Green Synthesis of Adipic Acid Over Tin-Substituted Polyoxometalates Series, H3−2xSnxPMo12O40 (x = 0.25–1.5)

Polyoxometalates (POMs) especially those of Keggin-type represent a potential alternative as catalysts in the processes of oxidation. POMs are known to possess oxidative and Bronsted acidity properties simultaneously. The oxidative power can be enhanced by the introduction of transition metal as a counter ion. In this work, the series of POMs of formula H3−2xSnxPMo12O40 (x = 0.25–1.5) are prepared using the cationic exchange method under stoichiometric conditions and characterized by various physico-chemical methods and tested in the cyclohexanone reaction in the presence of hydrogen peroxide (30%). Elemental analysis and X-ray diffraction showed the reliability of the synthesis method, infrared spectroscopy showed a preservation of the Keggin anion, and UV–visible spectroscopy showed that the tin salts are in a partially reduced state. The POMs are found to be active in the cyclohexanone oxidation reaction, and among them, the most efficient is H1Sn1PMo12O40 with 44% of adipic acid yield.