Thermo-responsive polymer micelle-based nanoreactors for intelligent polyoxometalate catalysis

Article abstract of DOI:10.1016/j.catcom.2014.09.016

A series of polymer micelle-based nanoreactor containing polyoxometalates and poly(N-isopropylacrylamide)-block-poly(L-lysine) (PNIPAM-b-PLys-POM) have been successfully constructed. By adjusting the molar ratio of NIPAM/Lys, the temperature-depend cataly

Full text of DOI:10.1016/j.catcom.2014.09.016

Catalysis Communications 58 (2015) 164–168
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Thermo-responsive polymer micelle-based nanoreactors for intelligent
polyoxometalate catalysis
Xiaowei Wu, Yanfang Hu, Xiaohong Wang, Li Chen ⁎
Department of Chemistry, Northeast Normal University, Changchun 130024, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 July 2014
Received in revised form 9 September 2014
Accepted 10 September 2014
Available online 19 September 2014
A series of polymer micelle-based nanoreactor containing polyoxometalates and poly(N-isopropylacrylamide)-
block-poly(L-lysine) (PNIPAM-b-PLys-POM) have been successfully constructed. By adjusting the molar ratio
of NIPAM/Lys, the temperature-depend catalytic activity of PNIPAM-b-PLys-POM towards catalytic wet hydrogen
peroxide oxidation can be controlled. This work highlights the potential of using thermo-responsive micelles as a
platform for developing smart catalyst system.
©
2014 Elsevier B.V. All rights reserved.
Keywords:
Thermo-responsive
Polyoxometalates
Smart catalysts
1
. Introduction
(N-isopropylacrylamide) (PNIPAM), poly (N, N-diethylacrylamide)
PDEAM) and their derivatives, undergo sharp and reversible phase tran-
(
Stimuli-responsive recyclable catalysts, namely “smart” catalysts,
sitions at their lower critical solution temperature (LCST), resulting in spe-
cial solubility behavior in aqueous solutions. Using thermo-responsive
polymers to load metal particles or to combine them with organic mole-
cules to prepare “smart” catalysts may switch on and off the catalytic
activities by adjusting the temperature below or above the LCST. A
thermo-responsive nanoreactor for catalyst shows a homogenerous na-
ture at a temperature below the LCST, whereas phase separation occurs
at a temperature above the LCST, which can be easily recycled from the
solution for the next cycle [8]. Hiromi Hamamoto et al. [9] reported a
have gained increasing attention in recent years [1]. Polymeric micelles
are useful functional assemblies due to their well-defined phase-
separated core–shell structure. The uses of micelle formed by amphi-
philic copolymers in the stabilization of metal nanoparticles have been
extensively studied and some metal-functionalized micelles have been
utilized in a range of catalysis reactions such as hydrogenations, oxida-
tions, reductions, and Heck reactions [2,3]. Although these catalytic sys-
tems are effective for many reactions, the colloidal nature of the catalyst
sites and the difficulty in recycling of catalysts have largely limited their
further development and applications. To improve the performance of
the micelle-based catalysis system, combing metal nanoparticles with
stimuli-responsive polymeric micelles may generate novel functions
in reaction processes [4].
Polyoxometalates (POMs), the oxo-clusters of early transition
metals in their highest oxidation states, are extensively used as homo-
geneous and heterogeneous catalysts [5]. Traditionally, to overcome
the disadvantages of the homogeneous POM-catalytic systems includ-
ing weak thermal stability, low surface area and separation problems
from reaction, many measures have been taken by supporting or encap-
sulating them on suitable materials [6]. However, few studies had
drawn the attention to the controllable and recyclable catalysis field
networked PNIPAAm-PW12
O40 catalyst, which showed an increase in cat-
alyst affinity for organic substrates in water at higher temperatures and a
loss of affinity at lower temperatures, allowing easy separation of the or-
ganic products upon completion of the reaction. This kind of catalyst
could be applied in organic/aqueous two-phase system. With the devel-
opments of green chemistry and biological engineering, the aqueous-
phase catalysis system has drawn much attention during the last decades,
and the exploitation of smart catalyst which can be implemented in an
aqueous solution became a useful strategy to achieve the goal of green
chemistry [10].
