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Scientific RePORTS | 7:44178 | DOI: 10.1038/srep44178
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Hierarchical Porous Interlocked
Polymeric Microcapsules: Sulfonic
Acid Functionalization as Acid
Catalysts
Xiaomei Wang, Jinyan Gu, Lei Tian & Xu Zhang
Owing to their unique structural and surface properties, mesoporous microspheres are widely applied in
the catalytic eld. Generally, increasing the surface area of the specic active phase of the catalyst is a
good method, which can achieve a higher catalytic activity through the fabrication of the corresponding
catalytic microspheres with the smaller size and hollow structure. However, one of the major challenges
in the use of hollow microspheres (microcapsules) as catalysts is their chemical and structural stability.
Herein, the grape-like hypercrosslinked polystyrene hierarchical porous interlocked microcapsule
(HPIM-HCL-PS) is fabricated by SiO
2
colloidal crystals templates, whose structure is the combination
of open mouthed structure, mesoporous nanostructure and interlocked architecture. Numerous
microcapsules assembling together and forming the roughly grape-like microcapsule aggregates
can enhance the structural stability and recyclability of these microcapsules. After undergoing the
sulfonation, the sulfonated HPIM-HCL-PS is served as recyclable acid catalyst for condensation reaction
between benzaldehyde and ethylene glycol (TOF = 793 h
−1
), moreover, exhibits superior activity,
selectivity and recyclability.
As one important functional polymeric material, polymeric microcapsules have attracted considerable atten-
tion due to their unique physicochemical properties
1,2
. eir unique properties make them valuable for many
potential applications, such as drug delivery
3
, catalytic carrier
4
, controlled release
5
, adsorption and separation
6,7
and stimuli-responsive material
8
. Motivated by their promising prospects, great eorts have been devoted to the
fabrication of the polymeric microcapsules, such as template-assisted methods
9,10
, self-assembly of block copoly-
mer process
11,12
, interfacial mini-emulsion polymerization methods
13
and microuidic approach
14
. Among such
methods, the template-assisted methods are ecient to prepare polymeric microcapsules with well-dened size
and morphology as a result of the desirable stability and monodispersity of the templates
1,2
.
For eectively decreasing mass transfer resistance and greatly enhancing catalytic activity of polymeric micro-
capsules, the following critical structural/morphological characteristics should be taken into consideration: the
size and shape of the entire microcapsule; the porosity on the capsule wall; the thickness, structure/morphology,
and the stability of the capsule wall
15,16
. Compared with the former two issues which are primarily related to mass
transfer rate for the substances, the latter one is related to the mechanical stability and recyclability of the micro-
capsules. Nevertheless, in most cases, the enclosed microcapsules with the dense and thick capsule wall would
cause the high mass transfer resistance between the capsule lumen and the bulk solution, thus decreasing the
apparent catalytic activity
17,18
. As reported previously, the hierarchical porous nanostructure that contain open
mouthed (macroporous) structure and mesoporous nanostructure can eectively decrease mass transfer resist-
ance from the capsule wall, improve the ow rate of the solution in the capsule lumen, make fully use the exposed
outer and inner surfaces, thus greatly enhance catalytic activity
17,18
.
Compared with other kinds of porous microcapsules, porous polymeric microcapsules gradually become an
important and attractive class of porous materials owing to the unique properties of large surface areas and high
chemical stabilities
19
. As we all know, crosslinking of swollen chloromethylated polystyrene via Friedel-Cras
alkylation is a facile and popular method for formation of the porous structure
19,20
. Meanwhile, this hyper-
crosslinked method can eectively improve mechanical strength and solvent resistance of porous polymeric
Department of Polymer Science and Engineering, Hebei University of Technology, Tianjin 300130, P.R. China.
