Highly selective oxidation of alcohols with hydrogen peroxide and polyethylene oxide-supported long-chain imidazolium polyoxometalate hybrid catalyst

Article abstract of DOI:10.1007/s13738-015-0651-7

A polyethylene oxide-supported long-chain imidazolium polyoxometalate hybrid catalyst ([PEO-didodecylimidazolium]₃[PW₁₂O₄₀]₂) was synthesized and employed as catalyst for the oxidation of various alcohols with H

Full text of DOI:10.1007/s13738-015-0651-7

J IRAN CHEM SOC
DOI 10.1007/s13738-015-0651-7
ORIGINAL PAPER
Highly selective oxidation of alcohols with hydrogen peroxide
and polyethylene oxide‑supported long‑chain imidazolium
polyoxometalate hybrid catalyst
Zhao‑gang Wang1 · Yao Yang1 · Xiao‑hua Cao1 · Ming Lu1
Received: 10 February 2015 / Accepted: 21 April 2015
© Iranian Chemical Society 2015
Abstract A polyethylene oxide-supported long-chain
imidazolium polyoxometalate hybrid catalyst ([PEO-
didodecylimidazolium]3[PW12O40]2) was synthesized and
employed as catalyst for the oxidation of various alcohols
with H2O2 at room temperature. Owing to the exsistence of
long chain, the catalyst had a good surface-active perfor-
mance. The catalyst showed high activity not only in the
oxidation of primary and secondary benzylic alcohols but
also in the oxidation of heterocyclic and allylic alcohols
to give good yields of aldehydes or ketones. The catalyst
can be reused for six times without significant loss of its
activity.
an economical and environmental viewpoint, the develop-
ment of catalytic oxidation systems with cheap and green
oxidants, such as molecular oxygen or hydrogen perox-
ide is particularly attractive and many efforts have been
devoted to the development of efficient procedures over the
last years. These attractive catalytic systems display high
atom economy and release only innocuous by-products
such as H2O [6–10].
Selective oxidation of organic substrates with polyoxo-
metalates (POMs)-based systems is of great importance
in industry and are widely used for manufacturing vari-
ous high-demand commodity chemicals. Recently, organic
hybrids of POMs are attractive catalyst for various oxida-
tion reactions. Examples which include surfactant-POM
combination [11, 12], coordination polymers [13, 14],
polymer-POM conjugates [15–17], and POM-ILs [18–23]
have been prepared and tested as promising catalysts for
the oxidation process.
Keywords [PEO-didodecylimidazolium]3[PW12O40]2 ·
Hybrid catalyst · Oxidation of alcohols · Room temperature
Introduction
In this work, we report a novel imidazolium polyoxo-
metalate composite in which peroxotungstate is immobi-
lized onto polyethylene oxide supported long alkyl chain
ionic liquids (Fig. 1). Owing to the existence of the poly-
ethylene oxide and long alkyl chain moiety, both improved
catalytic performance, easy separation and excellent recov-
erability were achieved. The protocol presented here pro-
vides an efficacious strategy to achieve both catalysts
recycling and product isolation from a viewpoint of green
chemistry.
The selective oxidation of primary and secondary alcohols
into the corresponding aldehydes or ketones is one of the
most important and challenging transformations for many
biological and organic reactions providing key intermedi-
ates and valuable pharmaceuticals [1–3]. Large variety of
traditional methods employing chemical reagents in stoi-
chiometric amounts have been reported to accomplish this
transformation with considerable drawbacks, such as use of
expensive reagents and volatile organic solvents, and dis-
charge of environmentally pernicious wastes [4, 5]. From
Results and discussion
* Ming Lu
luming302@126.com
The catalysts could be easily prepared in a four-step pro-
cedure. First, polyethylene glycol was treated with thionyl
chloride in anhydrous toluene at 0 °C under an inert gas
1
Nanjing University of Science and Technology, Nanjing,
Jiangsu, China
1 3

