On the nature of acid sites in substituted aluminophosphate molecular sieves with the AEL topology

A series of substituted AlPO4 molecular sieves with the AEL topology were prepared and characterized by different techniques, to evaluate the amount and nature of the additional elements and their effect on the acidity and catalytic properties of the resulting solids. It was found that a one-to-one interaction between Lewis (L) and Bronsted (B) sites occurs after calcination of the substituted materials, leading to an enhanced acidity and causing, as a result, an increase in the selectivity towards isobutene during the transformation of I-butene. Substitution models are discussed in order to rationalise the origin and nature of such B-L synergism.