Efficient mineralization of phenol by a temperature-responsive polyoxometalate catalyst under wet peroxide oxidation at lower temperatures

Article abstract of DOI:10.1039/c7ra08409a

Herein, a temperature-responsive polyoxometalate (POM) catalyst [C₁₆H₃₃(CH₃)₃N]₃[PO₄{WO(O₂)₂}₄]/poly(N-isopropylacrylamide) (abbreviated as (C₁₆PW(O₂)₂/PNIPAM) was prepared and used in the catalytic wet peroxide oxidation (CWPO) of phenol under mild conditions. The POM catalyst C₁₆PW(O₂)₂/PNIPAM showed a higher degradation efficiency and mineralization of phenol with H₂O₂ at room temperature or even at lower temperature (0 °C) within a short time (120 min). The high efficiency at lower temperature was attributed to its temperature-responsive property, wherein the lattice of the temperature-sensitive polymer relaxed at lower temperature and then wrinkled at higher temperature. These characteristics also permitted C₁₆PW(O₂)₂/PNIPAM to be easily separated for recycling. The leaching test indicated that the POM catalyst exhibited excellent stability and little leaching and can be used as a thermosensitive catalyst for about six times. C₁₆PW(O₂)₂/PNIPAM has potential application in the CWPO of phenol without the limitation of temperature and pH conditions.

Full text of DOI:10.1039/c7ra08409a

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PAPER
Efficient mineralization of phenol by
a temperature-responsive polyoxometalate
catalyst under wet peroxide oxidation at lower
temperatures†
Cite this: RSC Adv., 2017, 7, 43681
Yiming Li, Xueyan Zhang, Dan Zhang, Yue Li, Xiaohong Wang
and Shengtian Wang*
*
Herein, a temperature-responsive polyoxometalate (POM) catalyst [C16
poly(N-isopropylacrylamide) (abbreviated as (C16PW(O
2 2
catalytic wet peroxide oxidation (CWPO) of phenol under mild conditions. The POM catalyst C16PW(O ) /
3 3 3 4 2 2 4
H33(CH ) N] [PO {WO(O ) } ]/
2 2
) /PNIPAM) was prepared and used in the
2 2
PNIPAM showed a higher degradation efficiency and mineralization of phenol with H O at room
ꢀ
temperature or even at lower temperature (0 C) within a short time (120 min). The high efficiency at
lower temperature was attributed to its temperature-responsive property, wherein the lattice of the
temperature-sensitive polymer relaxed at lower temperature and then wrinkled at higher temperature.
Received 31st July 2017
Accepted 29th August 2017
These characteristics also permitted C16PW(O
leaching test indicated that the POM catalyst exhibited excellent stability and little leaching and can be
used as a thermosensitive catalyst for about six times. C16PW(O /PNIPAM has potential application in
the CWPO of phenol without the limitation of temperature and pH conditions.
2 2
) /PNIPAM to be easily separated for recycling. The
DOI: 10.1039/c7ra08409a
2 2
)
rsc.li/rsc-advances
homogeneous catalyst, narrow acidic range of pH (ꢁ3.0), and low
utilization of hydrogen peroxide and low degree of mineraliza-
Agricultural processes and industrial manufacturing produce tion, which lead to the limitation of their application. POM has
1
. Introduction
1
11,12
a large amount of wastewater including organic pollutants,
which are toxic and hardly biodegradable. Therefore, a wide degradation, CWPO or as anchoring sites for single-atom metal
a broad use in catalysis such as in wet air oxidation,
photo-
13
14,15
range of scientic studies have been conducted to develop new catalysts.
technologies, such as physical adsorption, photo-catalysis, and
33(CH
Our group developed the polyoxometalate [C16
SiV 40 for the CWPO of phenol, and highest
-
H
) N] H
3 3 4 2
2 10
W O
chemical oxidation, to achieve the effective purication of catalytic activity was obtained (a 91.6% degradation efficiency,
wastewater polluted with organic compounds such as phenol 93.2% COD removal, and 85.5% TOC reduction with the phenol
2
–5
and its derivatives.
