Modelling of Brønsted acidity in AFI and CHA zeotypes

Abstract

The Brønsted acidity of AFI and CHA zeotypes in their aluminosilicate (zeolite) and silico-aluminophosphate (SAPO) forms has been modelled using potential-based computational methods. We find that the gradient of the electrostatic potential at the proton correlates with the OH stretching frequency, providing a method for characterising the acidity of these materials. Structural effects are shown to be one of the factors influencing acidity through the study of acid centres on the different oxygen atoms attached to the same T site. The influence of chemical composition on acidity is also analysed through the simulation of Brønsted acid pairs, silicon islands and aluminosilicate regions in SAPOs. We present a general discussion on the relative acidity of these aggregates, demonstrating how strong acid centres in SAPOs can appear in SiAl regions, although the highest acidity in SAPOs is found at the borders of the silicon islands.