One-stage synthesis of cagelike porous polymeric microspheres and application as catalyst scaffold of pd nanoparticles

Article abstract of DOI:10.1021/ma200320a

One-stage synthesis of cagelike porous polymeric microspheres and their application as catalyst scaffold of Pd nanoparticles are discussed. The synthesis of cagelike porous polymeric microspheres is achieved by W/O/W emulsion polymerization of a surfactan

Full text of DOI:10.1021/ma200320a

ARTICLE
pubs.acs.org/Macromolecules
One-Stage Synthesis of Cagelike Porous Polymeric Microspheres and
Application as Catalyst Scaffold of Pd Nanoparticles
Jianxiong Xu, Guojun Chen, Rui Yan, Da Wang, Minchao Zhang, Wangqing Zhang,* and Pingchuan Sun
Key Laboratory of Functional Polymer Materials of the Ministry of Education, Institute of Polymer Chemistry,
Nankai University, Tianjin 300071, China
S Supporting Information
b
ABSTRACT: One-stage synthesis of cagelike porous polymeric
microspheres and their application as catalyst scaffold of Pd
nanoparticles are discussed. The synthesis of cagelike porous
polymeric microspheres is achieved by W/O/W emulsion
polymerization of a surfactant monomer of N-(4-vinylbenzyl)-N,
N-dibutylamine hydrochloride and a hydrophobic monomer of
styrene in water. Ascribed to the surfactant monomer, con-
venient one-stage synthesis of cagelike porous polymeric micro-
spheres is afforded, and cagelike porous polymeric microspheres
with size ranging from 300 to 600 nm and pore volume as large
as 0.31 cm3/g are fabricated. The porous character and the inherent quaternary ammonium moieties make the cagelike porous
polymeric microspheres to be the promising catalyst scaffold of Pd nanoparticles, and up to 10 wt % Pd catalyst with the metal
particle size ranging from 2.1 to 5.7 nm is loaded. The catalytic hydrogenation of nitrobenzene by H2 demonstrates that the
heterogeneous Pd catalyst is efficient and reusable.
template by solvent etching.34ꢀ42 Thus, convenient synthesis
1. INTRODUCTION
of porous polymeric microspheres is urgently needed in catalysis.
Interfacial polymerization through emulsion system such as
The application of polymer as catalyst scaffold can be traced to
as early as the 1960s.1 Since then, various polymeric materials
oil-in-water-in-oil (O/W/O) emulsion and water-in-oil-in-water
including resins,2ꢀ5 porous polymer,6ꢀ9 nano- and micro-
(W/O/W) emulsion has been proved to be an efficient method
spheres,10ꢀ18 dendrimer,19ꢀ21 and even some soluble poly-
to produce porous microspheres, in which a binary surfactant
mers22,23 have been designed to support catalyst of noble metal
mixture or a unitary surfactant is usually needed.47ꢀ54 For
nanoparticles. Compared with the inorganic support such as
example, the typical methods for making W/O/W emulsions
charcoal,24 clay,25 and metal oxides including silica and zeolites,26
use a two-step process of initial formation of an inverse water-in-
the polymeric scaffold has the advantages that the chemical
oil emulsion and subsequent emulsification in water using a
composition, size, morphology, and even the penetrability or
combination of two surfactants.49 Improved methods using a
accessibility of the polymeric scaffold can be tailored.27ꢀ29 Of all
unitary surfactant to construct W/O/W emulsion are also
the polymeric catalyst scaffolds, the porous microspheres may be
one of the most interesting candidates since the porous character
provides good accessibility for reactants just as the micro- and
proposed, and various porous polymeric microspheres have been
synthesized.51,52 Different from the template polymerization
employing a hard template to prepare hollow microspheres, a
mesoporous inorganic support.30ꢀ35 Up to now, various porous
soft template such as surfactant micelles or oil drops is used in the
polymeric microspheres including hollow microspheres have
W/O/W emulsion polymerization, and thus template elimina-
been fabricated and employed as scaffold for Pd, Au, Pt, Ag,
tion is avoided.47ꢀ54 However, the removal of the surfactants and
and Rh nanocatalysts.30ꢀ36 We have demonstrated that hollow
isolation of the resultant porous microspheres often involve
repetitious centrifugation, washing, and redispersion, which
limits their practical application.
Herein, a convenient one-stage synthesis of cagelike porous
polymeric microspheres containing the polystyrene (PS)
and poly[N-(4-vinylbenzyl)-N,N-dibutylamine hydrochloride]
polymeric microspheres act as not only the catalyst scaffold for
noble metal nanoparticles but also the microcapsules for reac-
tants; thus, the hollow polymeric microspheres containing the
immobilized catalyst can be used as microreactor within which
catalysis runs highly efficient.34ꢀ36 However, the synthesis of
porous polymeric microspheres generally involves multiple and
(PVBAH) segments through W/O/W emulsion polymerization
complex procedures.34ꢀ46 For examples, the general method to
fabricate hollow microspheres by template polymerization in-
cludes synthesis of a suitable template particle, polymerization of
the shell-forming monomer on the surface of the template
particle to form coated microspheres, and removal of the
Received: February 11, 2011
Revised: April 18, 2011
Published: April 22, 2011
r
2011 American Chemical Society
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Scheme 1. Chemical Structure of VBA
2.4. Immobilization of Pd Nanoparticles on the PS-co-
PVBAH Cagelike Porous Microspheres. A given weight of PdCl2
was initially dissolved in 15.0 mL of ethanol. Then, 0.133 g of the PS-co-
PVBAH cagelike porous microspheres, which were synthesized at pH = 7
with the weight ratio of the feeding monomers to the solvent of water
(O/W) = 2/100 and the weight ratio of VBA/St = 10/100 as discussed
above, was added. The dispersion was kept at room temperature for 2 h
with magnetic stirring, and then cool NaBH4 aqueous solution (7.5 mL,
5ꢀ50 mmol/mL) was added, in which the molar ratio of NaBH4/PdCl2
was set at 5:1. The resultant black dispersion was initially dialyzed
against water for 3 days to remove the organic solvent of ethanol and the
excessive NaBH4, and then the aqueous dispersion was diluted with
water to a predetermined volume to obtain the aqueous dispersion of Pd
nanoparticles immobilized on PS-co-PVBAH cagelike porous micro-
spheres (Pd@PS-co-PVBAH).
