Light-induced synthesis of triazine N-oxide-based cross-linked polymers for effective photocatalytic degradation of methyl orange

Article abstract of DOI:10.1039/c6ra25532a

The scarcity of clean water and increasing environmental pollution are critical issues owing to the rapid development of industrialisation, population growth and long-term droughts. Research and development of robust photocatalysts is very important to treat recalcitrant organic dye wastewater. Porous organic polymers are favored as a new type of photocatalyst owing to their high specific surface area and porosity. Here, two novel 2D cross-linked polymers LCP-1 (Light-induced Cross-linked Polymer 1) and LCP-2 (Light-induced Cross-linked Polymer 2) were synthesized through light-induced click cross-coupling reaction. Starting from LCP-1 and LCP-2 as supports, the polymeric aromatic N-oxides (LCPO-1 and LCPO-2) were prepared through the reaction of the nitrogen-containing aromatic heterocycle and peroxyacetic acid at 333 K, respectively. The structure and morphology were studied by IR, solid-state ¹³C NMR, TEM and SEM. The polymeric aromatic N-oxides (LCPO-1 and LCPO-2) as organic metal-free photocatalysts showed great potential in the photodegradation of methyl orange (MO) in solution. The turn over number (TON) and the turn over frequency (TOF) of the catalysts varied and depended on the structures. For LCPO-1, the calculated TOF of the catalyst for the three cycles were determined, which were 4.83 × 10¹⁶, 2.50 × 10¹⁶ and 2.00 × 10¹⁶ molecules per g per s, while for LCPO-2 were 4.00 × 10¹⁶, 2.40 × 10¹⁶ and 1.67 × 10¹⁶ molecules per g per s, respectively.

Full text of DOI:10.1039/c6ra25532a

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Light-induced synthesis of triazine N-oxide-based
cross-linked polymers for effective photocatalytic
degradation of methyl orange†
Cite this: RSC Adv., 2017, 7, 9309
Yangxue Li, a Wei Zhang,b Jian Wang,d Haojie Lu,a Yu Liu,d Zhi Liuc and Zhigang Xieb
*
The scarcity of clean water and increasing environmental pollution are critical issues owing to the rapid
development of industrialisation, population growth and long-term droughts. Research and development
of robust photocatalysts is very important to treat recalcitrant organic dye wastewater. Porous organic
polymers are favored as a new type of photocatalyst owing to their high specific surface area and
porosity. Here, two novel 2D cross-linked polymers LCP-1 (Light-induced Cross-linked Polymer 1) and
LCP-2 (Light-induced Cross-linked Polymer 2) were synthesized through light-induced click cross-
coupling reaction. Starting from LCP-1 and LCP-2 as supports, the polymeric aromatic N-oxides (LCPO-
1 and LCPO-2) were prepared through the reaction of the nitrogen-containing aromatic heterocycle and
peroxyacetic acid at 333 K, respectively. The structure and morphology were studied by IR, solid-state
13C NMR, TEM and SEM. The polymeric aromatic N-oxides (LCPO-1 and LCPO-2) as organic metal-free
photocatalysts showed great potential in the photodegradation of methyl orange (MO) in solution. The
turn over number (TON) and the turn over frequency (TOF) of the catalysts varied and depended on the
structures. For LCPO-1, the calculated TOF of the catalyst for the three cycles were determined, which
were 4.83 ꢀ 1016, 2.50 ꢀ 1016 and 2.00 ꢀ 1016 molecules per g per s, while for LCPO-2 were 4.00 ꢀ
1016, 2.40 ꢀ 1016 and 1.67 ꢀ 1016 molecules per g per s, respectively.
Received 20th October 2016
Accepted 19th January 2017
DOI: 10.1039/c6ra25532a
rsc.li/rsc-advances
environment, posing a great threat to the health of human
beings and other living creatures.8 Thus, the removal of these
Introduction
organic contaminants has become one of the most important
environmental issues worldwide.
