Measurements of Acidic Property of 12-Tungstophosphates

Abstract

The acidic properties of H₃PW₁₂O₄₀ and its metal salts were measured by means of the temperature-programmed desorption (TPD) method and the indicator method. There was no relationship between the order of acid strength of these salts measured by TPD method and that measured by the indicator method. There were also curious phenomena observed in TPD that the desorption peaks appeared at the same temperature regardre ss of the employed probe molecules, NH₃ and pyridine, which have different basisity. Acidcatalyzed reactions, dehydration of ethanol and isomerization of cis-2 -butene, were carried out in order to see which method was appropriate for explanation of acid catalysis. The catalytic activity of various salts in both reactions had a good correlation with th e acid strength measured by the indicator method.
The fact that no relationship was observ ed between the activity and the acid strength measure by TPD, which is widely utilized and regarded as a reliable and convenient method for the measurement of acidity, indicates that the adsorption of basic molecules was ruled by facters other than the interaction between the absorbed basic probe mol ecules and acid sites.
This res ult was attributed to the absorption property of heteropoly compounds; that is, heteropoly compounds absorb polarized molecules such as NH₃ and pyridine into their bulk and stabilize them in the lattice accompanied by the rearrangement of secondary structure of heteropoly compounds so called pseudo-liquid phase. This is one of the significant differences between solid acid with “rigid” structure such as zeolite, and Si0₂-Al₂0₃, etc. and that with “flexible” structure of heteropoly compounds.