of a surfactant monomer of N-(4-vinylbenzyl)-N,N-dibutylamine
hydrochloride (VBAH) and a hydrophobic monomer of styrene
is reported. Ascribed to the VBAH surfactant monomer, the PS-
co-PVBAH cagelike porous microspheres can be easily prepared
without additional surfactant. Also ascribed to the VBAH surfac-
tant monomer, the palladium precursor can be immobilized
within the PS-co-PVBAH cagelike porous microspheres through
ion exchange in the aqueous phase, and therefore the catalyst of
Pd nanoparticles immobilized on cagelike porous microspheres
is fabricated by the subsequent reduction with NaBH4 aqueous
solution. Catalyzed hydrogenation of nitrobenzene demon-
strated that the resultant heterogeneous Pd catalyst is efficient
and reusable.
By varying the weight ratio of PdCl2 to the PS-co-PVBAH cagelike
porous microspheres, four Pd@PS-co-PVBAH catalysts called 1-Pd@
PS-co-PVBAH, 2-Pd@PS-co-PVBAH, 5-Pd@PS-co-PVBAH, and 10-
Pd@PS-co-PVBAH with the weight ratio at 1.0%, 2.0%, 5.0%, and
10.0% were fabricated.
2.5. Catalyst Testing. The hydrogenation of nitrobenzene at
30 °C in aqueous dispersion by bubbling H2 under a atmosphere
pressure was employed to test the typical 10-Pd@PS-co-PVBAH cata-
lyst. In the hydrogenation of nitrobenzene, 10 mL of aqueous dispersion
of the 10-Pd@PS-co-PVBAH catalyst containing 5.0 ꢁ 10ꢀ3 mmol of Pd
was added into a 25 mL tubelike glass reactor equipped with a reflux
condenser and a magnetic stirrer, and then the 10-Pd@PS-co-PVBAH
catalyst was activated at 30 °C by bubbling H2 (0.01 cm3/min) for 1 h.
Hydrogenation by bubbling H2 (0.01 cm3/min) was started immedi-
ately after addition of a given amount of nitrobenzene (0.06ꢀ0.30 g,
0.5ꢀ2.5 mmol). The molar ratio of C/S, in which C represents the Pd
catalyst and S represents the substrate of nitrobenzene, ranges from 0.2%
to 1.0%. To monitor the conversion of nitrobenzene and the yield of
phenylamine, aliquots of the reaction mixture were withdrawn at given
time intervals and analyzed by high-performance liquid chromatography
(HPLC).
To detect Pd leaching from the typical 10-Pd@PS-co-PVBAH catalyst
into the aqueous phase, the hydrogenation in aqueous dispersion at
30 °C with the molar ratio C/S = 0.5% was checked. After 20 min
hydrogenation, the reaction mixture was filtered at the reaction tem-
perature and the filtrate was collected. Part of the filtrate was further
bubbled with H2 (0.01 cm3/min) at 30 °C and the hydrogenation was
monitored with HPLC, and part of the filtrate was analyzed by atomic
absorption spectroscopy (AAS analysis).
To check the reuse of the typical 10-Pd@PS-co-PVBAH catalyst, after
the hydrogenation of nitrobenzene at 30 °C with the molar ratio C/S =
0.5% was just completed in 90 min, the mixture was cooled to room
temperature instantly. Subsequently, the product was extracted from the
reaction mixture with cyclohexane (20 mL ꢁ 3), and then the organic
phase was removed. The 10-Pd@PS-co-PVBAH catalyst remaining in
the aqueous phase was reused in another run of hydrogenation of
nitrobenzene under the same experiment conditions as in the fresh run.
To diagnose possible aggregation of the Pd nanoparticles in the
10-Pd@PS-co-PVBAH catalyst, the aqueous dispersion of the recycled
catalyst was detected by transmission electron microscopy (TEM).
2.6. Characterization. The scanning electron microscopy (SEM)
images were obtained with a Shimadzu SS-550 microscope. Transmis-
sion electron microscopy (TEM) observation was performed using a
Philips T20ST electron microscope at an acceleration of 200 kV,
whereby a small drop of the aqueous dispersion of the synthesized
microspheres was deposited onto a piece of copper grid and then dried at
room temperature under vacuum. The nitrogen adsorption was per-
formed on a Micromeritics Gemini V system at 77 K. The specific
surface area was calculated by BET (BrunauerꢀEmmettꢀTeller)
2. EXPERIMENTAL SECTION
2.1. Materials. Styrene (St, >98%, Tianjin Chemical Co.) was
distilled under vacuum before being used. The reagents including
n-dibutylamine(DBA, >98%, Alfa), chloromethylstyrene(CMS, >97%, Alfa),
polyvinylpyrrolidone (PVP, Mw = 10000, BASF), K2S2O8 (>99.5%,
Tianjin Chemical Co.), PdCl2 (>99%, Alfa), NaBH4 (>99%, Tianjin
Chemical Co.), nitrobenzene (99%, Tianjin Chemical Co.), and pheny-
lamine (99%, Tianjin Chemical Co.) were used as received. All the other
chemical reagents were analytic grade and used as received.
