Incorporation of decavanadate ions into silica gels and mesostructured silica walls

Decavanadate (V₁₀O₂₈^6-) polyoxometalate (POM) ions in water decompose into smaller POM ions or monomeric H ₂VO₄^- when the pH is raised to higher than 6. In this report, however, we have found that silicate ions can kinetically stabilize decavanadate ions for up to 8 h at pH 12 conditions. Gelation of these solutions by lowering the pH to 4-5 followed by aging produced monolithic vanadia-silica composite gels with a wide range of vanadium contents (V/(V + Si) up to 48%). Solid state ⁵¹V NMR and infrared spectroscopic data show that the vanadium species is predominantly V₁₀O ₂₈^6- when the vanadium content is 2.3% ≤ V/(V + Si) ≤ 32% and another V(V) species in distorted octahedral coordination starts to appear in addition to V₁₀O₂₈^6- for high vanadium content of V/(V + Si) = 48%. The kinetic stabilization of decavanadate by silicate ions suggests that the silicate ions condense around the decavanadate ions to form a core-shell like structure that may behave as pure silica in some chemical reactions. Thus, upon processing as in the synthesis of mesoporous silica using a cationic surfactant myristyltrimethylammonium bromide we have synthesized hexagonal mesostructured materials with vanadia-silica composite walls.