Evaluation of the Activity of Potassium Dodecatungstate Salts in Alcohol Oxidation Reactions: Cobalt(II), Cobalt(III), or Silicon(IV) as the Heteroatom.

Silicotungstic acid potassium salts had the silicon heteroatom substituted by Co(III) or Co(II) cations and were evaluated as catalysts in oxidation reactions of terpenic alcohols with hydrogen peroxide. Borneol was the model molecule selected. All the catalyst salts were characterized by Fourier-transformed infrared, ultraviolet–visible and Raman spectroscopies, thermal analyses, powder X-rays diffraction and cyclic voltammetry analyses, energy dispersive X-ray spectroscopy and scam electronic microscopy (SEM) analyses, potentiometric titration. This set of techniques revealed that salts containing Co(II) or Co(III) as the heteroatom were successfully obtained. The activity of potassium dodecatungstocobaltate(III) (i.e., K₅Co^IIIW₁₂O₄₀) was compared to other tungsten heteropoly catalysts (i.e., H₄SiW₁₂O₄₀, K₄SiW₁₂O₄₀, K₆Co^IIW₁₂O₄₀, Co₂SiW₁₂O₄₀) and the precursor CoCl₂. Among the various heteropoly compound investigated, the K₅Co^IIIW₁₂O₄₀ salt was the most active and selective catalyst toward oxidation product (i.e., camphor). Conversion and selectivity higher than 90% were achieved, indicating the high efficiency of this catalyst. The effects of the main reaction parameters such as temperature, the stoichiometry of reactants, type and catalyst load, were investigated. Special attention was given to correlating the structural properties of catalysts with their activity. Based on experimental data, a reaction pathway was proposed. As far as we know, it is the first time that silicotungstic acid-derived salts having heteroatom Co(II) or Co(III) cations, were used as catalysts in oxidation reactions of terpene alcohols. The effect of the oxidation state and position of Co on the activity of silicotungstic acid was evaluated. The H₄SiW₁₂O₄₀ had the Si(IV) heteroatom exchanged with Co(II) or Co(III). Their potassium salts were tested in borneol oxidation reactions with H₂O₂ being the K₅Co^IIIW₁₂O₄₀ the most active and selective catalyst. [ABSTRACT FROM AUTHOR]