2.2. Synthesis of N-(4-Vinylbenzyl)-N,N-dibutylamine
(VBA). Into a flask CMS (30.4 g, 0.20 mol), DBA (51.6 g, 0.40 mol),
K2CO3 (48.8 g, 0.40 mol), and 200 mL of CHCl3 were sequentially
added. The flask content was initially degassed by nitrogen purge and
then heated at 50 °C for 24 h with magnetic stirring. After completion of
the reaction, the mixture was poured into 500 mL of water, and then the
organic phase was collected. The collected organic phase was initially
dried over anhydrous magnesium sulfate, and then the solvent is
removed by rotary evaporation under vacuum at room temperature to
obtain a crude product. Lastly, the crude product was purified by column
chromatography using the solvent mixture of petroleum ether and ethyl
acetate (6:1 by volume) to afford 30.1 g (69% yield) of a light-yellow
liquid of VBA (Scheme 1). 1H NMR (CDCl3): δ 7.41 (d, J = 8.3 Hz,
2H), 7.32 (d, J = 8.3 Hz, 2H), 6.72 (dd, 11.0, 17.8 Hz, 1H), 5.77
(dd, 17.8, 0.8 Hz, 1H), 5.27 (dd, 0.8,11 Hz, 1H), 3.62 (s, 2H), 2.36 (t, J =
8 Hz, 4H), 1.42ꢀ1.31 (m, 8H), 0.96 (t, J = 7.4 Hz, 6H).
2.3. Synthesis of the PS-co-PVBAH Cagelike Porous Micro-
spheres. Into a glass flask a given amount of VBA and a given volume of
water ranging from 100 to 400 mL were initially added. Subsequently,
HCl aqueous solution (10 mL, 0.1ꢀ1 mol/L) was added dropwise with
magnetic stirring until a clear solution was observed at pH = 7. Then, St
(5.2 g, 50 mmol) and PVP (0.3 g) were added with vigorous stirring for
30 min to form a stable emulsion. The flask content was initially
degassed under nitrogen purge to remove oxygen, then heated at
80 °C, and followed by addition of K2S2O8 (0.15 g, 0.55 mmol). The
polymerization was performed under a nitrogen atmosphere with
vigorous stirring for 20 h at 80 °C. After completion of the polymeri-
zation, the product was washed with ethanol (20 mL ꢁ 3), collected by
centrifugation, and dried under vacuum at 40 °C to obtain the PS-co-
PVBAH cagelike porous microspheres.
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Scheme 2. Schematic One-Stage Synthesis of the
PS-co-PVBAH Cagelike Porous Microspheres through the
W/O/W Emulsion Polymerization
Figure 1. SEM (A) and TEM (B, C, D) images of the PS-co-PVBAH
cagelike porous microspheres synthesized at pH = 7, VBA/St = 10/100,
and O/W = 2/100.
After the polymerization initiated by K2S2O8, these emulsifying
W/O/W droplets are converted into the PS-co-PVBAH cagelike
porous microspheres which includes several macropores within
the body as shown in Scheme 2.
method, the pore-size distribution was calculated from the adsorption
branch using BJH (BarettꢀJoynerꢀHalenda) method, and total pore
1
volume was obtained at p/p0 of 0.99. The H NMR spectrum was
The parameters including the pH value of the aqueous phase,
the monomer percent, or the weight ratio of the feeding
monomers (St and VBA) to the solvent of water (O/W) and
the weight ratio of VBA/St will affect the formation and the
structure of the emulsifying W/O/W droplets and therefore
further affect the synthesis of the PS-co-PVBAH cagelike porous
microspheres. Thus, in the next discussion, synthesis of the
PS-co-PVBAH cagelike porous microspheres under the optimized
conditions is initially introduced, and subsequently the effect by
the pH, the O/W value, and the VBA/St value on synthesis of the
PS-co-PVBAH cagelike porous microspheres is discussed.
Figure 1 shows the SEM (A) and the TEM (BꢀD) images of
the PS-co-PVBAH cagelike porous microspheres synthesized
under the optimized conditions at pH = 7, O/W = 2/100, and
VBA/St = 10/100. The SEM image (Figure 1A) shows that the
synthesized PS-co-PVBAH cagelike porous microspheres have a
uniform size distribution, and the average diameter is 350 nm.
From the TEM images (Figures 1BꢀD), it is concluded that the
PS-co-PVBAH cagelike porous microspheres containing lots of
20ꢀ50 nm macropores. The number of the macropores included
in single PS-co-PVBAH cagelike porous microspheres ranges
from 60 to 80. It is believed that the number of the macropores
approximately corresponds to the amount of the surfactant
monomer micelles in the W/O/W droplet. Besides, it is observed
that some of the macropores are just located on the periphery of
the cagelike porous microspheres (as indicated by the cycles in
Figure 1D). These peripheral macropores together with the
inner macropores make the cagelike porous microspheres acces-
sible for reactants when they are used as catalyst scaffold.