Correspondence and requests for materials should be addressed to X.Z. (email: xuzhang@hebut.edu.cn)
Received: 30 September 2016
accepted: 03 February 2017
Published: 16 March 2017
OPEN
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Scientific RePORTS | 7:44178 | DOI: 10.1038/srep44178
microcapsules
19
. For enhancing catalytic eciency, porous polymeric microcapsules with the ultrathin shell
are desirable
17
. However, the recyclability of these microcapsules represented a challenge in the eld of catalyst.
Introducing magnetic property is a proven and eective way to improve the cycling performance of polymeric
microcapsules. Unfortunately, the loss of the magnetic properties and poor recyclability still appear during the
repeated catalytic runs, which restricts the practical applications
21
. In order to solve this formidable challenge, the
interlocked polymeric microcapsules, which integrate numerous microcapsules into a targeted nanocomposite,
have attracted intensive attention owing to their excellent recyclability and mechanical stability
22,23
. As catalyst
supports, we can introduce open mouthed structure and interlocked architecture in the multifunctional poly-
meric microcapsules. is way may open a novel interdisciplinary area between catalysis and polymer science.
In this paper, we present a facile method to fabricate grape-like hypercrosslinked polystyrene hierarchical
porous interlocked microcapsule (HPIM-HCL-PS) by silica colloidal crystals templates (CCTs). eir structure
is the integration of open mouthed structure, hierarchical porous nanostructure and interlocked architecture.
e presence of interlocked architecture makes numerous microcapsules assemble together and form the roughly
grape-like microcapsule aggregates. is interlocked structure endows HPIM-HCL-PS with excellent mechanical
stability and recyclability. Hierarchical porous nanostructure contains open mouthed (macroporous) structure
and mesoporous nanostructure provided not only large specic surface area for high catalytic activity but also
highly developed hierarchical macro/mesoporosity for rapid mass transport. erefore, hierarchical porous pol-
ymeric microcapsules have enhanced properties compared with single-sized porous structure. Herein, as recycla-
ble acid catalysts, the sulfonated HPIM-HCL-PS (HPIM-HCL-SPS) for condensation reaction between ethylene
glycol and benzaldehydeas is illustrated as an example. The detailed fabrication process of HPIM-HCL-PS
has been illustrated in Fig.1. Linear polystyrene shells are rst grown onto the surface of the silica CCTs by
surface-initiated atom transfer radical polymerization (SI-ATRP), functionalized with chloromethyl groups
and then hypercrosslinked via Friedel-Cras alkylation. Aer the silica CCTs are removed, HPIM-HCL-PS is
achieved and followed by a sequential sulfonation resulting in sulfonic groups (HPIM-HCL-SPS). As we all know,
liquid acids (e.g., H
2
SO
4
, HF, and H
3
PO
4
) used as catalyst in the acid-catalyzed reactions will cause diculties in
product separation, equipment corrosion and environmental pollution, thus, environmentally recoverable solid
acid catalysts are highly desirable. e HPIM-HCL-SPS is served as recyclable acid catalyst for condensation
reaction between ethylene glycol and benzaldehyde, moreover, exhibits high catalytic activity.
Results and Discussion
e SiO
2
microsphere with an approximate diameter size of 550 nm that constructed SiO
2
opals (the sacricial
template) was rstly prepared through Stöber method
24
. e polystyrene ATRP from the silica particles exhib-
ited the characteristics of a controlled/“living” polymerization, had a narrow molecular weight distribution
(M
w
/M
n
< 1.36), the number average molecular weight is 6.33 × 10
4
. e distribution plot was shown in FigureS1
(SupportingInformation). e SEM photograph of SiO
2
opals was shown in the Fig.2a. It is observed that SiO
2
opals had a hexagonal close-packed structure, which indicated that the template microspheres had a very uniform
shape and exhibited the monodisperse property (Fig.2a). Subsequently, the Br-containing SI-ATRP initiation
sites were introduced on the surface of the SiO
2
microspheres by BITS
22
. e energy-disperse X-ray (EDX) pat-
tern was shown in the FigureS2 (SupportingInformation). e weak but evenly distributed signals from C and N
element demonstrated that the silica opal was modied by the BITS. is result further conrmed the existence of
ATRP initiation sites on the surface of SiO
2
microspheres. e SI-ATRP graing of linear polystyrene (LPS) from
the SiO
2
-Br microspheres was carried out to form SiO
2
@LPS composite opals. In the cross-sectional SEM image of
the SiO
2
@LPS composite opals (Fig.2b), we can clearly observe the core/shell structure. e content of PS graed
layer aer chloromethylation is about 32% according to the TGA curve (FigureS3, SupportingInformation).
