Synthesis of novel magnetic chitosan supported protonated peroxotungstate and its catalytic performance for oxidation

Article abstract of DOI:10.1039/c2nj40753a

A novel magnetically recoverable catalyst in which protonated peroxotungstate was immobilized into a network of cross-linked chitosan with a superparamagnetic Fe₃O₄ core (Fe₃O ₄-CS/HWO) was prepared, characterized and used in oxidation reactions. With H₂O₂ as oxidant, a wide range of substrates including olefins, sulfides, amines and allylic alcohols could be oxidized selectively, exhibiting a relatively high utilization percentage of H₂O₂. Due to the existence of peroxotungstate as well as the magnetic core, both improved catalytic performance and facilitated separation were achieved for the reaction process. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012.

Full text of DOI:10.1039/c2nj40753a

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PAPER
Synthesis of novel magnetic chitosan supported protonated
peroxotungstate and its catalytic performance for oxidationw
Jie Zhu, Peng Cheng Wang and Ming Lu*
Received (in Montpellier, France) 25th August 2012, Accepted 29th September 2012
DOI: 10.1039/c2nj40753a
A novel magnetically recoverable catalyst in which protonated peroxotungstate was immobilized
into a network of cross-linked chitosan with a superparamagnetic Fe3O4 core (Fe3O4–CS/HWO)
was prepared, characterized and used in oxidation reactions. With H2O2 as oxidant, a wide range
of substrates including olefins, sulfides, amines and allylic alcohols could be oxidized selectively,
exhibiting a relatively high utilization percentage of H2O2. Due to the existence of
peroxotungstate as well as the magnetic core, both improved catalytic performance and
facilitated separation were achieved for the reaction process.
benign catalysts. For this purpose, several new technologies
Introduction
have been developed such as temperature-responsive7 and
surfactant combined catalysts.8 Recently, employing magnetic
Fe3O4 to construct a recyclable heterogeneous catalyst has led
to new research interests.9 Considerable efforts have been
devoted to the field and magnetic systems for oxidation have
also been reported. Tucker10 and Karimi11 anchored TEMPO
onto silica-coated Fe3O4 nanoparticles to obtain oxidation
catalysts. And Miao12 described a bimagnetic ionic liquid
based on Fe and TEMPO which displayed paramagnetic
behaviour and catalytic activity for the selective aerobic
oxidation of aromatic alcohols. In particular, unlike silica
coated Fe3O4 magnetic core, cross-linked chitosan (CS) was
reported to be able to envelop Fe3O4 and immobilize
12-silicotungstic acid molecules into the network, achieving
great stability.13
Selective oxygen transfer to organic substrates with H2O2 as a
terminal oxidant is of great importance1 in industry since
products such as epoxy compounds, aldehyde/ketone, sulfoxide/
sulfones, nitro, nitroso, hydroxamate and so on are widely used
for manufacturing various high demand commodity chemicals.2
It also agrees with the idea of green synthesis in synthetic
chemistry because of the high content of active oxygen species
in H2O2 and co-production of only water. Many efforts have
been devoted to the field with a variety of catalysts being
developed including Ti, V, Mn, Mo, W, Re, Pt complexes.3
Among them, tungstate-based epoxidation systems with H2O2
have attracted much attention due to their high reactivities and
inherent poor activity for the decomposition of H2O2. Notable
examples include the synthesis of peroxotungstates to catalyze
the oxidation of olefins, arenes, phenols, alcohols, phosphines
and sulfides.4
The W(O2)2O core, recognized as most stable5 and a
common motif in oxoperoxo-tungsten systems, has a high
formation tendency but is also reactive to perform substrate
oxidation, converting itself into WO(O2)2+ core.6 Despite the
high activity, the utilization of H2O2 is relativity low since
excessive H2O2 was needed. Recently, to circumvent the
problem, a protonated peroxotungstate TPA3[H{W2O2(O2)4-
(m-O)}2] (TPA = [(n-C3H7)4N]+) was developed, which also
showed high catalytic activity for epoxidation.4c
In this work, we report a novel magnetically recoverable
catalyst in which protonated peroxotungstate is immobilized
into the network of the cross-linked chitosan with superpar-
amagnetic Fe3O4 cores (noted as Fe3O4–CS/HWO, Scheme 1).