Nitrogen adsorption is usually utilized to characterize porous
materials.55 To diagnose the porosity, the PS-co-PVBAH cagelike
porous microspheres and the reference sample of the solid PS
microspheres (Figure S1) which have the similar size and
obtained on a 400 MHz spectrometer using CDCl3 as solvent. The solid
state 13C CP/MAS NMR with TOSS was performed on a Varian
Infinityplus-400 wide-bore (89 mm) NMR spectrometer equipped with
a 4 mm double-resonance HX CP/MAS probe at a proton frequency of
400 MHz. The FTIR measurement was performed on a Bio-Rad FTS-
6000 IR spectrometer. The powder X-ray diffraction (XRD) measure-
ment was performed on a Rigaku D/max 2500 X-ray diffractometer.
HPLC analysis was performed on a LabAlliance PC2001 system
equipped with a C18 column and a UVꢀvis detector using a mixture
of CH3CN and water (6/4 by volume) as eluent. AAS analysis was
performed on a Solaar AAS 2 atomic absorption spectrometer. The
X-ray photoelectron spectroscopy (XPS) analyses were performed with
a Kratos Axis Ultra DLD spectrometer employing a monochromated Al
KR X-ray source (1486.6 eV) and a delay line detector (DLD).
3. RESULTS AND DISCUSSION
3.1. Synthesis of the PS-co-PVBAH Cagelike Porous Micro-
spheres. The synthesis of the PS-co-PVBAH cagelike porous
microspheres includes the construction of the W/O/W emulsion
and the polymerization of the W/O/W emulsion initiated by the
hydrophilic K2S2O8 to form colloids stabilized by PVP, which is
schematically shown in Scheme 2. After mixing the surfactant
monomer of VBAH and the hydrophobic monomer of St in
water and followed vigorous stirring, the emulsifying W/O/W
droplets, which are composed of the hydrophobic monomer of St
and several encapsulated surfactant monomer micelles of VBAH
(indicated by the large red cycle) as shown in Scheme 2, are
formed. Furthermore, it is expected that some of the surfactant
monomers locate on the surface of the emulsifying W/O/W
droplets together with PVP to act as stabilizer, and a few of the
surfactant monomers (indicated by the small red cycle in
Scheme 2) are soluble in the emulsifying W/O/W droplets.
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Figure 2. Nitrogen adsorptionꢀdesorption isotherms of the PS-co-
PVBAH cagelike porous microspheres (A) and the reference sample of
the solid PS microspheres (B), in which the isotherm A was offset
vertically by 50 cm3/g.
Figure 4. TEM images of the PS-co-PVBAH microspheres synthesized
at pH = 1 (A), pH = 3 (B), pH = 7 (C), and pH > 7 (D) under the
constant values of O/W = 3/100 and VBA/St = 10/100.
just shows an ichnographic image of the 3D cagelike porous
microspheres.
To detect the chemical composition, the PS-co-PVBAH cage-
like porous microspheres are characterized by solid 13C CP/MAS
NMR. As indicated by Figure 3, the characteristic chemical shifts
due to the PS and the PVBAH segments are well recorded,
confirming the chemical composition of the PS-co-PVBAH
cagelike porous microspheres. In addition, the residual spinning
side bands indicated as asterisks are also observed. The chemical
composition of the PS-co-PVBAH cagelike porous microspheres
is further confirmed by FTIR analysis (Figure S2).
3.2. pH Effect on Synthesis of the Cagelike Porous Micro-
spheres. Because of the three long alkyl chains, the VBA
monomer is insoluble in neutral water at room temperature. It
is founded that when VBA is added into water, a thin yellow layer
of the VBA monomer floating on water is optically observed even
after vigorous stirring. Whereas, when HCl aqueous solution is
added, VBA is acidified to form the surfactant monomer of
VBAH, and the yellow layer floating on water gradually disappear
until a pale opal dispersion is observed at pH = 7. Thus, the
dispersion of VBA in water and therefore the construction of the
W/O/W emulsion and the formation of the PS-co-PVBAH
cagelike porous microspheres are pH-dependent. Figure 4 shows
the TEM images of the PS-co-PVBAH colloids synthesized at
different pH under the constant values of O/W = 3/100 and
VBA/St = 10/100. Herein, it should be pointed out, when the
VBA monomer is added into neutral water, the pH value of the
aqueous phase is almost as same as those of the neutral water
(pH = 7) since the VBA monomer cannot be dispersed in neutral
water at all. To distinguish with the aqueous dispersion of the
acidified monomer VBAH at pH = 7, the biphasic system of VBA
and the neutral water is referred to pH > 7. As indicated in
Figure 4A, coreꢀshell porous microspheres are synthesized at
pH = 1. The average size of the coreꢀshell porous microspheres
is 300 nm, and the average shell thickness is 40 nm. The
Figure 3. Solid-state 13C CP/MAS NMR spectra of the PS-co-PVBAH
cagelike porous microspheres.
chemical composition with the PS-co-PVBAH cagelike porous
microspheres are characterized by nitrogen adsorption analysis.
From the N2 adsorptionꢀdesorption isotherms shown in Fig-
ure 2, an obvious hysteresis loop at 0.7ꢀ0.9 p/p0 is observed for
the PS-co-PVBAH cagelike porous microspheres (Figure 2A),
whereas the reference sample of the solid PS microspheres
exhibits a type I isotherm (Figure 2B). These results further
confirm the porous character of the PS-co-PVBAH cagelike
porous microspheres.55 Ascribed to the porous character, the
PS-co-PVBAH cagelike porous microspheres have a much larger
pore volume (0.31 cm3/g vs 0.067 cm3/g) and a larger specific
surface area (26.4 m2/g vs 10.6 m2/g) than those of the solid PS
microspheres. The average pore size of the macropores calcu-
lated by the BJH method is about 46 nm, which is well-consistent
with those measured by TEM observation shown in Figure 1.
