Electronic Structure and Stability of Double Six-Membered Rings of Oxygen-Bridged Silicon and Aluminum Atoms Related to Cation Site Occupancy in FAU Zeolites: a DFT Study

Abstract

The electronic structure and relative stability of clusters with general composition (Mn+)x/nH12Si12-xAlxO18 with x = 0, 2, 4, M = H+, Li+, Na+, K+, Ca2+ forming double-six-membered rings (D6R), are studied by the B3LYP method. In Ca2H12Si8Al4O18 clusters, maximum separation of the Al atoms is favored, which implies a minimum number of Al−O−Si−O−Al linkages. A large number of isomers with Si,Al distributions strongly related to the type and location of the charge-compensating cations were determined for the ratio Si/Al = 5. All cations favor maximum separation of Al atoms within the T6O6 rings. Concurrent occupation of extraframework cation sites in the center of the D6R and above the T6O6 window is energetically unfavorable for Li+ and Na+ cations, whereas this is achieved with K+, in agreement with experimental data. The Si,Al ordering with one Al atom per T6O6 ring and at the largest distance within the D6R is the most stable one with Li+ and K+ as extraframework cations, whereas Na+ and Ca2+ favor the configuration with two Al atoms in one T6O6 ring. The small and medium size cations Li+, Na+, and Ca2+ approach the regions of high electron density in the D6R. The HOMOs of both the siliceous fragments and the aluminum-containing fragments with the extraframework cations located near the four-membered ring walls consist mainly of O 2p nonbonding AOs. When the extraframework cations take up positions at the D6R center, or near the T6O6 rings, the HOMOs O 2p nonbonding character is reduced: Al 3s and Al 3p AOs are admixed to the O 2p AOs. The maximum electron density is concentrated at the Al atoms and the Al-bonded oxygen atoms. When the Al atoms are in one T6O6 ring of a D6R, the oxygen atoms of this ring bear a higher electron density. Lewis basic sites appear in proximity to the Brønsted acid sites in proton balanced clusters.