A calorimetric study of the acidity of bulk and silica-supported heteropoly acid H3PW12O40

The acid strength of bulk and SiO2-supported (10–60 wt%) H3PW12O40 (PW) is characterised by differential scanning calorimetry of ammonia adsorption in a gas–solid system and by microcalorimetry of pyridine adsorption in a liquid–solid system (cyclohexane slurry). In both systems, the strength of PW in terms of the heat of adsorption of NH3 or pyridine increases with increasing PW loading on silica, the bulk PW being the strongest acid. The temperature and the heat of NH3 desorption are shown to indicate the acid strength, in full agreement with the heats of NH3 adsorption. Attempts to measure the acidity of PW included in a sol–gel silica matrix have failed due to a large amount of water present in such systems.