New Evidence on the Structure of Potassium Salts of 12-Tungstophosphoric Acid, K_xH_3-xPW₁₂O₄₀

Abstract

Physicochemical properties and compositions of KxH(3-x)PW12O40 salts, where 2 < or = x < or = 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 KxH(3-x)PW12O40 salts, where 2 < or = x < 3, from 313 K to higher temperatures induces the transformation of the heteropolyacid-covering K(3) 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.

Methanolysis of Castor Oil Catalysed by Solid Potassium and Cesium Salts of 12-Tungstophosphoric Acid

“…No change was observed for K3-CO 3 sample. As described before, these changes in surface areas were related to the change in secondary structure of K-salts identified as enhanced spreading of HPA onto the K3-core [28].…”

“…Such a dehydrated HPW is not stable as a separate phase and can exist only because it is stabilized by the K3-core support. The coverage of the K3-core by HPW in the K2-salt was estimated to be equal to one monolayer while in K2.5 this coverage was much lower than 1 monolayer [28]. To the best of our knowledge, no information concerning the role of aging in the morphology of Cs-salts has been reported.…”

“…This resulted in the increase of specific surface area of the K2 and K2.5 salts. DTA-TG experiments revealed that HPW existed on the surface of K3-core in the form of at least partially dehydrated phase [28]. Such a dehydrated HPW is not stable as a separate phase and can exist only because it is stabilized by the K3-core support.…”

“…The Cs or K-salts were prepared as described before [28]. The stoichiometric quantities of cesium carbonate, cesium chloride or potassium carbonate were added to aqueous solutions of the 12-tungstophosphoric acid.…”

“…cations changing within the range 2 B x \ 3 consists of K 3 PW 12 O 40 core covered by surface layer of amorphous H 3 PW 12 O 40 [28,30]. It has been shown that in fresh K2 and K2.5 salts obtained after drying the salts precipitates at 313 K the heteropolyacid is deposited in the bulk form on the neutral K3 core.…”