Cross-linked reverse micelles with embedded water pools: A novel catalytic system based on amphiphilic block copolymers

Article abstract of DOI:10.1039/c4ra04358h

Based on the idea of structural design, a novel catalytic system was developed from a block copolymer for the oxidation reaction of 2,3,6-trimethylphenol (TMP). The block copolymer, poly(4-vinylpyridine)-block- poly(ethylene glycol)-block-poly(4-vinylpyridine) (P4VP-PEG-P4VP), was synthesized via anionic polymerization. After self-assembly in water-1-hexanol solution and shell cross linking, the block copolymer formed the shell cross-linked reverse micelles (SCRMs). The CuCl₂ complexed SCRMs were used in the catalytic oxidation reaction of TMP. Through coordinating with metal ions, regulating the distribution of metal catalytic active centers, and with the cocatalysis effect of the immobilized water droplets, this polymer-supported catalyst system demonstrated an efficient catalytic activity and recoverability. This work not only provides a promising catalyst based on mesoscale structure design using block copolymers but is also an example for deeper understanding on the structure effect in catalysis. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra04358h

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Cross-linked reverse micelles with embedded water
pools: a novel catalytic system based on
amphiphilic block copolymers†
Cite this: RSC Adv., 2014, 4, 38234
a
b
a
Ruihan Liu, Shiping Wang, Jia Yao, Weiwei Xua and Haoran Lia
*
Based on the idea of structural design, a novel catalytic system was developed from a block copolymer for
the oxidation reaction of 2,3,6-trimethylphenol (TMP). The block copolymer, poly(4-vinylpyridine)-block-
poly(ethylene glycol)-block-poly(4-vinylpyridine) (P4VP-PEG-P4VP), was synthesized via anionic
polymerization. After self-assembly in water–1-hexanol solution and shell cross linking, the block
copolymer formed the shell cross-linked reverse micelles (SCRMs). The CuCl2 complexed SCRMs were
used in the catalytic oxidation reaction of TMP. Through coordinating with metal ions, regulating the
distribution of metal catalytic active centers, and with the cocatalysis effect of the immobilized water
droplets, this polymer-supported catalyst system demonstrated an efficient catalytic activity and
recoverability. This work not only provides a promising catalyst based on mesoscale structure design
using block copolymers but is also an example for deeper understanding on the structure effect in catalysis.
Received 10th May 2014
Accepted 6th August 2014
DOI: 10.1039/c4ra04358h
www.rsc.org/advances
and avors in the chemical industry.17 The homogeneous
copper catalyst system can achieve high conversion and selec-
tivity,18–20 but a serious drawback is that almost or even more
Introduction
Over the recent years, the development of polymer-supported
catalysts1–4 has attracted widespread attention with the
consideration of structure at nanoscopic or mesoscopic
levels,5–8 offering a viable method for catalyst design. The
designed catalysts can dramatically alter their reactivity, selec-
tivity, and efficiency by impacting the location and activity of
catalytic active centers. However, designing a more effective
polymer-supported catalyst with a specic structure is still a
challenge because of the lack of knowledge regarding the
reaction mechanism and the mesoscale structure.
Micelles from amphiphilic block copolymers have already
been developed in catalysis applications.9–11 In general, micelles
are considered to have a hydrophobic core and a hydrophilic
shell in polar solvents. However, if the solvent system is less
hydrophilic, the core and the shell may be inverted to give
reverse micelles (RMs).12,13 In addition, compared with the self-
assembled micelles formed through noncovalent interactions,
the formation of shell cross-linked reverse micelles
(SCRMs)7,14–16 can provide reinforcement and stability, even if
the environmental conditions change.
than a stoichiometric amount of catalyst is required, which is
environmentally unfriendly. Polymer-supported metal cata-
lysts21–25 are considered to be more friendly to the environment.
Takaki et al.17 succeeded in the oxidation of TMP with polymer-
supported copper catalysts. However, it took a long time to
complete the reaction and the products could only be obtained
under high O2 pressure (10 atm). The relatively low activity
might be because of the simple structure of Takaki's catalyst.
