Porous heterogeneous organic photocatalyst prepared by HIPE polymerization for oxidation of sulfides under visible light

Article abstract of DOI:10.1039/c2jm32778c

A porous heterogeneous photocatalyst was prepared by high internal phase emulsion (HIPE) polymerization. Such porous materials have interconnected pores and enough active moieties for photocatalysis. The material demonstrated a very high catalytic efficiency and can be reused for photocatalyzed oxidation of thioanisole under visible light.

Full text of DOI:10.1039/c2jm32778c

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Porous heterogeneous organic photocatalyst prepared by HIPE
polymerization for oxidation of sulfides under visible light†
Wenliang Li,abc Wenjing Zhang,ad Xiaoqing Dong,ab Lesan Yan,ab Ruogu Qi,ab Weicai Wang,a Zhigang Xiea
a
*
and Xiabin Jing
Received 3rd May 2012, Accepted 13th July 2012
DOI: 10.1039/c2jm32778c
catalyst for further use. For this purpose, many inorganic materials
have been used to support photocatalysts, such as silica gels,2,5a
sepiolites,5b zeolites,5c carbon nanotubes5d and mesoporous titanium
dioxide.5e One of the main drawbacks of inorganic supports is that
photocatalysts are usually loaded by physical adsorption, so it is
difficult to control their contents and to obtain high enough content
of them. HIPE polymerization is a relatively new method for the
production of high-porosity materials. The tunable architecture of
these porous materials makes them attractive candidates for catalyst
supports.6 However, to the best of our knowledge, there is no report
on photocatalyst supports prepared by HIPE polymerization.
On the basis of our previous study on functionalized porous
materials by HIPE polymerization,7 we report here the preparation
and application of a BODIPY-functionalized porous heterogeneous
photocatalyst.
In this study, 1-(azidomethyl)-4-vinylbenzene (VBA)8 and BDP-
Yn9 were synthesized as previously described. BDP-St was synthe-
sized by ‘‘click’’ reaction between VBA and BDP-Yn (Scheme 1).
BDP-St is a pink powder. A solution of BDP-St in methanol
exhibits a bright green fluorescence emission. The optical properties
of BDP-St in methanol are presented in Table 1 and Fig. S1 (in ESI†).
It is evident that BDP-St possesses extremely sharp absorption and
fluorescence spectral bands (full width at half-maximum height
(fwhm) ¼ 24 nm), and an outstanding bright fluorescence emission
(F ¼ 0.72) at 509 nm. The BDP-St dissolved in MeOH retained a
strong fluorescence emission after a 2-month storage under ambient
conditions with daylight exposure. These excellent optical properties
indicate the usefulness of BDP-St for photocatalysis.
A porous heterogeneous photocatalyst was prepared by high internal
phase emulsion (HIPE) polymerization. Such porous materials have
interconnected pores and enough active moieties for photocatalysis.
The material demonstrated a very high catalytic efficiency and can
be reused for photocatalyzed oxidation of thioanisole under visible
light.
The development of organic photocatalysts driven by visible light is
gaining increasing interest from synthetic chemists because of their
advantages over semiconductor or organometallic photocatalysts,
e.g., no noble or toxic metals are needed, and larger absorption in the
visible light region. Many recent studies have been reported on the
application of metal-free organic dyes as photocatalysts.1 However,
one of the major problems associated with the use of organic
photocatalysts is that they are less robust than semiconductor and
organometallic photocatalysts, as they always suffer from photo-
bleaching or solvolytic attack in the reaction medium.2 Thus, an
organic photocatalyst with good photostability is highly desirable.
Boron-dipyrromethene (BODIPY or BDP) compounds have been
recognized as some of the most outstanding dyes due to their excel-
lent characteristics, such as higher fluorescence quantum yields, larger
molar absorption coefficients, and especially their excellent thermal
and photochemical stability in both solution and the solid state.3 We
have recently focused on BODIPY-based photocatalysts and proved
that BODIPYs are an alternative organic photocatalyst.4
Heterogeneous catalysts can be easily removed after the reaction,
so it is possible to run in a continuous mode and to recycle the
aState Key Laboratory of Polymer Physics and Chemistry, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun 130022, P. R. China. E-mail: xbjing@ciac.jl.cn; Fax: +86-
431-85262775; Tel: +86-431-85262775
bGraduate University of Chinese Academy of Sciences, Beijing 100049,
P. R. China
cJilin Medical College, Jilin 132013, P. R. China
dSchool of Chemistry and Environmental Engineering, Changchun
University of Science and Technology, Changchun 130022, P. R. China
† Electronic supplementary information (ESI) available: synthesis
procedure of BDP-St and PHP, 1H NMR, 13C NMR and MS spectra
of BDP-St (Fig. S3–S5), measurement of the optical properties of
BDP-St and PHP, procedure for photocatalysis, 1H NMR of
photocatalysis reaction mixtures (Fig. S6–S10). See DOI:
10.1039/c2jm32778c
Scheme 1 Synthesis of BDP-St.
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J. Mater. Chem., 2012, 22, 17445–17448 | 17445

