BMIm-PF6@SiO2 Microcapsules: Particulated ionic liquid as a new material for the heterogenization of catalysts

Article abstract of DOI:10.1021/cm501840d

A method for the preparation of silica microcapsules containing a high loading of ionic liquids (ILs) is described. The method paves the way to the conversion of ionic liquids into particulated materials, which results in ILs with new properties without changing their molecular structures. The synthesis of these new materials is based on the emulsification of ionic liquids in water, using surfactants or dispersants, and the resulting ionic liquid droplets are then confined in a silica shell formed via interfacial hydrolysis and polycondensation of tetraethoxysilane. This material can be isolated by centrifugation, followed by drying to yield a fine powder of ionic liquid-silica microcapsules, which are water and organic solvents redispersible. These new materials are utilized in the heterogenization of palladium catalyst and then applied in the hydrogenation of alkynes. The catalyst shows chemoselectivity in the hydrogenation of internal alkynes such as 4-octyne. Comparative studies have shown that the same catalyst loses this selectivity when it is applied under homogeneous conditions.

Full text of DOI:10.1021/cm501840d

Article
pubs.acs.org/cm
BMIm-PF6@SiO2 Microcapsules: Particulated Ionic Liquid as A New
Material for the Heterogenization of Catalysts
Ester Weiss,† Bishnu Dutta,† Andreas Kirschning,‡ and Raed Abu-Reziq*,†
†Institute of Chemistry, Casali Center of Applied Chemistry and the Center for Nanoscience and Nanotechnology, The Hebrew
University of Jerusalem, Jerusalem 91904, Israel
‡Institute of Organic Chemistry and Center of Biomolecular Drug Research (BMWZ), Leibniz University Hannover, Schneiderberg
1b, 30167 Hannover, Germany
S
* Supporting Information
ABSTRACT: A method for the preparation of silica micro-
capsules containing a high loading of ionic liquids (ILs) is
described. The method paves the way to the conversion of
ionic liquids into particulated materials, which results in ILs
with new properties without changing their molecular
structures. The synthesis of these new materials is based on
the emulsification of ionic liquids in water, using surfactants or
dispersants, and the resulting ionic liquid droplets are then
confined in a silica shell formed via interfacial hydrolysis and polycondensation of tetraethoxysilane. This material can be isolated
by centrifugation, followed by drying to yield a fine powder of ionic liquid-silica microcapsules, which are water and organic
solvents redispersible. These new materials are utilized in the heterogenization of palladium catalyst and then applied in the
hydrogenation of alkynes. The catalyst shows chemoselectivity in the hydrogenation of internal alkynes such as 4-octyne.
Comparative studies have shown that the same catalyst loses this selectivity when it is applied under homogeneous conditions.
groups.14 In addition, ionic liquids have been recently
1. INTRODUCTION
encapsulated in polymeric shells15 or carbon shells.16
In the last three decades, intensive investigations have been
carried out aimed at developing methods and materials for
Recently, silica microcapsules prepared by the sol−gel
method have been tested in controlled release formulations
heterogenization of homogeneous catalysts.1 The goal of these
as an alternative to polymeric microcapsules.17 The main
studies was to combine the advantages of homogeneous and
heterogeneous catalysis. For this purpose, different materials,
method employed for the synthesis of these microcapsules is
usually based on an emulsion/sol−gel approach. Thus, an oil
phase containing silicon tetraalkoxide monomers (e.g.,
tetraethoxysilane) is emulsified in water using surfactants and
then an interfacial polycondensation process takes place to
generate a silica shell around the oil droplets. The morphology
and the properties of the silica shell can be controlled easily by
performing the interfacial polymerization of the silane
monomers under different pH or temperature conditions. In
addition, the type of the silane monomer employed as a
precursor for creating the silica shell has a significant effect on
its morphology and porosity.
