[α-PW12O40]3- immobilized on ionic liquid-modified polymer as a heterogeneous catalyst for alcohol oxidation with hydrogen peroxide

Article abstract of DOI:10.1080/00397910801929564

Ionic liquid-modified polystyrene resin beads were demonstrated to be an appropriate support for polyoxometalate. In this heterogeneous catalytic system, alcohols can be efficiently oxidized to corresponding carbonyl groups with H2O2 in CH3CN. The catalyst can be easily recovered by filtration and recycled without apparent loss of catalytic performance. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910801929564

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[α‐PW12O40]3- Immobilized on Ionic
Liquid–Modified Polymer as a Heterogeneous
Catalyst for Alcohol Oxidation with Hydrogen
Peroxide
Xianjun Lang a b , Zhen Li a & Chungu Xia a
a State Key Laboratory for Oxo Synthesis and Selective Oxidation , Lanzhou
Institute of Chemical Physics, Chinese Academy of Sciences , Lanzhou, China
b Graduate University of Chinese Academy of Sciences , Beijing, China
Published online: 24 Apr 2008.
To cite this article: Xianjun Lang , Zhen Li & Chungu Xia (2008) [α‐PW12O40]3- Immobilized on Ionic
Liquid–Modified Polymer as a Heterogeneous Catalyst for Alcohol Oxidation with Hydrogen Peroxide, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 38:10,
1610-1616, DOI: 10.1080/00397910801929564
To link to this article: http://dx.doi.org/10.1080/00397910801929564
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Synthetic Communicationsw, 38: 1610–1616, 2008
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910801929564
[a-PW12O40]32 Immobilized on Ionic
Liquid–Modified Polymer as a
Heterogeneous Catalyst for Alcohol
Oxidation with Hydrogen Peroxide
Xianjun Lang,1,2 Zhen Li,1 and Chungu Xia1
1State Key Laboratory for Oxo Synthesis and Selective Oxidation,
Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences,
Lanzhou, China
2Graduate University of Chinese Academy of Sciences, Beijing, China
Abstract: Ionic liquid–modified polystyrene resin beads were demonstrated to be an
appropriate support for polyoxometalate. In this heterogeneous catalytic system,
alcohols can be efficiently oxidized to corresponding carbonyl groups with H2O2 in
CH3CN. The catalyst can be easily recovered by filtration and recycled without
apparent loss of catalytic performance.
Keywords: Alcohol oxidation, heterogeneous catalysis, hydrogen peroxide,
polyoxometalate
INTRODUCTION
Polyoxometalates (POMs) constitute a versatile class of inorganic, anionic
clusters that have applications involving medicine, catalysis, materials
science, geochemistry, nuclear waste processing, and photochemical or elec-
trochemical response.[1] However, the majority of the applications of POMs is
in the area of catalysis; about 80–85% of patent and applied literature claims
or investigates POMs for their catalytic activity.[2] POMs have received much
Received in Japan November 5, 2007
Address correspondence to Chungu Xia, State Key Laboratory for Oxo Synthesis
and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy
of Sciences, Lanzhou, China. E-mail: cgxia@lzb.ac.cn
1610

