Transition Metal-Substituted Potassium Silicotungstate Salts as Catalysts for Oxidation of Terpene Alcohols with Hydrogen Peroxide

Article abstract of DOI:10.1007/s10562-020-03449-9

Abstract: In this work, the catalytic activity of the transition metal-substituted potassium silicotungstate salts (i.e. K_8-nSiMⁿ⁺W₁₁O₃₉ (Mⁿ⁺ = Cu²⁺, Co²⁺, Ni²⁺, Zn²⁺ and Fe³⁺) was investigated on the oxidation reactions of the terpene alcohols with H₂O₂ aqueous solution. The metal-substituted silicotungstate salts were easily synthesized in one-pot reactions of the precursor metal solutions (i.e. Na₂WO₄, Na₂SiO₃ and MCl_n) with KCl added in stoichiometric amount; after this precipitation step, the solid heteropoly salts were filtered and dried in an oven. This procedure of synthesis avoids multi-step processes that starts from the pristine heteropolyacid. The substituted metal heteropoly salts were characterized by infrared spectroscopy, measurements of the specific surface area (BET) and porosimetry by isotherms of adsorption/desorption of N₂, X-rays diffraction, thermal analyses, dispersive X-rays spectroscopy, scanning electronic microscopy. The acidity strength was estimated by potentiometric titration with n-butylamine. All the salts were evaluated as catalysts in terpenic alcohols oxidation reactions with H₂O₂. The K₅SiFeW₁₁O₃₉ was the most active and selective catalyst toward oxidation products. The impacts of the main reaction variables such as catalyst concentration, temperature, oxidant load, and nature of the terpene substrate were assessed. The highest activity of the K₅SiFeW₁₁O₃₉ catalyst was assigned to the highest Lewis acidity. Graphic Abstract: [Figure not available: see fulltext.].