E. Nikbakht et al. / Inorganic Chemistry Communications 55 (2015) 135–138
137
capsules, PMo12O40 ⊂ {(Mo)Mo5}12{FeIII}30, according to the literature,
their synthesis was confirmed by elemental analysis, TG, FTIR, XRD, and
UV–vis spectroscopy (Figs. S1–S4) [27]. Catalytic experiments were initi-
ated by the oxidation of benzyl alcohols (1 mmol), as a model compound,
with H2O2 (4.5 mmol) in the presence of 1 μmol POM nano capsule in
bidistilled water at 45 °C [35]. Only after 45 min, using a few amounts
of catalyst (1 μmol), benzyl alcohol completely converted and benzalde-
hyde was produced exclusively in 100% yield by an easy isolation with
ethyl acetate as a safe solvent.
aqueous solution of catalyst was reused directly for the next round of re-
actions without further purification. The solid catalyst HxPMo12O40
⊂
H4Mo72Fe30(CH3COO)15O254 could also be obtained easily by removing
the water followed by washing with ethyl acetate or ethanol and drying
under vacuum. The ease of recovery, combined with the intrinsic stability
of the HxPMo12O40 ⊂ H4Mo72Fe30(CH3COO)15O254, allows for the catalyst
to be recovered efficiently over 10 times in the oxidation of benzyl alcohol
(Fig. 2). Only after the ninth run was a negligible decrease in catalyst
performance (b2%) observed. Therefore, these POM catalysts showed
high stability, activity and selectivity in oxidation reaction runs.
Comparison between FTIR, XRD, and UV–vis spectra of the used cat-
alyst with those of fresh one (Figs. S1–S3) illustrated that the structure
and morphology of the catalyst remained completely intact. Therefore,
the title methodology is environmentally benign because of the use of
hydrogen peroxide as an oxygen source, water as a reaction media, re-
usability of an active catalyst, very low catalyst loading, easy isolation
of hydrophobic organic products, and as a final point no need for toxic
reagents or solvents. These advantages make this catalytic method
readily amenable to scalability.
In summary, green oxidation of different alcohols into aldehydes and
ketones by a water soluble POM nano capsule was developed. High to
excellent yields were obtained in the oxidation reactions by
HxPMo12O40 ⊂ H4Mo72Fe30(CH3COO)15O254 as catalyst. This catalytic
system is valuable because of easy and safe procedure and also from
the environmental point of view. As a result, this method could have
high potential for industrial purposes.
Since the structure of POMs is pH dependent, the oxidation of enzyl
alcohol was run at different pH values (Fig. 1). When the pH value was
adjusted to less than or equal to 6.6, exactly the same conversions and
selectivity were obtained. Nevertheless, since the HxPMo12O40
⊂
H4Mo72Fe30(CH3COO)15O254 nano capsule became less stable at
higher pH values, low oxidation activity was observed; consequently,
the clusters are broken as established by UV–vis study. Note that all
POMs decompose at high pH values, while smaller species are formed
[27].
Using the best reaction conditions and to establish the general appli-
cability of the method, various alcohols were subjected to the oxidation
protocol using H2O2 and HxPMo12O40 ⊂ H4Mo72Fe30(CH3COO)15O254
.
By these very mild reaction conditions – 1 μmol nano capsule catalyst,
4.5 mmol H2O2, 45 °C, and 0.5–2.5 h oxidation reaction in water – to a
reasonable extent various benzylic alcohols produced target aldehydes
and/or ketones in excellent yields (Table 1). This catalytic system
efficiently worked both in the presence of electron-donating or
electron-withdrawing substituent as well as in less sterically favored
positions on the aromatic rings (Table 1, entries 1 to 13). Our catalytic
process was also amenable to cyclic alcohols (Table 1, entries 14 and
15) and high yields were observed for linear alcohols and allyl alcohol
(Table 1, entries 16 to 18). By increasing the time of reaction, complete
conversion was obtained for most substrates with the exception of aro-
matic alcohols with electron donating groups (Table 1, entries 11 to 13)
that only provided high yields at moderate times. As everyone could see
in Table 1, this catalytic system possesses novelty regarding selectivity
only for aldehydes and ketones. It was shown that secondary and pri-
mary linear and aromatic alcohols were oxidized selectively (greater
than 99%) to ketones and aldehydes, respectively, with no further
oxidation of aldehydes to carboxylic acids.
Acknowledgment
Support for this research by the University of Isfahan is acknowledged.
Appendix A. Supplementary material
Supplementary data to this article can be found online at http://dx.
References
For comparison the catalytic activity of H3PMo12O40 and
H4Mo72Fe30(CH3COO)15O254 in the oxidation of benzyl alcohol at the
same conditions were also investigated. Results for H3PMo12O40 showed
only 20% conversion after 60 min and for H4Mo72Fe30(CH3COO)15O254
demonstrated that 120 min oxidation reaction is needed for complete
conversion of alcohol.
In this procedure, the POM nano capsule catalyst was reused several
times without any loss of activity. Recovery of the catalyst was easy and
efficient. When the reaction was completed, hydrophobic organic
products were isolated by adding ethyl acetate as a safe solvent. Then
Fig. 2. Recyclability of the POM nano capsule catalyst in the oxidation of benzyl alcohol
with hydrogen peroxide in water according to the procedure mentioned in Table 1.