processes, the CWPO system has emerged as a clean and conditions for 90 min). A higher usage of H
2 2
Among the reported degradation to catalyst to H O molar ratio of 1 : 60 : 156.4 under ambient
16
2 2
O was required to
6–9
effective alternative because it has signicant oxidizing ability achieve high efficiency, and it did not exhibit activity at lower
with harmless by-products and non-toxic substances and high temperatures. Despite the higher efficiency and higher degree of
degradation efficiency under mild conditions (20–60 C, atmo- mineralization, the separation was relatively difficult due to its
ꢀ
spheric pressure). To date, some studies have been reported on nanostructure. In addition, our city, Changchun, is located in the
the CWPO of phenol containing the catalysts of Fenton agents, Northeast of China, where for up to six months, the temperature
ꢀ
metal oxides, and polyoxometalates (Table S1†). At present, the remains below 10 C. Normal CWPO treating temperature does
highest efficiency was obtained with nano-metallic particles not favor the degradation of phenol in our city. Therefore,
with 100% degradation of phenol under ultrasound power procedures with high efficiency and deep mineralization at low
(
500 W) for 120 min and the molar ratio of phenol to catalyst to temperatures are of great value and highly desirable.
10
H O
2 2
was 1 : 12.5 : 170. However, there are three major draw-
Thermoresponsive polymers exhibit very sensitive and
backs for Fenton or metal oxides: the need for the removal of the reversible temperature-dependant water solubility, which have
attracted signicant attention in terms of the catalytic
17,18
loading.
In our desulfurization process based on POMs, we
Key Lab of Polyoxometalate Science of Ministry of Education, Northeast Normal
University, Changchun 130024, P. R. China. E-mail: wangxh665@nenu.edu.cn; Tel: have designed temperature-responsive POMs [C16
33
H -
+
†
1
86-431-85099759
N(CH
)
]
[PO
{MO(O
)
}
]/PNIPAM (M ¼ Mo and W) using poly-
3
3
3
4
2
2
4
Electronic supplementary information (ESI) available. See DOI:
0.1039/c7ra08409a
19
N-isopropylacrylamide (PNIPAM) as the support in the H
2 2
O
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oxidation of organic sulfurs. It was found that the thermores- The mixture was stirred magnetically at room temperature for
ponsive hybrid [C H N(CH ) ] [PO {MO(O ) } ]/PNIPAM 30 min. The reaction mixture was poured into a 200 mm long
1
6
33
3 3 3
4
2 2 4
exhibited a novel switchable property based on a change of test tube (inside diameter 12.5 mm). The tube was clamped to
ꢀ
temperature: it dissolved in organic solvents at higher temper- a ring standing in a chamber drying oven at 25 C. Then, 0.1 g
atures and became gradually insoluble upon decreasing the
2 2
C16PW(O ) was added to the upper layer of the mixture under
temperature; we speculated that these thermo-responsive POM stirring for 30 min. The mixed solutions were cast on glass
ꢀ
hybrids could exhibit good activity in the CWPO of phenol, substrates, followed by gelation and drying at 60 C. The
especially at lower temperatures.
2 2
C16PW(O ) /PNIPAM was obtained in 80.3% yield.
2 2
Herein, we used a thermo-sensitive POM C16PW(O ) /PNI-
PAM hybrid in the CWPO of phenol at various temperatures. It
exhibited fast adsorption and higher activity than its parent at
room temperature and even at lower temperatures, showing
great potential in the cold temperature region. In addition, the
degradation procedure of phenol under CWPO using the
abovementioned POM was studied (Scheme S1†).
2
.4 Catalytic procedure
For the degradation of phenol in a water system, 0.01 g of the
catalyst was suspended in a fresh aqueous phenol solution (C0
¼
0.72 mM, 5 mL, pH ¼ 1.7) in a 25 mL three-neck ask at
ambient temperature. Then, 0.15 mL of hydrogen peroxide
30.0%) was added to the mixture under stirring to start the
(
reaction. Aer the reaction, the catalyst was separated by
centrifugation at 1600 rpm for 10 min, which resulted in two
layers: the upper layer was the aqueous phase and the bottom
layer was the solid phase containing the catalyst. The aqueous
sample was obtained at different times and immediately tested
and measured using a 4-aminoantipyrine spectrophotometric
method via a 756 CRT UV-vis spectrophotometer at a wave-
length of 505 nm, which detected the degradation of phenol.