Hence, in this paper, a novel catalyst system was designed for
the oxidation of 2,3,6-trimethylphenol (TMP) by dioxygen. The
catalyst carrier is a kind of cross-linked reverse micelle formed
by the self-assembly of block copolymers with an encapsulated
water pool. The relationship between the mesoscale structure
and catalysis efficiency was investigated. It is worth mentioning
that a small amount of water can act as an activator in some
catalytic reactions,26,27 such that the structure of the aqueous
cores of RMs, even with water pools embedded, can have a
positive effect on promoting reactions.
To prepare the SCRM structure, in this work, an ABA
amphiphilic block copolymer poly(4-vinylpyridine)-block-poly-
(ethylene glycol)-block-poly(4-vinylpyridine) (P4VP-PEG-P4VP)
was synthesized via anionic polymerization. P4VP-PEG-P4VP
formed RMs in a mixed biphase solution (Scheme 1). The
hydrophilic core of P4VP-PEG-P4VP RM consists of PEG blocks,
enclosing a water pool, and the P4VP blocks constituting the
shell. 1,2-Bis(2-iodoethoxy) ethane (BIEE) was used as a cross-
linking agent to selectively quarternize the nitrogen atoms on
the pyridine groups in the micelle shell, thus forming SCRMs.
The catalytic oxidation of TMP using molecular oxygen as the
oxidant is a key step to synthesize vitamins, pharmaceuticals,
aState Key Laboratory of Chemical Engineering, Department of Chemistry, Zhejiang
University, Hangzhou 310027, PR. China. E-mail: yaojia@zju.edu.cn
bKey Lab of Applied Chemistry of Zhejiang Province, Department of Chemistry,
Zhejiang University, Hangzhou 310027, PR. China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra04358h
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Self-assembly and shell cross-linking of P4VP-PEG-P4VP
reverse micelles
Water was added into 1-hexanol to achieve a water content of
50%, forming 100 mL of biphase mixed solution. Then, the
block copolymer P4VP-PEG-P4VP was added to the solution at a
pH of 4 (polymer weight: 0.15%) and stirred at room tempera-
ture for 24 h.
Scheme 1 Structure of the amphiphilic block copolymer P4VP-PEG-
P4VP reverse micelles in 1-hexanol–water mixed solution.
1 mmol of BIEE was dissolved in 10 mL of 1-hexanol.
Aer being stirred and le standing for 24 h, the BIEE solu-
tion was added into the abovementioned P4VP-PEG-P4VP
solution in a dropwise manner to achieve shell cross link-
ing. The solution was then stirred at a low speed for at least 3
days at room temperature. Several degrees of cross linking
were studied, among which the highest was 30%. The solvent
was stripped by freeze drying, and the SCRMs dried powder
was obtained.
Through the coordination with metal ions,28–34 CuCl2–(P4VP-
PEG-P4VP) SCRMs were successfully applied in the catalytic
oxidation reaction of TMP in the 1-hexanol media, followed by
reusing the catalyst in several cycles.
Preparation of the CuCl2–(P4VP-PEG-P4VP) SCRMs complex
Experimental section
Materials
P4VP-PEG-P4VP SCRMs (0.5 mmol) were dissolved in 100 mL 1-
hexanol. A solution of the appropriate amount of copper salts
(CuCl2$2H2O, Aldrich) in 10 mL 1-hexanol was slowly added,
and then the combined solution was mixed under moderate
stirring for 12 h. Products were obtained by centrifugation,
washing and freeze drying.
Poly(ethylene glycol) (PEG, Mn ¼ 2000 g molꢀ1, Aldrich) was
dried under vacuum at 55 ꢁC for 1 h and then stored under
ꢁ
nitrogen at ꢀ15 C. 4-Vinylpyridine (4VP, Aldrich) was passed
through an activated basic alumina column to remove the
stabilizing agents and stirred with a small amount of CaH2
overnight at room temperature, followed by vacuum distilla-
tion. The 4VP was lled with nitrogen and stored in a refriger-
ator at ꢀ15 ꢁC. Tetrahydrofuran (THF) was dried under
reuxing over sodium and distilled in a nitrogen atmosphere
just before use. Potassium naphthalene was prepared by dis-
solving naphthalene in dried THF and reuxing the solution
over potassium until it turned dark blue. The feed ratio of
potassium and naphthalene was 1 : 1. The 2,3,6-trimethylphe-
nol was donated by Zhejiang NHU Company, Ltd. All the other
reagents and solvents were of analytical grade (Aldrich) and
were used without further purication.