In this report, a series of polymer micelle-based nanoreactor con-
taining molybdovanadophosphoric polyoxometalates and poly(N-
isopropylacrylamide)-block-poly(L-lysine) (PNIPAM-b-PLys-POM)
was prepared via the electrostatic force between POMs and the amino
groups of poly(L-lysine). And then, the catalytic wet hydrogen peroxide
oxidation (CWPO) of phenol was chosen as a catalysis model. The cata-
lytic activity of PNIPAM-b-PLys-POM could be controlled by not only the
temperature but also the molar ratio of NIPAM/Lys. This work highlights
the potential of using these temperature-responsive micelles as a plat-
form for intelligent controllable heterogeneous catalysts.
[
7].
Catalysts combined with thermo-responsive polymers have attracted
much interest to date. Typical thermo-responsive polymers, such as poly
⁎
Corresponding author. Tel.: +86 431 85099667; fax: +86 431 85099668.
E-mail address: chenl686@nenu.edu.cn (L. Chen).
http://dx.doi.org/10.1016/j.catcom.2014.09.016
566-7367/© 2014 Elsevier B.V. All rights reserved.
1

X. Wu et al. / Catalysis Communications 58 (2015) 164–168
165
Scheme 1. Synthesis routes of PNIPAM-NH2 (a), PNIPAM-b-PZL (b) and PNIPAM-b-PLys (c).
2
. Methods
2.2. Preparation of catalysts
2
.1. Material and reagent
2.2.1. Synthesis of PNIPAM-b-PZL
Amine-terminated PNIPAM (PNIPAM-NH
2
) was synthesized and
ε-(Benzyloxycarbonyl)-L-lysine and trifluoroacetic acid were pur-
characterized according to Ref. [16]. The PNIPAM-b-PZL copolymers
were synthesized by the ROP of Lys (Z)-NCA using PNIPAM-NH as a
macro-initiator. Typically, 1.0 g of PNIPAM-NH and 2.0 g of Lys(Z)-
NCA were dissolved in anhydrous DMF in a dried flask, respectively.
The DMF solution of PNIPAM-NH was then added into the DMF solu-
chased from GL Biochem (Shanghai) Ltd., China. Lys (Z)-NCA was pre-
pared according to the method described by the Fuchs-Farthing using
triphosgene [16]. 33% HBr solution in acetic acid (HBr/AcOH) was pur-
chased from Acros. N,N-Dimethylformamide (DMF) was obtained
from the Shanghai Chemical Reagent Co. and used after distillation
2
2
2
tion of Lys(Z)-NCA under stirring. The solution was stirred for 72 h at
40 °C under argon atmosphere. Then DMF was evaporated under re-
duced pressure and the resulting product was dissolved in 40 mL of
chloroform, and then precipitated into an excess of diethyl ether. The
yield was 2.4 g (89.2%). The composition of PNIPAM-b-PZLL was deter-
5 2
under reduced pressure. H PV Mo10O40 was synthesized according to
the previous reference [17]. Other chemicals were obtained from com-
mercial suppliers, they were of AR grade and used without further
purification.
1
mined by H NMR in trifluoroacetic acid.
2.2.2. Preparation of PNIPAM-b-PLys
PNIPAM-b-PLys was prepared by the deprotection of PNIPAM-b-
PZys using HBr. Typically, PNIPAM-b-PZL of 1.0 g was dissolved in
.0 mL of trifluoroacetic acid. Then, 4 equiv of 33% HBr/AcOH with re-
8
spect to the benzyl carbamate (Z) group was added, and the solution
was stirred for 1 h at room temperature. To precipitate the polymer,
the solution was slowly poured into an excess of diethyl ether under
vigorous stirring. Then the product was washed with diethyl ether 5
times, and the polymer was isolated and dried at room temperature.
Subsequently, the product was dissolved in double distilled water and
dialyzed with a cellulose membrane (cutoff Mn = 3500) for 3 days.