The degradation efficiency of phenol was calculated using the
following formula:
2
. Experimental
2
.1 Materials
All reagents were of AR grade and used without further puri-
cation. A 0.72 mM phenol solution was prepared. C16PW(O
2 2
) /
PANIPAM was synthesized according to ref. 20 and character-
ized by IR spectroscopy.
21
2.2 Physical measurement
Elemental analysis was carried out using a Leeman Plasma Spec
(
I) ICP-ES and a P-E 2400 CHN elemental analyzer. The FT-IR
Phenol degradation (%) ¼ (C ꢂ C )/C
0
i
0
ꢂ1
spectra (4000–400 cm ) were obtained using KBr discs via
a Nicolet Magna 560 IR spectrometer. The P MAS NMR spectra where C
of the samples were obtained using a Bruker AVANCE III sample and C
3
1
0
is the initial phenol concentration in the liquid
is the nal phenol concentration in the liquid
i
4
00 WB spectrometer equipped with a 4 mm standard bore sample.
CPMAS probe with the X channel tuned to 162 MHz. Scanning
The utility of H
2
O
2
was detected via the titration method with
22
electron microscopy (SEM) and energy-dispersive X-ray spec- Ce(SO
)
.
The temperature-control C16PW(O /PNIPAM cata-
)
2 2
4
2
troscopy (EDAX) were performed using a XL30 ESEM FEG at lyst was le in the bottom of the reactor and was obtained by
5 kV (PhilipsXL-30). Calorimetric measurements were per- decanting and washing three times with water. The obtained
2
ꢀ
ꢀ
ꢀ
formed using an SDT Q600 instrument from 16 C to 80 C at solid was desiccated under vacuum at 60 C for 120 min for
ꢀ
ꢂ1
a scan rate of 0.5 C min . The analysis of phenol during the recycling. The leaching of POMs from the polymer was tested
reaction was performed using a closed reux colorimetric using ICP analysis.
method via a 756 CRT UV-vis spectrophotometer at a wave-
length of 505 nm. Total organic carbon (TOC) was monitored
2.5 Adsorption experiment
using a Shimadzu TOC-VCPH total organic carbon analysis
system. Chemical oxygen demand (COD) was determined using
a closed reux colorimetric method via a 756 CRT UV-vis spec-
trophotometer at a wavelength of 600 nm. The intermediates
were analyzed via high pressure liquid chromatography (HPLC)
with a UV detector using a YWG (4.6 mm ꢃ 150 mm, 10 mm)
column. The mobile phase was a solution of H PO (0.02% w/w)
An adsorption experiment was carried out to determine the
adsorption capacity of the catalyst for an aqueous phenol
solution. In the adsorption experiment, 0.01 g of catalyst was
suspended in a fresh aqueous phenol solution (C ¼ 0.72 mM,
0
5
mL, pH ¼ 1.7) in a 25 mL three-neck ask for 2 h at ambient
temperature. Then, the solid phase was separated by ltration.
The catalyst was dried under vacuum at 60 C and then used in
3
4
ꢀ
ꢂ1
in H O/acetonitrile (95/5 v/v) with a ow rate of 0.7 mL min
.
2
determining the adsorption effect of phenol by IR spectroscopy.
2
.3 Preparation of C PW(O ) /PNIPAM
16 2 2
2.6 Differential scanning calorimetry (DSC)
In a typical synthesis, N-isopropylacrylamide (5 g) and N,N-
methylenebisacrylamide (MBA) (0.04 g) were dissolved in Calorimetric measurements were carried out using an SDT
ꢀ
ꢀ
a dimethyl sulfoxide (DMSO) mixed solution (4 mL) containing Q600 instrument from 16 C to 80 C at a scan rate of
1
ꢀ
ꢂ1
.5 g of polyvinylpyrrolidone (PVP) and 0.02 g of Pluronic F-127 0.5 C min . Herein, 0.0006 mg of the catalyst was used. The
(
(
PF127). Then, 0.03 g of the initiator ammonium persulfate DSC curve was integral to obtain the latent heat of the phase
APS) was added to the abovementioned monomer solution. change.
