SiW12O404

SiW12O404 is commonly written as [SiW12O40]4−, a Keggin-type polyoxometalate (POM) based on a silicotungsten oxide framework. It is a well-studied inorganic anion in contemporary inorganic and catalysis chemistry, renowned for its structural regularity, high negative charge, and chemical stability in aqueous and acidic media.

The core of the ion consists of a central silicon tetrahedron (SiO4) surrounded by twelve edge-sharing WO6 octahedra. The twelve tungsten centers form a protective, highly regular shell around the central silicon, yielding a compact anion with high symmetry. The overall 4− charge makes it soluble as various salt forms, and in solution the species is typically described as H4SiW12O40 under strongly acidic conditions.

Synthesis and derivatives: [SiW12O40]4− is commonly prepared via acidic condensation of tungstic acid (H2WO4) with silica sources under controlled pH, heat, and ionic strength to yield the silicotungstic acid H4SiW12O40. Neutralization or ion exchange then affords salts such as Na4[SiW12O40], K4[SiW12O40], or tetrabutylammonium derivatives. Many derivatives exist by partial or complete substitution of counter-ions, enabling immobilization on supports or incorporation into composite materials.

Applications: SiW12O40-type POMs function as homogeneous catalysts for selective oxidation of organic substrates (often using H2O2 or O2 as oxidants), as redox mediators in electrochemical systems, and as building blocks for heterogenized catalysts when anchored to solid supports. Their robustness in acidic media and tunable redox properties make them versatile in catalysis, electrochemistry, and materials science. Related heteroatom variants, such as [PW12O40]3−, form part of the broader family of Keggin-type polyoxometalates.