J IRAN CHEM SOC
for 24 h to afford PEO-dichloride. PEO-dichloride was
subsequently added to sodium imidazole salts in toluene at
80 °C for 24 h to generate the PEO-diimidazolium. Then,
PEO-diimidazolium was reacted with 1-bromododecane
C12H25Br at 70 °C for 20 h to give PEO-didodecylimidazo-
lium. In the next step, the PEO-didodecylimidazolium was
then treated with H3[PW12O40] in dry ethanol at 40 °C for
6 h. The [PEO-diimidazolium]3[PW12O40]2 was obtained as
a white viscous solid (Scheme 1).
945 cm−1 v(W=O), and 900–725 cm−1 v(W–O–W) were
exhibited clearly in its Keggin structure (curve b). For
[PEO-didodecylimidazolium]3[PW12O40]2, minor differ-
ence could be detected when compared the IR spectrum
of catalyst [PEO-didodecylimidazolium]3[PW12O40]2 with
that of H3PW4O12. In spite of the decrease of peak inten-
sities and slight shift of peak positions, the characteristic
peaks appeared distinctively, which indicated that the salts
retained the well Keggin structure even after protons in
H3PW12O40 were substituted by organic cations.
Figure 2 showed the IR spectra of [PEO-didodecyli
midazolium]3[PW12O40]2,
(PEO-didodecylimidazolium
With benzyl alcohol as the model substrate, the catalyst
PEO-didodecylimidazolium]3[PW12O40]2 was designed
for the oxidation of alcohols. The exploratory experiments
were started by screening the heteropoly acid salts. The
and H3PW12O40 raw material, respectively. IR spectrum
of PEO-didodecylimidazolium(curve a) and [PEO-dido
decylimidazolium]3[PW12O40]2 (curve c) showed detect-
able changes that were characteristic of C–O–C stretch-
ing vibrations in around 1100 cm−1 C–H stretching vibra-
tions around 2800 cm−1. The characteristic peaks that
belong to H3PW12O40 around 1079–1040 cm−1 v(P–O),
Fig. 1 Selective oxidation of alcohols with hydrogen peroxide cata-
Fig. 2 IR spectra (a) PEO-didodecylimidazolium (b) H3PW12O40 (c)
lyzed by [PEO-didodecylimidazolium]3[PW12O40]2
PEO-didodecylimidazolium]3[PW12O40]2
NH
N
SOCl2
Cl
Cl
OH
HO
1
C12
N
C12Br
N
N
N
C12
N
N
N
N
3
2
C12
H3PW12O40
3-
N
[PW12O40
]
2
N
N
C12
N
3
4
Scheme 1 Synthesis of PEO-didodecylimidazolium]3[PW12O40]2
1 3

J IRAN CHEM SOC
results are summarized in Table 1, entries 1-3. It is found
that phosphotungstic acid gave best performance for the
oxidation of benzyl alcohol. Possible reason was that tung-
stic ion was easier to form polyoxometalate which was the
main active species in the process of oxidation of alcohols.
The lengths of carbon chain also have a great impact on
oxidation of alcohols. The results using different lengths of
carbon chain are summarized in Table 1. It was obviously
found that the advantage effect appeared with increase of
the length of carbon (n = 4, 8, 12), which may attribute
to stronger lipophilic ability and better surface-active per-
formance, but the conversion and yield have no obvious
increase when the number of carbon atom exceeded n = 12
(Table 1, entries 3–7).
In order to investigate the surface-active effect of [PEO-
didodecylimidazolium]3[PW12O40]2 upon the reactions,
control experiments using different surfacants for oxida-
tions of benzyl alcohol were studied. As can be seen in
Table 2, [PEO-didodecylimidazolium]3[PW12O40]2 system
gave the best result over cationic surfactant cetyl trimethyl-
ammonium bromide(CTAB), acetonitrile, neutral surfactant
TritonX-10, anionic surfactant sodium dodecyl sulfate
(SDS) (Table 2, entries 1–4). Possible reasons are as fol-
lows. Firstly, with attraction of POM anion by cationic sur-
factant, the conversion rate in cationic surfactant micelles
was significantly higher than anionic surfactant under the
same conditions. It may be due to strong electrostatic attrac-
tion between the imidazolium cation head group on micelle
shell and phosphotungstic acid anion ([PW12O40]3−), which
leads to concentration of [PW12O40]3− in the surface of the
micelles significantly and promotes the oxidation reaction
effectively. To the contrary, strong electrostatic repulsion
exists between the anionic surfactant anionic head group
and [PW12O40]3−on micelle shell therefore the oxidation
reaction was strongly suppressed. Since micelle shell of
the nonionic surfactant is uncharged, there is no electro-
static attraction or the electrostatic repulsion between the
Table 1 Screening of catalysts for oxidation of benzyl alcohol
Entry Heteropoly acid
CnH2n+1 Conv (%) Yeild (%)
1
2
3
Phosphotungstic acid
Phosphomolybdic acid
n = 12 100
98
64
55
n = 12
69
60
Molybdovanadophosphoric n = 12
acid
4
5
6
7
Phosphotungstic acid
Phosphotungstic acid
Phosphotungstic acid
Phosphotungstic acid
n = 0
n = 4
n = 8
50
65
89
48
64
85
98
n = 16 100
Table 2 Oxidation of benzyl alcohol under different surfactant con-
ditions
Entry
Surfactant
Conv (%)
Yeild (%)
1
2
3
4
PEO-didodecylimidazolium
100
90
98
85
56
67
CTAB
SDS
59
TX-10
70
Table 3 Reaction conditions on oxidation of benzyl alcohol under micelle conditionsa
0.05 mol% of cat
CHO
OH
30% aq H2O2
(2 equiv)
Entry
Catalysts (mol %)
H2O2 (equiv)
T/ °C
Conv (%)a
Yeild (%)b
1
2
3
4
5
6
7
8
9
10
–
3
2
3
2
2
2
2
2
2
2
rt
–
–
TPA(0.05)
rt
80
75
–
PEO-didodecylimidazolium(0.05)
rt
–
[PEO-didodecylimidazolium]3[PW12O40]2(0.05)
[PEO-didodecylimidazolium]3[PW12O40]2 (0.01)
[PEO-didodecylimidazolium]3[PW12O40]2 (0.1)
[PEO-didodecylimidazolium]3[PW12O40]2 (0.20)
[PEO-didodecylimidazolium]3[PW12O40]2 (0.05)
[PEO-didodecylimidazolium]3[PW12O40]2 (0.05)
[PEO-didodecylimidazolium]3[PW12O40]2 (0.05)
rt
100
89
98
84
96
96
95
90
85
rt
rt
>99
>99
99
rt
40
60
80
100
100
Reaction conditions: benzyl alcohol (10 mmol), [PEO-didodecylimidazolium]3[PW12O40]2 (0.05 mol % of benzyl acohol), 30 % aq H2O2 (2
equiv), rt
a
Determined by HPLC
b
Isolated yield
1 3

