Investigation of the thermal decomposition of some metal-substituted Keggin tungstophosphates

The thermal decomposition of the tetrabutylammonium (TBA) salts (TBA)(4)H-3 [PW11O39] and (TBA)(4)H-x, [PW11M(H2O)O-39]. nH(2)O, x = 3-(oxidation numbe r of M), M = Mn(II), Co(LI), Ni(II), Cu(II) or Fe(III), n = 0-3, has been s tudied by thermal analyses and the decomposition products identified by pow der X-ray diffraction, FTIR and NMR spectroscopy. The organic cation starte d to decompose in the 150-200 degrees C range. The release of the metal M f rom the substituted polyoxoanions accompanied the initial degradation of th e organic cations and [PW12O40](3-) was formed as an intermediate at ca. 30 0 degrees C. For a comparison, the thermal decomposition of (TBA)(3)[PW12O4 0] was also investigated. The thermal decomposition of the potassium salts of the lacunary and metal-substituted anions was also studied, but the form ation of [PW12O40](3-) was not observed. The temperature of decomposition o f the [PW11M(H2O)O-39]((4+x)-) anions was at least 150 degrees C higher for the potassium than for the tetrabutylammonium salts. This study exemplifie d that the thermal stability of some Keggin anions was dependent on the cou nter-cation present. (C) 1999 Elsevier Science B.V. All rights reserved.