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3. Results and discussion
3
.1 Evaluation of the degradation efficiency
A preliminary experiment was performed at 25 C and under
atmospheric pressure in the presence of H without the
catalyst and with different catalysts including C16PW(O
PNIPAM and C16PW(O /PNIPAM (Fig. 1). Without the catalyst,
ꢀ
2 2
O
2 2
) ,
2 2
)
phenol was degraded negligibly (less than 8.0%); this indicated
that the oxidative ability of H O was weak at room temperature
Fig. 2 Images of the phase changes observed for C16PW(O
PAM at (a) 0 C, (b) 10 C, (c) 25 C, (d) 32 C, (e) 45 C, and (f) 60 C.
2
)
2
/PNI-
2
2
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
and pressure (Fig. 1a). In addition, the degradation of phenol by
was enhanced to a maximum of 46.0% using PNIPAM
Fig. 1b) at 120 min, whereas the phenol degradation reached
0.0% at 30 min and then hardly increased upon prolonging the
reaction time. This may be attributed to the adsorption of
phenol and no catalytic degradation for the support PNIPAM
2 2
H O
(
4
It can be seen that C PW(O ) /PNIPAM exhibits a tempera-
1
6
2 2
ture-dependent relaxation in water (Fig. 2): at lower tempera-
ture, relaxation occurs in water, whereas at temperatures higher
ꢀ
than 32 C, the catalyst wrinkles. In addition, the heat capacity
(Fig. S1a†). Fig. S1† displays the adsorption of phenol by PNI-
of C16PW(O
2
)
2
/PNIPAM was measured using a DSC instrument.
PAM and C PW(O ) /PNIPAM, which shows some difference
ꢀ
1
6
2 2
It can be seen that the peak at 32.93 C is the temperature point
for these two species in the amount of adsorbed phenol.
However, the adsorption amount of phenol per gram of the
polymer was almost the same for C16PW(O ) /PNIPAM and
2 2
ꢂ1
of phase change with DH ¼ 113.5 J g (Fig. S2†). The above-
mentioned results indicated that C PW(O ) /PNIPAM could
1
6
2 2
exhibit different degrees of swelling, as also indicated in the UV-
visible spectra of C16PW(O /PNIPAM in water upon varying the
temperature (Fig. 3). It can be seen that the transmittance of
16PW(O /PNIPAM increased gradually upon increasing the
temperature to 32 C and decreased at high temperature (60 C).
Therefore, the phase changes in C16PW(O /PNIPAM occur
PNIPAM (Fig. S1b†). This indicated that the decrease of phenol
in the PNIPAM system was mainly attributed to the adsorption
effect. In addition, the product distribution of the reaction
system using PNIPAM was tested with only phenol; this also
determined the adsorption of phenol and no oxidation
occurred. These two aspects indicate that PNIPAM was the only
support. Therefore, the decrease in phenol was mainly attrib-
uted to the adsorption of phenol by the polymer support,
whereas the adsorption effect mainly depended on the polymer.
Addition of POMs can signicantly accelerate the degradation
2 2
)
C
2 2
)
ꢀ
ꢀ
2 2
)
upon varying the temperature; this is mainly attributed to
PNIPAM.
Moreover, the higher activity of C PW(O ) /PNIPAM was
1
6
2 2
attributed to the stronger adsorption of phenol (Fig. 4) by
C
C
(
16PW(O
16PW(O
2
)
)
2
2
/PNIPAM. First, the adsorption of phenol by
/PNIPAM was determined using IR spectra at 1 h
2 2
rates; C16PW(O ) exhibited a 70.3% degradation efficiency for
2
about 120 min (Fig. 1c), whereas C PW(O ) /PNIPAM can
1
6
2 2
2 2
Fig. 4). In comparison with those of C16PW(O ) /PNIPAM, the
catalyze the complete decomposition of phenol under the same
reaction conditions (Fig. 1d). Therefore, the activity of the
ꢂ1
peaks originating from phenol at 1640 cm
shied to
ꢂ1
ꢂ1
ꢂ1
ꢂ1
1
9
659 cm , 1292 cm shied to 1285 cm , 981 cm shied to
69 cm , and 826 cm shied to 816 cm ; this suggested
C
16PW(O
2
)
2
/PNIPAM hybrid originated from C16PW(O
2
)
2
. The
ꢂ1
ꢂ1
ꢂ1
higher efficiency observed for C16PW(O )
2 2
/PNIPAM was attrib-
that some interaction occurred between the O atom of phenolic
uted to its temperature-responsive property in the water phase.
