Linear amphiphilic TEMPO-grafted poly(ether sulfone) as polymeric interfacial catalyst: Synthesis, self-assembly behavior, and application

Article abstract of DOI:10.1016/j.reactfunctpolym.2016.06.005

In this study, we report the development of a novel recyclable polymer-supported interfacial catalyst for Montanari oxidation. The catalyst was prepared by immobilization of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) on poly(ether sulfone) (PES) bridged by imidazole groups and characterized by ¹H nuclear magnetic resonance (NMR) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, elemental analysis, contact angle measurement, and transmission electron microscopy (TEM). This well-designed polymer was self-assembled into nanoaggregates in CH₂Cl₂. It was then used as Pickering emulsifier in Montanari oxidation system (NaClO/NaBr/immobilized TEMPO) for selective oxidation of different alcohols, which exhibited higher activity due to the enhanced mass transfer through microreactor mechanism. Moreover, this polymeric interfacial catalyst was pH-sensitive and could be easily recycled by adding small amount of acid, and subsequent cycles of alcohol oxidation showed no loss on either conversion or selectivity. This study represents an innovative approach for developing polymeric interfacial catalyst.

Full text of DOI:10.1016/j.reactfunctpolym.2016.06.005

Reactive and Functional Polymers 105 (2016) 134–139
Contents lists available at ScienceDirect
Reactive and Functional Polymers
journal homepage: www.elsevier.com/locate/react
Linear amphiphilic TEMPO-grafted poly(ether sulfone) as polymeric
interfacial catalyst: Synthesis, self-assembly behavior, and application
⁎
⁎
Liang Chen, Jun Tang, Qi Zhang , Jianli Wang
State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 April 2016
Received in revised form 10 June 2016
Accepted 12 June 2016
Available online 15 June 2016
In this study, we report the development of a novel recyclable polymer-supported interfacial catalyst for
Montanari oxidation. The catalyst was prepared by immobilization of 2,2,6,6-tetramethylpiperidine-1-oxyl
(TEMPO) on poly(ether sulfone) (PES) bridged by imidazole groups and characterized by 1H nuclear magnetic
resonance (NMR) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, elemental analysis, contact
angle measurement, and transmission electron microscopy (TEM). This well-designed polymer was self-assem-
bled into nanoaggregates in CH2Cl2. It was then used as Pickering emulsifier in Montanari oxidation system
(NaClO/NaBr/immobilized TEMPO) for selective oxidation of different alcohols, which exhibited higher activity
due to the enhanced mass transfer through microreactor mechanism. Moreover, this polymeric interfacial cata-
lyst was pH-sensitive and could be easily recycled by adding small amount of acid, and subsequent cycles of al-
cohol oxidation showed no loss on either conversion or selectivity. This study represents an innovative approach
for developing polymeric interfacial catalyst.
Keywords:
Montanari oxidation
Pickering emulsion
Poly(ether sulfone)
TEMPO
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
a liquid–liquid heterogeneous system, triphase catalysis often suffers
from low efficiency of reaction [20–22].
Mild and selective oxidation of primary and secondary alcohols into
the corresponding aldehydes or ketones is one of the most important
and challenging chemical transformations in organic synthesis [1–4].
Using stable nitroxyl radicals, such as 2,2,6,6-tetramethylpiperidine-1-
oxyl (TEMPO), as metal-free organic catalyst for alcohol oxidation has
attracted considerable research interest in the past decades, because
of its low toxicity, reversible redox behavior, and high efficiency and se-
lectivity [5–12]. However, TEMPO is expensive, and it is difficult to sep-
arate catalysts from the products by simple processing, which
significantly limits the large-scale application of TEMPO. Efficient
recycling of TEMPO is thus highly desirable for sustainability of the
industry.
To date, several strategies have been developed to recycle TEMPO.