We describe herein, a method for the synthesis of
particulated ionic liquids by preparing silica microcapsules
that contain high loading of ionic liquids in their cores. This
approach of surrounding microdroplets of ionic liquids with
silica shells is regarded to be a powerful tool for designing ionic
liquids with new properties. We also demonstrate that the new
material, composed of ionic liquid core-silica shell micro-
particles, can be utilized in entrapment of palladium catalysts.
such as organic polymer solids,2 polymeric microcapsules,3
inorganic solids,4 metal organic frameworks (MOFs)5 and
nanoparticulated materials6 were designed and utilized
efficiently as support in the immobilization of homogeneous
catalysts. In many cases, these heterogenized catalysts could
easily be recycled. However, tuning the reactivity and the
selectivity of the heterogenized catalysts in organic trans-
formations is still a major challenge.
In recent years, ionic liquids (ILs)7 have attracted
considerable attention due to their potential for application in
diverse fields including catalysis,8 organic synthesis,9 electro-
chemistry,10 and separations.11 They are nonvolatile, nonflam-
mable, thermally and chemically stable, easily handled and their
polar noncoordinating properties allow them to dissolve a
broad spectrum of organic, inorganic and polymeric materials.
Because of these unique properties, they have been considered
as green alternative media to conventional organic solvents.
However, their relatively high viscosities lead to difficulties in
their separation and practical applications. To overcome or
circumvent these limitations, solid state ionic liquid materials
have been prepared by immobilization of ionic liquids on
porous solids12 or polymer membranes,13 and by direct
polymerization of ionic liquids functionalized with vinyl
Received: May 21, 2014
Revised: August 7, 2014
© XXXX American Chemical Society
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of chloroform. The mixture was stirred at room temperature for 6 h
and then the chloroform was evaporated to dryness affording an
orange fine powder. The adsorbed palladium acetate on silica was
reduced by 500 psi H2 and a black powder was obtained.
These “heterogenized” catalysts are applied in the hydro-
genation of alkynes.
2. EXPERIMENTAL SECTION
2.6. General Procedure for the Hydrogenation Reactions
with Pd/BMIm-PF6@SiO2 Microcapsules. Pd/BMIm-PF6@SiO2
(150 mg) containing 6 × 10−3 mmol Pd, was dispersed in 2 mL of
proper solvent and 1.0 mmol of substrate were placed in a 25 mL
glass-lined autoclave. After sealing, the autoclave was purged three
times with hydrogen and pressurized to 500 psi. The mixture was
stirred at room temperature for 24 h and then the hydrogen was
released. The microcapsules were separated by centrifuge, washed
twice with the same medium, and used for subsequent cycles. The
2.1. Materials. Polyoxyethylene (20) sorbitan monooleate (Tween
80), polyethylene glycol octylphenyl ether (Triton X-100), polyoxy-
ethylene (20) stearyl ether (Brij 78), sodium dodecyl sulfate (SDS),
the triblock copolymer poly(ethylene glycol)-poly(propylene glycol)-
poly(ethylene glycol) (Pluronic P123), Pd/C (10%), the alkyne
substrates, trans-4-octene, and palladium acetate were purchased from
Sigma-Aldrich. Sodium lignosulfonate (Reax 88B), lignosulfonic acid
(Reax 88A) and butylated polyvinylpyrrolidone (Bu-PVP) were
contributed from FMC Corporation. Tetraethoxysilane (TEOS) was
contributed from Sol−Gel Technologies. The ionic liquid 1-butyl-3-
methylimidazolium hexafluorophosphate (BMIm-PF6) was purchased
from Chemada Fine Chemicals.