From the average size of the macropores measured by TEM and
the pore volume calculated by BJH method, the number of
the macropores including in single PS-co-PVBAH cagelike
porous microspheres is evaluated to be ∼100. The number is
larger than that by TEM observation, since the TEM microscope
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have a coreꢀshell structure, in which the core size is 80 nm and
the shell thickness is 35 nm. We think the formation of the solid
coreꢀshell microspheres can be ascribed to the phase separation
between the PS and the PVBA segments, since the PVBA and PS
segments are incompatible due to their different hydrophilic/
hydrophobic character. It is expected that the solid PS-co-PVBA
coreꢀshell microspheres have a PVBA-rich shell and PS-rich
core, which are just as those coreꢀshell microspheres synthe-
sized by one-stage soap-free emulsion polymerization as dis-
cussed elsewhere.56
3.3. Effect of the Oil/Water Weight Ratio on Synthesis of
Cagelike Porous Microspheres. Herein, the oil (O) represents
the total feeding monomers of VBA and St. It is found that stable
colloids can be synthesized when the O/W value is below 6/100
with pH = 7 and VBA/St = 10/100. Figure 5 shows the TEM
images of the resultant PS-co-PVBAH microspheres synthesized
with O/W ranging from 6/100 to 1.5/100 at pH = 7 and
VBA/St = 10/100. The TEM observation confirms uniform
cagelike porous microspheres can be fabricated within the O/W
range from 6/100 to 2/100 (Figure 5AꢀC). When O/W
decreases from 6/100 to 3/100 and further to 2/100, the average
size of the resultant cagelike porous microspheres decreases from
600 nm (Figure 5A) to 400 nm (Figure 5B) and further to
350 nm (Figure 5C). Besides, it seems that the macropores in the
cagelike porous microspheres synthesized at O/W = 6/100 are
larger than those synthesized at O/W = 3/100 and O/W = 2/100.
The formation of the large-sized macropores is possibly ascribed
to the fusion of the single surfactant micelles during the W/O/W
emulsion polymerization. Despite the different size of the
macropores, the BJH analysis indicates that these three cagelike
porous microspheres have the similar pore volume of
0.2ꢀ0.3 cm3/g. When O/W decreases to 1.5/100, coreꢀshell
porous microspheres (Figure 5D) similar to those shown in
Figure 4B are fabricated. The formation of the coreꢀshell porous
microspheres under low O/W value of 1.5/100 at pH = 7 is
similar to those under high O/W value of 3/100 at pH = 1ꢀ3,
since the percent of the VBAH unimer in aqueous phase is high
under the both synthesis cases.
3.4. Effect of the VBA/St Weight Ratio on Synthesis of the
Cagelike Porous Microspheres. Clearly, the VBA/St weight
ratio represents the percent of the surfactant monomer in the
total monomers, and high VBA/St value represents high surfac-
tant monomer percent in the resultant W/O/W emulsion.
Polymerization under different VBA/St value at pH = 7 with a
constant O/W = 3/100 value is checked. Figure 6 shows the
TEM images of the resultant colloids synthesized with the VBA/
St value ranging from 5/100 to 40/100. When VBA/St is 5/100,
uniform solid PS-co-PVBAH microspheres (Figure 6A and
Figure S3) without obvious porous structure are fabricated.
The average size of the solid PS-co-PVBAH microspheres is
300 nm. The BJH analysis shows that the pore volume of the
solid PS-co-PVBAH microspheres, 0.12 cm3/g, is little higher
than those of the solid PS microspheres (Figure S1) but is lower
than the PS-co-PVBAH cagelike porous microspheres synthe-
sized at high VBA/St value shown in Figure 1. When the VBA/St
value increases to 10/100 and further to 20/100, uniform cage-
like porous microspheres with the average size of 400 nm
(Figure 6B) and 500 nm (Figure 6C) are synthesized. The pore
volume values of these two PS-co-PVBAH cagelike porous
microspheres are estimated to be 0.22 and 0.30 cm3/g, which
are larger than the solid PS-co-PVBAH microspheres (Figure 6A)
as discussed above. Whereas, the latter PS-co-PVBAH cagelike
Figure 5. TEM images of the PS-co-PVBAH microspheres synthesized
with weight ratio O/W of 6/100 (A), 3/100 (B), 2/100 (C), and 1.5/
100 (D) at pH = 7 and VBA/St = 10/100.
coreꢀshell porous microspheres are similar to the general
coreꢀshell microspheres synthesized by one-stage soap-free
emulsion polymerization of the binary mixture of a hydrophobic
monomer and a hydrophilic monomer in aqueous phase.56
However, different form the general coreꢀshell microspheres,
macropores with size ranging from 10 to 30 nm are clearly
observed in the present coreꢀshell porous microspheres. The
reason for the formation of coreꢀshell porous microspheres at
pH = 1 is discussed. At pH = 1, the VBA monomer is fully
acidified, and it exists as both the unimer in aqueous phase and
the surfactant micelles of VBAH encapsulated within the emul-
sifying W/O/W droplets. During the polymerization, the emul-
sifying W/O/W droplet converts into the porous core and the
polymer resultant from the VBAH unimer deposits on the core to
form the shell, and therefore coreꢀshell porous microspheres as
shown in Figure 4A are fabricated. When the pH value increases
to 3, the balance between the unimer and the surfactant micelles
of VBAH changes to the latter, and therefore 250 nm coreꢀshell
porous microspheres (Figure 4B) with a much more thin shell
than those shown in Figure 4A (10 nm vs 40 nm) are produced.