Extraordinary catalytic performance for the oxidation of
olefins, sulfides and alcohols was exhibited with both high
conversion and selectivity. Owning to the existence of the
Fe3O4 core, easy separation and excellent recyclability were
achieved by applying an external magnet.
Experimental
On the other hand, easy separation and effective recycling
are also important factors when developing environmentally
Materials and methods
All solvents and reagents employed in the work were used as
received from Sinopharm Chemical Reagent Co., Ltd unless
otherwise stated. NMR spectra were recorded on a Bruker
DRX300 spectrometer. IR spectra were recorded on a
NICOLET NEXUS870. Products were identified using a
6820 gas chromatograph (GC) with an Agilent Technologies
School of Chemical Engineering, Nanjing University of Science and
Technology, Xiaolingwei 200, Nanjing, Jiangsu Province, China.
E-mail: luming@mail.njust.edu.cn; Fax: +86 25 84315514;
Tel: +86 25 84315030
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c2nj40753a
c
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washed with deionized water and ethanol and then dried over-
night at 50 1C.
Typical procedure for catalytic oxidation
Substrate (1.5 mmol), CH3CN (10 mL), 30% H2O2 (3 mmol)
were introduced into a 50 mL three-neck flask. The reaction
was initiated by addition of the catalyst Fe3O4/CS/HWO
(5 mmol, about 4.5 mg), and allowed to proceed at a certain
temperature for a certain time period, during which the
reaction solution was periodically analyzed by GC. Before
the GC analysis, the remaining H2O2 was decomposed at
room temperature by addition of Ru(OH)x/Al2O3.16
Scheme 1 The synthesis of Fe3O4/CS/HWO.
Results and discussion
HP-Innowax (30 m  0.32 mm  0.5 mm). XRD data were
collected with CuKa radiation on Bruker C8 ADVANCE.
TEM images were taken on JEM-H-7650. Magnetic properties
were measured at room temperature using a vibrating sample
magnetometer (VSM) at a maximum field of 1 T. Thermo-
gravimetric analysis (TGA) was performed on TGA/
SDTA851e under air atmosphere from 50 1C to 800 1C, with
a heating rate of 20 1C minÀ1 and an air flowing rate of 30 mL
minÀ1. XRF analysis was carried out with ARL-9800.
Fabrication of the catalysts
The synthetic process of Fe3O4/CS/HWO is shown in
Scheme 1. Magnetic nanoparticles Fe3O4 were chosen as the
core magnetic support because of their simple synthesis, low
cost, and relatively large magnetic susceptibility. Chitosan, a
linear polymer of (1-4)-linked 2-amino-2-deoxy-gucopyranose,
is generally regarded to be non-toxic, biocompatible and
biodegradable. The free amines in chitosan provide various
opportunities for chemical modification to obtain different
chitosan derivatives when reacting with acid, hydroxyl, aldehyde
and so on. In our case, the amines act as not only basic
functional groups to anchor peroxotungstate, but also nucleo-
philic functional groups attacking the carbonyl in glutaralde-
hyde to form a polymer network layer on the surface of Fe3O4
nanoparticles, which make chitosan an excellent candidate for
building up magnetically recoverable catalysts containing
protonated peroxotungstate. Except for the role of preventing
agglomeration, the cross-linked chitosan polymer layer also
protects the Fe3O4 core from the acidic and catalytic properties
of protonated peroxotungstate, ensuring the stability of both
Fe3O4 core and peroxotungstate.
Synthesis of chitosan coated Fe3O4 (Fe3O4/CS)
The nanosized magnetic particles were synthesized by a chemical
coprecipitation method14 under alkaline conditions. Chitosan
(deacetylated degree 85–95 wt%, viscosity 50–800 mPa s, 0.75 g)
was dissolved in acetic acid solution (0.05 M, 150 mL) followed
by the addition of freshly prepared Fe3O4 nanoparticles (0.75 g).
