Catalysts Based on Vanadium-Containing Heteropoly Compounds: Effect of Outer-Sphere Cations on the Physicochemical and Oxidative Properties of a Si–W–V Heteropoly Acid

The effect of catalytically active vanadium(V) atoms on the physicochemical properties of a silicotungstic heteropoly acid has been studied. It has been shown that introducing vanadium(V) atoms into a framework of monovacant W-containing heteropoly anions in the form of a H₆V₁₀O₂₈ solution prepared using the environmentally friendly peroxide approach ensures the formation of mixed Si–W–V anions with the retention of structural integrity. It has been demonstrated that the partial substituting of such large cations as Cs⁺ and nBu₄N⁺ for protons results in the precipitation of insoluble acidic А_4.5H_0.5SiW₁₁VO₄₀ salts, the properties of which change substantially depending on the type of the introduced counterion (А⁺). The textural characteristics of the synthesized salts have been compared, their TG/DTG/DSC profiles have been obtained, and their phase compositions have been analyzed. IR-ATR, XRD, and ICP-AES characterizations of samples after hydrothermal treatment have proved that the synthesized salts are highly stable. Samples display activity as reversible oxidizers in the reaction of 5-hydroxymethylfurfural conversion in an aqueous medium with a yield of 2,5-diformylfuran up to 89% while retaining their efficiency upon repeated use.