Oxidation of TMP by O2 using CuCl2–(P4VP-PEG-P4VP) as
catalyst
Catalytic oxidation of TMP was conducted in all-glass reactor
vessels consisting of a 50 mL round-bottomed ask con-
nected reux condensing tube and ventilation capillary. TMP
(136.2 mg, 1 mmol) was dissolved in 1-hexanol (20 mL), and
stirring was continued at room temperature under O2 (1 atm).
Aer the addition of an appropriate amount of H2O into the
CuCl2–(P4VP-PEG-P4VP) SCRMs complex, the morphological
characteristic of the complex changed from powder to paste.
The catalyst, moist CuCl2–(P4VP-PEG-P4VP) SCRMs, was
added to the TMP solution and the mixture was stirred
under the conditions indicated in Table 1, which was moni-
tored by gas chromatography (GC). The precipitated
catalyst was recovered through centrifugation and thorough
washing in 1-hexanol. Aer being dried by lyophilization
and wetted again, the catalyst was reused in the next cycle
(Table 2).
Synthesis of P4VP-PEG-P4VP
Dried PEG (2.0 g) was dissolved in 100 mL of dried THF, and the
solution was transferred to a 250 mL round-bottomed ask
under nitrogen. 4 mL of a freshly prepared solution of potas-
sium naphthalene in THF (0.5 mol Lꢀ1) was injected into the
round-bottomed ask via a nitrogen-washed injector and stir-
red until the color of the solution turned light green. The mixed
solution was stirred at room temperature for 0.5 h to form the
Characterization methods
alcoholate macroinitiator. A known amount of freshly distilled Transmission electron microscopy (TEM) was performed on a
4VP was then charged to the reactor under nitrogen atmosphere Hitachi HT-7700 at an acceleration voltage of 120 kV. A drop of
ꢁ
at ꢀ78 C. Polymerization was carried out at this temperature very dilute solution was applied onto a carbon-coated TEM
for 24 h. The crude copolymer was dissolved in ethanol at 60 ꢁC copper grid. The sample was then immediately frozen by liquid
and precipitated at room temperature. The copolymer was dis- nitrogen and dried by lyophilization.
solved in ethanol at 60 ꢁC, and then transferred to a dialysis
Dynamic light scattering (DLS) measurements were per-
tube to dialyze against water for 1 week to remove small mole- formed on a Malvern Zetasizer Nano-ZS equipped with a He–Ne
cules. The diblock copolymer, poly(ethylene glycol)-block-poly(4- laser at a wavelength of 633 nm. The experimental data were
vinylpyridine) (P4VP-PEG), was synthesized using a similar analyzed by the CONTIN method, which is based on an inverse-
method for comparison.
Laplace transformation of the data and provides access to a size
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Table 1 Oxidation of TMP with CuCl2–amine liganda
Product and yield/%
Amine ligandb (mmol)
CuCl2 mol%
H2Od mol%
Conv.%
2
3
4
Mass balancee %
c
Run
1
2
3
4
5
6
7
8
—
—
—
P4VP
P4VP-PEG-P4VP
P4VP-PEG-P4VP
P4VP-PEG-P4VP
P4VP-PEG-P4VP
10
10
10
10
2
10
50
100
0
100
No reaction
tr
100
49
39
100
100
100
—
tr
44
3
—
tr
17
0
6
tr
6
—
tr
tr
tr
7
tr
tr
tr
—
tr
61
6
38
46
68
81
1000
1000
1000
1000
1000
1000
2
46
62
74
7
a
Reaction conditions: 1 (1 mmol), 1-hexanol (20 mL), O2 (1 atm, pure oxygen bubbled in from a gas cylinder), reaction time (24 h), 90 ꢁC. b Ratio of
4VP unit of P4VP to CuCl2 is unity. c Based on 1. d Based on 1. e Total yield of 2–4.
