Charge sensitivity analysis of intrinsic basicity of faujasite-type zeolites using the electronegativity equalization method (EEM)

The Electronegativity Equalization Method (EEM) is used to study the dependence of the basicity of faujasite-type zeolites on the Si:Al ratio, the cation type, and the type of oxygen. The model structures were obtained by distance least-squares optimization within the F222 space group, and cations were located with the Monte Carlo technique, starting from experimental XRD positions. The main effect determining basicity, as indicated by the average negative charge on the oxygens, is the chemical composition of the framework. The average negative oxygen charge increases with decreasing Si:Al ratio, in agreement with the experimentally found increase of basicity. The secondary structure effect identifies the supercage oxygens O1 and O4 as the most basic. The details of the cation distribution further controls the basicity of these oxygens, which increases when Nat is replaced by Csf on sites II and, especially, site III, also in agreement with experiment. Besides charges on oxygen, Fukui indices and regional sensitivities are also good indicators of basicity.