Organic dyes are widely used in food, medicine, printing and
dyeing, cosmetics and so on, with more than 1 ꢀ 105 types in
commercial use.1 According to statistics, Chinese dye produc-
tion of 1.5 ꢀ 105 tons is ranked at the forefront of world dye
production of 8 ꢀ 105 to 9 ꢀ 105 tons per year.2 With the
extensive use of various dyes, approximately 10–15% of them
will nally end up in discharged effluents.3,4 Most of these dyes
are extremely stable, so it is difficult for them to be degraded
naturally, increasing the color of polluted water, inuencing the
amount of incident light, and then affecting the normal life
activities of aquatic animals and plants, causing eutrophication
in stagnant water bodies.5–7 More serious is that dyes with
carcinogenic and mutagenic effects are discharged into the
As solar energy, radiant light and heat from the sun are
considered to be the most abundant clean energy sources
available, utilizing and conversion of solar energy is the most
promising way to solve the energy crisis and environmental
pollution in the world.9 An effective tool to overcome this
obstacle is to develop novel photocatalytic materials that
convert light energy into chemical energy and promote the
decomposition and synthesis of organic compounds.10–13
Specically, ever since the discovery of water photolysis on
a TiO2 electrode by Fujishima, inorganic materials, metal oxide
or sulde semiconductor materials, such as TiO2, ZnO, CdS,
SnO2 and WO3, have proved to be efficient in depollution of
water from toxic chemicals as photocatalysts.14–19 However,
there are several key problems that inuence their large-scale
industrial practical application including: (1) the light
response range is narrow, the large band gap (3.2 eV) of TiO2
largely limits its photocatalytic activity in the UV light region,
which is less than 5% of the solar spectrum to the ground; (2)
the quantum efficiency is low, not more than 28% at most; (3)
retrievability and regeneration are difficult. Therefore, devel-
oping photocatalysts with high efficiency, good stability and
aCollege of Environment and Resources, Jilin University, Changchun 130026, P. R.
China. E-mail: yangxueli@jlu.edu.cn
bState Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of
Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun
130022, P. R. China
cJilin Province ShunFood Technology Services Limited Liability Company, Changchun
13000, P. R. China
dState Key Laboratory of Theoretical and Computational Chemistry, Institute of
Theoretical Chemistry, Jilin University, Changchun 130023, P. R. China
† Electronic supplementary information (ESI) available: Characterization gure
and supporting gures. See DOI: 10.1039/c6ra25532a
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good reusability simultaneously has become one of the noble hydrophilicity and electronic structure improving the harvest-
missions of material science. ing of solar light and the photocatalytic ability. As a result, the
On the other hand, cross-linked polymers represent a new polymeric aromatic N-oxides (LCPO-1 and LCPO-2) could be
class of robust and porous materials composed of organic used as novel organic heterogeneous photocatalysts for degra-
molecules connected by covalent bonds, and with their low dation of pollutants in wastewater.
density and stable structure have attracted widespread atten-
tion.20–34 Unlike inorganic materials, cross-linked polymers,
with higher reactivity and quantum yield owing to their high
specic surface area and porosity, are favored as a new type of
photocatalyst.35–40 More importantly, the properties of cross-
Experimental
Materials and measurements
All starting materials were purchased from commercial
linked polymers can be facilely tuned and modied by
modular synthesis and selection of monomers. Synthesis of
cross-linked polymers usually requires harsh conditions and
the use of catalysts.41,42 Developing simple, mild, environmen-
tally friendly and efficient reactions is still a big challenge in
this eld. Light as a common energy donor has been applied in
driving many chemical reactions.43–48 Herein, two cross-linked
polymers LCP-1 (Light-induced Cross-linked Polymer 1) and
LCP-2 (Light-induced Cross-linked Polymer 2) were prepared
through light irradiation; then, polymeric aromatic N-oxides
(LCPO-1 and LCPO-2) were afforded by introducing N–O
bonds on the polymers (Scheme 1). The changed polarity,
suppliers and were used without further purication unless
otherwise noted.
