DFT Study of Zeolite LTA Structural Fragments:  Double Four-Member Rings of Oxygen-Bridged Silicon and Aluminum Atoms

Abstract

The electronic structure and relative stability of [H 8 Si 8-x Al x O 12 ] xstructural fragments with Si and Al atoms located at tetrahedral sites and connected by bridging oxygen atoms so as to form a tetragonal prism, together with oxygen-coordinated charge-compensating cations, are studied by the B3LYP/6-31G(d) method. Clusters with Al/Si ratios in the 0-1 range and different orderings of the extraframework cations and Si and Al at tetrahedral sites are proper models for the double four-member rings, which are secondary building units of the LTA structure. The stability of the structure fragments is higher when the framework's negative charges are at maximum distance, in agreement with Dempsey's rule. The HOMO orbitals of H 8 Si 8 O 12 , the anionic clusters [H 8 Si 8-x Al x O 12 ] x-, and the proton-balanced ones are mainly O 2p nonbonding AO. In fragments with Al/Si > 0 and Na + at extraframework cation positions, the oxygen nonbonding nature of the HOMO orbital decreases and certain electron density is shifted to the Al centers. The LUMO orbitals of Na x H 8 Si 8-x Al x O 12 clusters consist of the charge-balancing cations' AO.

Evolution and Reversibility of Host/Guest Interactions with Temperature Changes in a Methyl Red@Palygorskite Polyfunctional Hybrid Nanocomposite

“…Accordingly, a (99 590) pruned grid was used for the B3-LYP calculations. As the importance of allowing free relaxation of the framework was demonstrated in previous theoretical studies, , no symmetry or geometrical constraints were imposed in the clusters ( C 1 symmetry). Basis set superposition error (BSSE) was evaluated following the a posteriori method proposed by Boys and Bernardi (1970) as implemented in Gaussian 09 .…”

Toward Understanding Extra-Large-Pore Zeolite Energetics and Topology: A Polyhedral Approach

“…Silsesquioxanes and related silica clusters have been widely used as both experimental − and theoretical − model-systems for zeolites, due to their similarities with their periodic analogues in terms of structure and rigidity . For these studies, ranging from investigations into the effect of ring-size and metal substitution on zeolite energetics to those focusing on vibrational modes of zeolites or on zeolite-based catalysis, the cluster approach with hydrogen termination was shown not to influence the outcome of these studies.…”

“…The DFT calculations were performed using the three parameter B3LYP , functional and a 6-31G** basis set, , as implemented in the program Gaussian98 . This combination of functional and basis set has previously been successfully used by us and other authors ,, for modeling similar silica systems. The geometric parameters for all clusters were fully optimized using the Berney algorithm , and tight optimization criteria (max force = 150 -6 au, RMS force = 100 -6 au, max displacement 60 -6 au, RMS displacement 40 -6 au).…”

Vibrational Modes of Double Four-Member Rings of Oxygen-Bridged Silicon and Aluminum Atoms: A DFT Study

“…The relative stability of the Na 2 H 8 Si 6 Al 2 O 12 isomeric fragments decreases in the order C 2 h > C 2 > C 2 v . A more detailed discussion of the electronic structure and energetics of H 8 T 8 O 12 clusters is presented elsewhere 1 Equilibrium geometries of D 4 R. Atoms in decreasing size are O, Na + , tetrahedral Al and Si.…”