2.2. Instrumentation. Scanning electron microscope (SEM) was
employed to analyze the morphology of the BMIm-PF6-silica
microcapsules. The experiments were performed on high resolution
scanning electron microscope (HR SEM) Sirion (FEI company) using
Shottky-type field emission source and secondary electron (SE)
detector. The images were scanned at voltage of 5kV. Transmission
electron microscope (TEM) and electron diffraction spectroscopy
(EDS) were performed with (S) TEM Tecnai F20 G2 (FEI Company,
USA) operated at 200 kV. Thermogravimetric analysis (TGA) was
performed on Mettler Toledo TG 50 analyzer. Measurements were
carried out at temperature range that extended from 25−950 °C at
heating rate of 10 °C/min under N2 atmosphere. Dynamic light
scattering (DLS) was utilized to determine the size distribution of the
BMIm-PF6-silica microcapsules. Those measurements were performed
on Nano Series instrument of model Nano-Zeta Sizer (Malvern
Instruments) model ZEN3600. Gas chromatography (GC) (Aglient
Technologies, 7890A) with a capillary column (HP-5, 30 m) was used
to analyze the hydrogenation products. 1H NMR and 13C NMR
spectra were recorded with Bruker DRX-400 instrument. Solid-state
29Si MAS, 13C MAS, and 31P MAS NMR spectra were recorded with
1
products were analyzed by H NMR and GC chromatography. When
the medium of the reaction was water, the products were first extracted
1
with ether and then analyzed by H NMR and GC chromatography.
3. RESULTS AND DISCUSSION
3.1. Preparation of Ionic Liquid-Silica Microcapsules.
The silica microcapsules filled with an ionic liquid were
prepared according to the emulsion/sol−gel approach. To
emulsify an ionic liquid in water, we chose an ionic liquid that is
immiscible with water. For that purpose, 1-butyl-3-methyl-
imidazolium hexafluorophosphate (BMIm-PF6) was selected as
the water emulsifiable ionic liquid. The formation process of
the BMIm-PF6 silica microcapsules (BMIm-PF6@SiO2 micro-
capsules) is illustrated in Scheme 1.
Scheme 1. Illustration of the Preparation Process of Ionic
Liquid−Silica Microcapsules
Bruker DRX-500 instrument. Emulsifications were performed using
Kinematica Polytron homogenizer PT-6100 equipped with dispersing
aggregate 3030/4EC. Inductively coupled plasma mass spectrometry
measurements (ICP) were performed on 7500cx (Agilent company)
using an external standard calibration.
2.3. General Procedure for the Preparation of BMIm-PF6@
SiO2 Microcapsules. 1-Butyl-3-methylimidazolium hexafluorophos-
phate (1.5−3.75 g) was emulsified with 15.9 g of water containing a
proper surfactant (0.6−2.5 g) by milling at 11000 rpm for 2 min.
Then, tetraethoxysilane (TEOS) (3.25−5.25 g) was dropped slowly
and the resulted mixture was stirred at room temperature for 12 h. The
resulted BMIm-PF6@SiO2 microcapsules were separated from the
mixture by centrifugation, washed three times with water and then
analyzed by scanning electron microscope (SEM).
2.4. Preparation of Pd/BMIm-PF6@SiO2 Microcapsules. Reax
88A (1.6 g) was dissolved in 15.9 g of water and homogenized for 1
min at 8900 rpm to give the water phase. Then, 30 mg (0.13 mmol) of
palladium acetate was dissolved in 3.75 g of 1-butyl-3-methylimida-
zolium hexafluorophosphate and then emulsified with the water phase
by homogenization at 11000 rpm for 2 min. After that, 3.75 g of TEOS
was added dropwise and homogenization continued for 2 min,
followed by stirring at room temperature for 12 h. The resulted silica
microcapsules were separated from the suspension by centrifugation
and the brownish precipitate was washed three times with 40 mL of
water to remove excess of reagents. The silica microcapsules were
dispersed in 50 mL of water and transferred to a glass-lined autoclave,
which was then pressurized with 500 psi hydrogen. The mixture was
stirred under hydrogen atmosphere for 24 h. Finally, the resulted Pd/
BMIm-PF6@SiO2 microcapsules were separated by centrifugation and
dried to yield 5 g of brown powder. The palladium loading in this
material was 0.04 mmol/g.