3ꢂ
hydroxyl and the terminal oxygen of [PO
4
{WO(O
2
)
2
}
4
]
. In
addition, the amount of phenol absorbed was measured
Fig. 1 The effect of different catalysts on the degradation of phenol.
Blank (a), PNIPAM (b), C16PW(O
Reaction conditions: PNIPAM (0.0084 g), C16PW(O )
2
)
2
(c), and C16PW(O
2
)
2
/PNIPAM (d).
(0.0016 g),
(0.1 mL), phenol solution Fig.
temperatures.
2 2
C
16PW(O
2
)
2
/PNIPAM (0.01 g),
H
2
O
2
6 2 2
3 The visible spectra of C1 PW(O ) /PNIPAM at different
ꢀ
(
0.72 mM, 5 mL), 25 C, 2 h.
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2 2
Fig. 6 The degradation efficiency between C16PW(O ) /PNIPAM and
Fig. 4 The IR spectra of (a) C16PW(O
)
2 2
/PNIPAM, (b) phenol, and (c)
C
16PW(O at different temperatures.
2 2
)
ꢀ
C
16PW(O
2
)
2
/PNIPAM-phenol at 25 C for 0.5 h.
contamination by a reductive substance, whereas the TOC
refers to the total carbon content. At 120 min, the COD and TOC
were reduced to 98.8% and ꢁ100%, respectively, indicating that
it was the oxidative degradation of phenol and not an adsorptive
effect. This indicates that the phenol molecules can be totally
(
2 2
Fig. S1b†). It can be seen that C16PW(O ) /PNIPAM can rapidly
adsorb phenol molecules in 30 min and then saturation is
reached aer 30 min. This adsorption of phenol can help them
access the active sites in the POM; this promotes its oxidative
degradation. Moreover, the adsorption of phenol by C16PW(O
and C16PW(O /PNIPAM depended on the temperature, and
the adsorption capacity of C PW(O ) /PNIPAM was larger than
mineralized into simple inorganic compounds by C16PW(O
PNIPAM/H
2 2
) /
2 2
)
2 2
O .
2 2
)
For further optimizing the degradation process with
C PW(O ) /PNIPAM, the reaction was performed at various
1
6
2 2
ꢀ
1
6
2 2
that for C PW(O ) below 25 C; in addition, at higher
1
6
2 2
temperatures, reaction times, H O concentrations, pH values,
2
2
temperatures, C16PW(O
than the C16PW(O /PNIPAM hybrid (Fig. 5). This difference
was also attributed to the temperature-responsive property of
16PW(O /PNIPAM. As a result, C16PW(O /PNIPAM exhibi-
at a lower tempera-
2 2
) showed a higher adsorption ability
catalyst dosages, and loading amounts, as shown in Fig. 7.
Fig. 7a shows the relationship between the degradation effi-
ciency and reaction temperature and time when the degrada-
2 2
)
C
2
)
2
2 2
)
2 2
tion of phenol is catalyzed by C16PW(O ) /PNIPAM. A little
ted 29.9% higher activity than C16PW(O
2
)
2
ꢀ
difference was found for various temperatures at 30 min, indi-
cating most of the adsorption of phenol by the hybrid occurred
during this period. Then, the degradation efficiency increased
upon prolonging the reaction time to 120 min at all the
temperatures studied herein, and the best phenol conversion
ture of 25 C (Fig. 6) and a signicant degradation efficiency of
8
ꢀ
9.2% at 0 C.
To clarify the exact effect of C PW(O ) /PNIPAM on the
1
6
2 2
2 2
conversion of phenol by H O , the COD and TOC changes were
tested. It is known that the COD of water reects the extent of
ꢀ
was found to be ꢁ100% at 25 C at 120 min. In addition, 100%
ꢀ
ꢀ
degradation at 10 C and 0 C was obtained at 180 min and
200 min, respectively. C16PW(O
2 2
) /PNIPAM showed excellent
performance at lower temperature; this was different from the
16
3 3 4 2 2 10 40
previously reported results of [C16H33(CH ) N] H SiV W O .
The changes in the COD and TOC are given in Fig. 7b, showing
that the removal of COD and TOC increased gradually to almost
100% at 120 min, in accordance with the phenol conversion.