One approach is the immobilization of TEMPO onto soluble supports
[13–16], which is often accompanied with complex recycling process
and loss of the catalyst mass. Another approach is to anchor TEMPO
onto insoluble supports, such as silica sol–gels [17], mesoporous silica
[18], and polystyrene beads [19]. Catalyst recycling is much simpler,
usually by filtration and centrifugation. However, because the most
studied TEMPO-catalyzed oxidation reaction (Montanari oxidation) is
Fortunately, the advent of particle-stabilized emulsion, the so-called
Pickering emulsion, offered an alternative route for increasing the reac-
tion efficiency. Submicrometer- and nanometer-sized particles with ap-
propriate wettability can be self-assembled at the liquid–liquid
interface to stabilize the interface between water and oil. The emulsion
reaction medium can thus be regarded as highly dispersed
microreactors, which could offer a large specific area of reaction inter-
face [23–25]. So far, several types of TEMPO-grafted Pickering emulsi-
fiers have been developed and have shown appreciable increase in the
efficiency of Montanari oxidation [26–28]. Among these, polymeric par-
ticles have been believed to be one of the best Pickering interfacial cat-
alysts, because the amphiphilicity of the particle surface could be better
regulated.
In this study, we report the manufacture of a unique pH-sensitive
polymeric interfacial catalyst (PIC) (Scheme 1). The catalyst was pre-
pared by immobilization of TEMPO on poly(ether sulfone) (PES)
bridged by imidazole groups. This well-designed polymer was self-as-
sembled into nanoaggregates in CH2Cl2, and was further used as Picker-
ing emulsifier in Montanari oxidation system for selective oxidation of
different alcohols. The catalyst showed superior catalytic activity for se-
lective oxidation of alcohols, because of the enhanced mass transfer
through microreactor mechanism. Moreover, because of the high pH-
responsiveness of the imidazole groups, the amphiphilicity of the cata-
lyst could be tuned by pH, leading to the easy recycling of this interfacial
⁎
Corresponding authors.
E-mail addresses: qizhang@zjut.edu.cn (Q. Zhang), wangjl@zjut.edu.cn (J. Wang).
http://dx.doi.org/10.1016/j.reactfunctpolym.2016.06.005
1381-5148/© 2016 Elsevier B.V. All rights reserved.

L. Chen et al. / Reactive and Functional Polymers 105 (2016) 134–139
135
Scheme 1. Chemical structure of polymeric interfacial catalyst, and sketch of its application in Montanari oxidation system for selective oxidation of alcohols.
catalyst. The subsequent cycles of alcohol oxidation showed no loss on
either conversion or selectivity. To the best of our knowledge, there
has been no report on using TEMPO-grafted linear polymer as Pickering
emulsifier for Montanari oxidation.
2.4. Synthesis of catalyst PES-im-TEMPO
PES-im (0.50 g), BrC4TEMPO (0.15 g, 0.49 mmol), and K2CO3 (0.08 g,
0.58 mmol) were dissolved in DMF (10 mL) at room temperature. The
mixture was heated to 80 °C and kept for 12 h. The obtained product
was collected by precipitation in ethanol, washed alternately with
water and ethanol thrice, and finally dried in a vacuum oven at 60 °C
overnight with a yield of 0.48 g (90%).
2. Materials and methods
2.1. Materials
2.5. Characterization
PES (E6020P, Mw
= 58,000) was obtained from BASF.
Aqueous sodium hypochlorite (NaClO, active chlorine 5%), imidaz-
ole, dibromobutane, tetrabutylammonium hydrogen sulfate
(Bu4NHSO4), and alcohols were used as-received from Aladdin
Co. Ltd. TEMPO, 4-hydroxy-2,2,6,6,-tetramethylpiperidine-1-oxyl
(4-OH-TEMPO), chloromethyl methyl ether (CMME), petroleum
ether, ethyl acetate, and diethyl ether were bought from Sigma-
Aldrich. Bromobutyl TEMPO (BrC4TEMPO) was prepared according
to the reported protocol [28]. All other chemicals were of analytical
grade and used as received.
Nuclear magnetic resonance (NMR) spectra were acquired on a
Bruker Advance III 500 MHz spectrometer using dimethyl sulfoxide
(DMSO)-d6 as solvent, and the sample was treated by phenyl hydrazine
before measurement. Infrared (IR) spectra were recorded by attenuated
total reflectance–Fourier transform infrared spectroscopy (ATR–FTIR)
using Nicolet iS10 FTIR spectrophotometer. Elemental analysis (EA)
was performed on a Vario MACRO cube elemental analyzer. The static
contact angle (CA) of water on the surface of the catalyst was deter-
mined by sessile drop method using DataPhysics (OCA-20). The mor-
phology of PES-im-TEMPO in Pickering emulsion was visualized using
a Hitachi (H-7500) transmission electron microscope (TEM) operating
at 80 kV, and the sample was prepared by placing a drop of Pickering
emulsion on a carbon-coated copper grid and the solvent was evaporat-
ed at room temperature. The conversion, purity, and relative yields of
the final products for Montanari oxidation were determined by using
Shimadzu GC-2014C equipped with a flame ionization detector and
high-purity hydrogen as the carrier gas.