When the pH value further increases to 7, most of the VBAH
surfactant monomer exists as surfactant micelles, and therefore
the cagelike porous microspheres as shown in Figure 4C are
synthesized. The present cagelike porous microspheres have
almost the same morphology as those shown in Figure 1, except
that the former has a slightly larger size (400 nm vs 350 nm).
These results possibly suggest that high-concentrated monomer
favors formation of large cagelike porous microspheres, which
will be further discussed subsequently. When the VBA monomer
is not acidified, it is insoluble in water but soluble in styrene, and
therefore the W/O emulsion forms through vigorous stirring.
After polymerization of the W/O emulsion, uniform solid micro-
spheres, the average size of which is 150 nm as shown in
Figure 4D, are fabricated. Interestingly, the TEM image
(Figure 4D) shows that the solid PS-co-PVBA microspheres
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Figure 6. TEM images of the PS-co-PVBAH microspheres synthesized
with weight ratio VBA/St of 5/100 (A), 10/100 (B), 20/100 (C) and
40/100 (D) at pH = 7 and O/W = 3/100.
Figure 7. TEM images of the 10-Pd@PS-co-PVBAH (A), 5-Pd@PS-
co-PVBAH (B), 2-Pd@PS-co-PVBAH (C), and 1-Pd@PS-co-PVBAH
(D) catalysts.
porous microspheres synthesized at VBA/St = 20/100 have a
larger average macropore size than the former ones synthesized
at VBA/St = 10/100 (50 nm vs 15 nm), which is also confirmed
by the TEM images shown in Figure 6B,C. These results suggest
that when the VBA/St value increases from 5/100 to 20/100,
colloids ranging from solid microspheres to cagelike porous
microspheres are fabricated and the average size of the macro-
pores increases with the increase in the VBA/St value. The
possible reason is discussed. Under low VBA/St value of 5/100,
most of the VBAH monomer exists as unimer in the emulsifying
droplets, and therefore solid microspheres as shown in Figure 6A
are fabricated. When the VBA/St value increases, the VBAH
surfactant micelles instead of the unimers are encapsulated
within the emulsifying W/O/W droplets, and therefore cagelike
porous microspheres are fabricated. However, when the VBA/St
value further increases to 40/100, the mixture of solid micro-
spheres and cagelike porous microsphere including macropores
as large as 200 nm (Figure 6D) are fabricated. Under a high
VBA/St value of 40/100, only part of the VBAH surfactant
monomer can be encapsulated within the emulsifying W/O/W
droplets which will be converted into cagelike porous micro-
spheres, and the residual VBAH surfactant monomer as well as
some of the hydrophobic monomer of St forms the emulsifying
W/O droplets which convert into solid microspheres after
polymerization. Thus, the mixture of solid microspheres and
cagelike porous microsphere as shown in Figure 6D are fabri-
cated at high VBA/St value of 40/100.
exchange between the quaternary ammonium moieties in the
PVBAH segment and the PdCl4 anions. Herein, the PS-co-
2ꢀ
PVBAH cagelike porous microspheres (Figure 1) synthesized at
pH = 7, O/W = 2/100, and VBA/St = 10/100 are used as typical
example to immobilize Pd nanoparticles. The present immobi-
lization of Pd nanoparticles on the PS-co-PVBAH cagelike
porous microspheres includes the ion exchange with the metal
precursor and the subsequent reduction with NaBH4 aqueous
solution, which is very similar to those on the scaffold of the
polymeric microspheres containing chelate ligands as discussed
elsewhere.34ꢀ36,57 By varying the weight ratio of PdCl2 to the PS-
co-PVBAH cagelike porous microspheres, four catalysts called
1-Pd@PS-co-PVBAH, 2-Pd@PS-co-PVBAH, 5-Pd@PS-co-
PVBAH, and 10-Pd@PS-co-PVBAH are fabricated. Figure 7
shows the TEM images of the four Pd catalysts. As indicated,
the resultant Pd nanoparticles are uniformly embedded in the
PS-co-PVBAH cagelike porous microspheres in all the four cases,
and almost no free Pd aggregates are observed even in the
10-Pd@PS-co-PVBAH catalyst (Figure 7A) with the weight ratio
of PdCl2 to the polymeric scaffold as high as 10%. However,
when the weight ratio of PdCl2 to the polymeric scaffold further
increases to 15%, the loading of Pd catalyst is beyond the
PS-co-PVBAH cagelike porous microspheres since black aggre-
gates can be optically observed during the reduction. The average
size of the immobilized Pd nanoparticles—2.1 nm for the
1-Pd@PS-co-PVBAH catalyst, 3.3 nm for the 2-Pd@
PS-co-PVBAH catalyst, 5.1 nm for the 5-Pd@PS-co-PVBAH
catalyst, and 5.7 nm for the 10-Pd@PS-co-PVBAH catalyst
(Figures S4 and S5)—increases with the increase in the Pd
content in the catalyst. The XPS analysis of the typical as-
prepared 10-Pd@PS-co-PVBAH catalyst confirms that the cata-
lytic metal exists as Pd nanoparticles (Figure S6).