This result demonstrated that C PW(O ) /PNIPAM was more
1
6
2 2
active in the CWPO treatment of phenol, whereas the interme-
diates formed during phenol degradation were oxidized to
simple inorganic species as soon as they were formed. Further,
the inuence of the pH values on the degradation of phenol was
investigated (Fig. 7c); it was observed that the degradation
efficiency of phenol over C PW(O ) /PNIPAM at the pH values
1
6
2 2
of 1.5, 1.7, 2.2, 3.0, 4.0, 5.0, 6.0, and 7.0 were 96.8, 100, 96.0,
4.2, 92.3, 88.5, 82.0, and 74.9%, respectively. The maximum
9
degradation efficiency (100%) was obtained at pH 1.7. Upon
2 2 2 2
Fig. 5 Phenol adsorption on C16PW(O ) /PNIPAM and C16PW(O ) at
different temperatures.
increasing the pH, the degradation of phenol decreased.
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Fig. 7 The main parameters affecting the degradation of phenol: temperature (a), COD and TOC (b), pH (c), H
2
O
2
concentration (d), catalyst
dosage, (e) and loading amount (f).
However, the degradation of phenol showed no signicant 14 mol of H
difference in the pH range of 0–5, with only about an 8% a high degradation of phenol, an excess of H
decrease. This result shows that C16PW(O /PNIPAM can added. The optimal hydrogen peroxide concentration was
2
O
2
is needed to oxidize 1 mol of phenol. To achieve
2 2
O
needs to be
2 2
)
perform over a wide pH range. Upon further increasing the pH 0.19 M in our experiment, which was 19 times higher than
to 7, the degradation is decreased to 74.9%, which is also active, stoichiometric concentration of 0.01 M. In addition, about
and upon prolonging the reaction time, 100% conversion of 98.9% H O was consumed at 120 min in the presence of
2
2
phenol can be achieved. This decrease may be attributed to an
C
16PW(O
2
)
2
/PNIPAM. This also indicated that some organic
increase in the pH that leads to the ionization of phenol to form intermediates were produced during the oxidation of phenol by
ꢂ
Ph-O . Because the surface charge of the POMs is negative, the
2 2 2 2
H O . Then, further increase in the H O used did not lead to
phenol molecules are repelled far away from the catalytic a higher degradation efficiency due to the decomposition of
centers; this results in a decrease in phenol adsorption by the . This usage was smaller than the previous results reported
catalyst and hence a decrease in the degradation efficiency. for [C H (CH ) N] H SiV W O .
2 2
H O
16
1
6
33
3 3
4
2
2
10 40
Fig. 7d shows the effect of H O concentration when the reac-
tion is carried out at 25 C for 120 min using 0.01 g of catalyst. dation of phenol. Upon increasing the catalyst dosage from
Fig. 7e shows the effect of the catalyst dosage on the degra-
2
2
ꢀ
Obviously, the degradation efficiency increased markedly when 0.001 to 0.01 g, the degradation efficiency was increased from 40
the concentration of H O was increased to 0.1 mL. In theory, and 100%, respectively. The result can be attributed to an
2 2
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increase in the number of catalytic active sites in the catalyst. catalyst via a simple treatment. Moreover, the total amount of
When the amount of catalyst was increased to 0.01 g, the C PW(O ) /PNIPAM leaching through the six runs of the reac-
1
6
2 2
degradation efficiency of phenol improved slowly. Therefore, tion reached only 7.4% of the starting amount, as ascertained by
.01 g was selected as the most appropriate amount of catalyst ICP (Table S2†) measurements (Fig. 8).
with a high degradation efficiency. The loading amount of
16PW(O had a signicant inuence on the degradation
0
C
2 2
)
3.3 Catalytic reaction mechanisms of the CWPO process
efficiency of phenol (Fig. 7f). The degradation efficiency
increased as the loading amount of POMs was increased. In
addition, the degradation efficiency was 55.6, 75.0, 83.5, 96.8,
and 100% at a loading amount of 2.0, 4.0, 8.0, 12.0, and
Generally, it has been shown that the oxidation process of
organic molecules by POMs occurs via a free radical mecha-
2
5,26
ꢂ
2
Superoxide radicals (O c ) are extremely high-
nism.
potential oxidizing agents with short lifetimes and can oxidize
16.0 wt%, respectively. If the loading amount continued to
27
organic substrates and generate other free radicals. To
conrm the main active radicals responsible for the degrada-
tion of phenol catalyzed by C PW(O ) /PNIPAM, a comparative
increase, C PW(O ) /PNIPAM was unable to display the
1
6
2 2
temperature-responsive property.