Thermogravimetric analysis (TGA) was performed using
a Netzch Sta 449c thermal analyzer system at A heating rate of
10 ꢁC minꢂ1 in air atmosphere. Elemental analyses were
measured by Elementar model Vario Micro analyzer. FTIR
spectra were measured using a Nicolet Impact 410 Fourier
transform infrared spectrometer. The nitrogen adsorption
isotherm was measured on an Autosorb iQ2 absorptometer
from Quantachrome Instruments. XRD was performed using
a Rigaku D/MAX2550 diffractometer with CuKa radiation, 40
kV, 200 mA with a scanning rate of 0.4ꢁ minꢂ1. TEM micro-
graphs were recorded using a FEI Tecnai G2F20 s-twin D573
with an acceleration voltage of 300 kV. SEM micrographs were
obtained on a JEOL JXA-840 under an accelerating voltage of
15 kV. 1H-NMR spectra were recorded at 400 MHz in DMSO-d6
as the internal standard with TMS, and solid-state 13C NMR
spectra were recorded at 5 KHz. Photoluminescence spectra
were recorded on a PerkinElmer LS55 spectrouorometer. The
electronic structures and characteristics for these compounds
were explored by density functional theory with Gaussian 09
soware at the B3LYP/6-31G(d) level of theory (refer to the ESI
for more details†), which has been reported as a good balance
between accuracy and computational efficiency.
Synthesis of 1,4-bis(2-phenyl-2H-tetrazol-5-yl)benzene (BPTB)
1,4-Bis(2-phenyl-2H-tetrazol-5-yl)benzene (BPTB) was synthe-
sized according to a previously described procedure.49,50 Into
a 50 mL Schlenck tube was added 5,50-(1,4-phenylene)bis(1H-
tetrazole) (2 mmol, 0.428 g), phenylboronic acid (8 mmol, 0.996
g), Cu2O (5 mol%, 0.1 mmol, 0.016 g) and DMSO (4 mL). The
reaction mixture was stirred under oxygen atmosphere at 100 ꢁC
until tetrazole had disappeared, as monitored by TLC. The
reaction mixture was then cooled to room temperature and
diluted with 100 mL of dichloromethane, washed consecutively
with 20 mL of 1 M aqueous HCl and 20 mL of brine (four times).
The organic layer was separated and dried over MgSO4, then
concentrated under reduced pressure and the pure product was
obtained aer recrystallization (0.512 g, 1.4 mmol, 70%). 1H
NMR (CDCl3, 400 MHz): d ¼ 8.44 (s, 4H), 8.22–8.24 ppm (d, 4H),
7.59–7.63 ppm (t, 4H), 7.51–7.55 ppm (d, 2H). E. A, calc. for
C
20H14N8 (%): C, 65.56; H, 3.85. Found: C, 65.00; H, 4.54.
Synthesis of LCP-1
The reaction mechanism for LCP-1 is based on the photo-
activatable 1,3-dipolar cycloaddition reaction.49 Into a 50 mL
Scheme 1 Synthesis of LCP polymers and schematic diagram of its
oxidation LCPO polymers.
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a quartz test tube was added 1,4-bis(2-phenyl-2H-tetrazol-5-yl) spinning nuclear magnetic resonance (CP/MAS NMR), compu-
benzene (BPTB) (0.5 mmol, 0.106 g), 2,4,6-tris(allyloxy)-1,3,5- tational calculation and powder X-ray diffraction (PXRD). TGA
triazine (TRAT) (0.3 mmol, 0.050 g) and ethyl acetate (10 mL). results showed that LCP-1 and LCP-2 are stable up to 210 ꢁC and
The reaction mixture was stirred under UV irradiation at 254 nm 340 ꢁC in air (Fig. S2†), respectively. The IR spectrum of the LCP
at room temperature for 48 h to afford a yellow precipitate. The polymers exhibits a vibration band characteristic of C]N bonds
resulting yellow solid was stirred in chloroform to remove the at 1665 cmꢂ1 and 1695 cmꢂ1, respectively. The original vibra-
unreacted reagents and then washed with various solvents to tional bands separate into double or tripartite peaks suggesting
afford LCP-1 in 85% yield. LCP-1 is insoluble in water and the inuence of the N–O bond on the C]N bond of LCP poly-
common organic solvents, like dimethyl sulfoxide, methanol, mers, consistent with a high degree of oxidation (Fig. S3†).53 CP/
tetrahydrofuran and acetone.
