Acidic Properties in the Solid State

As for the acidic properties (amount, strength and type of acid sites) of solid heteropoly compounds, bulk acidity as well as surface acidity must be considered, since acid catalysis often occurs in the solid bulk. These acidic properties are sensitive to the counter cations as well as to the constituent elements of polyanion. The acid forms are protonic acids and the acid strength reflects that in solution. In the case of salts, there are several possible mechanisms for the origins of the acidity. 

Typical TD results for the samples prepared as described above are shown in Fig. 3.75. Those data demonstrate that heteropoly compounds in the acid form are strong protonic acids and that all the protons contribute to the acidity. For quantitative discussion it is necessary to conflrm the establishment of equilibrium and the stability of the polyanion structure. The acid strength estimated by the TD of NH3 for Si02-supported heteropoly acids paralleled that in solution.  

The following five mechanisms are present for the acidity of the salts. 

The third mechanism was assumed for the salts of divalent metals. Lewis acidity has been reported for A1 salt from the IR spectra of absorbed NH3, but it was not detected in the case of Cu salt. The last mechanism (eq. (6)) was proposed for the salts of Ag and Cu.  

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