Polarizability model of acidity of zeolites

Abstract

A new model has been developed for the estimation of structural acidity parameters and the acid amount of zeolites using valence electron density, atomic polarizability, and chemical composition. Based on this model, the computed zeolite acidity has been compared with the data obtainable from Sanderson's electronegativity model and also with available experimental data. The general agreement between our calculated values and those reported in the literature has been found to be very good in the case of mordenites. It has been established that electronegativity equalization (EE_q) in zeolites could be equally explained via the electronic polarization process governed by the proposed equation: EE_q=1.67 [(N_P/α_P)N_Q/α_Q)N_R/α_R)]^1/(p+q+r), where N is the valence electron density and a the atomic polarizabilities of the constituting atoms expressed in the unit of volume, and p, q, r are the number of atoms representing a molecule of general formula P_inpQ_qR_r The relative polarizing power of structures has also been reported as a function of Al content. This means that increased bond polarization will result in enhanced acidic behavior of the protons upon interaction with basic substrate molecule. This is why relative polarizing powers, α'_p', are directly related to the Al content, the acidity [H⁺], and shrinkage effect of bond lengths (D_t−0).