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Alcohol Oxidation with Hydrogen Peroxide
1611
attention in catalysis because of their hydrolytic and thermal stability,
solubility in various media, tunable acid and redox properties at the
molecular or atomic level, and so on.[3] Molybdenum- and tungsten contain-
ing-POMs have been well established as efficient catalysts for oxidation of
various organic substrates with aqueous H2O2, such as epoxidation of
olefins[4,5] and allylic alcohols,[6] oxidation of alcohols,[7,8] and so on in homo-
geneous or two-phase systems. Although the efficiency of these systems is
rather high, they share common drawbacks such as difficult catalyst/product
separation and poor catalyst reusability. To avoid this problem, one of the
best choices is to immobilize POMs on an appropriate support without loss
of the intrinsic activity and selectivity.[9]
The unique properties of ionic liquids, including high thermal stability,
tunability of their acidities, and excellent retention of polar or charged
catalysts, make them appealing media for a broad range of catalytic appli-
cations.[10] Supported ionic liquid catalysis is a concept that combines
the advantages of ionic liquids with those of heterogeneous support
materials.[11] Tungsten-containing POMs [W2O3(O2)4(H2O)]22[12] and
[g-l,2-H2SiV2W10O40]42[13] have been successfully immobilized on ionic
liquid–modified SiO2 as efficient heterogeneous oxidation catalysts with
H2O2. However, to our knowledge, polymeric materials–based support for
polyoxometalate modified by ionic liquid has never been reported before.
Herein we present a heterogeneous catalytic system for a range of alcohol oxi-
dations with 30% aqueous H2O2 catalyzed by [a-PW12O40]32 immobilized on
ionic liquid–modified polystyrene resin beads (PS-IL-PW). The catalytically
active polyoxometalate anion [a-PW12O40]32 was immobilized onto the
support by stoichiometric anion exchange via a very simple preparation
procedure under relatively benign and easy-to-handle conditions. The
activity of the recovered catalyst is almost intact upon recycling by a
simple filtration step.
RESULTS AND DISCUSSION
Catalyst preparation process is shown in Scheme 1. In the first step, chlori-
nated polystyrene resin beads were vigorously stirred in N-methylimidazole
at 90 8C to anchor a covalently ionic liquid fragment to the beads’ surface
(PS-IL). IR spectrum shows the characteristic bands of the parent imidazole
salt, for example, v(C-H) 3150, v(-CH3, -CH2-) 2950, 2850, and v(C55N)
1570, 1460 cm21. After anion exchange with polyoxometalate anion, the
support catalyst was characterized with IR. Apart from the bands assigned
for ionic liquid skeleton, the IR spectrum shows v(P-Oa) 1080, v(W-Od)
982, v(W-Ob-W) 891, and v(W-Oc-W) 801 cm21, which are the characteristic
bands of Keggin-type polyoxometalate, indicating that [a-PW12O40]32 had
been successfully immobilized on ionic liquid–modified polymer (PS-IL).
The potential of this supported catalyst for oxidation of alcohols to

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1612
X. Lang, Z. Li, and C. Xia
Scheme 1. Catalyst preparation process; PS ¼ polymer support.
corresponding carbonyl compounds with H2O2 was examined with cyclohex-
anol as the probe substrate. The catalytic activity of the N-methylimidazole
salt of H3PW12O40 ([Hmim]3PW12O40) was also investigated for a comparison
between homogeneous and heterogeneous catalysis systems. The results are
presented in Table 1 (entries 1–5). At first, we examined the catalytic
activity of PS-IL-PW in water (entry 3). The conversion of cyclohexanol
was rather low. That may be attributed to the low surface area of polystyrene
resin beads, and cyclohexanol is insoluble in water. It is reasonable for low
conversion as a result of low contact area between substrate and catalyst.
However, when [Hmim]3PW12O40 was used as the catalyst (entry 5), the
reaction proceeded by the substrate/water biphasic manner, and the conver-
sion of cyclohexanol was satisfied. To improve the reaction efficiency of the
current heterogeneous catalytic system, we used CH3CN instead of water as
the reaction medium, and the yield of cyclohexanone apparently increased
(entry 1) to reach 83.75%. Nevertheless, when the catalyst of
[Hmim]3PW12O40 was used in this manner (entry 4), the yield of cyclo-
hexanone was much lower than the biphase system (entry 5), even lower than
the heterogeneous catalysis system (entry 1). That may be because CH3CN
was not a suitable solvent for alcohol oxidation when [Hmim]3PW12O40
was used as the catalyst. However, PS-IL-PW catalyzed the reaction
through the solid/liquid interface, which can surpass this limitation. The
catalyst can be easily recovered and recycled for five rounds without apparent
loss of its catalytic activity (entry 2). The IR spectrum of the recovered
catalyst showed the absorption in the region 1050 cm21 (peaks at 1111,
1078, and 1018 cm21), which were assigned to P-O stretching vibrations. A
strong peak at 978 cm21 indicated the presence of W ¼ O, and peaks at
896, 817 (O-O), and 620 cm21 (W-O-O asym), which were the characteristic
bands of [PO4fW2O2(m-O2)2(O2)2g2]32, indicating the PW12O was decom-
32
40
posed to this active intermediate in the presence of H2O2.
The present system could be applied to oxidation various kinds of
alcohols. Representative results are summarized in Table 1 (entries 6–16).
Several trends should be noted. First, liquid secondary alcohols were all selec-
tively oxidized to the corresponding ketones in high yields (entries 7–12).