The first step in this process was based on the emulsification
of BMIm-PF6 in water using a proper surfactant to stabilize the
dispersed ionic liquid droplets. Numerous surfactants and
dispersants were examined: Tween 80, Triton X-100, Brij 78,
SDS, Pluronic P123, Reax 88B, and Reax 88A. The desired
spherical BMIm-PF6@SiO2 capsules were observed only in the
presence of Reax 88A as visualized in the SEM images
(Supporting Information, Figure S1). The surfactant should be
suitable to interact with both the aqueous phase and the BMIm-
PF6 phase. BMIm-PF6 is more polar than organic solvents.
Therefore, a surfactant with long hydrophobic chains decreases
the ability to stabilize the ionic liquid droplets. In the second
step, the silane monomer, tetraethoxysilane (TEOS), was
slowly added to the ionic liquid-water emulsion, and silica shells
confining the ionic liquid droplets were formed after stirring for
12 h, at room temperature. The confinement of BMIm-PF6
droplets with a silica layer is based on electrostatic interactions.
The encapsulation procedure is carried out under pH = 4.
Above pH = 2 the silanol groups are partially deprotonated
forming negatively charged silanolates. These groups electri-
cally interact with the positively charged imidazolium groups in
the ionic liquid. The resulting BMIm-PF6@SiO2 microcapsules
were isolated and washed by simple centrifugation. This
separation process also sheds light on the stability of the
formed microcapsules and the robustness of their silica shells.
Destruction of the microcapsules yielded a separated layer of
2.5. Procedure for the Preparation Pd/SiO2 (5%). Palladium
acetate (527.4 mg, 2.35 mmol) and 5 g of silica were mixed in 150 mL
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the ionic liquid after centrifugation, while the desired products
were obtained as a fine brown precipitate. After it was dried, a
fine powder of BMIm-PF6@SiO2 microcapsules, as particulated
ionic liquid, was collected (Figure 1). This powder could be
redispersed easily in water or in organic solvents.
Figure 1. Picture of the ionic liquid BMIm-PF6 (left) versus powder of
BMIm PF6@SiO2 microcapsules (right).
Figure 4. IR spectrum of BMIm-PF6@SiO2 microcapsules.
Various analyses were performed to characterize the BMIm-
PF6@SiO2 microcapsules. The size of the microcapsules was
determined using dynamic light scattering (DLS) analysis,
which showed an average size of 3.6 μm (Figure 2). To reveal
attributed to the C−H ring stretching vibration of the
imidazolium indicates the presence of the ionic liquid BMIm-
PF6 within the silica microcapsules. Thermal gravimetric
analysis (TGA) was used to investigate the decomposition of
BMIm-PF6, Reax 88A in comparison to BMIm-PF6@SiO2
microcapsules in order to evaluate the loading percentage of
the ionic liquid (Supporting Information, Figure S2). The
analyses were performed under N2 atmosphere at temperature
range of 25−950 °C with 10 °C/min heating rate. It can be
seen in the weight loss curve of BMIm-PF6@SiO2 micro-
capsules (Supporting Information, Figure S2) that there are five
stages of weight losses. The first, a small weight loss centered at
80 °C, which probably belongs to the desorption of physically
adsorbed water molecules on the silica shell. The other four
weight losses centered at 185, 240, 330, and 650 °C are
ascribed to the decomposition of the encapsulated ionic liquid
and the dispersant Reax 88A. This analysis demonstrated a
decomposition of about 70% of organic material. To estimate
the percentage of ionic liquid in the capsules, additional
method was required to determine the proportion of Reax 88A.
Therefore, elementary analysis of sulfur was performed, and
1.3% was obtained. Furthermore, elementary analysis of pure
Reax 88A detected 11.71% of sulfur. These results indicate that
the percentage of Reax 88A within the capsules is 3.6%. Thus,
the loading of ionic liquid within the capsule is 66.4% and the
percentage of SiO2 in the capsule is 28%. In the 13C MAS NMR
of the BMIm-PF6@SiO2 microcapsules (Figure 5b), the
Figure 2. Size distribution of BMIm-PF6@SiO2 microcapsules.
