Elsevier

Catalysis Communications

Volume 12, Issue 8, 31 March 2011, Pages 726-730
Catalysis Communications

Short Communication
Keggin heteropolycompounds as catalysts for liquid-phase oxidation of sulfides to sulfoxides/sulfones by hydrogen peroxide

https://doi.org/10.1016/j.catcom.2010.12.023Get rights and content

Abstract

H4PMo11VO40, H5PMo10V2O40 and H9PMo6V6O40 acids and an acidic pyridinium salt of H4PMo11VO40 were synthesized. They were characterized by FT-IR and the variations of their acid properties were determined by titration with n-butylamine. They proved to be highly active and selective catalysts for the hydrogen peroxide oxidation of methyl phenyl sulfide to the corresponding sulfoxide or sulfone. The conversion and selectivity results may be explained in terms of the co-existence of acidic and oxidative properties in the catalysts. On the other hand, a convenient catalytic homogeneous procedure has been found to oxidize different sulfides to sulfoxides or sulfones, with 35% aqueous H2O2, using (PyH)H3PMo11VO40 as catalyst. The oxidation reaction is carried out at room temperature for sulfoxides or 40 °C for sulfones and requires a short time. The sulfoxides or sulfones were obtained with excellent yields by controlling the amount of H2O2.

Graphical Abstract

Research Highlights

►Keggin heteropolycompounds as catalysts for ecofriendly liquid phase oxidation. ►Pyridinium salt of H4PMo11VO40, H4PMo11VO40, H5PMo10V2O40 and H9PMo6V6O40 acids. ►Aqueous hydrogen peroxide as a clean oxidant. ►Selective oxidation of sulfides to sulfoxides or sulfones.

Introduction

Sulfoxides and sulfones are important intermediates in organic chemistry due to their application in fundamental research and other extended usage, especially because chiral sulfoxides are versatile intermediates for the preparation of biologically and medically important products [1]. Omeprazole and the pesticide Fipronil are two typical examples of the extensive application of these intermediates in pharmaceutical and fine chemical industries [2]. The most widely used method for the preparation of sulfoxides and sulfones is the oxidation of the corresponding sulfide. For this reason, sulfide oxidation to sulfoxides and sulfones has been the subject of many studies, and several methods for this transformation have been reported in the literature [3], [4], [5], [6], [7], [8]. Some traditional oxidizing reagents used for this purpose include nitric acid, trifluoroperacetic acid, hydrogen peroxide, nitromethane solution in dilute NO3H/H2SO4, iodic acid, other hypervalent iodine reagents, and CAN (cerium ammonium nitrate), among others [1], [8], [9], [10], [11].

On the other hand, the introduction of a catalytic method to synthesize aliphatic and aromatic sulfoxides is still needed. The catalytic conversion has been accomplished using reagents such as binuclear manganese complexes periodic acid, N-hydroxyphthalimide-molecular oxygen, sodium perborate and/or sodium percarbonate and silica sulfuric acid in the presence of KBr, manganese(III) complexes with bidentate O,N-donor oxazoline ligand and UHP oxidizing agent, supported nitric acid on silica gel and polyvinylpyrrolidone (PVP) catalyzed by KBr and/or NaBr [12], [13], [14], [15], [16], [17]. However, some of these methods have drawbacks such as the use of corrosive acids, hazardous peracids and toxic metallic compounds that generate waste streams. Consequently, it is necessary to develop environmentally benign methods.

In the last decades, very useful procedures involving catalysis and hydrogen peroxide, as oxidant, have been developed. They promote the oxidation of organic substrates due to their effective oxygen content, low cost, safety in storage and operation and, mainly, the environmentally friendly character of hydrogen peroxide. These obvious advantages have encouraged the development of useful procedures for hydrogen peroxide oxidation of sulfides, including the use of a wide range of catalysts based on metal or semimetals, for example the previously mentioned binuclear manganese complex [18] or heteropolyacids [19], [20].

Heteropolyacids (HPA) with Keggin structures show activity as both acid and redox catalysts. Keggin structures are interesting catalysts; due to their complex structure and reactive properties they give ample opportunity for novel scientific study [21], [22], [23].

Keggin vanadium-based structures have also been reported [24]. More typical than pure vanadium-addenda complexes are the mixed addenda complexes of molybdenum and/or tungsten, in which one to three of the addenda are replaced by vanadia. They could be either electron oxidants or strong acids, with an acid strength higher than that of the classical acids.

These vanadium heteropolycompounds have recently been studied due to their importance as catalysts in the oxidation reaction, for example, hydroxylation of benzene, oxidation of toluene, nitrobenzene and norbornene using aqueous hydrogen peroxide [25], benzyl alcohol oxidation [26], oxidation of benzoins to benzyls, aldehydes and esters by dioxygen [27], and liquid-phase oxidation of methane with hydrogen peroxide [28]. Recently Leng and co-workers [29] reported the use of pyridine modified molybdovanadophosphate hybrid catalyst for the direct hydroxylation of benzene by hydrogen peroxide in the acetic acid and acetonitrile mixed solvent.

The objective of the present work is to investigate the influence of Keggin-type molybdovanadophosphoric acids and their acidic pyridine salt on the selective homogeneous oxidation of sulfides with aqueous hydrogen peroxide, at room temperature for sulfoxides and 40 °C for sulfones.

Section snippets

Catalyst preparation

The heteropolyacids H4PMo11VO40 (M11PV1), H5PMo10V2O40 (M10PV2) and H9PMo6V6O40 (M6PV6) were prepared by a hydrothermal synthesis method [24]. As example is presented M11PV1 synthesis according to the next procedure: a stoichiometric mixture of 0.98 g of phosphoric acid, 0.91 g of vanadium pentoxide and 14.4 g of molydenum trioxide was suspended in 150 mL of distilled water. The mixture was stirred for 6 h at 80 °C. After cooling down to 20 °C and removal of insoluble molybdates and vanadates, the

Characterization of catalysts

FT-IR spectra of Keggin-type molybdovanadophosphoric acids and M11PV1Py1 pyridinium salt are presented in Fig. 1. The main characteristic features of bulk H3PMo12O40 (M12P) in FT-IR [24] are observed at 1064 (P–Oa), 964 (Mo–Od), 871 (Mo–Ob–Mo) and 784 (Mo–Oc–Mo) cm 1. The characteristic stretches associated with the Keggin structure of M11PV1 are shown in Fig. 1. In these spectra, a splitting of the P–Oa band and a shift of the Mo–Od band were observed compared to those of V-free HPA [24]. The

Conclusions

The substituted vanadium atoms in Keggin-structure heteropolyacids are essentially active sites with high performance for the oxidation of sulfides to sulfoxides/sulfones. High conversion (100%) within a very short time was observed using molybdovanadophosphate heteropolycompounds, H4PMo11VO40, H5PMo10V2O40, H9PMo6V6O40 and (PyH)H3PMo11VO40, as catalysts in homogeneous conditions. The highest selectivity to sulfoxide was obtained when (PyH)H3PMo11VO40 was used as catalyst.

We have found a clean

Acknowledgments

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.

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