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., (K5CoW12O40)-W-III) was compared to other tungsten heteropoly catalysts (i.e., H4SiW12O40, K4SiW12O40, (K6CoW12O40)-W-II, Co2SiW12O40) and the precursor CoCl2. Among the various heteropoly compound investigated, the (K5CoW12O40)-W-III 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.

Automated summary

Silicotungstic acid-derived salts with Co(II) or Co(III) cations were used as catalysts in the oxidation reactions of terpenic alcohols. The (K5CoW12O40)-W-III salt exhibited the highest activity and selectivity towards the oxidation product camphor, achieving conversion and selectivity higher than 90%. The effects of temperature, stoichiometry of reactants, and catalyst load on the reaction were investigated, and a reaction pathway was proposed based on the experimental data.