J IRAN CHEM SOC
imidazolium cation head group and phosphotungstic acid
anion [PW12O40]3−, so that the catalytic activity of non-
ionic surfactant for oxidation was between the cationic sur-
factant and anionic surfactant.
system to a variety of alcohols as shown in Table 4. Sub-
strate scope was extended to benzylic, allylic, heterocyclic,
and aliphatic alcohols. In all cases, ketones or aldehydes
were the only detected products. Obviously, the activities
of primary benzylic alcohols were best, and the activities
of those primary benzylic alcohols were also not signifi-
cantly affected by the electronic properties (electron-with-
drawing or electron-donating group) and steric hindrance
of the substituents on the benzene ring. Secondary alcohols
such as benzhydrol and 2-phenylethanol also gave a mod-
erate yield by prolonging the reaction time. It was found
that the oxidative efficiency was not affected with the exist-
ence of double bond and the double bond remained stable
during the oxidation process. Heterocyclic alcohols such as
furfuryl alcohol would be also suitable to the system. Fur-
thermore, oxidations of aliphatic alcohols were also tested.
Unfortunately, in the case of aliphatic alcohols such as
dodecyl alcohol, the results were unsatisfactory even after
elongating the reaction time.
To further optimize the reaction conditions, control
experiments were carried out with benzyl alcohol as sub-
strate and H2O2 as oxidant. The comparable results were
summarized in Table 3. It was found that almost no product
was obtained in aqueous solution without catalyst (Table 3,
entry 1) or without POMs (Table 3, entry 3). All these facts
indicated that it was still Keggion anions that served as the
active species for oxidation reactions. The encapsulation
process would not change either reaction activity or mecha-
nism (Table 3, entry 2, and entry 4). In addition, catalytic
amount was enough to proceed the reaction and the reac-
tion termination could be observed with the removal of it.
Further increase of catalyst amount would be meaningless
(Table 3, entries 5–7). The reaction was performed well
even at room temperature. High temperature obviously
accelerates the decomposition of H2O2 and decreases the
selectivity of benaldehyde (Table 3, entries 8–10).
The role of polyoxometalate might act as a hydrogen
acceptor during the proposed catalytic cycle. Firstly, per-
oxometal species are generated by hydrogen peroxide-
mediated oxidation of polyoxometalate. Then, alcohol
To examine the utility and generality of this methodol-
ogy for the oxidation of alcohols, we applied the present
Table 4 Oxidation of various
alcohols
OH
R2
0.05 mol% of cat
O
R1
R1
R2
30% aq H2O2
(2 equiv)
OH
OH
OH
OH
OH
OMe
OMe
100%
99%
6 h
Me
OMe
(98%)
98%
6 h
(96%)
8 h
(94%)
>99%
>99%
(99%)
5 h
5 h
(98%)
OH
OH
OH
OH
Ph
OH
Ph
90%
81%
10 h
NO2
Cl
(84%)
16 h
84%
(80%)
10 h
96%
95%
(82%)
8 h
8 h
(95%)
(93%)
OH
Reaction conditions: alcohols
(10 mmol), [PEO-didodecy
limidazolium]3[PW12O40]2
(0.05 mol % of acohol), 30 %
aq H2O2(2 equiv), rt
OH
OH
Ph
Me
O
N
OH
70%
(68%)
90%
86%
20 h
83%
12 h
16 h
14 h
(84%)
82%)
(81%)
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J IRAN CHEM SOC
Conclusion
HOOH
C O
H2O
+
M
O
In conclusion, we have developed an efficient and environ-
ment-friendly method for the aerobic oxidation of alcohols
with [PEO-didodecylimidazolium]3[PW12O40]2 as cata-
lyst and H2O2 as oxidant. Owing to the exsistence of long
chain, the catalyst had a good surface-active performance
to promote reaction effectively. Moreover, it could be recy-
cled and reused without significant loss of catalytic activity
after six runs. For this environmentally catalyst, it would
