, 819, 752, 698,
689, 656; elemental microanalysis (%): C, 9.32; H, 0.19; N, 0.60.
General procedure
Pd@IL@Co/C catalysts
for
the
hydrogenation
using
To a schlenk tube Pd@IL@Co/C (1 mol% Pd, 20 µmol), sub-
strate (2 mmol), iPrOH (20 mL), and dodecane (1 mmol) as
115 internal standard were introduced. The reaction mixture was
sonicated in an ultrasonic bath for 10 min and subsequently
stirred vigorously under 1 atm H2-pressure (balloon). The pro-
gress of the reaction was monitored by GC analysis. For recy-
cling experiments the particles were separated by an external
Benzamidopropyl-butyl-imidazolium bromide functionalized
55 carbon coated cobalt nanoparticles (12, Spacer-IL@Co/C)
900 mg (0.063 mmol of imidazole units) of 11 were sonicated in
15 mL freshly distilled 1-bromobutane under N2-atmosphere
using an ultrasonic bath. The reaction mixture was stirred for 24 h
at 80 °C. The particles were separated by an external magnet and
8
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i
Song, J. Jiang and Z. Jin, Catal. Lett., 2013, 143(2), 200. d) W. Zhu,
Y. Yu, H. Yang, L. Hua, Y. Qiao, X. Zhao and Z. Hou, Chem. Eur.
J., 2013, 19(6), 2059.
magnet and after magnetic decantation washed with PrOH (2 x 5
mL) and dichloromethane (2 x 5 mL). After drying in vacuo the
catalyst was reused for further runs.
13 a) Y. Kume, K. Qiao, D. Tomida and C. Yokoyama, Catal.
Commun., 2008, 9(3), 369. b) B. Karimi, D. Elhamifar, J. H. Clark
and A. J. Hunt, Org. Biomol. Chem., 2011, 9(21), 7420. c) M.
Gruttadauria, L. F. Liotta, A. M. P. Salvo, F. Giacalone, V. La
Parola, C. Aprile and R. Noto, Adv. Synth. Catal., 2011, 353(11-12),
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70
Acknowledgements
5
This work was funded by the Deutsche Forschungsgemeinschaft
(Re 948/8-1, “GLOBUCAT”), the IDK NANOCAT (Eliten-
etzwerk Bayern), and PITN-GA-2012-290248_Magneticfun. We
thank Turbobeads Llc for generously providing the magnetic
nanobeads and gratefully acknowledge the help of Joachim Re-
75 14 Y. S. Chun, J. Y. Shin, C. E. Song and S.-g. Lee, Chem. Commun.,
2008(8), 942.
15 a) A. J. Amali and R. K. Rana, Green Chem., 2009, 11(11), 1781. b)
Y. Kim and M.-J. Kim, Bull. Korean Chem. Soc., 2010. c) F. Zhang,
J. Jin, X. Zhong, S. Li, J. Niu, R. Li and J. Ma, Green Chem., 2011,
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J. Stark and O. Reiser, Adv. Funct. Mat., 2013, DOI:
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Palladium Nanoparticles Supported on Ionic Liquid Modified,
Magnetic Nanobeads – Recyclable, High-Capacity Catalysts for
Alkene Hydrogenation
TOC one sentence
Ionic liquid modified magnetic nanobeads allow the support of high concentrations of
palladium nanoparticles (34 wt%) with consistent high activity in alkene
hydrogenations.