1
6
2 2
experiment was carried out by adding 10 wt% of the catalyst
quenchers into the initial solution under the same conditions
3.2 Reusability of catalyst
(
Fig. 9). The possible oxidative intermediate radicals, such as
The stability of a catalyst is one of the most important factors in
the heterogeneous CWPO process. The FTIR spectra of the
1
ꢂ
2 2 2
), superoxide (O c ), hydroperoxy (HO c) or
singlet oxygen ( O
OHc), were demonstrated indirectly through the use of appro-
(
recovered C16PW(O
2 2
) /PNIPAM was investigated (Fig. S3†). In
priate quenchers for these radicals based on otherwise identical
conditions. Certain quenchers include KI (a quencher of OHc
radicals on the catalyst surface ), sodium azide (NaN , a singlet
oxygen quencher, but may also interact with OHc radicals ),
and p-benzoquinone (C , BQ, a quencher of the superoxide
radical ). The addition of p-benzoquinone had an inuence on
the process of phenol degradation; this suggested that the
active oxidative species in this system was a combination of
the IR spectrum of fresh C16PW(O
2
)
2
/PNIPAM, the peaks
ꢂ1
(Fig. S3a†) at 1166, 981, 826, and 723 cm
were usually
28
3
assigned to n(P–O), n(W]O), n(O–O), and n[W(O )], respectively.
2
29
30
3ꢂ
These peaks suggest the existence of the [PO {WO(O ) } ]
4
2 2 4
6
4 2
H O
group in the hybrid. In addition, the peaks at 2930 and
31
ꢂ1
+
1
3 3
455 cm were due to N–H, C–H, and C–N in [C16H33N(CH ) ] .
ꢂ1
The three characteristic peaks at 3270, 1640, and 1290 cm can
be attributed to n(–NH–C]O), which are the intense vibrations
ꢂ
1
O
2
c
2
and singlet oxygen ( O ) free radicals. The effect of
23
of PNIPAM. As compared to the IR spectrum of C16PW(O
2
)
2
,
p-benzoquinone was much higher than that of KI and higher
than that of NaN ; this indicated that O c was formed as
a possible oxidative intermediate species during the reaction.
However, KI promotes the degradation of phenol. It might be I
enhancing the transmission of electrons and accordingly
improving the transmission of the peroxy-chain. The above-
mentioned results indicate that the reaction of phenol
oxidation underwent a superoxide radical and single oxygen
mechanism.
some red shis of the n(W]O), n(O–O), and nasym[W(O
2
)]
ꢂ
3
2
occurred; this indicated the existence of some interactions
24
between C PW(O ) and PNIPAM. These interactions were
1
6
2 2
ꢂ
attributed to the hydrogen bonds formed between the terminal
oxygens of C16PW(O and the NH groups in PNIPAM, which
conrmed the little leaching of POMs from PNIPAM. The IR
spectrum of the recovered C16PW(O /PNIPAM showed no
changes in the characteristic peaks for [C16
2 2
)
2 2
)
+
H
33N(CH
, and PNIPAM; this indicated the stability of
C PW(O ) /PNIPAM during the oxidative reaction. The DR-UV-
3 3
) ] ,
3
ꢂ
[
PO {WO(O ) } ]
4 2 2 4
Therefore, the mechanism of CWPO degradation of phenol
over C PW(O ) /PNIPAM catalyst was proposed as follows:
1
6
2 2
1
6
2 2
vis spectra of C PW(O ) /PNIPAM (Fig. S4a†) had a signicant
1
6
2 2
2 2
characteristic peak at 240 nm. In addition, C16PW(O ) /PNIPAM
aer the degradation of phenol (Fig. S4b†) was the same as the
original. The stability of the catalyst was also demonstrated by
3
1
P magic-angle spinning (MAS) NMR spectroscopy (Fig. S5†).