the structure of the resulting microcapsules, they were analyzed
by scanning electron microscopy (SEM) and transmission
electron microscopy (TEM). The results of the TEM analysis
(Figure 3a) confirmed the formation of a core−shell structure
Figure 3. (a) TEM image of the BMIm-PF6@SiO2 microcapsules. (b)
SEM image of BMIm-PF6@SiO2 microcapsules.
of the microcapsules, with an average shell thickness of 90−110
nm. On the other hand, SEM analysis (Figure 3b) indicated
that the BMIm-PF6@SiO2 microcapsules were spherical, and
their surface showing some degree of roughness in addition to
high porosity. BET measurements were performed and the
surface area obtained was 47.4 m2/g. Infrared analysis (IR)
(Figure 4) revealed the presence of characteristic band at
wavelength of 3400 cm−1, which belongs to the Si−OH stretch,
and of 1090 cm−1, which was assigned to the Si−O−Si stretch.
In addition, the bands at 3120 and 3160 cm−1 that are
Figure 5. (a) 13C NMR spectrum of the ionic liquid BMIm-PF6. (b)
13C MAS NMR spectrum of BMIm-PF6@SiO2 microcapsules.
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characteristic peaks of the ionic liquid BMIm-PF6 were all
present, as was judged by comparing the solid NMR spectrum
with the 13C NMR spectrum of the nonencapsulated BMIm-
PF6 (Figure 5a). Furthermore, 29Si MAS NMR analysis
indicated the presence of Q3 ([SiO3(OH)], −97.8 ppm) and
Q4 ([SiO4], −109 ppm) species in the structure of the
microcapsules shell (Figure 6). In addition, the peak at −106
TEOS was increased from 20:80 up to 50:50 (Supporting
Information, Figure S5). However, it was very difficult to obtain
microcapsules when the w/w percentage of TEOS was greater
than 50% (Supporting Information, Figure S5). Furthermore,
varying the concentration of BMIm-PF6 in the emulsion from
13% to 21% (w/w) could lead to the production of
microcapsules with spherical morphology (Supporting In-
formation, Figure S6). Nevertheless, stable microcapsules
without leakage of the ionic liquid were produced only when
the concentration of the ionic liquid was 21%.
3.2. Heterogenization of Palladium Catalyst Using
Ionic Liquid−Silica Microcapsules. The new materials,
based on BMIm-PF6@SiO2 microcapsules, were utilized in the
heterogepnization of a palladium catalyst. The procedure for
the preparation of this entrapped catalyst was based on
dissolving palladium acetate in BMIm-PF6, followed by
emulsification in water and construction of the silica shell
around the ionic liquid droplets by polycondensation of TEOS.
The resulting microcapsules were treated with hydrogen to
reduce Pd(II) to Pd(0), which yielded the catalyst Pd/BMIm-
PF6@SiO2. TEM analysis (Figure 7) of the resulting
Figure 6. 29Si MAS NMR spectrum of BMIm-PF6@SiO2 micro-
capsules.
ppm belongs to a [SiO3(OEt)] species, which indicates that
some of the ethoxide groups were not hydrolyzed. This
observation was also confirmed by 13C MAS NMR analysis in
which the characteristic peaks of the ethoxide groups appear at
18.8 and 56.5 ppm. The peak of the methyl in the ethoxide
groups at 18.8 ppm overlaps with one carbon peak of the ionic
liquid BMIm-PF6. Preparing silica by sol−gel process that
usually performed under ambient conditions can produce
materials that contain alkoxide groups in their structure. These
groups can be eliminated by condensation reaction with silanol
groups during aging process or treatment at high temperatures.
In the case of BMIm-PF6@SiO2 microcapsules, heating at high
temperature can affect the structure of the ionic liquid−silica
microcapsules and therefore was avoided. The peak at 147.4 is
attributed to the aromatic carbons of Reax 88A indicating that
some of the Reax 88A is entrapped within the capsule shell.
The other characteristic peaks of Reax 88A overlap with the
BMIm-PF6 and the ethoxy groups of the silica shell. These
results correlate with the elementary analysis mentioned above.