1
After dialysis, the solution was immediately lyophilized. The H NMR
measurement of the obtained PNIPAM-b-PLys was carried out in
trifluoroacetic acid to determine the composition.
2.2.3. Loading of POMs
5 2
PNIPAM-b-PLys and H PV Mo10O40 (5 mol equiv. as an ammonium
unit) were added together into 20 mL aqueous solution. A self-
assembly process of PNIPAM-b-PLys with H PV Mo10 40 in water at
room temperature resulted in the formation of PNIPAM-b-PLys-POM,
5
2
O
Fig. 1. 1H NMR spectrum of PNIPAM-b-PZL (a) and PNIPAM-b-PLys (PNL-2) (b).

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X. Wu et al. / Catalysis Communications 58 (2015) 164–168
Fig. 2. TEM image of spherical aggregates formed from PNL-2 (a) and PNL-2-POM (b) at pH 5.0 and 45 °C.
then dialyzed with a cellulose membrane (cutoff Mn = 3500) for
recycled catalyst was also assessed under an identical condition. Phenol
concentrations were monitored by means of 4-aminoantipyrine (4-AAP)
spectrophotometric method using UV–vis spectroscopy at 510 nm.
3
days. After dialysis, the solution was immediately lyophilized.
2
.3. Catalyzed oxidation of phenol
3. Results and discussion
0
.50 g of catalysts was suspended by magnetic stirring in a fresh
aqueous phenol solution (50 mg/L, 100 mL) in a three-neck glass flask
at 25 °C, 0.50 ml of H (30 wt.%, 4.89 mmol) was dropwise added at
an interval of 20 min (after a certain sample was withdrawn). A 1 mL so-
lution was sampled at an interval of 20 min, then diluted to 10 mL and
mixed with 0.1 g of thiosulfate for at least 15 min to eliminate residual
Poly(N-isopropylacrylamide)-block-poly(L-lysine) was synthesized
O
2 2
according to the previous work [11] as shown in Scheme. 1. PNIPAM-
block-PZL copolymers were firstly prepared by the ring-opening poly-
merization (ROP) of ε-benzyloxycarbonyl-lysine N-carboxyanhydride
2
(Lys(Z)-NCA) using PNIPAM-NH as a macro-initiator. And after the
H
2
O
2
, catalyst was separated by centrifugation (10,000 rpm) and filtered
de-protection of PNIPAM-b-PZL by HBr, PNIPAM-b-PLys was successful-
ly prepared. The structures of PNIPAM-b-PZL and PNIPAM-b-PLys were
confirmed by H NMR spectra (Fig. 1). The disappearance of the peaks at
by 0.22 μm membranes before analysis. During the first 60 min, the ex-
periment was conducted at room temperature, then in the following
1
6
H
0 min, the temperature was heated to 50 °C slowly without adding
and the last 60 min was carried out at 50 °C with H adding.
All experiments were repeated three times. The catalytic activity of the
5.02 and 7.27 ppm assigned to benzyl group indicated that the protec-
tive group in the polymers was completely removed. Estimating the
peak intensities of the signal of ε-methylene protons (5.02 ppm)
2
O
2
2 2
O
Fig. 3. Phenol degradation obtained with PNL-1-POM (a) and PNL-3-POM (b). Scheme of the complexation reaction of phenol with 4-AAP to form the p-quinoid compound (antipyrine
dyes) (c).

X. Wu et al. / Catalysis Communications 58 (2015) 164–168
167
Fig. 4. Illustration of the self-assembly process of PNIPAM-b-PLys-POM in aqueous solution above and below LCST. Long chains of PNIPAM and short chains of PLys in PNIPAM-b-PLys-POM
10:1) (a) comparable molar ratio of NIPAM/Lys in PNIPAM-b-PLys-POM (1:1) (b).