Moreover, in the Fig.2b, it is also denitely found that there exist several clear pits on the LPS shell, marked by
the red arrow. At the contacting sites, few BITS modied the silica microspheres led to few LPS forming. Aer
hypercrosslinking reaction, the LPS shell became coarse and showed some shrinkage owing to the crosslinking of
chloromethylated LPS molecular chains (Fig.2c). e SEM and STEM images of HPIM-HCL-PS were shown in
the Fig.2d and e, respectively. e ultrathin shell of microcapsules was around 60 nm. ese microcapsules were
interconnected with each other via the open mouths and presented a grape-like interlocked architecture. During
the Friedel-Cras reaction, the SiO
2
@LPS composite opal structure (numerous microspheres contacted with
Figure 1. Schematic illustration of preparation process for HPIM-HCL-PS.
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Scientific RePORTS | 7:44178 | DOI: 10.1038/srep44178
each other) facilitates the interparticle cross-linking, which makes the nal microcapsules interlock with each
other by covalent bond and form the interlocked architecture (Fig.2f)
9
. In principle, each microcapsule should
contain twelve open mouths, since every template microsphere is in interconnected with twelve neighboring
microspheres in hexagonal close-packing (Fig.2f)
25,26
. Furthermore, the pre-existing open mouths can facilitate
the template removal compared to the traditional microcapsules prepared by hard templating method.
For the hypercrosslinked porous polymer, Friedel-Cras reaction is an excellent method to form mesopores
without adding pore forming agents
20
. e permanent porosity in hypercrosslinked porous polymer is a result of
extensive crosslinking reactions. is way can eectively prevent the polymer chains from collapsing into a dense,
nonporous state
19
. e mesostructure of the HPIM-HCL-PS formed via hypercrosslinking the chloromethylated
LPS was conrmed by nitrogen adsorption measurements (FigureS4, SupportingInformation). e adsorption
and desorption isotherms of the obtained HPIM-HCL-PS show a type II isotherm, indicating the presence of
mesopores. However, the adsorption and desorption branches of HPIM-HCL-PS did not close completely in the
region of low relative pressure, presumably due to a typical nature of these mesopolymers
27–29
.
e chemical compositions of the corresponding products were also characterized by FT-IR (Fig.3). In
Fig.3a–c, it can be found that the 1100 cm
−1
corresponding to a Si-O-Si stretch. Aer graing of linear polysty-
rene from the SiO
2
microspheres, peaks arising from the C-H vibrating signals of a mono-substituted benzene
at 698 and 758 cm
−1
were observed (Fig.3b). In the spectrum of the chloromethylated SiO
2
@LPS compos-
ites (Fig.3c), the characteristic bands at 675 cm
−1
was assigned to the derived chloromethyl groups (-CH
2
Cl),
indicating that the chloromethylation reaction successfully occurred in the LPS shells
25
. e FT-IR spectra of
HPIM-HCL-PS indicated that almost all the silica templates were removed, as indicated by the disappearance of
the characteristic peak at 1100 cm
−1
(Fig.3d).
Figure 2. (a) SEM image of SiO
2
opals and average size distri-bution of SiO
2
microspheres (inset); (b) SEM
image of SiO
2
@LPS composites; (c) SEM image of SiO
2
@HCL-PS composites; (d) SEM (1.00 kV) image and
TEM image (inset) of HPIM-HCL-PS; (e) STEM (8.00 kV) image of HPIM-HCL-PS; (f) schematic illustration
of preparation process of the HPIM-HCL-PS. All the scale bars are 1 µ m.
