Acid-Base Properties of Amorphous Metal Oxides

The acid-base properties of amorphous mixed metal oxides can be varied by choosing different metal oxide constituents at diflerent concentrations and by changing the treatment of the sample (44). Thus, it appears that, by properly choosing the aforementioned variables, mixed oxides could be used to develop new catalysts with desired acid-base properties. The use of micro-calorimetric adsorption measurements to quantify the acid-base properties of metal oxides and mixed metal oxides has been limited, to date, to a few systems. However, for some of these solids, for example, silica, alumina, and silica-alumina, several investigations have led to a satisfactory description of their acidity and acid strength. We present here a compendium of those measurements and discuss some of the important properties observed. 

In contrast, the surface of alumina has strong acid and basic sites, as demonstrated by the differential heats of adsorption of basic probe molecules such as ammonia 140,147,153, 180) and n-butylamine 48,177) or of acidic probe molecules such as carbon dioxide 147,180) and hexafluoroisopropanol 179). The temperature dependence of the heat of adsorption for alumina is characteristic of a strong acidic surface, with the initial differential heat increasing and the adsorption capacity decreasing with increasing adsorption temperature. 

Inspection of Tables XIII-XV shows that the basic strength of the probe molecules most used to determine the acid strength of metal oxides seems to be ammonia n-butylamine pyridine trimethylamine piperidine  

Probe molecule T (K) 9lnlllal (kJ mor ) 9max (kJ mol ) 9final (kJmor ) (/rniolg- l Ref. 

Amount adsorbed in ftmol m the surface area was not specified. The sample was outgassed at 673 K. 

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