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Physicochemical properties and compositions of K(sub x)H(sub 3-x)PW12O40 salts where 2 less than or equal to x less than or equal to 3 are investigated. Results reveal that freshly prepared K2HPW12O40 salt contains particles of heteropolyacid and particles of the neutral potassium salt, the sample being in 78.6% amorphous.

Abstract: The Ag3PW12O40·3H2O salt forms two different structures: the first one, existing at temperature up to 473 K, contains one water molecule hydrating each silver cation, whereas the other one appears above 573 K after the loss of the water molecule. It results in the shortening of the lattice parameter a from 11.93 to 11.70 Å. This dehydration–hydration process of silver salt is reversible. In the structure at 473 K, the [Ag(H2O)]+ complex is centered in the octahedral space between the Keggin anions. Each silver cation is bonded to two terminal oxygens of different Keggin anions while water molecule forms weak hydrogen bonds with other two Keggin anions. Unexpectedly, in dehydrated structure existing at 573 K, Ag+ cations are not centered in the octahedral space between Keggin anions and are connected to two terminal oxygens of Keggin anions only. Probably, the formation of two various structures of silver tungstophosphate influences its catalytic activity with changing temperature. [Copyright &y& Elsevier]

The cesium salts Cs xH3− xPW12O40 of Cs content x = 2 up to x = 3 were tested as the catalysts in the gas and liquid phase reactions. Dehydration of ethanol and transesterification of triglycerides with methanol were selected as the catalytic reactions. Apart from the standard preparation, the catalysts were prepared by two-stage procedure with methanol or water as a solvent. The Cs-salts were characterized by FT-IR, XRD, scanning electron microscopy and energy dispersive X-ray techniques. In turn, the influence of Cs-salts composition on the pH and conductivity of their aqueous colloidal solutions was investigated. The results obtained by the latter techniques were also characteristic for acidity of surface layer of colloidal particles because of surface layer-solution equilibrium. It has been shown that the secondary structure of acidic cesium salts existing in crystalline samples (solid solution of H3PW12O40 in Cs3PW12O40) changes after contacting with polar medium to the system consisting most probably of Cs3PW12O40 core with epitaxial layer of heteropolyacid. This is result of the protons migration from bulk to surface layer of primary particles enhanced by polar medium. It strongly influences the surface acidity of primary particles as well as the activity of Cs-salts in transesterification of triglycerides with methanol. In such polar medium, Cs2HPW12O40 salt becomes the most active catalyst, more active than Cs2.5H0.5PW12O40. An accumulation of partial glycerides and in particular glycerol on the surface of primary particles of Cs-salts resulted in relatively low maximum conversion of triglycerides, most probably due to partial blockage of the catalytic centers. This effect and the almost constant activity of Cs-salts under recycling use in the transesterification experiments are considered to be experimental evidences that methanolysis over Cs-salts was accomplished with the participation of surface protons. [ABSTRACT FROM AUTHOR]

Physicochemical properties and compositions of Kξ-H3-xPW12O40 salts, where 2 ≤ x ≤ 3, have been investigated. It has been found that freshly prepared K2HPW12O40 salt (drying at 313 K) contains particles of heteropolyacid and particles of the neutral potassium salt, the sample being in 78.6% amorphous. On aging at room temperature, the heteropolyacid spreads to form a surface layer covering the neutral potassium salt particles K3PW12O40. Heat treatment of KξH3-xPW12O40 salts, where 2 ≤ x « 3, from 313 K to higher temperatures induces the transformation of the heteropolyacid-covering K3 core into a well-dispersed, amorphous surface layer. On further heating of the acidic potassium salts, the surface layer decomposes between 855 and 915 K with the formation of a PW8O26-type bronze as a new phase, the K3PW12O40 salt remaining unchanged, The latter starts to decompose at 1093 K, and in the case of all samples, the process is completed at about 1183 K. Rietveld structure refinement, XPS, and 31p NMR measurements of acidic potassium salts indicate that the core of these salts is always formed by the K3PW12O40 salt, which is covered by a heteropolyacid. Comparison of lattice parameters of the K3 salt and HPW leads to the conclusion that the layer is composed of partially or completely dehydrated heteropolyacid molecules. The coverage of the core by HPW in the K2 sample was estimated to be equal to one monolayer. [ABSTRACT FROM AUTHOR]