(
attributed to PZL block and the signal of methenyl group proton
4.05 ppm) of the isopropyl group in the PNIPAM block in the spectrum,
Herein, H
5 2
PV Mo10O40 catalyzed oxidation of phenol in the presence
(
of H was chosen as a model reaction to test the thermo-responsive
2 2
O
the composition of PNIPAM-b-PZL could be obtained as listed in
Table S1. Generally, adjusting the feed molar ratios of monomer to
macro-initiator, the molecular weights could be controlled. Compared
catalytic property of the PNIPAM-b-PLys-POM [12]. A typical procedure
was carried out listed in experimental. Oxidative coupling of phenol
with 4-AAP in alkaline solution leads to formation of the antipyrine
dyes (Fig. 3c), which show maximum absorbance at 510 nm [13,14].
As shown in Fig. 3, both PNL-1-POM and PNL-3-POM exhibited the com-
parable catalytic performance compared with homogeneous catalysis of
1
with the H NMR spectra before de-protection, there was no change in
1
the number average DP (degree of polymerization, calculated from H
NMR spectra) after de-protection, suggesting that there was no cleavage
of the polymers that occurred during the de-protection.
5 2
H PV Mo10O40 during the first 120 min. However, when the tempera-
The self-assemble behaviors of PNIPAM-b-PLys were investigated by
a fluorescence technique using pyrene as the probe. The obtained criti-
cal micelle concentrations (CMCs) of PNIPAM-b-PLys were listed in
Table S1.
We know that PLys is a kind of cationic polymer considered to be ca-
pable of electrostatic interaction with polyanion. It is hydrophilic when
the amine groups are charged at low pH value. When the temperature is
up to LCST, PNIPAM became hydrophobic, thus PNIPAM-b-PLys would
self-assembly into the micelle-like aggregates with PNIPAM as cores
and PLys-POM as shells. The TEM measurement was performed at
pH 5.0 and 45 °C. As can be seen in Fig. 2, the aggregates were uniform
spheres and the mean diameter was about 200 nm.
Fig. S1 shows the temperature dependence of optical transmittance
at 500 nm for 0.3 mg/mL aqueous suspension of PNIPAM-b-PLys at
pH = 5.0. To obtain temperature-dependent spectra, the initial temper-
ature was kept at 25 °C and increased to 50 °C at a rate of 0.5 °C/min. The
mixture was a slightly white clear solution at 25 °C and was set as 100%
transmittance. It was found that at pH 5.0 with the lysine content in-
creasing, the LCST of the copolymer solutions became higher, suggesting
that the hydrophilicity of the PLys block would significantly affect the
solution behavior of the corresponding copolymer.
ture was increased above LCST of PNIPAM to 50 °C, the reaction rate
of PNL-1-POM dropped obviously, much lower than that of the homoge-
neous catalysis (Fig. 3a). As to PNL-3-POM, a remarkably faster catalytic
activity than that of homogeneous catalysis was observed in Fig. 3b. Fur-
thermore, the experiments of PNL-2-POM and PNL-4-POM also showed
the improved catalytic activity compared with homogeneous catalysis
(Fig. S2). For PNL-0-POM, it showed a catalytic activity similar to H O
2 2
oxidation of phenol due to without PLys to carry more POMs to catalyze
phenol (Fig. S3). According to our previous work, the catalyzed oxida-
5 2
tion of phenol with H PV Mo10O40 would lead to total mineralization
[15]. The degradation of phenol with PNL-1-POM and PNL-3-POM for
several runs was conducted and the date was listed in Table S2 and
Table S3 of Supplementary Data. As shown, the recycled catalyst showed
the same activity and thermo-responsiveness, indicating that such
temperature-responsive behavior was stable and reversible.
A comparison of PNL-i-POM (i = 1, 2, 3, 4) that applied in catalyzed
oxidation of phenol during the period of 50 °C (Fig. S3) revealed notably
that the polymer micelle-based nanoreactor PNIPAM-b-PLys-POM ex-
hibited a better catalytic activity than homogeneous catalysis if the
molar ratio of NIPAM/Lys was in the certain range (about 3:1 to 1:3
for this work). When the molar ratio of NIPAM/Lys was higher than a

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X. Wu et al. / Catalysis Communications 58 (2015) 164–168
certain value (10:1 for this work), the reaction rate would drop quickly.
