Macro- and mesoporous polymers synthesized by a CO2-in-ionic liquid emulsion-templating route

Article abstract of DOI:10.1002/anie.201209255

Poring over polymers: Highly porous polymers with hierarchical macro- and mesoporous structures were synthesized by using the title method and UV radiation. The porosity properties of the polymers can be easily tuned by controlling the CO₂ pres

Full text of DOI:10.1002/anie.201209255

.
Angewandte
Communications
DOI: 10.1002/anie.201209255
Heterogeneous Catalysis
Macro- and Mesoporous Polymers Synthesized by a CO -in-Ionic
2
Liquid Emulsion-Templating Route**
Li Peng, Jianling Zhang,* Jianshen Li, Buxing Han, Zhimin Xue, and Guanying Yang
Porous polymers have received much interest because of their
wide range of applications in catalysis, gas separation,
easily tuned by the control of the CO pressure. The as-
2
synthesized polymers combine the advantages of both meso-
and macropores, and have shown application potential in
catalysis.
[
1]
structure replication, electrode materials, etc. It is partic-
ularly appealing to synthesize macro- and mesoporous
polymers because the hierarchical porous structures have
the advantages of each class of the hierarchical pores and
offer transport and diffusion pathways, especially for larger
The emulsion formation in the CO /N-ethyl perfluoro-
2
octylsulfonamide (N-EtFOSA)/1-butyl-3-methylimidazolium
nitrate ([bmim]NO ) system was studied in the pressure range
3
[
2]
guest species. Generally, macro- and mesoporous polymers
are prepared using the direct synthesis methodology by
of 10–16 MPa at 258C (for structures of N-EtFOSA and IL
[bmim]NO see Figure S1 in the Supporting Information).
3
[3]
reaction-induced phase separation.
Emulsion templating is a useful method for the prepara-
The volume fraction of IL was fixed at 35 vol% and the
surfactant concentration was 5.0 wt% based on the IL. A
white, opaque emulsion was formed upon stirring, and filled
the entire optical vessel (see Figure S2 in the Supporting
Information). The emulsions were stable for about 2 hours
after stirring was stopped. The electrical conductivity meas-
urements show that the emulsions are highly conducting (see
Figure S3 in the Supporting Information), thus representing
[4]
tion of porous polymers. Usually, oil-in-water or water-in-oil
[5]
emulsions are used for emulsion polymerization. Interest-
ingly, Cooper et al. developed a CO -in-water emulsion-
2
[
6]
templating route for the synthesis of porous polymers. The
removal of the droplet phase is simple because CO can revert
2
to the gaseous state upon depressurization. Moreover, the
[
7]
CO -in-water emulsion is more attractive than the conven-
IL-continuous (CO -in-IL) emulsions. Because the surfactant
2
2
tional emulsions consisting of water and oil because CO is
N-EtFOSA is much more soluble in the IL than in CO , the
2
2
[8]
nontoxic, inexpensive, and nonflammable. The CO -in-
IL-continuous emulsion is favored. When acrylamide (mono-
mer), benzophenone (initiator), and N,N’-methylenebisacryl-
amide (crosslinker) for the PAM synthesis were added to the
emulsion, no remarkable change in the stability of the
emulsion was observed.
2
water emulsion allows the production of hydrophilic polymers
[
6]
with porous structure.
In recent years, ionic liquids (ILs) have received tremen-
dous attention owing to their negligible vapor pressures, high
chemical and thermal stability, wide liquid temperature range,
The CO -in-IL emulsion containing acrylamide, benzo-
2
[9]
and wide electrochemical windows. Most importantly, ILs
phenone, and N,N’-methylenebisacrylamide was exposed to
[10]
can solvate a wide range of organic and inorganic reagents.
ILs have been widely utilized as promising media in different
UV irradiation for 1 hour. Then CO was released and the
2
product was obtained after washing and drying. As an
example, Figure 1 shows the SEM images of the PAM
synthesized within the emulsion at 12.4 MPa. The macro-
pores, which are about 3.5 mm in diameter, are distributed
[
11]
fields, such as chemical reactions,
including polymeri-
[12]
zation.
polymers is not widely reported.
However, their use for the synthesis of porous
[13]
Herein, we propose for the first time a CO -in-IL
2
emulsion templating route for the synthesis of porous
polymers. The polymerization is initiated by UV irradiation
[14]
to attain a fast polymerization rate. Highly porous poly-
acrylamide (PAM) and poly(trimethylolpropane trimethacry-
late) (PTRM) having hierarchical macro- and mesoporous
structures were obtained, and the porosity properties can be
[
*] L. Peng, Prof. J. Zhang, J. Li, Prof. B. Han, Z. M. Xue, G. Yang
Beijing National Laboratory for Molecular Sciences, CAS Key
Laboratory of Colloid and Interface and Thermodynamics, Institute
of Chemistry, Chinese Academy of Sciences (China)
E-mail: zhangjl@iccas.ac.cn
[
**] The authors thank the National Natural Science Foundation of
China (21173238, 21133009, 21073207), Ministry of Science and
Technology of China (2009CB930802), and the Chinese Academy of
Sciences (KJCX2.YW.H16) for support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201209255.