The resulting mixture was then sonicated for 30 min before
being heated to 40 1C. Glutaraldehyde aqueous solution (4 wt%,
18 mL) was added dropwise under mechanical agitation, after
which the mixture was kept stirring at 40 1C for 1 h and 60 1C
for 1 h to give cross-linked chitosan coated Fe3O4.
During the fabrication process, the amount of chitosan and
glutaraldehyde would have a great influence on the catalytic
performance of the catalyst. With small amounts, the polymer
network layer was inadequate and could hardly anchor the
protonated peroxotungstate, during which the Fe3O4 core was
destroyed. On the other hand, despite the stability of the
Fe3O4 core attributed to the higher degree of cross-linking,
a larger amount of polymer immobilized the free amines and
resulted in lower catalytic activity. Considering both the
stability for magnetic recovery and the activity for catalytic
oxidation, the catalyst used in the following procedure was
prepared with Fe3O4 1.5 g, chitosan 1.5 g and 4 wt% glutar-
aldehyde 36 mL. The composition of the catalyst could be
detected with XRF analysis (see ESIw).
Synthesis of immobilized protonated peroxotungstate
(Fe3O4/CS/HWO)
A suspension of H2WO4 (1 g, 4 mmol) in 30% aqueous H2O2
(4 mL, 40 mmol) was stirred at 32 1C for 90 min until a pale
yellow solution was obtained. Water (35 mL), 65% HNO3
(0.39 g, 4 mmol), 30% H2O2 (3.5 mL) were then added and the
resulting solution was mixed with the above reactant mixtures
and stirred for 0.5 h at 60 1C. The solid product was separated
with an external magnet, washed with deionized water and
ethanol and then dried overnight at 50 1C.
Synthesis of TBA3[H{W2O2(O2)2(l-O)}2] (TBA = [(n-C4H9)4N]+
TBA3[H{W2O2(O2)2(m-O)}2] was prepared according to ref. 15
with TBABr instead of TPAOH.
Characterization of the catalysts
Synthesis of Fe3O4/CS/WO
IR spectrum (Fig. 1) of chitosan coated Fe3O4 (Fe3O4/CS)
shows detectable changes that are characteristic of chitosan
around 1150 cmÀ1 and 3460 cmÀ1, which clearly differs from
that of the bare magnetic nanoparticles. The characteristic IR
bands at 803 cmÀ1 and 895 cmÀ1 belong to the W–O vibrations.17
These facts are consistent with the synthesis design, and confirm
A suspension of H2WO4 (1 g, 4 mmol) in 30% aqueous H2O2
(4 mL, 40 mmol) was stirred at 32 1C for 90 min until a pale
yellow solution was obtained. The resulting solution was
mixed with Fe3O4–CS mixtures and stirred for 0.5 h at 60 1C.
The solid product was separated with an external magnet,
c
2588 New J. Chem., 2012, 36, 2587–2592
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the successful anchoring of protonated peroxotungstate onto the
chitosan coated Fe3O4 particles.
XRD analysis was also applied to the prepared Fe3O4/CS/
HWO, TBA3[H{W2O2(O2)2(m-O)}2] and chitosan raw material.
As shown in Fig. 2, the XRD patterns confirm that the coating
process did not induce any phase change of Fe3O4. Chitosan
derivatives existed in Fe3O4/CS/HWO catalyst comparing
curves a and c. The diffraction peaks of protonated peroxo-
tungstate, however, were not as obvious as that in the IR
spectra, probably due to the folding of cross-linked chitosan,
which was also consistent with the synthetic design.
TEM images of Fe3O4/CS/HWO were obtained as shown in
Fig. 3. It is clear that the synthesized catalysts are well
dispersed, and some bigger structures existed that were more
likely due to the aggregation/coalescence of individual nano-
particles. It can be seen that dark Fe3O4 cores were surrounded
by grey CS/HWO shells, suggesting the successful coating
process of CS/HWO materials. After the catalyst had been
used 5 times the aggregation phenomenon is clearly presented,
which may be a problem that inhibits further recycling.
