Thermal dehydration of H3+xPVxM12-xO40 center dot yH(2)O Keggin type heteropolyacids; formation, thermal stability and structure of the anhydrous acids H3PM12O40, of the corresponding anhydrides PM12O38.5 and of a novel trihydrate H3PW12O40 center dot 3H(2)O

The formation and thermal stability of the catalytically important anhydrou s acids H3PM12O40 (M = Mo,W,V), of the corresponding anhydrides PM12O38.5 a nd of a novel first time isolated trihydrate H3PW12O40. 3H(2)O have been st udied by high temperature X-ray diffraction (XRD) and thermogravimetry (TG) . The corresponding crystal structures were determined from X-ray powder da ta using the Rietveld method and by deriving three-dimensional structure mo dels from the experimental radial distribution functions. The true unit cell symmetry of the anhydrous acid H3PMo12O40 is rhombohedra l, space group R3, a = 11.48 Angstrom, alpha = 87.46 degrees, Z = 2. Its cr ystal structure is built up from distorted (PMo12O40) Keggin units. The mol ecular centers of the Keggin units form a rhombohedral distorted cubic body centered lattice. The structure resembles that of the cubic ammonium/potas sium salts. However, due to the formation of hydrogen bridges between the p olyanions, the orientation of the Keggin units is different. Heat treatment of the anhydrous acid between 673 K and 733 K leads to the r elease of the constitutional water molecules with the formation of an amorp hous 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 o f the polyions is largely disordered. Dehydration of H3PW12O40. 6H(2)O between 413 K and 453 K results in the for mation of a new cubic acid containing three molecules of water of crystalli zation. Its structure is closely related to that of the well known hexahydr ate. Both structures may be thought of as being made up of two different in terpenetrating substructures of anions and cations with space group symmetr y Pn3m. The anionic substructures are similar; the cationic substructures, however, are different. In the hexahydrate structure the polyanions are lin ked by nearly planar H5O2+ dioxonium cations and the oxygen atoms occupy th e x, 1/4, 3/4 crystallographic sites. The release of water leads to the for mation of H3O+ oxonium cations which occupy the regular cationic positions 1/4, 3/4, 3/4. The molecular formula of the new trihydrate should be writte n as (H3O+)(3)PW12O40.