MAS NMR spectroscopy gave more information about the
structure of the polymers. The signals were assigned to the
carbon atoms of the N-heterocyclic rings, triazine rings, oxetane
rings and phenyl linkers (Fig. 1c and 2c). For LCP-1, the four
intense signals at 168.35, 139.22, 130.98 and 123.01 ppm can be
unambiguously assigned to C1/C5, C6/C8, C7/C10 and C9/C11,
respectively, in the triazine and benzene rings, while the two
relatively weaker signals at 30.08 and 78.51 ppm are attributed
to C4 and C2/C3 in the N-heterocyclic rings and –CH2 alkyl
groups. For LCP-2, the two intense signals at 172.78 and
128.65 ppm can be unambiguously assigned to C1 and C6/C7,
respectively, in the triazine and benzene rings, while the two
relatively weaker signals at 33.79 and 82.51 ppm are attributed
to C4 and C2/C3/C5 in the oxetane rings and –CH2 alkyl groups.
FE-SEM was used to investigate the morphology of the LCP
polymers that were cast from ethyl acetate suspension on Si
wafer. The FE-SEM images revealed that the resulting solid was
irregular in shape without a well-dened morphology (Fig. 2a).
Transmission electron microscopy (TEM) was performed. The
TEM images (Fig. 2b) indicated that the texture was largely
amorphous. To characterize the nature of the pores, the N2
sorption of the LCP polymers was measured at 77 K, and
exhibited a typical type I isotherm (Fig. 3).
Synthesis of LCP-2
The reaction mechanism for LCP-2 is based on the [2 + 2]
51
¨
Paterno–Buchi reaction. Into a 50 mL a quartz test tube was
added terephthalaldehyde (TPA) (0.5 mmol, 0.106 g), 2,4,6-
tris(allyloxy)-1,3,5-triazine (TRAT) (0.3 mmol, 0.050 g) and ethyl
acetate (10 mL). The reaction mixture was stirred under UV
irradiation at 254 nm at room temperature for 48 h to afford
a gray precipitate. The resulting gray solid was stirred in chlo-
roform to remove the unreacted reagents and then washed with
various solvents to afford LCP-2 in 80% yield. LCP-2 is insoluble
in water and common organic solvents, like dimethyl sulfoxide,
methanol, tetrahydrofuran and acetone.
Synthesis of LCPO-1 and LCPO-2
LCPO polymers were prepared using a previously described
method.52 2 g of LCP-1 and 10 mL CH3COOH and 20 mL of 30%
H2O2 were mixed. Then the system was sealed to semi-closed,
heated to 333 K and kept for 3 hours under stirring condition.
Aer that, the mixture was ltered and the solid was washed
with pure water, till the avor of CH3COOH could not be smelt.
Then the solid was dried at 393 K overnight.
LCPO-2 was also synthesized using the same method with
LCP-2 as the reactant.