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Scientific RePORTS | 7:44178 | DOI: 10.1038/srep44178
Mesoporous microspheres have attracted much attention in the eld of catalysis due to their unique structural
and surface properties. In catalytic reactions, two key factors for the catalytic activity are mass transfer rate as
well as utilization ratio of active sites. Compared to the traditional solid mesoporous microspheres, theoretically,
the mesoporous microcapsules with smaller size can eectively shorten the transmission path, improve the mass
transfer rate and increase the utilization ratio of active sites. erefore, we propose a model that the solid mes-
oporous microspheres can be peeled o layer by layer to form numerous smaller microcapsules with ultrathin
shell. ese microcapsules can provide shorter transmission path, and eectively raise the utilization ratio of
active sites (Fig.4). Meanwhile, preparation of the open-mouthed microcapsules with ultrathin capsule wall is
a good choice to further decrease the mass transfer resistance between the capsule lumen and the bulk solution,
improve the ow rate of the solution in the capsule lumen. However, these microcapsules are dicult to recycle
and reuse, resulting in low catalytic performance due to their weakly chemical and structural stability. Herein,
numerous mesoporous microcapsules with ultrathin shell and open mouths were integrated into a targeted nano-
composite (HPIM). In this case, on the premise of guaranteeing the recyclability and mechanical stability, the
catalytic activities of these microcapsules were still improved considerably.
To further investigate the relationship between catalytic properties and structural characteristics, the activity
of the HPIM-HCL-SPS as the recyclable acid catalysts is evaluated using the condensation reaction of benzalde-
hyde and ethylene glycol (Table1). In the HPIM-HCL-SPS spectrum (FigureS5, SupportingInformation), the
characteristic bands at 1126, 1178 and 1220 cm
−1
are assigned to the derived sulfonic acid groups (− SO
3
H)
30
.
e number of immobilized protons on the HPIM-HCL-SPS is 1.45 mmol/g based on acid-base titration. is
H
+
content is lower than other samples in the literature
21,27
, which likely owe to the hypercrosslinking reaction
that leads to the sulfonation reaction sites reducing sharply. As shown in Table1, the conversion of benzaldehyde
is around 70% with a high selectivity (98%) toward 2-phenyl-1,3-dioxolane. is conversion is higher than the
Figure 3. FT-IR spectra of (a) SiO
2
opals, (b) SiO
2
@LPS composite opals, (c) chloromethylated SiO
2
@LPS
composite opals, and (d) HPIM-HCL-PS.
Figure 4. Schematic illustration of HIPM structure.
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Scientific RePORTS | 7:44178 | DOI: 10.1038/srep44178
other catalyst for this condensation reaction
21
. Moreover, on the basis of the number of immobilized protons on
the HPIM-HCL-SPS, the average turnover frequency (TOF) was about 793 h
−1
when the conversion reached 70%.
is result is also higher than the literature values for the same model catalysis (Table2)
21,27
. For a fair compar-
ison, the TOFs were calculated based on the H
+
contents in the system (the same is done for literature values).
e outstanding catalytic performance of HPIM-HCL-SPS was attributed to their ultrathin shell and hierarchical
porous structure. e open-mouthed structure can eectively improve the mass transfer rate of the capsule lumen
and made full use of the exposed outer and inner surfaces. e ultrathin shell greatly shortened the transmis-
sion path and increased the utilization ratio of active sites for catalysis reactions. erefore, the HPIM-HCL-SPS
exhibited higher activity in the same reaction time (1 h) though the H
+
content (1.45 mmol/g) is lower than other
samples in the literatures.