Table 2 Recycling of catalyst CuCl2–(P4VP-PEG-P4VP) in the oxidation of TMPa
Product and yield/%
Catalyst
Recycle
CuCl2 mol%
Conv.%
2
3
4
Mass balanceb%
Non-cross-linked CuCl2–(P4VP-PEG-
P4VP)
First
100c
68.4
52.7
100
95.9
83.3
81.6
63.3
100
100
98
100
100
100
100
100
78
66
57
74
88
87
79
68
5
3
2
2
tr
8
4
1
1
86
68
60
81
98
94
86
69
Second
Third
First
Second
Third
Fourth
Fih
tr
tr
7
2
3
30% Cross-linked CuCl2–(P4VP-PEG-
P4VP)
7
tr
a
Reaction conditions: 1 (1 mmol), 1-hexanol (20 mL), O2 (1 atm), reaction time (24 h), 90 ꢁC. Ratio of H2O to CuCl2 is 10 in the rst reaction, and the
amount is xed in following reactions. Total yield of 2–4. c Ratio of 4VP unit of P4VP to CuCl2 is 1 in the rst reaction.
b
distribution histogram for the analyzed micellar solutions. The
z potential of RM nanoparticles was determined by laser
Results and discussion
Synthesis and characterization of P4VP-PEG-P4VP
Doppler anemometry using a Malvern Zetasizer Nano-ZS. The
amphiphilic block copolymer
ꢁ
temperature was set to 25 C, and the results were normalized
with respect to the polystyrene standard solution.
The coordination of Cu(II) and SCRMs was measured by
electron paramagnetic resonance (EPR) on a Bruker A300 with
X-band frequencies at 108 K. The solution of 10ꢀ4 M copper-
containing SCRMs in DMSO was transferred to an EPR tube
and shock frozen in liquid nitrogen to obtain a transparent,
glassy sample.
The yields of all the products obtained from TMP oxidation
were determined by GC (Agilent GC-2014) using ethyl benzoate
as an internal standard.
P4VP-PEG-P4ꢁVP was synthesized by anionic polymerization in
THF at ꢀ78 C using potassium naphthalene as the initiator.
Vamvakaki et al.35 synthesized the poly(ethylene oxide-tertiary
amine methacrylate) with potassium naphthalene as a func-
tional initiator. In our study, anionic polymerization was initi-
ated with potassium naphthalene from PEG, and P4VP-PEG-
P4VP was obtained by the polymerization of 4VP in a
controlled mechanism (Scheme 2).
1
Analysis of the polymer including H NMR and GPC can be
found in ESI Fig. S1 and S2.† The ratio of polymer to monomer
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A small amount of CuCl2 was used as a staining agent to
determine the location of the pyridine group in the RMs. The
TEM image in Fig. 1c indicates that the pyridine groups were
located on the shell. The small dots near the exterior surface
were the coils of P4VP molecular chain, generated by the coor-
dination of Cu2+ and pyridine groups.
The size of the water pool in the RMs could be controlled by
the component ratio of the solution.15 The morphologies of
P4VP-PEG-P4VP self-assemblies at different component ratios
of solution are illustrated in Fig. 2. ESI Fig. S3† shows the
particle size distribution of P4VP-PEG-P4VP copolymer self-
assemblies according to DLS measurements, and the result is
consistent with that obtained from TEM. When only 1-hexanol
was used, the polymers could not form a stable aggregation. On
increasing the ratio of water in the mixture of water–1-hexanol,
micelles can form. When the ratio of water was higher, the water
pool in RMs could form more easily. When the mass ratio of
water–1-hexanol reached 1 and above (Fig. 2d and e), the core–
shell structure micelles could be obtained. As the ratio of water–
1-hexanol increased from 1.5 to 4, the water pool inside the RM
became bigger. When the solution was pure water, the polymers
formed large thin lms.