31P MAS NMR was performed in order to establish if hydrolysis
of BMIm-PF6@SiO2 occurs during encapsulation. The 31P
MAS NMR spectra (Supporting Information, Figure S3) show
that the encapsulation process does not affect the BMIm-PF6
chemical structure.
To optimize the preparation of the BMIm-PF6@SiO2
microcapsules, different parameters such as surfactant concen-
tration, ionic liquid/water phase ratio and TEOS/ionic liquid
ratio, were examined to evaluate their effect on the structure of
the microcapsules. Thus, when Reax 88A was applied at
concentrations of 2.5%, 5%, 7.5%, and 10% (weight-to-weight,
w/w), spherical microcapsules were obtained (Supporting
Information, Figure S4). However, the formation of silica
particles as byproducts was minimized significantly when the
percentage of Reax 88A was 7.5% (w/w). Interestingly, the w/
w ratio of the ionic liquid to TEOS had great effect on the
synthesis of BMIm-PF6@SiO2 microcapsules. Spherical micro-
capsules could be achieved when the ratio of BMIm-PF6 to
Figure 7. TEM image of Pd/BMIm-PF6@SiO2.
heterogeneous catalyst indicated the formation of palladium
nanoparticles, with an average size of 7 nm (Supporting
Information, Figure S7). In addition, electron diffraction
spectroscopy (EDS) measurements supported the presence of
palladium in the BMIm-PF6@SiO2 microcapsules (Supporting
Information, Figure S8). When the catalyst Pd/BMIm-PF6@
SiO2 was analyzed by X-ray powder diffraction (XRD), only a
broad peak between 2θ = 10 and 2θ = 33 was obtained, which
is attributed to the amorphous silica shell of the microcapsules
(Supporting Information, Figure S9). No characteristic peaks of
palladium nanoparticles could be detected by the XRD analysis
due to their low concentration within the microcapsules. DLS
analysis showed an average size of 2.0 μm (Supporting
Information, Figure S10). BET measurement demonstrated
an increase in the capsule surface area to 97.7 m2/g after
encapsulation of the Pd nanoparticles. This can be explained by
the decrease in the capsule size with the palladium
encapsulation.
3.3. Catalytic Performance of Pd/BMIm PF6@SiO2. The
performance of the catalyst Pd/BMIm-PF6@SiO2 was tested in
the hydrogenation of terminal and internal alkynes. The
catalyst, Pd/BMIm-PF6 was reacted with different alkynes in
diethyl ether under 500 psi of hydrogen for 24 h. Table 1
summarizes the hydrogenation of several of aromatic and
aliphatic alkynes. When terminal aromatic alkynes were used,
no selectivity was achieved (Table 1, entries 1−3). On the
other hand, when terminal aliphatic alkynes were hydrogenated,
partial selectivity toward alkenes was observed (Table 1, entries
4−5). In addition reduction of 1-octyne was accompanied by
isomerization of the formed 1-octene. Furthermore, hydro-
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selectively as the main product (Table 2, entry 1). In addition,
only 6% n-octane was detected when the same catalyst was
utilized to reduce trans-4-octene, under the same conditions.