The size of Fe3O4 could achieve the nanoscale, as also reflected
in Fig. 3. During the reaction process, nano-sized Fe3O4, carrying
protonated peroxotungstate, was well-distributed in the solvent to
form a pseudo-homogeneous system rather than a heterogeneous
one, which alleviated the disadvantage of heterogeneous immo-
bilization. Furthermore, in contrast to the homogeneous catalytic
system of protonated peroxotungstate, the advantage of magnetic
recycling was afforded.
Fig. 2 XRD patterns of (a) chitosan; (b) Fe3O4; (c) TBA3[H{W2O2-
(O2)2(m-O)}2]; (d) Fe3O4/CS/HWO and (e) Fe3O4/CS/HWO after
being used 5 times.
Catalytic performance of the catalyst
Fig. 3 TEM images of (a) Fe3O4/CS/HWO and (b) Fe3O4/CS/HWO
after being used 5 times.
The catalytic performance of immobilized protonated perox-
otungstate was first tested in the oxidation of diphenyl sulfide
with H2O2 as oxidant. To screen the optimal reaction conditions,
exploratory experiments were conducted and the comparable
results are summarized in Table 1. It was found that almost no
product was obtained in acetonitrile without any catalyst (entry 1)
or with Fe3O4/CS only without peroxotungstate (entry 2).
A catalytic amount of Fe3O4/CS/HWO was enough to perform
the reaction and the reaction termination could be observed with
the removal of Fe3O4/CS/HWO. The conversion of diphenyl
sulfide could achieve 61% in the presence of Fe3O4/CS/HWO
by using a stoichiometric amount of H2O2 (entry 3). Almost
complete conversion of diphenyl sulfide is obtained with 2 equiva-
lents of H2O2 comparing entries 3–5. Furthermore, excessive H2O2
Table
1 Oxidation of diphenyl sulfide with different reaction
conditionsa
30% H2O2
(mmol)
T
t
Conv. Yield
(%)
Entry Catalyst
(1C) (h) (%)
1
2
—
Fe3O4/CSb
3
3
25 12
25 12
3
3
3
3
3
4
5
6
7
8
9
10
11
12
Fe3O4/CS/HWO
Fe3O4/CS/HWO
Fe3O4/CS/HWO
Fe3O4/CS/HWO
Fe3O4/CS/HWO
Fe3O4/CS/HWO
Fe3O4/CS/HWOc
Fe3O4/CS/WOd
Fe3O4/CS/H2WO4
1.5
2
3
3
3
4
3
3
3
25
25
25
40
80
80
25
25
3 61
3 83
3 98
2 98
2 93
2 98
5 95
8 89
61
82
97
97
92
97
93
88
83
97
25 10 84
25 2 98
TBA3[H{W2O2(O2)2(m- 3
O)}2]
a
Reaction conditions: diphenyl sulfide 1.5 mmol, catalyst 5 mmol,
CH3CN 10 mL, under atmosphere. Adjusting the pH value with
b
c
0.1 mL H2SO4 solution. Fe3O4/CS/HWO was prepared with double
d
amount of chitosan and glutaraldehyde. Fe3O4/CS/WO = Fe3O4/
Fig. 1 IR spectra of (a) chitosan, (b) Fe3O4, (c) Fe3O4/CS/HWO and
CS/W2O2(O2)2(m-O)}2.
(d) Fe3O4/CS/HWO after being used 5 times.
c
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and high temperature were obviously beneficial to the reaction,
but at the same time, high temperature would accelerate the
decomposition of H2O2 and more H2O2 was required to complete
the reaction (entries 4–8). In addition, non-enveloped peroxotung-
state TBA3[H{W2O2(O2)2(m-O)}2] was prepared to see the influence
of immobilization. It was found that though immobilized, similar
results were obtained by Fe3O4/CS/HWO with some extra time
(entry 12). A decrease of conversion, however, was observed with
Fe3O4/CS/HWO with a higher degree of cross-linked chitosan
(entry 9), probably due to the difficulty in contact between catalyst
and substrate. The catalytic activity of protonated peroxotungstate
was also much higher compared with that of Fe3O4/CS enveloped
H2WO4 (entry 11) or normal peroxotungstate (entry 10).
