The H 3 PW 12 O 40 and H 4 SiW 12 O 40 heteropolyacids supported on silica were studied by 1 H MAS NMR as a function of both the coverage and the dehydroxylation temperature. The signal at ca. 8 ppm attributed to the highly acidic protons of the anhydrous form of the heteropolyacid was never observed in the case of the phosphotungstic polyanion supported on silica while it was clearly present in the case of the silicotungstic species. These results explain the different activities of the two supported heteropolyacids in acid reactions involving strong acid sites. .
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Thomas et al. (2005) studied this question.
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