Photodegradation of methyl orange (MO)
MO solution was prepared in 0.05 mol Lꢂ1 NaH2PO4, 0.05 mol
Lꢂ1 H3PO4 and 0.05 mol Lꢂ1 NaCl buffer solution. A certain
amount of catalyst (LCP-1, LCPO-1: 0.100 g, LCP-2 and LCPO-2:
0.100 g) was dispersed in 100 mL of 5 mg Lꢂ1 MO solution in
a Pyrex reactor. Aer stirring in the dark for 0.5 h, visible light
irradiation was provided by a 400 W LED lamp with 420 nm
wavelength. During the experiment, the reactor was kept at 298
K by a water bath. An N2 control experiment was conducted in
the same way except N2 was pumped into the Pyrex reactor from
the beginning of the experiment. At suitable time intervals, the
mixture was centrifuged and the supernatant was extracted. The
concentration of the solution was further analysed by UV at an
absorbance wavelength of 465 nm for MO.
Results and discussion
The LCP polymers were characterized by thermogravimetric
analysis (TGA), infrared spectroscopy (IR), eld-scanning elec-
tron microscopy (FE-SEM), transmission electron microscopy
Fig. 1 SEM image (a), TEM image (b), and solid-state 13C NMR spec-
(TEM), nitrogen adsorption, cross polarization/magic angle trum (c) of LCP-1.
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Fig. 4 (a) and (c) UV spectra of LCP and LCPO polymers. (b) and (d)
Fluorescence spectra of LCP and LCPO polymers.
in the dark and irradiated with visible light indicate that the
degradation of MO on these polymers is indeed driven by visible
light. The degradation efficiency (%) can be calculated as effi-
ciency (%) ¼ (C0 ꢂ C)/C0 ꢀ 100% (where C0 is the initial
concentration of MO and C is the concentration aer photo
irradiation).56 For LCPO-1, when irradiated by visible light, the
degradation efficiency of MO was as high as 30%, but the
degradation efficiency of MO was as low as 2% treated in the
dark in 90 minutes. The photocatalytic activity under light was
almost 15 times higher than that in the dark. For LCPO-2, when
irradiated by visible light, the degradation efficiency of MO was
as high as 24%, but the degradation efficiency of MO was as low
as 1% when treated in the dark for 90 minutes. The photo-
catalytic ability in light was almost 24 times higher than that in
dark. Moreover, these results indicate that LCPO-1 has better
performance prepared under the same experimental condi-
tions. The XRD patterns of the used LCPO polymers and fresh
LCPO polymers were in good agreement, which demonstrates
Fig. 2 SEM image (a), TEM image (b), and solid-state 13C NMR spec-
trum (c) of LCP-2.
In order to conrm the formation of the N–O bond in these
polymers, we examined their UV and FL spectra. As displayed in
Fig. 4, the introduction of the N–O bond into LCP polymers
enhances light harvesting. The possible mechanism is that the
p electron conjugation of the whole periodic framework of the
LCP polymers is increased by the introduction of the N–O bond
into the triazine ring of the LCP polymers.54,55 Moreover, the FL
intensity of the LCP polymers is weaker than that of LCPO
polymers, indicating the effective suppressing of the recombi-
nation of electron–hole pairs. Therefore, LCPO polymers not
only appropriate band gap for the absorption of light to
generate charge carriers (electron–hole pairs), but also afford
the ability to efficiently separate the carriers, making them
suitable photocatalysts.
As shown in Fig. 5, the photocatalytic activities of LCPO-1
and LCPO-2 were evaluated via the photodegradation of MO
under visible light irradiation. The control experiment was
carried out by studying the photolysis of MO for the same
duration under visible light irradiation in the absence of
a catalyst. It can be seen that MO is stable under visible light
irradiation. The different degradation activities for MO treated
Fig. 5 (a) and (b) Photocatalytic degradation of MO (5 mg Lꢂ1) over
LCP polymers and LCPO polymers. (c) and (d) PXRD patterns for fresh
and used LCPO polymers.
Fig. 3 Nitrogen sorption isotherms of LCP-1 (a) and LCP-2 (b).
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Fig. 6 Cyclic runs of MO degradation by LCPO-1 (a) and LCPO-2 (b).
the stability of the photocatalysts and that they can be reused
multiple times.