Another key parameter to evaluate the catalytic performance is recyclability and mechanical stability of the
HPIM-HCL-SPS. Aer ve successive cycles, the HPIM-HCL-SPS was still stable and active, with a conver-
sion of 63.5%. Aer reaction, the HPIM-HCL-SPS was collected and characterized again with SEM to check
their structural stability. As shown in the SEM image (FigureS6, SupportingInformation), aer ve cycles of
catalytic tests, the HPIM-HCL-SPS maintained the roughly grape-like morphology and the mesoporous struc-
ture hardly changed (FigureS7, SupportingInformation). Furthermore, aer the h cycle, acid-base titration
demonstrateed the protons content of HPIM-HCL-SPS (1.2 mmol/g) is still suciently high concentration to
catalyze the condensation reaction. e above results can be attributed to the existence of interlocked architec-
ture. Interlocked architecture makes numerous microcapsules assemble together, forms the roughly grape-like
microcapsule aggregates and thus greatly improves the recyclability and mechanical stability. Moreover, com-
pared to the traditional organic polymeric microcapsules without crosslinking, the highly crosslinked nature of
the HPIM-HCL-PS confers them higher chemical and thermal stability.
In summary, the grape-like HPIM-HCL-PS was fabricated by colloidal crystals templates method, whose
structure is the integration of open mouthed structure, mesoporous nanostructure and interlocked architec-
ture. Attributed to the structural eect, HPIM-HCL-SPS has exhibited signicantly enhanced catalytic activity
and recyclability in the condensation reaction of benzaldehyde and ethylene glycol with superior activity and
selectivity. Beneting from the merits of easy functionalization due to the existence of abundant benzene rings,
high utilization ratio of active sites and rapid mass transport, excellent chemical stability and recyclability, the
HPIM-HCL-PS has tremendous application potential in the catalytic eld.
Methods
Materials. Chloromethyl ether (CME, Henan Wanxiang Chemical-technical Factory, chlorine content 41%)
was dried by calcium chloride. Monodisperse silica microsphere that constructed colloidal crystal template (SiO
2
-
CCT) was synthesized according to Stöber method
24
. [3-(2-Bromoisobutyryl) propyl]-trimethoxysilane (BITS)
was synthesized according to Yang et al.
23
. Styrene (St), dimethyl Formamide (DMF) were distilled under reduced
pressure, then stored under argon at − 10 °C. N, N, N′ , N″ , N″ -pentamethyldiethylenetriamine (PMDETA, 98%),
benzaldehyde and 1,2-dichloroethane (DCE) were purchased from Aladdin Industrial Corporation. All other
reagents were used as received.
Preparation of Silica@Linear Polystyrene (SiO
2
@LPS) Composite Opals. Monodisperse disper-
sions of silica microspheres were centrifuged at 1000 rpm for 13 h to form the silica opal, then allowed to air-dry
25
.
Aer sintering at 500 °C for 3 h to enhance the interconnectivity between the microspheres, the dried silica opals
were heated at 60 °C in the mixture of ethanol, ammonia and deionized water. en, an excess of BITS was added,
and the mixture was kept to reux for 24 h. Finally, the modied silica opals were washed with ethanol to remove
the residual BITS and dried under vacuum
23
.
Catalyst Cycle
Benzaldehyde
Conversion, %
2-phenyl-1,3,dioxane
selectivity, % Side products, %
Blank — 35.6 96.2 1.3
HPIM-HCL-SPS
1 62.9 99.2 0.5
2 69.1 98.8 0.8
3 60.8 98.5 0.9
4 76.7 99.1 0.6
5 63.5 99.0 0.6
Table 1. Conversion of benzaldehyde catalyzed by HPIM-HCL-SPS.
Catalyst
TOF
(h
−1
)
S
BET
a
[m
2
/g]
V
p
b
[cm
3
/g]
D
p
c
[nm]
HPIM-HCL-SPS 793 604 0.27 3.0
FUD-14-SO
3
H
27
548 539 0.34 3.2
Fe
3
O
4
@DVB-2-H
21
433 45 — —
Table 2. Comparison of the turnover frequency (TOF) in the condensation reaction of benzaldehyde and
ethylene glycol as reported in the literature.