Hollow and sulfonated microporous organic polymers: Versatile platforms for non-covalent fixation of molecular photocatalysts

Article abstract of DOI:10.1039/c5ra06633f

Sulfonated hollow microporous organic polymers (S-HMOP) were prepared by template synthesis and post synthetic approach. The S-HMOP showed excellent non-covalent fixation ability towards various cationic dyes through ionic interaction. Among various dye systems, Zn-porphyrin loaded S-HMOP showed promising activity and stability in the decomposition of 4-chlorophenol under visible light irradiation.

Full text of DOI:10.1039/c5ra06633f

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Hollow and sulfonated microporous organic
polymers: versatile platforms for non-covalent
Cite this: RSC Adv., 2015, 5, 47270
fixation of molecular photocatalysts†
Received 13th April 2015
Accepted 20th May 2015
a
a
a
a
a
Nojin Park, Daye Kang, Min Cheol Ahn, Sungah Kang, Sang Moon Lee,
a
b
a
a
Tae Kyu Ahn, Jae Yun Jaung, Hee-Won Shin* and Seung Uk Son*
DOI: 10.1039/c5ra06633f
www.rsc.org/advances
Sulfonated hollow microporous organic polymers (S-HMOP) were ionic interaction is a very efficient strategy for various hetero-
5
prepared by template synthesis and post synthetic approach. The S- geneous catalytic systems.
HMOP showed excellent non-covalent fixation ability towards
Recently, various microporous organic polymers (MOPs)
6
various cationic dyes through ionic interaction. Among various dye have been prepared via coupling reactions of building blocks.
systems, Zn-porphyrin loaded S-HMOP showed promising activity Due to their high surface area and robustness, these materials
and stability in the decomposition of 4-chlorophenol under visible have been applied to various purposes such as adsorbents and
7
8
light irradiation.
catalysts. In addition, MOP materials with photocatalytic
9
activities have been studied. However, the studies including
those from our research group were based on limited dye
species. Usually, photoactive species were introduced to MOP
materials by coupling of pre-designed dye building blocks. As
far as we are aware, a MOP based heterogenization strategy
applicable to various dye systems was not reported. Moreover,
MOP materials usually have hydrophobic properties due to their
organic characteristics, which can limit the application to
aqueous systems. However, it has been veried that the post
synthetic approach for MOP materials can change their chem-
For environmental reasons, the removal of organic pollutants
from aqueous solution has attracted great attention from
scientists. Among the various removal strategies, photochem-
1
ical transformation based on visible light absorbing dyes is an
2
attractive method that addresses energy concerns. Various
organic dyes and metal complexes have been studied for this
purpose. As the reports accumulated, the dyes used in the
3
homogeneous systems were reviewed recently by Miranda et al.
As mentioned in the conclusion of the review paper, one of the
future issues requiring further efforts is the heterogenization of
systems. Recently, our research group has sought new hetero-
10
ical properties.
Our research group has studied functional MOP materials
11
based on Sonogashira coupling. Recently, we have shown that
4
genization strategy for the photocatalysts.
the outer shape of MOP materials can be engineered using
Through the structural analysis of known molecular photo-
catalysts used for organic pollutant removal, we gured out that
although dyes have structural diversity, a signicant number of
them have ionic characteristics due to water-solubility. Thus, we
thought that the heterogenization strategy should be applicable
to various dyes and ionic interaction based xation is consid-
erable. It has been reported that non-covalent xation based on
12
various templates. In this work, we report the synthesis of
water dispersible hollow MOP materials via post-modication,
the non-covalent xation of various dyes, and their photo-
catalytic performance in the visible light-driven decomposition
of 4-chlorophenol. Fig. 1 shows the heterogenization strategy
for ionic dyes.
Hollow MOP (HMOP) was prepared using tetrakis(4-
ethynylphenyl)methane and 1,4-diiodobenzene as building
12
blocks for Sonogashira coupling. Silica spheres (83 ꢀ 4 nm
a
Department of Chemistry, Department of Energy Science, Sungkyunkwan University, diameter) were used as templates. For the introduction of the
Suwon 440-746, Korea. E-mail: sson@skku.edu; hwshin91@gamil.com; Fax: +82-
anchoring group, the HMOP was post-modied by sulfonation
0
31-290-4572
13
with chlorosulfonic acid. The sulfonated HMOP (S-HMOP)
materials were characterized by various methods. First,
according to the transmission electron microscopy (TEM), the
b
Department of Organic and Nanoengineering, Hanyang University, Seoul 133-791,
Korea
†
Electronic supplementary information (ESI) available: Experimental details, low
magnication TEM images, PXRD patterns, N
sorption isotherms of materials, materials showed a hollow structure with very thin shell thick-
2
singlet oxygen detection in the photocatalytic reaction by ZnTPP loaded ness of 8.9 ꢀ 0.6 nm. There was no difference in the outer shape
S-HMOP, photocatalytic activity of ZnTPP loaded nonhollow S-MOP, and
of HMOP and S-HMOP (Fig. 2a and S1 in the ESI†). According to
analysis of the recovered ZnTPP/S-HMOP. See DOI: 10.1039/c5ra06633f
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Fig. 1 Synthesis of the sulfonated hollow microporous organic poly-
mer (S-HMOP) and the heterogenization of ionic dyes.
2
ꢁ1
ꢁ1
N
2
isotherm analysis, S-HMOP materials showed a 529 m g
2
surface area and microporosity, compared with the 733 m g
surface area of HMOP (Fig. 2i). According to powder X-ray
diffraction studies, the hollow materials in this work were all
6–12
amorphous (Fig. S2 in the ESI†), as reported in the literature.
The chemical properties of the S-HMOP were characterized
by infrared (IR) absorption and solid phase C nuclear
Fig. 2 TEM images of S-MOP (a) and dye loaded S-MOP (b–h). N
2
isotherm curves (i) at 77 K, pore size distribution diagrams (inset), IR
1
3
13
spectra (j), and (k) solid phase C NMR spectra of HMOP, S-HMOP and
magnetic resonance (NMR) spectroscopy. As shown in Fig. 2j ZnTPP loaded S-HMOP.
and S3 in the ESI,† the vibration peaks at 3500, 1182, 1040, and
ꢁ
1
14
6
52 cm were attributed to sulfonic acid groups. The solid
1
3
phase C NMR spectrum of S-HMOP showed change of (Table 1 and Fig. S4 in the ESI†). Fig. 3b shows the photographs
1
3
15
aromatic C peaks, compared with that of HMOP, supporting and absorption spectra of dye-loaded S-HMOP, showing visible
that sulfonic acid was introduced at phenyl rings (Fig. 2k). light absorption properties. The dye loading was in a range of
ꢁ1
According to the elemental analysis for sulfur contents 93–423 mmol g , which was calculated by elemental analysis for
(
5.66 w%), the amount of sulfonic acid groups in S-HMOP was the N contents in the materials. Compared with the loading
ꢁ
1
ꢁ1
calculated as 1.76 mmol g
.
amount (0.48–67 mmol g ) of dyes in non-porous polymer or
17
The xation ability of the S-HMOP for the dyes was investi- silica in the literature, the dye-loaded S-HMOP showed a very
gated. We discovered that the S-HMOP can trap cationic dyes efficient xation due to the porosity, hollow structure and thin
from the aqueous solution with vivid color changes through dye shell of supports. The loading amount of dyes in S-HMOP