By virtue of their reusability and stability, heterogeneous
catalysts are generally found to be advantageous over homoge-
neous catalysts. To study the reusability and the stability of the
photocatalysts, three successive photocatalytic experiments
were carried out by adding used photocatalysts to fresh MO
solutions (Fig. 6). No distinct difference in activity was observed,
which demonstrates the stability of the catalyst and implies its
recyclability. Furthermore, information about the turn over
number (TON) and the turn over frequency (TOF) of the catalyst
is very important, which tells us about the efficiency of the
catalyst.57 Since the concentration of the reactant (MO) was 1.5
ꢀ 10ꢂ2 M and the amount of the catalyst was kept at 1 g Lꢂ1, for
LCPO-1, the calculated TOF of the catalyst for the three cycles
were 4.83 ꢀ 1016, 2.50 ꢀ 1016 and 2.00 ꢀ 1016 molecules per g
per s, while for LCPO-2 they were 4.00 ꢀ 1016, 2.40 ꢀ 1016 and
1.67 ꢀ 1016 molecules per g per s. Therefore, LCPO-1 and LCPO-
2 can be used as high-performance and stable visible-light
photocatalysts and they have potential applications in envi-
ronmental protection.
Fig. 8 LUMO (above) and HOMO (below) of LCP-2 (left) and LCPO-2
(right) samples. Carbon, nitrogen and oxygen in the structural models
are shown as gray, blue and red spheres, respectively.
To improve our understanding of the electronic structure
change, theoretical calculations have been carried out. The
HOMO and LUMO analyses reveal that the energy of the LUMO
is ꢂ1.01 eV in LCP-1 and ꢂ2.18 eV in LCPO-1, while the energy
of the LUMO is ꢂ0.39 eV in LCP-2 and ꢂ1.99 eV in LCPO-2. In
addition, from the HOMO and LUMO positions of the LCP
polymers and the LCPO polymers (Fig. 7 and 8), it is obvious
that the HOMO does not change, both in LCP-1/LCPO-1 and
LCP-2/LCPO-2, while the LUMO has changed. Briey, compared
to LCP polymers, LCPO polymers have a lower HOMO–LUMO
band, thus the activity goes up under visible light irradiation,
meaning that the electrons are excited from the HOMO to the
LUMO of the photocatalyst easier. Meanwhile, the N–O bond
interacts strongly ꢂwith polar compounds owing to the strong
polar bond (+N–O ),58 which endows the LCPO polymers with
an adsorption-photocatalysis synergistic effect, and leads to the
effective degradation of the pollutant. This result indicates that
the introduced N–O bond plays an important role in improving
the photocatalytic activity. The photocatalytic efficiencies
(LCPO-1 > LCPO-2) may be owing to the energy gap between the
LUMO and HOMO.59 Such differences may result in the faster
generation speed of photo-generated holes and electrons in
LCPO-1 and the faster decomposition of MO initiated by
LCPO-1.
Conclusions
In conclusion, we have synthesized two new cross-linked poly-
mers (LCP-1 and LCP-2) based on light-induced click cross-
coupling reaction strategy. The polymeric aromatic N-oxides
(LCPO-1 and LCPO-2) as organic photocatalysts showed great
potential in the photodegradation of MO in solution. The strong
polar N–O bond enhances the oxidation ability of the photo-
induced hole, making an excellent synergistic effect with the
unique electronic structure of the parent polymers. This work
Fig. 7 LUMO (above) and HOMO (below) of LCP-1 (left) and LCPO-1
(right) samples. Carbon, nitrogen and oxygen in the structural models
are shown as gray, blue and red spheres, respectively.
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opens a new avenue to develop efficient heterogeneous catalytic 26 X. W. Yang, X. D. Zhuang, Y. J. Huang, J. Z. Jiang, H. Tian,
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This work was supported by the National Natural Science
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30 X. D. Li, J. H. Guo, H. Zhang, X. L. Cheng and X. Y. Liu, RSC
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