The exterior surface properties of P4VP-PEG-P4VP RMs were
investigated by measuring z potential36–38 (see ESI Fig. S4 and
S5†). At a positive potential, the RMs showed a stable structure.
With increasing pH, the potential declined for decreasing
degree of protonation of P4VP. The z potential became negative
at pH above 5.4, and the structure of RMs was unstable.
Following self-assembly, BIEE was used to crosslink the shell
layer, reinforcing the RM structure through covalent bonding.
Scheme 3 describes the formation procedure of P4VP-PEG-P4VP
SCRMs. As a cross-linking reagent, BIEE could quarternize the
nitrogen atoms on the pyridine group.
Therefore, the self-assemblies of the P4VP-PEG-P4VP RMs in
the mixed solution of water–1-hexanol at a concentration of 2 ꢃ
10ꢀ4 M were stabilized through cross linking reactions between
the pyridine groups within the peripheral shell. The morphol-
ogies of SCRMs with a different extent of cross linking are
shown in ESI Fig. S6.† With increasing content of cross linking
at the peripheral shell, the binding among the P4VP-PEG-P4VP
micelles became stronger. At 30% degree of cross linking,
partial peripheral P4VP segments formed a smooth thin lm
with interior structures of P4VP-PEG-P4VP micelles located on it
separately. The SCRMs remained spherical and well dispersed.
In addition, large aggregations of SCRMs could be achieved by
increasing the concentration of micelles or BIEE (see ESI
Fig. S7†).
Scheme 2 Synthesis of the P4VP-PEG-P4VP block copolymer.
and the number-average molecular weight could be calculated
through 1H NMR spectrum. Based on the calculation of the
NMR spectrum, we found that one P4VP-PEG-P4VP chain had
45 EG units and 50 4VP units, with a Mcopolymer of 7200 g molꢀ1
.
The Mn values obtained from NMR and GPC measurements
were consistent.
Formation of P4VP-PEG-P4VP SCRMs
The shell cross-linked reverse micelles were prepared in two
steps. First, P4VP-PEG-P4VP self-assembled into RMs in water–
1-hexanol solution (Scheme 1). Then, the solution of cross-
linker BIEE was added to the micelle solution under stirring
(Scheme 3), forming P4VP-PEG-P4VP SCRMs.
In the rst step, the mixed solution of water–1-hexanol was
used as the media for P4VP-PEG-P4VP RMs self-assembly in
acidic conditions. Because water is a good solvent for the PEG
block but not for the P4VP block, the formed core–shell RMs
had PEG chains as the core and P4VP chains as the shell. The
morphologies of the copolymer self-assemblies were charac-
terized by TEM.
For comparison, P4VP-PEG self-assemblies were prepared
under the same conditions, as described in the experimental
section. As shown in Fig. 1a, the P4VP-PEG diblock copolymer
self-assembled into vesicles with a large exterior diameter of
about 2 mm, which were loose and unstable. P4VP-PEG-P4VP
formed core–shell RMs, partially with a hollow structure (actu-
ally a water pool) (Fig. 1b). The folding of the ABA copolymer
chain of P4VP-PEG-P4VP enhanced the rigidity of the shell,
providing the P4VP-PEG-P4VP micelle with a higher stability
than the P4VP-PEG micelle. The P4VP-PEG-P4VP RMs had an
average exterior diameter of 100 ꢂ 20 nm with a shell thickness
of approximately 50 nm.
Catalytic application of P4VP-PEG-P4VP SCRMs in oxidation
of TMP
To achieve a preferable catalyst for further oxidation of TMP,
CuCl2$2H2O was added to the maximum micelle loading
(Cu : 4VP ¼ 1 : 1) corresponding to pyridine units in the P4VP-
PEG-P4VP SCRM solution (0.2 mmol Lꢀ1) (note that the
concentration of 4VP in the sample was calculated using Mn of
P4VP-PEG-P4VP, same as the value used for the block ratio
Scheme 3 Intermicellar and intramicellar cross linking of PEG-P4VP-
PEG RMs by BIEE.
