Abstract

The formation and thermal stability of the catalytically important anhydrous acids H₃PM₁₂O₄₀ (M = Mo,W,V), of the corresponding anhydrides PM₁₂O_38.5 and of a novel first time isolated trihydrate H₃PW₁₂O₄₀·3H₂O have been studied by high temperature X-ray diffraction (XRD) and thermogravimetry (TG). The corresponding crystal structures were determined from X-ray powder data using the Rietveld method and by deriving three-dimensional structure models from the experimental radial distribution functions.

The true unit cell symmetry of the anhydrous acid H₃PMo₁₂O₄₀ is rhombohedral, space group R3, a = 11.48 Å, α = 87.46°, Z = 2. Its crystal structure is built up from distorted (PMo₁₂O₄₀) Keggin units. The molecular centers of the Keggin units form a rhombohedral distorted cubic body centered lattice. The structure resembles that of the cubic ammonium/potassium salts. However, due to the formation of hydrogen bridges between the polyanions, the orientation of the Keggin units is different.

Heat treatment of the anhydrous acid between 673 K and 733 K leads to the release of the constitutional water molecules with the formation of an amorphous anhydride. The anhydride structure is built up from lacunary polyions PMo₁₂O₃₈⁺ and PMo₁₂O₃₉⁻ in a three dimensional arrangement very similar to that of the corresponding anhydrous acid. However, the mutual orientation of the polyions is largely disordered.

Dehydration of H₃PW₁₂O₄₀·6H₂O between 413 K and 453 K results in the formation of a new cubic acid containing three molecules of water of crystallization. Its structure is closely related to that of the well known hexahydrate. Both structures may be thought of as being made up of two different interpenetrating substructures of anions and cations with space group symmetry Pn3m. The anionic substructures are similar; the cationic substructures, however, are different. In the hexahydrate structure the polyanions are linked by nearly planar H₅O₂⁺ dioxonium cations and the oxygen atoms occupy the x, ¼, ¾ crystallographic sites. The release of water leads to the formation of H₃O⁺ oxonium cations which occupy the regular cationic positions ¼, ¾, ¾. The molecular formula of the new trihydrate should be written as (H₃O⁺)₃PW₁₂O₄₀.