Figure 1. SEM images of the PAM synthesized in a CO -in-IL emulsion
at 12.4 MPa and 258C.
2
1
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Angewandte
Chemie
uniformly throughout the product. These macropores result
The mesoporosity of the PAM was determined by an N2
from the templating effect of the CO droplets during the
adsorption/desorption method. As shown in Figure 2B, the N2
adsorption/desorption isotherm exhibits a mode of the
type IV, which is related to mesoporous materials. The BET
(Brunauer, Emmett, and Teller) surface area and total pore
2
polymerization, which occurs in the continuous IL phase. The
polymer particles (100–300 nm) are packed loosely, thus
generating smaller and irregular pores with sizes in the
hundreds of nanometers range. The FT-IR spectra indicate
the successful formation of the PAM and the thermogravi-
metric analysis shows that it is stable up to 2608C (see
Figures S4 and S5 in the Supporting Information).
The macroporosity of the PAM was determined by the
mercury porosimetry method. The macropore size distribu-
tion curve of the PAM shows a pore size distribution centered
at 3.4 mm (Figure 2A), which is consistent with that observed
2
À1
3
À1
volume are 143.0 m g and 0.473 cm g , respectively, and
the mesopore size distribution curve, calculated from Barrett-
Joyner-Halenda method, shows a pore size distribution
centered at around 12.6 nm (inset of Figure 2B). The
mesopores in the PAM may result from the templating
[15]
effect of the micelles formed from the surfactant in the IL.
The PAMs were synthesized in CO -in-IL emulsions at
2
different pressures, with all other experimental parameters
being the same as those above. The products were charac-
terized by FT-IR, mercury porosimetry, and N adsorption/
2
desorption methods. The results show that all the PAMs
present hierarchical macro- and mesoporous structures (see
Figures S6–S8 in the Supporting Information). Table 1 lists
Table 1: Effect of pressure on the porosity, total macropore volume
(Vpore), median macropore diameter (Dmacro), BET surface area (SBET),
mesopore volume (V ), and mesopore diameter (Dmeso) of the PAMs
t
synthesized in CO -in-IL emulsion.
2
[
a]
[a]
[a]
[b]
[b]
[b]
Pressure Porosity Vpore
Dmacro
[mm]
S
Vt
Dmeso
[nm]
BET
2
À3 À1
À1
À3 À1
[MPa]
[%]
[cm
g
]
[m g
]
[cm
g
]
1
1
1
1
0.3
2.4
4.0
6.0
86.4
84.8
86.5
87.5
3.80
4.03
5.28
6.84
2.1
3.4
7.2
152.3
143.0
89.0
0.491
0.473
0.258
0.105
13.1
12.6
6.4
13.9
46.8
4.7
[a] Measured by mercury intrusion porosimetry. [b] Measured by N2
adsorption/desorption isotherm analysis.
the properties of the macro- and mesopores of the PAMs
synthesized at different pressures. All these PAMs have a high
degree of porosity (> 80%). Interestingly, the pressure has
opposing effects on the properties of the macro- and
mesopores, that is, the higher pressure favors the formation
of a PAM with larger macropores and smaller mesopores.
From 10.3 to 16.0 MPa, the macropore diameter is increased
from 2.1 to 13.9 mm, while the mesopore diameter reduces
from 13.1 to 4.7 nm. The total macropore volume of the PAM
Figure 2. A) Macropore size distribution and B) the N adsorption/
desorption isotherm of the PAM synthesized at 12.4 MPa and 258C.
The inset in B) shows the mesopore size distribution curve.
2
3
À1
synthesized at 16.0 MPa can be as large as 6.84 cm g ,
whereas the mesopore volume and surface area fall to
3
À1
2
À1
0
.105 cm g and 46.8 m g , respectively. The mechanism
from the SEM images. The PAM has a total macropore
volume of 4.03 cm g and a porosity degree of 84.8%. It is
very interesting that the porosity degree of PAM is consid-
for the effect of pressure on the porosities of PAMs is
discussed below. The above results indicate that the macro-
3
À1
and mesopore properties of PAMs synthesized in CO -in-IL
2
erably higher than the volume fraction of CO in the emulsion
emulsions can be easily tuned by controlling the CO2
pressure.