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Fig. 1 TEM images of (a) PEG-P4VP, (b) P4VP-PEG-P4VP reverse micelles in mixed solution and (c) with 5 mol% CuCl2 added into the mixed
solution (polymer weight: 0.15%, mass ratio of 1-hexanol–water ¼ 1).
Fig. 2 TEM images of P4VP-PEG-P4VP self-assemblies prepared in a mixture solution of water–1-hexanol: (a) 100% 1-hexanol, (b) 80% 1-
hexanol + 20% water, (c) 60% 1-hexanol + 40% water, (d) 40% 1-hexanol + 60% water, (e) 20% 1-hexanol + 80% water, (f) 100% water (polymer
weight: 0.15%).
calculation). In general, Cu(II) can form 4, 5 or 6 coordinated 4-chloro-2,3,6-trimethylphenol (CTP)
3
and 2,2-3,3-5,5-
complexes with a wide range of ligands. However, because of hexamethyl-4,4-biphenyldiol (HBD) 4 in the process as detect-
steric hindrance, a mixture of lower coordination numbers are able compounds by GC. The control experiment without the
produced in general.32 Therefore, the maximum micelle loading catalyst did not proceed to any appreciable extent (Table 1, Run
at 1 : 1 ratio should be a combination of Cu(II) coordination and
the copper ions dissolved in the core of P4VP-PEG-P4VP SCRMs,
which was revealed by TEM in ESI Fig. S8a.†
The coordination of the Cu(II) can be conrmed directly by
EPR in DMSO. When measured at room temperature, the EPR
peaks of CuCl2 were not clearly separated (Fig. 3a). Therefore,
the temperature was adjusted to 108 K to improve Cu(II) signals.
As shown in Fig. 3b and c, g ¼ 2.3593 and gt ¼ 2.0906 were
k
observed for Cu(II) and g ¼ 2.4050 and gt ¼ 2.0914 for the
k
complexes of CuCl2–(P4VP-PEG-P4VP). These indicated the
presence of copper(II) d9 paramagnetic species in this system, as
expected. Similar EPR behavior was observed on Cu/P4VP,39
which validated that the copper ions were indeed located in the
P4VP-PEG-P4VP SCRMs.
CuCl2–(P4VP-PEG-P4VP) SCRMs were used as catalysts for
Fig. 3 X-band EPR spectra in the 2.6976–1.9274 g-factor region of (a)
TMP 1 oxidized to trimethyl-p-benzoquinone (TMBQ) 2 by
dioxygen (Table 1). In general, the reaction also produces and (c) CuCl2–(P4VP-PEG-P4VP) at 108 K in DMSO (red line).
CuCl2 at room temperature (black line), (b) CuCl2 at 108 K (blue line)
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1). Because of the important role played by H2O in the oxida- treated with nitric acid, the amount of copper in the catalyst was
tion, reaction could proceed with the addition of a small measured by atomic absorption spectroscopy (AAS). The
amount of water (Table 1, Run 2 and 3). Then, the oxidation of 1 amount of copper in the non-cross-linked CuCl2–(P4VP-PEG-
with CuCl2–P4VP and CuCl2–(P4VP-PEG-P4VP) (Cu : 4VP ¼ P4VP) declined signicantly. In the third reaction, only 52.7%
1 : 1) was carried out under similar conditions (Table 1, Run 4 copper was le and the yield obviously decreased, which was
and 6). Aer 24 h, only a low yield of 2 was obtained with CuCl2– because of the destruction in multiple trials at high tempera-
P4VP, indicating a signicantly higher activity of CuCl2–(P4VP- ture. The results of 30% cross-linked CuCl2–(P4VP-PEG-P4VP)
PEG-P4VP). It was suggested that the hydrophilic segments show that the catalyst could be easily recovered and reused
and the core domain containing water had a positive effect on without great decrease in the activity. The copper was proven to
oxidation. The water pool could dissolve more copper ions, such remain in the catalyst (by the ratio of copper/P4VP-PEG-P4VP)
that the catalytic centers were concentrated and the catalyst without considerable loss aer every recycle. Aer the h
became harder to wash off. The mesoscale structure of the reaction, the remaining copper dropped to 63.3% at the same
SCRM decreased the entropy of the reaction by reducing level with non-cross-linked CuCl2–(P4VP-PEG-P4VP). The
distances among the substrates, catalytic centers and H2O, recycle 2–4 (Table 2) brought about reasonable reductions in
resulting in efficient catalytic activity for this reaction.