Table
2
Selective oxidation of various substrates with H2O2
catalyzed by Fe3O4/CS/HWOa
Entry Substrates
Products
t/h Conv. (%) Yield (%)
1
3 98
4 85
4 64
97
83
63
2
3
All these facts indicated that it was still protonated perox-
otungstate that served as the active species for the oxidation
reactions. The immobilization process would not change the
reaction mechanism and the activity decrease was not obvious
either probably due to the pseudo-homogeneous process which
provides the large surface for readily accessible substrate mole-
cules. The merits of introducing magnetic nanoparticles also lie
here to achieve both good catalytic performance and facilitated
recycling.
4
5
6
3 75
3 80
3 76
75
79
73
The scope of this catalytic system for oxidation can be
extended to various alkenes, sulfides and amines as shown in
Table 2. In addition to diphenyl sulfide, thioanisole and
methyl n-octyl sulfide could be oxidized into the corresponding
sulfoxides generally within 3 h with high conversion and
selectivity (entries 1–6). Several cyclic alkenes were tested including
cyclopentene, cycloheptene, cyclooctene and norbornene, and high
yields of corresponding epoxides were obtained (entries 7–10). For
the oxidation of linear alkenes, both heptylene and octylene could
be transformed to the corresponding epoxides selectively (entries
11–14). The position of the double bond in the olefin seemed to
play a role in the oxidation efficiency as the reaction rates followed
the order of 1-octylene, 2-octylene and 3-octylene (entries 12–14),
probably due to the difference in their own activity. Aniline showed
a conversion of 85% to nitroso compounds in the dark (entry 15).
It was found that the efficiency of the hydrogen peroxide utilization
during the epoxidation process was the highest compared with that
of sulfides and amine, with 2 equivalents of H2O2.
7b
12 86
81
79
8c
9 88
Encouraged by these results, we continued to study the
chemoselectivity of the catalytic system. As shown in Table 3
some alcohols tested can also be oxidized selectively to give the
corresponding aldehyde or ketone, but a higher temperature
was needed probably since alcohols are harder to oxidize
compared with sulfides (entries 1–2). Interestingly, when an
equimolar olefin and alcohol were tested in the system, olefin
was always the first to be oxidized while alcohol remained
almost completely (entries 3–4). So we applied the catalyst to
the epoxidation of allylic alcohols and the epoxy alcohols were
afforded almost quantitatively and chemoselectively without
formation of the corresponding aldehydes and carboxylic acids
(entries 5–6).
9b
12 88
86
10c
11d
12d
13d
10 78
10 76
11 73
15 72
77
72
71
71
The recycling study
The advantage of the magnetic catalyst Fe3O4/CS/HWO lies in
not only the high catalytic activity attributed to the protonated
peroxotungstate, but also in the ease of separation and
c
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Table 2 (continued )
Table 3 The chemoselective studya
Entry Substrates Products
Entry Substrates
Products
t/h Conv. (%) Yield (%)
t/h Conv. (%) Yield (%)
14d
18 73
73
85
1
12 83
81
85
15d,e
3 85
2
12 86
a
Reaction conditions: substrate 1.5 mmol, CH3CN 10 mL, Fe3O4/CS/
HWO 5 mmol, 30% H2O2 3 mmol, room temperature for entries 1–3,
b
c
9–12, 40 1C for entries 4–8. H2O2 1.5 mmol. Fe3O4/CS/HWO
d
e
10 mmol. Fe3O4/CS/HWO 10 mmol, 30% H2O2 3 mmol. In the dark.
3b
12 2 + 90
2 + 82
recyclability provided by the Fe3O4/CS support. Simply by
applying an external magnet to the reaction vessel a separation
of the catalyst is achieved within seconds and the resulting
clear supernatant can be decanted, which minimizes the loss of
catalyst during separation. More than 98 percent of the
catalyst could be recovered from the reaction system with
only a trace remaining in CH3CN, which was also proved
by comparing the solution state before and after magnetic
separation as shown in Fig. 4. It also ensured the possibility to
reuse the catalyst.
