Theoretical studies on acidity and site selectivity of cations in faujasite zeolite

The influence of the silicon to aluminum ratio (1, 1.4, and 2) on the acidity and site selectivity of cations in faujasite-type zeolite is explained by using calculated atomic net charges and cation-binding energies. The chemical formulae of model compounds are K₉₆⁺Al₉₆Si₉₆O₃₈₄ (1:1), K₈₀⁺Al₈₀Si₁₁₂O₃₈₄ (1.4:1), and K₆₄⁺Al₆₄Si₁₂₈O₃₈₄ (2:1). Many experimental results (X-ray data, infrared spectra, and dealumination experiments) suggest that a covalent character is dominant in the zeolite framework. The net charges of framework atoms are derived by using the assumption of equalization of electronegativity in all the atoms. The atomic net charges of hydrogen atoms in fully decationated faujasite-type zeolite (H_xAl_xSi_1-xO₂) increase with a decrease of the aluminum content. The net charges of aluminum atoms in K_x⁺Al_xSi_1-xO₂ also increase with a decrease of aluminum content. It is easily explained from the calculated atomic net charges that both Brønsted acidity and Lewis acidity of the X-type zeolites are less than those of Y-type zeolite. The distribition of cations in model compounds is determined by calculating the binding energies for several possible cases of cation arrangements. Generally, the binding energies of cations decrease with a decrease of the aluminum content. The calculated binding energies indicate that sites I and I′ which belong to the same hexagonal prism normally cannot be occupied simultaneously. When the binding energies of cations in sites I and I′ are larger than those in sites II and III, the sum of the occupancy factors of sites I and I′ is nearly equal to one. The site preference series are obtained as follows: for 1:1 model, I > III′ > II > III > I′; for the 1.4:1 model, I > I′ > II > III′ > III; and for the 2:1 model, I′ > I > II > III′ > III.