Based on these results, we conclude a mechanism illustrated in Fig. 4. If
the chain length of NIPAM was much bigger than that of Lys, when the
catalytic system was conducted in a temperature above LCST of NIPAM,
the long NIPAM chain would become lyophobic and coat PLys-POM,
leading the catalyst to be precipitated from the aqueous solution, thus
preventing the contact between POMs and subtracts (Fig. 4a). In a cer-
tain range of NIPAM/Lys, the amphiphilic block copolymers could self-
assemble into the micelle, the hydrophilic PLys-POM became the shell
in the micelle-based nanoreactor, thus increasing the contact area
with substrates and achieving a faster catalytic rate than that of homo-
geneous catalysis (Fig. 4b). When PNIPAM-b-PLys-POM was in aqueous
solution at room temperature, they expanded largely and demonstrated
a similar form with homogeneous catalyst, showing a comparable cata-
lytic performance compared with homogeneous catalysis.
Acknowledgments
This research was financially supported by the Jilin Human Re-
sources and Social Security Bureau (201125020), the National Natural
Science Foundation of China (Projects 51273037 and 50903012), and
the Jilin Science and Technology Bureau (20130206074GX, Internation-
al Cooperation Project 20120729).
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2014.09.016.
References
[
[
[
1] J. Zhang, M. Zhang, K. Tang, F. Verpoort, T. Sun, Small 10 (2014) 32.
2] B.M. Rossbach, K. Leopold, R. Weberskirch, Angew. Chem. Int. Ed. 45 (2006) 1309.
3] Y. Liu, Y. Wang, Y. Wang, J. Lu, V. Piñón, M. Weck, J. Am. Chem. Soc. 133 (2011)
1
4260.
4
. Conclusion
[
[
4] A. Doring, W. Birnbaum, D. Kuckling, Chem. Soc. Rev. 42 (2013) 7391.
5] C.L. Hill, Chem. Rev. 98 (1998) 1.
[6] L. Cronin, A. Muller, Chem. Soc. Rev. 98 (2012) 417333-.
A series of polymer micelle-based nanoreactor containing thermo-
responsive copolymers and polyoxometalates (PNIPAM-b-PLys-POM)
was successfully fabricated. The molar ratio of NIPAM/Lys in PNIPAM-
b-PLys-POM had a great effect on the morphology of micelle formed
in aqueous solution (above LCST of copolymers, low pH), resulting in
the significantly different catalytic activity in catalytic wet hydrogen
peroxide oxidation of phenol. It should be noted that these thermo-
responsive polymer micelle-based nanoreactor could be applied in an
aquatic environment, meanwhile, the catalytic rate can be regulated
by the molar ratio of NIPAM/Lys and the temperature, outlining a poten-
tial for a promising platform for intelligent controllable heterogeneous
catalysts.
[7] J. Rieger, T. Antoun, S.H. Lee, M. Chenal, G. Pembouong, J. Lesage de la Haye, I.
Azcarate, B. Hasenknopf, E. Lacôte, Chem. Eur. J. 18 (2012) 3355.
[
[
8] S. Ikegami, H. Hamamoto, Chem. Rev. 109 (2009) 583.
9] H. Hamamoto, Y. Suzuki, H. Takahashi, S. Ikegami, Adv. Synth. Catal. 349 (2007)
2685.
[
[
10] Z. Guo, B. Liu, Q. Zhang, W. Deng, Y. Wang, Y. Yang, Chem. Soc. Rev. 43 (2014) 3480.
11] C. Zhao, X. Zhuang, C. He, X. Chen, X. Jing, Macromol. Rapid Commun. 29 (2008)
1810.
[12] S. Zhao, X. Wang, M. Huo, Appl. Catal. B Environ. 97 (2010) 127.
13] E. Emerson, J. Org. Chem. 08 (1943) 417.
14] R. Martin, Anal. Chem. 21 (1949) 1419.
15] X. Wu, X. Chen, H. Guan, X. Wang, L. Chen, Catal. Commun. 51 (2014) 29.
[16] W.H. Daly, D. Poche, Tetrahedron Lett. 29 (1988) 5859.
17] R.W. Martin, Anal. Chem. 2 (1) (1949) 1419.
[
[
[
[