4b
12 3 + 77
2 + 71
Both synthesized particles have small coercivities, which
indicate that they are superparamagnetic in nature. As shown
in Fig. 4, saturation magnetizations for Fe3O4 and Fe3O4/CS/
HWO are 62.56 and 26.98 emu gÀ1 respectively. The order is
due to the increasing amount of non-magnetic material on the
particle surface, which makes up a larger percentage of the
non-magnetic fraction. A direct result of this effect is that it
takes a longer time to separate Fe3O4/CS/HWO than bare
Fe3O4 from the particle solution.
5
8
6
79
83
78
80
6
Leaching and aggregation may be two major problems
hindering the recycling process. In order to further investigate
the stability of the catalyst during the catalytic process,
TGA measurements for the freshly prepared Fe3O4/CS/
HWO and the catalyst reused 5 times with the same mass
has been carried out. As shown in Fig. 5, the TGA curve for
the reused catalyst reveals a similar weight loss process to that
of fresh Fe3O4/CS/HWO, demonstrating the stability of the
catalyst even after reusing 5 times. But a slight decrease of
final thermal decomposition amount also appears in the curve
of the reused catalyst, which is attributed to the leaching
process. The leaching of a small amount of chitosan or
protonated peroxotungstate during the reaction process led
to an increasing ratio of Fe3O4, so as to reduce the weight loss
in TGA.
a
Reaction conditions: substrate 1.5 mmol, CH3CN 10 mL, Fe3O4/CS/
b
HWO 10 mmol, 30% H2O2 4 mmol, 60 1C. 0.75 mmol alcohol and
0.75 mmol olefin respectively.
The XRF study in the ESIw, IR spectrum in Fig. 1, XRD
pattern in Fig. 2 and TEM images in Fig. 3(b) of the reused
catalyst also testified the conclusion. According to the XRF
results, the contents of W in the catalyst reduced only
3.80 wt% after being used 5 times. The peaks of both chitosan
and protonated peroxotungstate showed some reduction in
the IR spectrum and XRD pattern. These facts prove that
the appearance and structure of Fe3O4/CS/HWO catalyst
remained intact after recycling.
Fig. 4 Magnetization curves for Fe3O4 and Fe3O4/CS/HWO.
With diphenyl sulfide as a substrate, the reusability of the
catalyst was further studied. As shown in Fig. 6, the recovered
c
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Acknowledgements
We thank the NSAF and NSFC (No: 11076017) of China
and Scientific Innovation Program of Jiangsu Province
(No: CXZZ11_0264) for support of this research.
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Fe3O4/CS/HWO can be reused at least 5 times without great
loss of activity. The coated cross-linked chitosan ensured the
stability of the Fe3O4 core during the reaction process, so as to
obtain a high recyclability. After 5 times of reuse, chitosan
may be destroyed. And Fe3O4 having lost the protection of
chitosan was easy to aggregate (as shown in TEM images
Fig. 3(b)) or oxidize. As the principal part of magnetism was
destroyed, the catalyst would be lost in the solution and
recovery efficiency decreased gradually.
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Conclusions
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In conclusion, we have synthesized a novel magnetically
recoverable catalyst with protonated peroxotungstate grafted
onto chitosan coated Fe3O4 (Fe3O4/CS/HWO). It was proven
to be efficient for the selective oxidation of a wide set of
substrates including olefins, sulfides, amines and allylic alco-
hols with relatively high utilization percentage of H2O2. In
particular, attributed to the magnetic Fe3O4/CS system, it was
a pseudo homogeneous process and after reaction, Fe3O4/CS/
HWO could be easily recovered with the help of an external
magnet, and could be reused with only a small amount of
leaching. The recovered catalyst could be reused at least 5
times without great loss of activity.
c
2592 New J. Chem., 2012, 36, 2587–2592
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012