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Table 1 Spectroscopic data of BDP-St measured in methanol at 298 K
fwhm
labs
(nm)
lemi
(nm)
3
nm
cmꢀ1
477
(Mꢀ1 cmꢀ1
)
Fa
497
509
24
is
65 000 ꢂ 2860
0.72 ꢂ 0.03
a
Reference
dye
1,3,5,7-tetramethyl-8-phenyl-4,4-difluoro-bora-
diazaindacene (see Chart S1, ESI).
Fig. 1 (a) Photograph of PHP5 in the dry solid state under day light. (b)
Fluorescence emission photograph of PHP5 in the solid state excited with
a 6 W UV lamp (lex ¼ 365 nm). (c) UV-vis DRS of PHP5.
There is a styryl group in BDP-St, and therefore, it can copoly-
merize with styrene (St). In order to obtain highly porous materials, a
HIPE polymerization procedure10 was adopted. The organic phase
consisted of BDP-St, St, divinylbenzene (DVB), and chlorobenzene
(CB). CB was used as a porogen and as a solvent of BDP-St. Because
BDP-St was less soluble in styrene and non-polar solvents, it was first
The average cavity size is ca. 10 mm in diameter, measured by
mercury intrusion porosimetry (Fig. 3a). The stress–strain curve of
PHP5 (Fig. 3b) indicates that it is a typical ductile material. Taking
both the mechanical property and BDP-St content into consider-
ation, PHP5 is selected for the following photocatalytic study.
The photocatalyzed sulfide oxidation was run in freshly distilled
methanol at room temperature without any additives. The substrate
concentration was about 3.2% (v/v). PHP5 was pulverized to a size of
ca. 1 mm and the porous catalyst was suspended in methanol at a
concentration of ca. 33 mg mLꢀ1 with stirring. A 24 W household
fluorescent lamp with a highpass filter (l ¼ 395 nm) was used as the
visible light source (400 to 700 nm).
Conversion of the reaction was determined by integrating the 1H
NMR peaks of the reaction mixtures assigned to the sulfide substrate
and sulfoxide product4 (ESI, Fig. S6–S10†). As shown in Table 3
(entry 1), both BDP-St and PHP5 are highly effective photocatalysts
for the aerobic oxidation of thioanisole to methyl phenyl sulfoxide.
The conversion was as high as 99%. No sulfone was detected in the
reaction products by 1H NMR, demonstrating a high degree of
selectivity of this reaction. We have also explored the scope of
substrates for this photocatalytic reaction (Table 3, entries 2–5). All
of the sulfides tested in the experiment gave reasonable conversions. It
was noticed that in the present experiments, BDP-St was a homo-
geneous catalyst while PHP was a heterogeneous catalyst. The
reaction rate with PHP5 is slower than that with BDP-St, probably
due to the heterogeneity in the case of PHP. Nevertheless, compa-
rable conversions could be obtained for most sulfide substrates by
properly prolonging the reaction time in the case of PHP catalyst.
A number of control experiments were carried out to demonstrate
the heterogeneous and photocatalytic nature of the reactions. The
reaction under light without PHP5 or in the dark with PHP5 showed
no conversion of the thioanisole to methyl phenyl sulfoxide (ESI,
Table S2, entries 1 and 2†), and only 4% thioanisole was converted
when the reaction was carried out under N2 protection (ESI,
dissolved in CB and then the BDP-St/CB solution (0.019 g mLꢀ1
)
was added to the mixture of St and DVB. The Span 80 was added to
the organic phase (0.3 mL mLꢀ1) as a surfactant for emulsion
formation. The inner phase was an aqueous solution of CaCl2 (ꢁ1
wt%) and K2S2O8 (ꢁ0.2 wt%). Its volume percent was 90% in the
emulsion system. The compositions of the organic phases are shown
in Table 2. After HIPE polymerization, highly and/or moderately
tough monolithic materials were obtained. Higher amounts of BDP-
St led to failure of the HIPE polymerization because the organic
phase after polymerization was not stable enough to ensure solidifi-
cation of the product due to the existence of too much CB.
The products from PHP2 to PHP5 are yellow. They exhibit a
bright green fluorescence emission in the dry solid state (Fig. 1a and
b). PHP5 and BDP-St have the same absorption peak at 507 nm in
UV-visible diffuse reflectance spectra (DRS) (Fig. 1c), confirming the
existence of BDP moieties on the surface of PHP5. PHP5 and BDP-
St in the solid state exhibit fluorescence at 544 and 614 nm (ESI,
Fig. S2†), respectively. Although red-shifted from that of BDP-St in
methanol solution at 509 nm due to stronger intermolecular inter-
actions, they still provide convincing evidence for photoactivity of
PHP5 and BDP-St in the solid state. Fig. 2 shows representative SEM
