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 (M n +) x / n H12Si12 - x Al x O18 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.

A DFT study of adsorption hydrogen on the Li-FAU zeolite

“…Therefore, we can conclude that clusters we used in the work are adequate to represent the interaction of H 2 and the acid sites in zeolite framework. From the relative stability of the Li + sitting sites in the Table 1, we found that site SI turned out to be the most stable location for Li + , which is in agreement with the study by D6Rs [24]. The SII site is preferred next and then follows the SIII site.…”

“…The selected bond distances and angles and the relative stability are reported in Table 1. Also shown in Table 1 for comparison are the results of experimental [29][30][31] and similar calculations with D6Rs [24] reported in the literature. It shows that the distance between the bridging oxygen and Li atom (R(Li-O)) of the present work has a better match to experiment than that in the D6Rs simulation.…”

“…The cluster we choose consists of a sodalite cage and D6Rs, which extend the study by a large series of bare extraframework cations or the clusters limited to S6Rs (a single six-membered ring) or D6Rs [22][23][24]. One framework Si atom was replaced by aluminium and the charge was compensated for by a Li + cation.…”

A Density Functional Theory Modeling on the Framework Stability of Al-Rich Cu-SSZ-13 Zeolite Modified by Metal Ions

“…When the alkaline-earth Ca 2+ enters the SSZ-13 zeolite framework, it still appears in the form of a divalent cation Ca 2+ , which balances the negative charges brought by the two framework Al atoms. According to a study on faujasite (FAU) zeolite by Uzunova et al, 35 there are two possible locations of Ca 2+ in zeolite containing dual six-membered rings: one is located in the plane of a six-membered ring, and the other is located in the center of a hexagonal prism. The structural optimization in Figure 6 shows that the stable location of Ca 2+ in the SSZ-13 zeolite is the same.…”

ACR announcements

“…[19] According to the plane‐wave results, the Cu(I) cations in the adsorption complexes remain coordinated to only two framework oxygen atoms even at the higher coordination sites, and they form relatively short bond lengths, which are more typical for Li cations than for transition metals. [41]…”