reaction yields. In the second recycle, 2 yielded a higher
The time course for the oxidation of TMP 1 with CuCl2– percentage than in the rst one, possibly because of the
(P4VP-PEG-P4VP) is shown in Fig. 4. With the consumption of absorption of SCRMs catalyst carrier and the redistribution of
substrate TMP 1, the CTP 3 appeared rst and nally changed to copper in the rst reaction. In addition, the change in the
the TMBQ 2. A small amount of the HBD 4 was detected aer morphologies of CuCl2–(P4VP-PEG-P4VP) was investigated in
the formation of 2. The reaction system essentially induces ESI Fig. S8.†
several competitive reactions such as C–C and C–O coupling
and peroxide formation from phenoxy radicals and dioxygen.
Thus, the yield and selectivity of 2 are very sensitive to reaction
Conclusion
conditions, particularly to the molar ratio of 4VP to CuCl2. At In this study, a novel catalytic system for the oxidation reaction of
low ratio, the results were unsatisfactory (Table 1, Run 5). TMP was designed and synthesized. First, an amphiphilic block
Higher copper loading in the ligand increased the conversion of copolymer P4VP-PEG-P4VP was synthesized via the anionic poly-
1 (Table 1, Run 6–8). This oxidation reaction at a relatively high merization technique. Aer addition into a water–1-hexanol
temperature produces a series of higher multimers (hard to be mixed solution, the block copolymer P4VP-PEG-P4VP self-
completely detected by GC). When the conversion reached assembled into reverse micelles. The reverse micelles consisted
100%, although the yield of 2 was not high (Table 1, Run 6), the of a hydrophobic shell of P4VP and a hydrophilic core of PEG,
selectivity of 2 increased with increasing copper loading.
partly with a water pool in the core. Demonstrated by TEM and z
With a low cross-linking extent of P4VP-PEG-P4VP RMs, the potential, the structure and morphologies of the reverse micelles
corresponding catalysts usually cannot be used more than twice changed along with the pH of the surroundings. To stabilize the
because of the mechanical degradation and/or elution of the structure, BIEE was used to cross-link the shell by quarternizing
catalyst. However, this limitation can be overcome by increasing the nitrogen atoms on the pyridine groups. SCRMs were rein-
the cross-linking content.25 Therefore, as a better reaction forced through covalent bonds between P4VP chains to maintain
condition compared to the one using non-cross-linked CuCl2– the structure when the environmental conditions changed.
(P4VP-PEG-P4VP) as catalyst, the reaction was repeated using
Aer coordinating with copper ions, the CuCl2–(P4VP-PEG-
30% cross-linking content of the CuCl2–(P4VP-PEG-P4VP) P4VP) SCRMs complex was obtained as a catalyst. The oxida-
(Table 2) for ve times. Aer being thoroughly dried and tion of TMP used the catalyst system with dioxygen under
ambient pressure. Taking advantage of the coordination with
metal ions, the distribution of metal catalytic active centers in
the structure, and the cocatalysis effect of the immobilized
water droplets, CuCl2–(P4VP-PEG-P4VP) demonstrated efficient
catalytic activity and recoverability. It is indicated that the
designed polymeric catalyst carrier has the potential to provide
a more reactive and recoverable catalytic system for catalytic
oxidation.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (no. 20873121), the National Basic
Research Program (no. 2009CB930104), and the Program for
Zhejiang Leading Team of S&T Innovation (2011R50007). We
also thank Dr Kexian Chen and Jun Guan for their valuable
discussions in this work.
Fig. 4 Oxidation of TMP with CuCl2–(P4VP-PEG-P4VP). For condi-
tions see Run 6 in Table 1.
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21 L. K. Shrestha, R. G. Shrestha, M. Abe and K. Ariga, So
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