Influence of cations on active sites of the alkaline earth salts of 12-tungstophosphoric acid: microcalorimetric study

The acidic properties of the alkaline earth salts of 12-tungstophosphoric acid were investigated using ammonia absorption microcalorimetry. The number and the strength of acid sites of these catalysts as well as differential heats of ammonia absorption were determined. As a result of substitution of two protons with one alkaline earth cation, the decrease of both the total number of acid sites and the number of the strongest acid sites characterized by differential heats higher then 150 kJ/mol, compared to the values found in the case of initial 12-tungstophosphoric acid, were noticed. While all protons seem accessible for ammonia in Mg, Ca and Ba salts, in the case of Sr salt less then stoichiometric ammonia sorption was detected which can be explained by steric effect of these divalent cations.