3.5. Immobilization of Pd Nanoparticles within the Cage-
like Porous Microspheres. Usually, immobilization of noble
metal nanoparticles on the polymeric scaffold can be achieved
initially by either coordination between the pendent ligands with
the metal precursor or ion-exchange followed reduction with
suitable reducing reactants.27ꢀ29 Since the PS-co-PVBAH cage-
like porous microspheres contain the PVBAH segment, it is
expected that Pd catalyst can be immobilized through ion
The four catalysts including the 10-Pd@PS-co-PVBAH,
5-Pd@PS-co-PVBAH, 2-Pd@PS-co-PVBAH, and 1-Pd@
PS-co-PVBAH are further characterized by XRD, and the XRD
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Figure 9. Time-dependent yield of phenylamine in the hydrogenation
of nitrobenzene in water with the molar ratio C/S of 0.20% (b), 0.50%
(2), and 1.0% (9). Reaction conditions: nitrobenzene (0.06ꢀ0.30 g,
0.5ꢀ2.5 mmol), 10 mL of the aqueous dispersion of 10-Pd@PS-co-
PVBAH containing 5.0 ꢁ 10ꢀ3 mmol of Pd catalyst, 30 °C, H2
(bubbling at atmospheres pressure, 0.01 cm3/min).
Figure 8. XRD patterns of the 10-Pd@PS-co-PVBAH (A), 5-Pd@PS-co-
PVBAH (B), 2-Pd@PS-co-PVBAH (C), 1-Pd@PS-co-PVBAH catalyst (D),
and the scaffold of the PS-co-PVBAH cagelike porous microspheres (E).
patterns are shown in Figure 8. For the 10-Pd@PS-co-PVBAH
(Figure 8A) and 5-Pd@PS-co-PVBAH (Figure 8B) catalysts,
three peaks at 2θ of 40.0°, 46.6°, and 68.0° corresponding to the
diffraction of the (111), (200), and (220) lattice planes of the Pd
nanoparticles and the broad peak centered at 2θ of 19.0°
corresponding to the amorphous polymeric scaffold of the PS-
co-PVBAH cagelike porous microspheres can be clearly ob-
served. According to the Scherrer equation,58 the average size
of the Pd nanoparticles is also evaluated utilizing the diffraction
peak of (111) lattice plane, and the values, 7.6 nm for the 10-
Pd@PS-co-PVBAH catalyst and 6.4 nm for the 5-Pd@PS-
co-PVBAH catalyst, are slightly larger than those observed by
TEM. For the 2-Pd@PS-co-PVBAH catalyst, possibly ascribed to
the small size of the Pd nanoparticles and the low Pd content, the
diffraction according to the immobilized Pd nanoparticles is very
weak, and only a pale peak at 2θ of 40.0° is observed. When the
size of the Pd nanoparticles and the Pd content further decrease,
the 1-Pd@PS-co-PVBAH shows a XRD pattern (Figure 8D)
similar to the polymeric scaffold itself (Figure 8E), from which no
Pd diffraction but only the broad peak corresponding to the
polymeric scaffold is observed.
3.6. Catalyst Testing. Hydrogenation of nitrobenzene to
phenylamine is an industrially important transformation, since
the product of phenylamine and its derivatives are widely used.
The hydrogenation of nitrobenzene is generally achieved in
organic solvent employing a heterogeneous Pd catalyst.59ꢀ65
Various Pd catalysts including the Pd nanoparticles immobilized
on inorganic support such as silica60,61 and metal oxides59,62 and
polymeric support such as poly(3,6-dibenzaldimino-N-vinylcar-
bazole),63 poly(vinylidene fluoride),64 and polysiloxane65 have
been proposed for this transformation. It is deemed that the
activity of the Pd catalyst is dependent on the support and the
size of the Pd nanoparticles,59,62 and generally small Pd nano-
particles present high catalytic activity.62
segment, the hydrogenation is performed in water. Figure 9
shows the time-dependent yield of phenylamine in the hydro-
genation of nitrobenzene with different molar ratio of C/S. It
indicates that the hydrogenation of nitrobenzene can be com-
pleted in 40ꢀ150 min with the molar ratio C/S ranging from
1.0% to 0.20% and affords almost a quantitive yield of phenyla-
mine. From Figure 9, the turnover frequency (TOF) value of the
10-Pd@PS-co-PVBAH catalyst, 578 hꢀ1 under the molar ratio
C/S = 1.0%, 576 hꢀ1 under the molar ratio C/S = 0.50%, and
606 hꢀ1 under the molar ratio C/S = 0.20%, is calculated.
Compared with the reference catalyst of Pd nanoaprticles
immobilized on the solid PS microspheres (Figure S7), the
present 10-Pd@PS-co-PVBAH catalyst is much more efficient
(TOF 576 hꢀ1 vs 129 hꢀ1). Compared with the hydrogenation
of nitrobenzene catalyzed by the catalyst of Pd/gel nanocompo-
site under very similar conditions, the present hydrogenation
runs much more fast (98% yield of phenylamine in 150 min vs
>99% yield in 40 min),64 confirming the highly efficient
10-Pd@PS-co-PVBAH catalyst.
The Pd catalyst leaching into aqueous phase has been further
checked employing the hydrogenation of nitrobenzene in water
at 30 °C and with C/S = 0.5%. After the yield of phenylamine
reaches 65.7% in 20 min, the 10-Pd@PS-co-PVBAH catalyst is
initially isolated from the reaction mixture by filtration, then the
liquid phase containing the reactant of nitrobenzene is bubbled
with H2 at 30 °C for a given time, and then the aqueous phase is
analyzed by HPLC. As shown in Figure 10, when the 10-Pd@PS-
co-PVBAH catalyst is removed from the reaction mixture, no
further yield of phenylamine is observed in the next 70 min.