3
1
2 2
The P MAS NMR spectrum of C16PW(O ) /PNIPAM aer the
reaction further determined its stability during the oxidation
reaction. The morphology of C PW(O ) /PNIPAM before and
1
6
2 2
aer the oxidative reaction was examined by SEM (Fig. S6a†),
showing that no changes occurred during the reaction. There-
fore, the C16PW(O
2
)
2
1
/PNIPAM, as characterized by IR spectra,
3
DR-UV-vis spectra, P MAS NMR spectra, and SEM before and
aer the degradation of phenol, kept its original structure and
morphology. The results showed that the catalyst remained
perfectly organized aer the reaction. Moreover, the catalyst
retained its high efficiency in the process of phenol degradation
aer six repeated experiments. Therefore, the leaching test
2 2
revealed that C16PW(O ) /PNIPAM had fantabulous stability
2 2
Fig. 8 The reuse of C16PMo(O ) /PNIPAM (a) and the amount of
and could be reused six times as a rapid thermosensitive
C
16PMo(O leached from the polymer (b).
2 2
)
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Fig. 9 The effects of scavenging agents including KI, NaN
3
, and 1,4- Fig. 10 The concentration of the intermediate products during the
benzoquinone (BQ) on the degradation of phenol. Reaction condi- reaction. Reaction conditions: C16PW(O
2
2
) /PNIPAM (0.01 g), phenol
ꢀ
tions: C16PW(O
catalyst, H
2
)
2
/PNIPAM (0.01 g), scavenging agents: 10 wt% of the solution (0.72 mM, 5 mL), 25 C, 2 h.
ꢀ
O
2 2
(0.1 mL), phenol solution (0.72 mM, 5 mL), 25 C, 2 h.
catalytic cycle was completed by the reoxidation of POMred to its
32
2 2
POMox form using H O .
2 2
(1) the reactants (phenol and H O ) were adsorbed on the
expansive membrane by the transmission peroxide chain via
the interaction between hydrogen peroxide and the O–O chain 4. Conclusions
in C PW(O ) . Then, the phenol molecules were accessible to
1
6
2 2
2 2
In this study, a POM thermosensitive catalyst C16PW(O ) /PNI-
the active sites anchored within micellar particles of
C PW(O ) in the lattice of PNIPAM (Scheme 1). This adsorp-
PAM was used in the CWPO of phenol under ambient conditions
1
6
2 2
ꢀ
and even at lower temperatures (0 C). This thermosensitive POM
tion reached an equilibrium aer the suspension was stirred for
ꢂ
catalyst exhibited high efficiency in active H O to degrade
2
2
3
0 min; (2) the POMred was formed. Then, O
2
c
can be formed
ꢀ
ꢀ
ꢀ
phenol at 25 C, 10 C, and even 0 C within a short time (100% of
degradation efficiency) due to the phase changes that occurred
upon varying the temperature. The high efficiency of the catalyst
was due to the catalytic center of C PW(O ) and the synergistic
via an O–O chain auto-oxidation process in the presence of
, such that the organic molecules can be degraded by the
2 2
H O
radical to accomplish the degradation of phenol. Phenol was
oxidized rst into catalysts and the peroxo-group was trans-
ferred to phenol molecules, while R$and reduced form of p-
benzoquinone (abbreviated as p-BQ), and seldom o-benzoqui-
none (abbreviated as o-BQ) and then into oxalic acid (abbrevi-
ated as OA), formic acid (abbreviated as FA) and propionic acid
1
6
2 2
effects of the polymer support, which enhanced the concentra-
tion of phenol molecules around the active sites. Moreover, the
temperature-responsive property and high stability are benecial
in the recovery of the catalyst, which exhibited consistent activity
for six reaction cycles. Therefore, CWPO based on the POM
C PW(O ) /PNIPAM catalyst is of great value for green chemistry
(abbreviated as PA) measured by HPLC (Fig. 10). In the begin-
1
6
2 2
ning of the reaction, the phenol molecules are mostly decom-
posed to OA and a small amount of FA and PA. Then, some of
the oxalic acid molecules degraded to FA, which was tested
during the degradation of oxalic acid under the same reaction
conditions (Fig. S8†). Oxalic acid was rapidly degraded at the
end of the reaction. In addition, they decomposed gradually and
and C PW(O ) /PNIPAM has potential for the CWPO of phenol
1
6
2 2
especially in low temperature regions.
Conflicts of interest
There are no conicts to declare.
2
were totally mineralized into CO and water. (3) Finally, the
Acknowledgements
This work was supported by the National Natural Science
Foundation of China [51578119].
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Scheme
1 A schematic of the orientation of the reactants in
2 2 2
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