FTIR study of hydration of dodecatungstosilicic acid

Abstract

The dehydration of H₄SiW₁₂ O₄₀·15.6 H₂O was studied in situ in the IR chamber. On evacuation at room temperature the departure of most loosely bonded water characterized by bands at 3550 and 1616 cm⁻¹ was observed. In the remaining hexahydrate the band at 3445 cm⁻¹ was ascribed to the hydrogen bond between the O^d oxygen atom of the Keggin unit and dioxonium H₅O ⁺₂ ion, the presence of which is manifested by the 1710 and 1100 cm⁻¹ vibrations. All these bands vanish in the case of anhydrous H₄SiW₁₂O₄₀, in which the band at 3106 cm⁻¹ ascribed to the hydrogen bond between neighbouring HPA anions O^d−H⁺−O_c is still present. The dehydration of hexahydrate is accompanied by splitting of the W=O_d band into 987 and 1010 cm⁻¹ reflecting the change of the kind of hydrogen bond in which the O_d oxygen atom is involved. Based on the above results it was concluded that protons forming oxonium ions in hydrated solid heteropoly acid are more strongly bonded than those in anhydrous one which are forming hydrogen bonds between neighbouring Keggin units.