xation. The loaded cationic dyes could not be removed by increased with a decrease in dye size.
sonication or washing with water and organic solvents. Anionic As described in the introduction section, the dyes in this
dyes such as eosin Y (sodium salt) were not loaded on the work have been used as photocatalysts in homogeneous
3
S-HMOP. When the HMOP was used instead of the S-HMOP, the systems. The introduction of sulfonic acid and dyes to HMOP
dyes were removed aer washing. These observations indicate resulted in the hydrophilic nature of materials (Fig. S5 in the
that the dye loading on the S-HMOP is based on ionic interac- ESI†). Thus, we tested the dye-loaded S-HMOP materials as a
16
tions between sulfonate and cationic dyes.
heterogeneous system for 4-chlorophenol decomposition in
Fig. 3a shows the cationic dyes used in this study. The water under visible light irradiation. Fig. 4 summarizes the
analysis of dye-loaded S-HMOP by TEM showed the complete results.
maintenance of hollow morphology (Fig. 2b–h). The surface area
was changed from 529 m g to 233–472 m g by dye loading tocatalytic decomposition of 4-chlorophenol. Without the dye
The 5 mol% of dyes in the S-HMOP was used for the pho-
2
ꢁ1
2
ꢁ1
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Fig. 4 (a) Photocatalytic and (b) recycle performance (4 h for each
run) for the decomposition of 4-chlorophenol by dye-loaded
S-HMOP. (c) Photocatalytic process. [4-Chlorophenol]
o
¼ 1 mM
(
(
25 mL), dye: 5 mol% of 4-chlorophenol, pHinitial ¼ 5.0, 200 W Xe lamp
ꢁ2
4.6 mW cm
intensity) with a cutoff filter (>420 nm), room
temperature. For the recycle tests, 5 mol% TPP, 5 mol% ZnTPP and
0 mol% RhB on S-HMOP were used.
1
Fig. 3 (a) Dyes used in this study. (b) Photographs and absorption
spectra of S-HMOP and dyes-loaded S-HMOP.
order of activities was methylene blue (MB) < Cu-tetrapyridium
ꢁ
1
porphyrin (CuTPP) < acridine yellow G (AYG, k ¼ 0.16 h ) <
obs
Table 1 Properties of materials in this work
ꢁ1
2+
rhodamine B (RhB, kobs ¼ 0.18 h ) < Ru(bpy)3 (RuBPy) <
ꢁ1
a
b
Zn-tetrapyridium porphyrin (ZnTPP, kobs ¼ 0.44 h ) < tetra-
ꢁ1
18
Surface area
Pore volume
Dye loading
pyridium porphyrin (TPP, kobs ¼ 0.71 h )-loaded S-HMOP.
2
ꢁ1
3
ꢁ1
ꢁ1
1
Material
(m g
)
(cm g
)
(mmol g
)
2
This order can be rationalized by the O generation ability of
the dyes in the systems. For the organic dyes and porphyrins, a
similar trend of photocatalytic activities was observed. In the
HMOP
S-HMOP
733
529
443
233
326
363
411
456
472
0.72
0.54
0.42
0.39
0.43
0.41
0.54
0.50
0.52
—
—
19
literature, porphyrin dyes have been xed on silica for the
RhB-S-HMOP
MB-S-HMOP
AYG-S-HMOP
RuBPy-S-HMOP
TPP-S-HMOP
CuTPP-S-HMOP
ZnTPP-S-HMOP
178
423
282
181
147
110
93
17a
development of heterogeneous photocatalytic systems. The
system showed ꢂ85% conversion of 4-chlorophenol aer 4 h
17a
using 50 mol% porphyrin dyes and 450 W Xe lamp (>420 nm).
In comparison, the ZnTPP-loaded S-HMOP showed superior
performance, with 86% decomposition of 4-chlorophenol aer
4
h using 5 mol% dyes and 200 W Xe lamp (>420 nm). The
a
b
Values at P/P
o
¼ 0.97. The amount of dye loading is based on the
enhanced activity of ZnTPP-loaded S-HMOP can be attributed to
the porosity and very thin (ꢂ9 nm shell thickness) hollow
structure of the materials, which can maximize the performance
elemental analysis for N contents in materials.
20
of the active dye species.
materials, 4-chlorophenol was not decomposed under visible
light irradiation (Fig. 4a). The dye-loaded S-HMOP materials
showed a dye-dependent performance. As shown in Fig. 4a, the
It has been well recognized that most organic dyes decom-
3
pose gradually in the photocatalytic process by self-oxidation.
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As shown in Fig. 4b, the organic dyes on the S-HMOP such as
RhB and TPP decomposed gradually in successive runs. Even in
these cases, according to the UV/visible absorption spectros-
copy of solution, the colored materials were not etched to
solution. The ZnTPP on the S-HMOP showed signicant main-
tenance of activities during three runs, which is attributable to
the enhanced stability of porphyrin against self-oxidation
5 J. M. Fraile, J. I. Garc ´ı a and J. A. Mayoral, Chem. Rev., 2009,
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21
through metallation.
The decomposition process of 4-chlorophenol by homoge-
1
neous photocatalysts has been suggested to be based on O
2
17c,22
mediated oxidative dechlorination (Fig. 4c).
In the photo-
catalytic performance of ZnTPP on the S-HMOP, the generation
1
of
2
O species and benzoquinone were directly detected by
1
emission (emission of O
2
at 1268 nm, Fig. S5 in the ESI†) and
UV/visible absorption spectroscopy (absorption of benzoqui-
none at 245 nm), respectively, suggesting that the photocatalytic
decomposition in this work follows the suggested process in the
8 Selected examples: (a) K. Thiel, R. Zehbe, J. Roeser,
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17c
literature.
In conclusion, this work shows that MOP chemistry can be
applied for the heterogenization of photocatalysts. The MOP
materials were engineered as hollow spheres using a silica
template. Sulfonated HMOP obtained by post synthetic modi-