2.2. Synthesis of chloromethyl poly(ethyl sulfone)
PES (10 g, 0.17 mmol) was dissolved in H2SO4 (100 mL, 98 wt%) by
mechanical stirring in a 250-mL three-neck flask. The temperature was
maintained at −5 °C and CMME (16 mL, 0.21 mol) was added dropwise
to the solution. After 30 min, the product was collected via precipitation
in deionized water, washed with deionized water thrice, and vacuum-
dried with a yield of 9.5 g (92%).
2.6. PIC-mediated Montanari oxidation of alcohols
2.3. Synthesis of PES-im
General procedure: alcohol substrate (1.0 mmol), TEMPO
(0.001 mmol, 0.068 mg) or PES-im-TEMPO (0.001 mmol, 5.0 mg), and
CH2Cl2 (3.5 mL) were charged into a 25-mL round-bottom flask. The
mixture was cooled to 10 °C, and NaBr (1.0 M, 0.15 mL) and NaClO
(0.37 M, 3.35 mL, pH ≈ 9.1) were added sequentially. The resulting mix-
ture was magnetically stirred at 1400 rpm. After a certain period of time,
the reaction was quenched by consuming hypochlorite with excess so-
dium sulfite, and the supernatant was analyzed by gas chromatography
(GC) after dried with anhydrous sodium sulfate. The catalyst
Imidazole (0.10 g, 1.47 mmol) was dissolved in 10 mL of dimethyl
fluoride (DMF) by magnetic stirring under nitrogen atmosphere. Then,
NaH (0.04 g, 1.67 mmol) was added to the solution and stirred for
30 min to obtain sodium imidazolate at ambient condition. After
chloromethyl poly(ethyl sulfone) (CMPES, 1.0 g) was dissolved in the
mixture, the temperature was raised to 60 °C and allowed to react for
30 min. The product was purified by using the same procedure as that
for CMPES. The yield of PES-im was 94%.

136
L. Chen et al. / Reactive and Functional Polymers 105 (2016) 134–139
precipitated after adding HCl (1.0 M) to adjust the pH to 2, and was col-
lected by centrifugation and used for next cycles after drying in vacuum.
The organic layer was separated and dried over anhydrous Na2SO4. The
solvent was removed under reduced pressure to result in the crude
product, which was purified by silica gel column chromatography
(ethyl acetate/petroleum ether 10/1). The yield of the product was cal-
culated by 1H NMR spectroscopy after careful determination of its
weight.
3. Results and discussion
3.1. Preparation of PES-im-TEMPO
TEMPO functionalities were grafted to PES through a three-step
modification, and the synthesis routes are shown in Scheme 2. The
chemical structures of the intermediates and final product PES-im–
TEMPO were characterized by 1H NMR (Figs. 1 and S1), 13C NMR (Fig.
S2), and FTIR (Fig. S3). Compared with PES (Fig. S1a), the 1H NMR spec-
trum of CMPES (Fig. S1b) showed an additional peak at around
4.85 ppm (c), because of the presence of the chloromethyl group
(\\CH2Cl); thus the degree of chloromethylation (DC) in CMPES was
calculated to be 15% from Eq. (1).
Fig. 1. 1H NMR spectrum of PES-im-TEMPO.
Eq. (1) is expressed as follows:
The data of EA were obtained from Eqs. (2) and (3), and the results
showed that the nitrogen weight in PES-im-TEMPO was 1.94%
(Table 1, 0.2 mmol/g), indicating that the graft ratio of TEMPO was 7%.
This result was consistent with the one determined by 1H NMR. All
these evidence have clearly shown the successful synthesis of PES-im-
TEMPO.
Ac=2
X
DC ¼
ꢀ 100%;
b;b =4
ð1Þ
0
A
where Ac and Ab,b′ are the integral area of the corresponding peaks in 1H
NMR spectrum of CMPES.