2
(
65 vol% internal phase). For the polymer synthesized in the
CO -in-water emulsion, the degree of porosity can be equal to
A mechanism for the formation of macro- and mesopo-
2
the volume ratio of the internal phase when the original
emulsion structure was completely retained during polymer-
ization. Herein, the excess porosity higher than the volume
rous polymers by a CO -in-IL emulsion-templating route is
2
proposed (Figure 3). In the emulsion, the CO2 droplets
(micron sized) are dispersed in the IL continuous phase,
and thus coexisting with the micelles which usually have sizes
on the nanometer scale (Figure 3a). Owing to the good
solvency of the IL towards the reactants and the poor
[
6b]
fraction of CO can be attributed to the templating effect of
2
[
13]
the IL during polymerization. The release of the IL after
polymerization generates excess macropores, which may
correspond to the pores with sizes in the hundreds of
nanometers range, as shown in the SEM images (Figure 1).
solvency of CO , the reactants are solubilized in the IL where
2
the polymerization will proceed. During the polymerization
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Angewandte
Communications
pressure of 2 MPa when using a substrate/Pd molar ratio of
3
1
200 (entry 1). The turnover frequency (TOF) was about
8000 h . The Pd/PAM catalyst was reused twice for this
À1
reaction, and there was almost no activity loss (entries 2 and
), thus indicating that the as-prepared catalyst is stable. For
3
comparison, the styrene hydrogenation was performed using
a commercial Pd/C catalyst under the same reaction con-
ditions (Table 1, entries 4–6). The TOFs of the Pd/PAM are
much higher than those of the commercial Pd/C catalyst.
Additionally, we also performed the styrene hydrogenation
Figure 3. Schematic illustration for the formation of macro- and
mesoporous polymers by using a CO -in-IL emulsion-templating route.
2
The large and small spheres in a) represent the CO droplets and
2
micelles, respectively.
using the Pd/PAM catalyst with an H pressure of 1 atm at
2
2
58C (entries 7 and 8). The TOFs are also higher than those
[
18]
reported for Pd/polymer catalyst (entry 9). These results
indicate that the as-synthesized Pd/PAM catalyst has a much
higher activity than the commercial Pd/C and the reported
Pd/polymer for the hydrogenation of styrene.
initiated by UV irradiation, the CO droplets and micelles act
2
as templates for the macro- and mesopore formation,
respectively. Additionally, the IL plays a templating role
[
13]
during polymerization. Therefore, the subsequent removal
The Pd/PAM catalyst was also utilized for the Suzuki
cross-coupling of iodobenzene with phenylboronic acid. The
yield of biphenyl was enhanced about 14% in comparison to
that obtained using the commercial Pd/C catalyst (see
Table S1 in the Supporting Information). The high catalytic
activity of the as-synthesized Pd/PAM catalyst may be
ascribed to the hierarchical structure of the PAM. The
macropores facilitate the mass transport and the mesopores
increase the surface area for contact of the reactants with the
palladium nanoparticles. Thus the Pd/PAM has high activity
for the catalytic reaction.
of CO , IL, and surfactant gives rise to the formation of
a highly porous polymer with a hierarchical macro- and
mesoporous structure (Figure 3b). At higher pressures, the
surfactant becomes more CO soluble, thus the interface is
less curved about CO and the CO droplet size is increased.
Consequently, the PAMs synthesized at higher pressure has
larger macropores because of the templating effect of the CO2
droplets (Table 1). As for the reduced mesopore size of the
PAMs made at higher CO2 pressure (Table 1), it can be
explained by the smaller micelle size at higher pressures,
2
2
[7]
2
2
a phenomena which has been reported for CO -in-water
Furthermore, the CO -in-IL emulsion-templating method
was applied for the synthesis of a hydrophobic polymer, that
is, the macro- and mesoporous PTRM was obtained (see SEM
2
2
[16]
[17]
micelles and CO -in-IL micelles.
2
The as-synthesized highly porous polymers have advan-
tages of both mesopores and macropores, and thus have
potential applications in catalysis. Herein we utilized the
PAM as a support for a palladium catalyst. The Pd/PAM
composite was synthesized and characterized by XRD, XPS,
and EDX spectra (see Figures S9–S11 in the Supporting
Information). The loading of palladium on the PAM was
images and N adsorption/desorption isotherm in Figures S12
2
and S13 in the Supporting Information). The data indicate
that the CO -in-IL emulsion-templating route is versatile in
2
synthesizing both hydrophilic and hydrophobic polymers, and
can be attributed to the good solvency of the IL towards
a wide range of reagents.
