Preparation, characterization of mixed-valence antimony-tin phosphomolybdic polyoxometalates and application in the cyclohexanone oxidation in the presence of hydrogen peroxide

Abstract

In this work, a series of polyoxometalates (POMs), as soluble salts of formula Sn1.5PMo12O40, SbPMo12O40 and $\mathrm{H}_{3-x}\mathrm{Sn}_{x1}\mathrm{Sb}_{x2}\mathrm{PMo}_{12} \mathrm{O}_{40}$ ($x=2x_{1}+3x_{2}$ and $x_{1}$, $x_{2}=$ 0–0.75) were synthesized using the cationic exchange method under stoichiometric conditions. They were characterized and tested in the oxidation of cyclohexanone using hydrogen peroxide in the absence of organic solvent, co-catalyst phase transfer compounds or surfactants. UV–Visible and X-ray photoelectron spectroscopic analyses showed a partially reduced state of POM with coexistence of the couples Mo(VI)/Mo(V), Sb(V)/Sb(III) and Sn(IV)/Sn(II), without affecting their structure, results confirmed by FT-IR and Raman spectroscopies. The reversible cyclic voltammetry analysis evidenced the valence change of the Mo species, Mo(VI)/Mo(V). The POMs are found to be active in the cyclohexanone oxidation reaction and among them the most efficient is $\mathrm{H}_{0.75}\mathrm{Sb}_{0.75}\mathrm{PMo}_{12}\mathrm{O}_{40}$ with 49% of adipic acid yield.

Keywords

• Polyoxometalates
• Mixed-valence salts
• Cyclohexanone
• Oxidation
• Adipic acid