In Fig. S1c, the emerging peak at 5.3 ppm (c′) and disappeared peak
at 4.85 ppm (c) indicated the complete reaction between chloromethyl
group and sodium imidazolate. The chemical shifts at 9.22 ppm (d′) and
5.54 ppm (c″) in Fig. 1 revealed the success of grafting TEMPO groups to
PES. The integration ratio of H(c″)/H(c′) was used to calculate the Y1
and Y2 values (Scheme 2). 13C NMR spectrum and peak assignment of
PES-im-TEMPO are shown in Fig. S2, which also revealed the success
of TEMPO immobilization. As shown in Fig. S3b, the FTIR spectrum of
CMPES showed a new band at 912 cm−1, which could be attributed to
the chloromethyl group (\\CH2Cl). The peak at 912 cm−1 shown in
Fig. S3c and S3d was attributed to the C\\H bond of the imidazole
group. The synthesis of PES-im was proved by the new peak at
1028 cm−1, which was attributed to C\\N bond (Fig. S3c). In Fig. S3d,
the new peak at 1630 cm−1 was attributed to the formation of
quaternized imidazole. In addition, the peak at 1360 cm−1 showed
the existence of the N\\O radical moiety in the sample.
Eqs. (2) and (3) are expressed as follows:
m1NMS
2m1SMN
GR1
GR2
¼
¼
ꢀ 100%
ð2Þ
ð3Þ
GR1ðm2NMS−2m2SMNGR1Þ
m2NMS−m2SMNGR1
ꢀ 100%
Where GR1 and GR2 are the graft ratio of PES-im and PES-im-TEMPO,
respectively, M is the molar mass of the element, and m is the mass frac-
tion of the element in Table 1.
3.2. Pickering emulsion-based Montanari oxidation of alcohols
PES-im-TEMPO was not soluble in either water or CH2Cl2, but it can
be well dispersed in CH2Cl2 with the help of ultrasonication, because of
Scheme 2. Synthesis routes of PES-im-TEMPO.

L. Chen et al. / Reactive and Functional Polymers 105 (2016) 134–139
137
Table 1
Results of the elemental analysis of PES-im-TEMPO and PES-im.
Elements [wt%]
Entry Sample
[C]
[H]
[N]
[S]
TEMPO loading [mmol·g−1
]
1
2
PES-im
59.41 3.88 1.60 11.89
–
PES-im-TEMPO 60.01 4.51 1.94 10.28 0.2
Fig. 4. Conversion results for Montanari oxidation of benzyl alcohol with (a) no catalyst,
(b) TEMPO, and (c) PIC.
Table 2
PIC-mediated Montanari oxidation of alcohols to the respective carbonyl derivatives.a
Fig. 2. CA of water on the surface of PES-im-TEMPO.
Time
(s)
Conversion
(%)
Selectivity
(%)b
Yield
(%)c
Entry Substrate
1
2
3
4
5
6
7
8
Benzyl alcohol
1-Octanol
90
60
60
60
15
60
180
15
N99
92
N99
71
N99
91
N99
N99
N99
N99
N99
N99
N99
N99
N99
N99
91
–
–
65
–
–
its amphiphilic structure. As shown in Fig. S4, the resulting dispersion
exhibited prominent Tyndall effect, indicating the existence of colloidal
particles. In a particle-stabilized emulsion, one of the main factors that
affect the formation of emulsions is the wettability of particles [29,30].
Thus, we measured the CA of water on the PES-im-TEMPO surface. As
shown in Fig. 2, the CA was approximately 102°, indicating a relative hy-
drophobicity, which would preferentially stabilize water-in-oil (w/o)
emulsions.
2-Phenylethanol
1-Phenylethanol
3-Pyridinemethanol
1-Nonanol
diphenylmethanol
4-Nitrobenzyl
alcohol
–
90
9
10
1-Hexanol
Cyclohexanol
60
30
N99
92
N99
N99
–
–
The same volume of water was added to the PES-im-TEMPO disper-
sion in CH2Cl2, and a stable Pickering emulsion was formed after mag-
netic stirring for 90 s at 1400 rpm, as shown in Fig. 3a. The droplet
diameter of the emulsion was between 50 and 200 μm, as observed
from microscopy (Fig. 3b). TEM characterization was used to visualize
the morphology of PES-im-TEMPO nanoparticles in the emulsion. It
was found that the particles were mostly located at the water–oil inter-
face, with an average particle diameter of 69 9 nm.