find a wider application in various reactions, which is an
ongoing project.
O OH
H
M O C R
H
OOH
OH
Peroxometal Pathway
M
H
R
C OH
H
H2O
Experimental section
Scheme 2 The mechanism of oxidation of alcohols
All starting materials were purchased from commercial
sources and used without further treatment. Analytical thin
layer chromatography (TLC) was performed on precoated
silica plates. Yields of the products refer to purification by
silica-gel column chromatography. IR spectrum analysis
was recorded on Nicolet IS-10 fourier transform infrared
spectroscopy. High performance liquid chromatography
experiments were performed on a liquid chromatograph
(Shimazhu LC-20AT, Japan).
General procedure for preparation of catalyst
To a solution of PEG-1000 (0.02 mol, 20 g) in toluene
(150 mL), pyridine (0.04 mol, 3.2 mL) was added, fol-
lowed by the addition of thionyl chloride (0.04 mol,
2.9 mL) within 30 min under N2 atmosphere at 0 °C. The
resulting slurry was stirred for 24 h at room temperature
and filtered. The toluene was then removed by rotator
evaporation under reduced pressure to give intermediate 1
as a gray viscous liquid. Next, intermediate 1 (0.015 mol)
was added to sodium imidazole salts solution 0.015 mol
(1.25 g) which was dissolved in toluene and the mixture
was stirred at 80 °C for 24 h. The reaction was deter-
mined by HPLC and evaporated under reduced pressure
to give intermediate 2 as an orange liquid with a yield of
98.0 %. In the next step, intermediate 2 (0.01 mol) was
added to 1-bromododecane C12H25Br 0.02 mol (1.25 g)
solution which was dissolved in toluene and the mixture
was stirred at 70 °C for 20 h. The reaction was determined
by HPLC and evaporated under reduced pressure to give
intermediate 3 as a white viscous solid. Finally, the inter-
mediate 3 (1.2 mmol) was mixed with H3[P(W3O10)4]
2.36 g (0.8 mmol) in dry ethanol with a mechanical stir-
ring at 40 °C for 6 h. The obtained white solid were fil-
tered and dried under vacuum to give the catalyst [PEO-
didodecylimidazolium]3[P(W3O10)4]2 with a yield of
95.0 %.
Fig. 3 Cycling reaction
coordinates to the peroxometal species to lead to alkoxy
peroxometal species. The intramolecular transfer of
β-hydrogen to peroxometal species leads to the aldehyde
and polyoxometalate (Scheme 2).
The advantage of the catalytic system lies in not only
the high catalytic activity with H2O2 as oxidant, but also
the easy recovery of both catalyst and products. Since both
the catalyst was immiscible with ethyl ether, the catalytic
system could be recovered through extraction. The catalyst
could be reused with the recharge of substrate without any
treatment. The organic layer containing the products was
analyzed after drying with anhydrous magnesium sulfate.
When benzyl alcohol was used as a model substrate, the
procedure was successfully performed for at least six times
without great loss of catalytic activity (Fig. 3).
1 3

J IRAN CHEM SOC
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Oxidation process of alcohols
In a typical process, benzyl alcohol (10 mmol), [PEO-dido
decylimidazolium]3[PW12O40]2 (0.05 mol %), 30 % aq
H2O2 (20 mmol) were then added. The mixture was stirred
and under room temperature. The progress of the reaction
was monitored by TLC with samples taken periodically.
After completion of reaction, the mixture was extracted
with ether for three times. The organic layer was dried over
anhydrous MgSO4 and evaporated under reduced pressure.
The product was analyzed by HPLC and purified by col-
umn chromatograph to give benaldehyde (98 % yield). The
next run was performed under identical reaction conditions.
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