images of PHP5, revealing a typical interconnected porous structure.
It is full of big cavities which are separated by very thin walls and
there are many small holes in the walls to ensure interconnection
between adjacent cavities. The big cavities correspond to the inner
aqueous phase and the walls correspond to the continuous phase
before HIPE polymerization. Formation of these small holes is due to
volume contraction of the cavity walls during polymerization and
subsequent solvent extraction.11 The biggest cavities have a diameter
of ca. 40 mm while the interconnecting pores are ca. 5 mm in diameter.
Table 2 Compositions of the organic phase for HIPE polymerization
BDP-St
Samples
St (mL)
DVB (mL)
Weight (g)
Contenta (wt%)
CB (mL)
PHP1
PHP2
PHP3
PHP4
PHP5
7.2
4.8
4.8
2.4
1.2
0.3
0.3
0.4
0.3
0.24
0
0
1
2
3.5
6.5
0
0.045
0.09
0.09
0.09
2.4
4.8
4.8
4.8
a
Mass ratio of BDP-St to the total of styrene, DVB and BDP-St.
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Fig. 2 Typical SEM images of PHP5.
Fig. 4 Repeated cycles with the same batch of PHP catalyst.
The recycling experiment was performed with PHP5 for thio-
anisole oxidation under visible light. After thioanisole was completely
consumed in each run, a fresh solution of thioanisole was supplied to
maintain its initial concentration at 0.25 M. A gradual decline in
conversion rate was observed (Fig. 4). After 9 cycles, the highest
conversion became 90% from 99% for the first run. This is because
some reaction intermediates or products were adsorbed and accu-
mulated on the catalyst surface, limiting further sorption and
conversion of the thioanisole substrate. Nevertheless, the catalyst
stability was relatively excellent due to a higher stability of BODIPY
dyes against the photobleaching.
Fig. 3 (a) Pore size distribution of PHP5. (b) Stress–strain curve of
PHP5 in compression mode.
Table 3 Photocatalytic oxidation of sulfidesa
In conclusion, a novel porous heterogeneous photocatalyst, PHP,
was developed via HIPE polymerization of styrylated BODIPY,
styrene and DVB. The catalyst was full of cavities of ca. 10 mm in
diameter and thin walls between the cavities. There were many small
pores in the walls to ensure interconnection between adjacent cavities.
BODIPY dyes were chemically attached to the polystyrene back-
bone. The PHP was a ductile material, not brittle, so it could be
recycled. Due to the high photocatalytic activity and photostability of
BODIPY dyes, the PHPs exhibited high photocatalytic efficiency in
the oxidation of sulfides and high photostability under visible light.
On the other hand, because of the heterogeneous nature of the PHP
catalyst and the limited availability of the BODIPY species on the
PHP surface, the reaction was a little slower than that under
homogeneous catalysis. Further improvement is being made in
reaction velocity and will be reported in due course.
Conversion (%)b
Entry
Ar
R1
BDP-St/24 h
PHP5/60 h
1
2
3
4
5
Ph
H
H
H
H
Me
>99
89
>99
59
>99
92
92
55
4-MeC6H4
4-ClC6H4
2-ClC6H4
Ph
83
77
a
b
All the reactions were run at room temperature. Conversions were
1
determined by H NMR.
We acknowledge the financial support from the National Natural
Science Foundation of China (Project no. 21104075) and from the
Department of Education of Jilin Province (Project no. 2012-325).
Table S2, entry 3†). These results showed that light, catalyst and
oxygen are essential for this reaction. In addition, a crossover
experiment was carried out to prove the heterogeneity of the PHP
catalyst. Thioanisole was used and a conversion of 99% was achieved
under the catalysis of PHP5 after 60 h. The PHP5 was then removed
by centrifugation and the supernatant was collected. Methyl p-tolyl
sulfide was added to the supernatant solution, and after the solution
was stirred under visible light for another 60 h, only 3% conversion
was obtained for the second substrate (ESI, Table S2, entry 4†). This
result proved that the catalytic centers were in the PHP solids, and
not in the liquid phase, indicating the heterogeneous nature of the
PHP photocatalyst. Furthermore, no appreciable fluorescence was
detected from the supernatant, indicating stability of PHP catalyst
was due to the covalent coupling of the BODIPY dyes on the poly-
styrene matrix. The oxidation of sulfides is likely mediated by
photochemically generated singlet oxygen (1O2) by the BODIPY
moieties on PHP. The corresponding mechanism has been proposed
in our previous work.4
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