On the other hand, when the hydrogenation of 4-octyne was
carried out homogeneously using palladium acetate dissolved in
BMIm-PF6, full saturated n-octane was generated as the main
product (Table 2, entry 10). At the end of this reaction, the
formation of palladium black precipitants was observed. We
believe the palladium catalyst loses its selectivity because it
aggregates to give the black precipitant. Previous reported
works demonstrated that selective hydrogenation of alkynes can
be obtained when palladium nanoparticles are formed in
specific functionalized ionic liquids.18 These investigations
support our assumption that silica microcapsules play a
significant role in the stabilization of palladium nanoparticles,
after their formation. It seems that producing a microenviron-
ment by confining the ionic liquid droplets with a silica shell
prevents the aggregation of the palladium nanoparticles and
leads to high selectivity in hydrogenation of 4-octyne. The
selectivity of Pd/BMIm-PF6@SiO2 in the hydrogenation of 4-
octyne was maintained to some extent when it was performed
in xylene or diethyl ether (Table 2, entries 3−4). For
comparison, hydrogenation of 4-octyne in diethyl ether with
palladium on charcoal (10%) and palladium on silica (5%) were
conducted, and as expected, only exhaustive hydrogenation
occurred (Table 2, entries 6−7). In comparison to BMIm-PF6,
reduction of 4-octyne with Pd/C and Pd/SiO2 in the presence
of BMIm-PF6 demonstrated lower selectivity (Table 2, entries
8−9). Interestingly, when the hydrogenation of 4-octyne
catalyzed by Pd/BMIm-PF6@SiO2 was carried out in hexane,
the catalyst lost its reactivity (Table 2, entry 2). This result
raises the possibility of tuning the reactivity of the same catalyst
when it is located in a microenvironment and dispersed in
different media. Leaching of the catalyst was examined by
separation of the catalytic microcapsules after hydrogenation of
4-octyne via centrifugation at 3000 rpm and reacting the
resulted supernatant liquid with 4-octyne. No products were
observed indicating that the palladium catalyst did not leach
from the silica microcapsules. In addition, ICP analysis was
performed, showing that Pd nanoparticles do not leach from
the capsules.
Table 1. Hydrogenation of Alkynes under Various
Conditions
a
conversion
b
c
entry
substrate
(%)
products (%)
1
2
phenylacetylene
100
100
ethylbenzene (100)
4-tert-
butylphenylacetylene
4-tert-butyl-4-vinylbenzene
(12), 4-tert-butyl-4-
ethylbenzene (88)
3
4
5
4-
100
100
91
1-chloro-4-vinylbenzene (4)
1-chloro-4-ethylbenzene
(96)
chlorophenylacetylene
1-octyne
1-octene (46), cis-2-octene
(22), trans-2-octene (11),
octane (18)
cyclohexylacetylene
vinyl cyclohexane (62), ethyl
cyclohexane (38)
6
7
diphenylacetylene
5-decyne
95
cis-stilbene (100)
100
cis-5-decene (90), trans-5-
decene (10)
8
4-ocyne
100
cis-4-octene (85), trans-4-
octene (6), n-octane (9)
a
1 mmol of substrate, 150 mg Pd/BMIm-PF6@SiO2 containing 6 ×
10−3 mmol Pd, 2 mL diethyl ether, 500 psi H2, room temperature, 24
h. Determined by H NMR and GC. Determined by GC.
b
c
1
genation of internal alkynes, (Table 1, entries 6−8), gave higher
selectivity to alkenes.
Further performance analysis of the hydrogenation was
examined using 4-octyne and different media. The results of
these hydrogenation reactions, in addition to results of
reactions performed under different condition for comparative
studies, are listed in Table 2. To prevent extraction of the ionic
liquid from the silica microcapsules, the hydrogenation
reactions were performed either in water or organic solvents
in which the ionic liquid is immiscible. Thus, the catalyst Pd/
BMIm-PF6@SiO2 was reacted with 4-octyne in water under
500 psi of hydrogen for 24 h and cis-4-octene was formed
Table 2. Hydrogenation of 4-Octyne under Various
a
Conditions.
