Characterization of silica-supported dodecatungstic heteropolyacids as a function of their dehydroxylation temperature.

Dodecatungsto-silicic H(4)SiW(12)O(40) and -phosphoric acids H(3)PW(12)O(40) were deposited on silica by a classical impregnation technique. The resulting materials were studied by in situ Raman and infrared spectroscopy, XPS and by solid-state (1)H MAS NMR as a function of their dehydroxylation temperature. The data show that in the case of H(3)PW(12)O(40) three silanol groups are protonated while in the case of H(4)SiW(12)O(40) at least one acidic proton remains. Upon heating this proton reacts leading to a disordered structure and a broadening of the W-O Raman bands.