a
Reaction conditions: alcohol (1 mmol), NaBr (0.15 mL, 1 M), NaClO (3.35 mL, 0.37 M,
pH ≈ 9.1), catalyst (0.001 mmol), CH2Cl2 (3.5 mL), T = 10 °C.
b
Determined by GC.
c
The yield was calculated by 1H NMR spectroscopy after careful determination of
weight.
blank test without the catalyst was first conducted, and the result
showed little conversion as expected (Fig. 4). Then, a reference run
was performed by using soluble TEMPO as the catalyst. As shown in
Fig. 4, it took approximately 4 min to attain full conversion of benzyl al-
cohol (TOF = 4.2 s−1). However, while using PES-im-TEMPO, the oxi-
dation reaction was much faster, only 90 s was needed for full
conversion (TOF = 11.1 s−1), and good selectivity was reserved
3.3. Catalytic activity evaluation of PES-im-TEMPO
Benzyl alcohol was selected as a model substrate because of its in-
trinsic fast oxidation characteristic in Montanari oxidation system. A
Fig. 3. (a) Image of PIC-stabilized emulsion (water in CH2Cl2); (b) Optical microscopy image of the emulsion droplets (scale bar: 100 μm); (c) TEM image of PIC nanoaggregates dispersed
in CH2Cl2 (scale bar: 200 nm).

138
L. Chen et al. / Reactive and Functional Polymers 105 (2016) 134–139
Fig. 5. Images of precipitation separation of PIC in the Montanari oxidation system by adding HCl solution.
(both N 99%) [31]. This could be attributed to the increase of the
biphase-specific interface [32], which could further enhance mass trans-
fer of the oxidation reaction, because of the formation of Pickering
emulsion (inset in Fig. 4). The designed polymer thus played the role
of both emulsifier and catalyst.
We also investigated the applicability of PES-im-TEMPO Pickering
interfacial catalyst in Montanari oxidation of other alcohols. Several dif-
ferent types of alcohols were selected, including primary benzylic, ali-
phatic heteroatoms containing alcohols and secondary benzylic or
aliphatic alcohols. Montanari oxidations of these alcohols were per-
formed under the same condition as benzyl alcohol, by using PES-im-
TEMPO as the interfacial catalyst. Their experimental results are sum-
marized in Table 2, which showed excellent activity and selectivity.
4. Conclusions
In summary, we have successfully developed a novel recyclable
polymer-supported interfacial catalyst for Montanari oxidation. The cat-
alyst was prepared by immobilization of TEMPO on PES bridged by im-
idazole groups. Its chemical structure and composition were confirmed
by 1H NMR, FTIR, and EA measurements. This well-designed polymer
was self-assembled to nanoaggregates in CH2Cl2 and then used as Pick-
ering emulsifier in Montanari oxidation system (NaClO/NaBr/
immobilized TEMPO) for selective oxidation of different alcohols. This
catalyst exhibited higher activity because of the enhanced mass transfer
through microreactor mechanism. Moreover, de-emulsification of the
Pickering emulsion could be achieved by adding a small amount of
acid, resulting in facile catalyst recycling. The following several cycles
of alcohol oxidation showed no loss on either conversion or selectivity.
This study represents an innovative approach for developing PIC.
3.4. Recycle of PES-im-TEMPO
In order to separate the product after Montanari oxidation reaction,
several drops of acid (1.0 M HCl) was added to achieve de-emulsifica-
tion. Clear phase separation was observed and the catalyst was precipi-
tated at the interface (Fig. 5). This was probably due to the alteration of
the amphiphilicity of PES-im-TEMPO nanoparticles by acid addition, as
the imidazole groups had high pH-responsiveness. The catalyst was
then collected by centrifuge and used in the next cycle of Montanari ox-
idation reaction after being vacuum-dried. In the following three cycles
of alcohol oxidation, the conversion of benzyl alcohol showed no loss on
either conversion or selectivity at 30 and 90 s, as shown in Figs. S5 and 6.
The 1H NMR spectra of recycled catalyst (after four cycles) did not
change, as compared with the original one (Fig. S6), indicating good re-
cyclability of this Pickering catalyst.
Acknowledgments
This study was financially supported by the National Natural Science
Foundation of China (Grants No. 21374103), Key Laboratory of Adsorp-
tion and Separation Materials & Technologies of Zhejiang Province (No.
XFFL2015), and Open foundation of Zhejiang Province Key Laboratory of
Biofuel.
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.reactfunctpolym.2016.06.005.
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