conversion
b
c
entry
1
catalyst
solvent
water
(%)
products (%)
The recyclability of the Pd/BMIm-PF6@SiO2 catalyst was
tested in three cycles of hydrogenation of 4-octyne. The catalyst
was active in all cycles and no significant loss in its selectivity
was obtained (Supporting Information, Figure S11). Moreover,
the catalytic microcapsules were characterized by SEM and
TEM after they were utilized in the hydrogenation reaction and
their structure or morphology was not changed (Figure 8a and
Figure 8b respectively). In addition, TEM analysis indicates
that after reaction, the Pd nanoparticles size does not change
(Supporting Information Figure S12). In addition, EDS
Pd/BMIm-
PF6@SiO2
100
cis-4-octene (88), trans-4-
octene (10), n-octane (2)
2
3
4
Pd/BMIm-
PF6@SiO2
hexane
xylene
10
cis-4-octene (10)
Pd/BMIm-
PF6@SiO2
100
100
100
100
100
100
100
100
cis-4-octene (84), trans-4-
octene (5), n-octane (11)
Pd/BMIm-
PF6@SiO2
diethyl
ether
cis-4-octene (85), trans-4-
octene (6), n-octane (9)
d
5
Pd/BMIm-
PF6@SiO2
diethyl
ether
cis-4-octene (84), trans-4-
octene (6), n-octane (10)
e
6
Pd/C
diethyl
ether
n-octane (100)
e
7
Pd/SiO2
Pd/C
diethyl
ether
n-octane (100)
8
BMIm-
PF6
cis-4-octene (20), trans-4-
octene (3), n-octane (77)
9
Pd/SiO2
Pd(OAc)2
BMIm-
PF6
cis-4-octene (68), trans-4-
octene (9), n-octane (23)
10
BMIm-
PF6
cis-4-octene (8), n-octane
(92)
a
2.7 mmol of 4-octyne, 6 × 10−3 mmol Pd, 2 mL solvent, 500 psi H2,
b
room temperature, 24 h. Determined by 1H NMR and GC.
Figure 8. (a) SEM image of Pd/BMIm-PF6@SiO2 microcapsules after
three cycles of hydrogenation. (b) TEM image of Pd/BMIm-PF6@
SiO2 microcapsules after three cycles of hydrogenation.
c
d
e
Determined by GC. Reaction was prolonged for 60 h. Reaction
was completed after 5 h.
E
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Chemistry of Materials
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4. CONCLUSIONS
We disclosed a method for preparing new materials based on
ionic liquids that are encapsulated in silica shells. This method
provides particulate ionic liquids in powdered form, and open
up opportunities to design ionic liquids with new properties
such as lower viscosity. Importantly, the new ionic liquid-silica
microcapsules were utilized in the heterogenization of
palladium catalyst that show chemoselectivity in the hydro-
genation of internal alkynes such as 4-octyne. Remarkably,
when the hydrogenation of 4-octyne was performed in ionic
liquid under homogeneous conditions, the palladium catalyst
lost its selectivity, due to the formation of black aggregates. We
believe that the materials developed in this work provide
excellent future prospects for many applications.
ASSOCIATED CONTENT
* Supporting Information
■
S
SEM images, XRD pattern, EDS, DLS, and TGA curves of the
ionic liquid−silica microcapsules, TGA curves of BMIm-PF6
and Reax 88A, size distribution of the palladium nanoparticles,
and results of the recycling of the catalyst in the hydrogenation
reaction. This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: Raed.Abu-Reziq@mail.huji.ac.il.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We acknowledge funding support from Niedersachsen−Israeli
Research Cooperation Program.
■
ABBREVIATIONS
■
Tween 80, polyoxyethylene (20) sorbitan monooleate; Triton
X-100, polyethylene glycol octylphenyl ether; Brij 78, polyoxy-
ethylene (20) stearyl ether; Bu-PVP, butylated polyvinylpyrro-
lidone; SDS, sodium dodecyl sulfate; Reax 88B, sodium
lignosulfonate; Reax 88A, lignosulfonic acid; Pluronic P123,
triblock copolymer poly(ethylene glycol)-poly(propylene gly-
col)-poly(ethylene glycol); ILs, ionic liquids; BMIm-PF6, 1-
butyl-3-methylimidazolium hexafluorophosphate; TEOS, tet-
raethoxysilane; BMIm-PF6@SiO2 microcapsules, BMIM-PF6-
silica microcapsules; Pd/BMIm-PF6@SiO2, entrapped catalyst
within BMIm-PF6-silica microcapsules; DLS, dynamic light
scattering; SEM, scanning electron microscopy; TEM, trans-
mission electron microscopy; EDX, energy dispersive X-ray
spectroscopy; XRD, X-ray powder diffraction
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