MODELING OF BRONSTED ACIDITY IN AFI AND CHA ZEOTYPES

The Bronsted acidity of AFI and CHA zeotypes in their aluminosilicate (zeolite) and silico-aluminophosphate (SAPO) forms has been modelled u sing potential-based computational methods. We find that the gradient of the electrostatic potential at the proton correlates with the OH st retching frequency, providing a method for characterising the acidity of these materials. Structural effects are shown to be one of the fact ors influencing acidity through the study of acid centres on the diffe rent oxygen atoms attached to the same T site. The influence of chemic al composition on acidity is also analysed through the simulation of B ronsted acid pairs, silicon islands and aluminosilicate regions in SAP Os. We present a general discussion on the relative acidity of these a ggregates, demonstrating how strong acid centres in SAPOs can appear i n SiAl regions, although the highest acidity in SAPOs is found at the borders of the silicon islands.