Na4SiW12O40

Na4SiW12O40 is the sodium salt of the Keggin-type polyoxometalate [SiW12O40]4-. The anion comprises a central silicon-centered tetrahedron surrounded by a shell of twelve tungsten oxide octahedra, giving a highly symmetric [SiW12O40]4- core. In solid state or aqueous solutions, four Na+ counterions balance the 4− charge, and the compound commonly exists as hydrated salts, Na4[SiW12O40]·nH2O.

Synthesis typically involves condensation of silicate and tungstate units under strongly acidic conditions in the presence of sodium cations. Aqueous solutions of sodium tungstate and a silicate source (e.g., SiO2 or silicate species) are acidified (pH around 1–2) and allowed to form the lacunary Keggin structure, which crystallizes as the sodium salt upon cooling or evaporation.

Properties: Na4SiW12O40 is water-soluble; the [SiW12O40]4- anion is redox-active, with tungsten centers able to undergo reversible reductions/oxidations. The compound is stable in acidic aqueous media and decomposes on heating (thermal stability typically several hundred degrees Celsius for hydrated salts depending on hydration level).

Applications: It is used as a homogeneous catalyst in oxidation reactions, often with hydrogen peroxide or dioxygen as oxidants; immobilized or supported forms are studied for heterogeneous catalysis and electrocatalysis. Polyoxometalates like Na4SiW12O40 serve as versatile building blocks in materials chemistry and catalysis.