730
G.P. Romanelli et al. / Catalysis Communications 12 (2011) 726–730
was tested and is shown in Fig. 3. Under these experimental
conditions, M11PV1 and M12P are dissolved in the solvent used. We
have observed that heteropolycompounds used as catalysts in this
reaction show acidity as well as oxidizing ability. The highest
conversion was obtained for the catalyst with one V atom in the
Keggin-type primary structure. The oxidation potential of this
heteropolyacid is higher than the one corresponding to M12P
oxidation of sulfides to sulfoxides/sulfones. High conversion (100%)
within a very short time was observed using molybdovanadopho-
sphate heteropolycompounds, H
PMo 40 and (PyH)H PMo11VO40, as catalysts in homogeneous
conditions. The highest selectivity to sulfoxide was obtained when
(PyH)H PMo11VO40 was used as catalyst.
We have found a clean and convenient procedure for oxidizing
sulfides to sulfoxides/sulfones, with 35% aqueous H (2 or 20 mmol,
4 5 2 40
PMo11VO40, H PMo10V O ,
H
9
6 6
V O
3
3
(
Table 1 [31]). On the other hand, the activity is very low with
2 2
O
nonacid catalysts. Conversion increases when the acid strength of the
catalysts (determined by potentiometric titration) increases (Fig. 2).
The reactivity of Keggin-type heteropolycompounds is governed by
their electronic structure. The addenda atoms (Mo in our case) accept
electron density from the terminal oxygen atoms via pπ–dπ
interactions. This effect renders terminal oxygen atoms acidic. In
respectively) and a catalytic amount of pyridinium salt from
heteropolyacids. Reagents and catalysts are cheap and easily available.
The oxidation reaction is carried out at room temperature for
sulfoxides and 40 °C for sulfones and requires a short time.
relation to H
O
2 2
, this is a viable oxidizing agent of moderate reactivity
Acknowledgments
at 20 °C. Some form of activation of the O–O bond such as a carbonyl
group in a peroxy acid or metal catalysis is often employed in order to
increase its efficacy as an oxidizing agent [32].
The authors thank the INCA, CONICET and UNLP for the financial
support and Mrs. G. Valle for their experimental contribution for the
measures of FT-IR.
M11PV1, M10PV2 and M6PV6 catalysts were tested under
experimental conditions previously used and the conversion results
are shown in Fig. 4a. The selectivity for all cases is presented in Fig. 4b,
according to Scheme 1 for Products 1 and 2. In homogeneous
conditions, for all catalysts used high conversion, to methyl phenyl
sulfoxide (Product 1) and methyl phenyl sulfone (Product 2) was
obtained. Total conversion was achieved after 1 h of reaction.
Comparing the three V-HPA used, M11PV1 was the most selective
to Product 1 (Fig. 4b), this was practically the only oxidation product
during the first hour of reaction. However, when the number of V
atoms per Keggin unit increases, the conversion increases as shown in
Fig. 4a. On the other hand, M10PV2 and M6PV6 were more selective to
Product 2. These results may be explained in terms of the co-existence
of acidic and oxidative properties in the catalysts. The catalysts with
lower acidity but with higher redox potential quickly oxidize Product
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The substituted vanadium atoms in Keggin-structure heteropo-
lyacids are essentially active sites with high performance for the