Furthermore, no Pd in the aqueous phase is detected by AAS
analysis (detection limit is 0.03 ppm). These results confirmed
that the Pd catalyst leaching in the present reaction condition can
be ignored. The much low Pd catalyst leaching into the aqueous
solution is discussed, and it is ascribed to two reasons. First, most
of the synthesized Pd nanoaprticles are encapsulated within the
matrix of the polymeric support of the PS-co-PVBAH cagelike
porous microspheres as shown in Scheme 2, which decreases the
catalyst leaching into aqueous solution. Second, in the present
Herein, the typical catalyst of 10-Pd@PS-co-PVBAH, which is
thought to be the most inefficient due to the largest size of the Pd
nanoparticles in all the four catalysts, is used to evaluate the
potential application of the PS-co-PVBAH cagelike porous
microspheres as catalyst scaffold. Besides, since the 10-Pd@PS-
co-PVBAH catalyst can be dispersed in water which is possibly
ascribed to the porous structure (low density) and the PVBAH
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Pd nanoparticles in the recycled catalyst is almost as the same as
those in the fresh one, confirming the reusability of the 10-Pd@PS-
co-PVBAH catalyst in the present conditions.
4. CONCLUSIONS
A strategy of one-stage synthesis of cagelike porous micro-
spheres containing macropores within the body through W/O/W
emulsion polymerization of a surfactant monomer of VBAH and
a hydrophobic monomer of St is proposed, and the application of
the cagelike porous microspheres as catalyst scaffold of Pd
nanoparticles is evaluated. The synthesis of cagelike porous
microspheres includes the construction of the W/O/W emulsion
in the presence of the VBAH surfactant monomer and the
hydrophobic monomer of St in water and the subsequent
polymerization initiated by the hydrophilic initiator of K2S2O8.
Ascribed to the VBAH surfactant monomer, formation of the
W/O/W emulsion avoids additional ionic/nonionic surfactant
and therefore affords a convenient one-stage synthesis of cagelike
porous microspheres. The parameters affecting the synthesis of
cagelike porous microspheres are explored. It is found that the
pH value of the aqueous phase, the oil content, and the percent of
the surfactant monomer in the W/O/W emulsion can greatly
affect the synthesis of cagelike porous microspheres. By tuning the
synthesis conditions, cagelike porous microspheres with size ranging
from 300 to 600 nm and pore volume as large as 0.31 cm3/g are
fabricated. The porous character and the inherent quaternary
ammonium moieties in the cagelike porous microspheres make
them to be the promising catalyst scaffold of Pd nanoparticles.
Initially through ion exchange followed reduction with NaBH4
aqueous solution, 2.1ꢀ5.7 nm Pd nanoparticles are immobilized on
the cagelike porous microspheres, and the loading of the Pd catalyst
on the cagelike porous microspheres is up to 10 wt %. The catalytic
performance of the prepared catalyst of Pd nanoparticles immobi-
lized on cagelike porous microspheres is tested by the catalytic
hydrogenation of nitrobenzene at 30 °C in water, and an efficient
and reusable heterogeneous Pd catalyst is demonstrated.
Figure 10. Time-dependent yield of phenylamine in the hydrogenation
of nitrobenzene with the molar ratio C/S = 0.5% (2) and after the
removal of the 10-Pd@PS-co-PVBAH catalyst (O). Reaction conditions
can be found in the caption for Figure 9.
Figure 11. TEM image of the recycled 10-Pd@PS-co-PVBAH catalyst.
’ ASSOCIATED CONTENT
catalyzed hydrogenation, the balance between Pd0 and Pd2þ
under bubbling H2 atmospheres favorably transfers to the former
and therefore the catalyst leaching into aqueous is blocked.
Lastly, the recycling of the typical 10-Pd@PS-co-PVBAH
catalyst is further studied. It is found that the 10-Pd@PS-co-
PVBAH catalyst prefers being dispersed in the aqueous phase
rather than in cyclohexane, while the substrate of nitrobenzene
and the product of phenylamine are soluble in cyclohexane. The
solubility difference of the Pd catalyst and the reactants makes an
easy separation of the 10-Pd@PS-co-PVBAH catalyst by cyclo-
hexane extraction and therefore affords convenient reuse of the
Pd catalyst. After the hydrogenation of nitrobenzene at 30 °C
with C/S = 0.5% is just completed in 90 min (seeing the
hydrogenation kinetics in Figure 9), the separated catalyst is
reused in the next run. It is found that that the yield of phenylamine
in 90 min keeps more or less the same (∼99%) even after four
cycles of hydrogenation (Figure S8). The recycled catalyst is further
characterized by TEM. As shown in Figure 11, the size and
morphology of the PS-co-PVBAH cagelike porous microspheres
in the recycled 10-Pd@PS-co-PVBAH catalyst remain as similar as
those of the fresh ones, suggesting the stability of the polymeric
scaffold. Furthermore, no clear Pd aggregation is observed in the
recycled 10-Pd@PS-co-PVBAH catalyst, and the average size of the
S
Supporting Information. Text as well as Figures S1ꢀS8
b
showing the TEM image of the solid PS microspheres, the FTIR
spectrum of the PS-co-PVBAH cagelike porous microspheres, the
magnified TEM image of the solid PS-co-PVBAH microspheres
synthesized with the weight ratio of VBA/St = 5/100, the TEM
image of Pd immobilized on solid PS microspheres, the XPS
spectrum of the 10-Pd@PS-co-PVBAH catalyst, the histogram of
size distribution of the Pd nanoparticles in the catalysts, and the
details on catalyst recycling. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: wqzhang@nankai.edu.cn. Tel: 86-22-23509794. Fax:
86-22-23503510.
’ ACKNOWLEDGMENT
The financial support by National Science Foundation of China
(Nos. 20974051 and 21074059) and Tianjin Natural Science
Foundation (No. 09JCYBJC02800) is gratefully acknowledged.
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