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Alcohol Oxidation with Hydrogen Peroxide
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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