cation showed excellent performance as a platform for the
heterogenization of photocatalysts. Among the various dyes
tested, Zn-porphyrin based heterogeneous system showed
promising activity and stability. We believe that more various
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Acknowledgements
49, 2353.
This
work
was
supported
by
grants
NRF-2012- 11 (a) H. C. Cho, H. S. Lee, J. Chun, S. M. Lee, H. J. Kim and
R1A2A2A01045064 (Midcareer Researcher Program) through
the National Research Foundation of Korea and by grants no.
S. U. Son, Chem. Commun., 2011, 47, 917; (b) J. Chun,
J. H. Park, J. Kim, S. M. Lee, H. J. Kim and S. U. Son,
Chem. Mater., 2012, 24, 3458; (c) H. S. Lee, J. Choi, J. Jin,
J. Chun, S. M. Lee, H. J. Kim and S. U. Son, Chem.
Commun., 2012, 48, 94; (d) N. Kang, J. H. Park, J. Choi,
J. Jin, J. Chun, I. G. Jung, J. Jeong, J.-G. Park, S. M. Lee,
H. J. Kim and S. U. Son, Angew. Chem., Int. Ed., 2012, 51,
6626; (e) J. Chun, S. Kang, N. Kang, S. M. Lee, H. J. Kim
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10047756 (Technology Innovation Program) funded by the
Ministry of Trade, Industry & Energy of Korea.
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1
3
1
prepared (See the Experimental Sections in the ESI†). The
ZnTPP/S-nonhollow-MOP showed less loading (42 mmmol/g
ZnTPP) of ZnTPP and lower photocatalytic activiy than
ZnTPP/S-HMOP (Fig. S7 in the ESI†).
1
1
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2
ꢁ1
2
ꢁ1
the materials decreased from 472 m
g
to 310 m g ,
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decomposition of 4-chlorophenol (Fig. S8 in the ESI†).
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¼ ꢁkobst.
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