3
.1 wt% as determined by ICP-AES analysis. The catalytic
In summary, macro- and mesoporous polymers with high
performance of Pd/PAM for hydrogenation of styrene is
shown in Table 2. Styrene was almost completely converted
into ethylbenzene within 10 minutes at 308C and an H2
porosity have been synthesized by a CO -in-IL emulsion-
templating route under UV radiation. The porosities of the
2
polymers are easily tuned by controlling the CO pressure.
2
These porous materials combine the advantages of both
meso- and macropores, and have potential applications in
catalysis, gas separation, and material fabrication. The CO2-
in-IL emulsion-templating method can be applied to the
synthesis of some other highly porous polymers.
[
a]
Table 2: Catalytic activity test for the hydrogenation of styrene.
[
b]
[c]
Entry
pH2
t
Yield [%]
TOF
[
[
[
[
[
[
[
[
[
d]
e]
e]
f]
1
2
3
4
5
6
7
8
9
2 MPa
2 MPa
2 MPa
2 MPa
2 MPa
2 MPa
1 atm
1 atm
1 atm
10 min
10 min
10 min
10 min
15 min
20 min
6 h
>99
>99
>99
85.93
93.45
>99
88.02
>99
–
18000
18000
18000
10300
7476
6000
1476
1250
Experimental Section
Emulsion formation and characterization: The apparatus consisted of
a view cell (9.5 mL) equipped with two quartz view windows, a high-
pressure pump, and a pressure gauge. In a typical experiment, 4.00 g
f]
f]
d]
d]
g]
8 h
–
of 5.0 wt% N-EtFOSA/[bmim]NO solution was loaded into the view
3
930
cell at 258C. The view cell was then charged with CO to the desired
2
[
a] Reaction conditions. Temperature: 308C for entries 1–6, 258C for
pressure with stirring. The emulsion formation was observed at
different pressures. The apparatus and procedure for conductivity
measurement are given in the Supporting Information.
entries 7–9. Substrate/Pd (mol/mol)=3200 for entries 1–3. Substrate/
Pd (mol/mol)=2000 for entries 4–6). [b] Yield of ethybenzene.
[
c] Turnover number (TON)=mol of product (ethylbenzene) per mole of
PAM synthesis and characterization: The monomer acrylamide
(0.8 g), crosslinker N,N’-methylenebisacrylamide (0.04 g) and initia-
tor benzophenone (0.04 g) were added into 5.0 wt% N-EtFOSA/
[bmim]NO3 solution (4.00 g), which was loaded in the view cell
À1
Pd; TOF=TONh . [d] The Pd/PAM synthesized in this work. [e] The
Pd/PAM was reused for 2 runs. [f] The commercial Pd/C catalyst. [g] The
Pd/poly(phenyltriazolylmethyl)styrene reported in Ref. [18].
1
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Angewandte
Chemie
(9.5 mL) equipped with quartz view windows at 258C. CO2 was
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2
by depressurization. The white product was washed with ethanol and
dried at 808C under vacuum for 24 h.
The morphologies of PAM were characterized by a HITACHI S-
800 SEM. The macroporosities were recorded by mercury intrusion
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porosimetry using a Micromeritics Autopore IV 9500 porosimeter.
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increasing to 44500 psia in predefined steps to give pore size/pore
volume information. The mesoporosities were determined by N2
adsorptio/desorption isotherms using a Quadrasorb SI-MP system.
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2
À1
4
1
0 mLmin
.
Pd/PAM synthesis and catalytic activity: The PAM synthesized at
2.4 MPa (0.15 g) and Pd(OAc)2 (0.016 g) was added into a flask
containing 100 mL acetone. The flask was placed in an oil bath at
08C and the mixture was stirred for 24 h. After centrifugation, the
7
product was dried and reduced at 1508C for 2 h with H . Pd/PAM was
2
obtained after cooling.
For the hydrogenation reaction, styrene (1 g), n-heptane (1 g),
and Pd/PAM (10 mg) were placed in a 20 mL stainless steel reactor.
The reactor was evacuated and filled with H (three times). The stirrer
2
was started with a rate of 300 rpm. H was added to suitable pressure
2
and kept to be constant during the reaction, which was monitored by
a pressure transducer (Foxboro/ICT model 930). After reaction for
a certain time, the product was separated with the catalyst by
centrifugation (1200 rpm) and the product was analyzed by a gas
chromatograph (Agilent 6820) equipped with a flame ionization
detector (FID) and a PEG-20M capillary column (0.25 mm in
diameter, 30 m in length). The experimental details for the Suzuki
cross-coupling reaction catalyzed by Pd/PAM are shown in the
Supporting Information.
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Received: November 19, 2012
Published online: January 4, 2013
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Keywords: heterogeneous catalysis · ionic liquids ·
mesoporous materials · palladium · polymers
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