Local Environment and Dynamic Behavior of Fluoride Anions in Silicogermanate Zeolites: A Computational Study of the AST Framework

Abstract

In silicogermanate zeolites containing double four-ring (d4r) building units, the germanium atoms preferentially occupy the corners of these cube-like units. While this general behaviour is well known, the absence of long-range order precludes a determination of the preferred arrangements of Si and Ge atoms at the corners of d4r cages by means of crystallographic methods. If fluoride anions are present during the synthesis, these are incorporated into the d4r cages. Due to the sensitivity of the 19 F chemical shift to the local environment, NMR experiments can provide indirect insights into the predominant (Si,Ge) arrangements. However, conflicting interpretations have been reported, both with regard to the preference for, or avoidance of, Ge-O-Ge linkages, and concerning the equilibrium position of fluorine inside the cage, where fluorine might either occupy the cage centre or participate in a partly covalent Ge-F bond. In order to shed light on the energetically preferred local arrangements, periodic electronic structure calculations in the framework of dispersion-corrected density functional theory (DFT) were performed. The AST framework was used as a suitable model system, as this zeolite is synthetically accessible across the range of (Si 1-n ,Ge n)O 2 compositions (0 ≤ n ≤ 1). DFT structure optimisations for (Si,Ge)-AST systems containing fluoride anions and organic cations revealed that arrangements of Si and Ge at the cage vertices which maximise the number of Ge-O-Ge linkages are energetically preferred, and that fluorine tends to form relatively short (~2.2 to 2.4 Å) bonds to Ge atoms that are surrounded by Ge-O-Ge linkages. The preference for Ge-O-Ge linkages disappears in the absence of fluorine, pointing to a "templating" effect of the anions. In addition to the prediction of equilibrium structures, DFT-based Molecular Dynamics calculations were performed for selected AST models in order to analyse the dynamics of fluoride anions confined to d4r cages. These calculations showed that the freedom of movement of fluorine varies markedly depending on the local environment, and that it correlates with the average Ge-F distance (short Ge-F bonds = restricted motion). An analysis of the Ge-F radial distribution functions provided no evidence for a coexistence of separate local energy minima at the cage centre and in the proximity of a germanium atom for any of the systems considered. The computational approach pursued in this work provides important new insights into the local structure of silicogermanate zeolites with d4r units, enhancing the atomic-level understanding of these materials. In particular, the findings presented here constitute valuable complementary information that can aid the interpretation of experimental data.

Synthesis and Post‐Synthesis Transformation of Germanosilicate Zeolites

“…TheG ed istribution in AST germanosilicates was thoroughly investigated at the DFT level, focusing on the equilibrium locations of fluoride and its dynamic behaviour. [30] Theresults clearly demonstrated that the formation of Ge-O-Ge linkages is energetically favourable in the fluoridecontained AST,but in the absence of the fluoride anion, there are no clear trends in Ge-O-Ge and Ge-O-Si energetic preferences.T hese calculations thus showed the templating effect of F À leading to d4r units with Ge-O-Ge linkages in AST germanosilicates,i nl ine with the EXAFS study of UTL, [31] which showed Ge atoms at asingle face of (4Ge,4Si) d4r units.H owever,adifferent Ge distribution in d4r was found in ITQ-22 (IWW)a nd ITQ-13 (ITH)b yN MR. [32] Therefore,t he Ge distribution in d4r germanosilicates is likely governed by the flexibility of T-O-T linkages in d4r units which, is framework dependent, as previously shown at various levels of theory. [33] In ar ecent computational investigation, the stability ranges for Si-O-Si and Ge-O-Ge angles in pure silica and pure germania cristobalite were calculated based on aD FT analysis employing aperiodic model.…”

Simulation-based evaluation of zeolite adsorbents for the removal of emerging contaminants

“…First of all, it needs to be emphasised that the DFT results depend on various parameters in the computational setup, especially the choice of exchangecorrelation functional and dispersion correction. While the PBE-D3 functional has been successfully used in studies of host-guest interactions between zeolites and organics, [79][80][81] it also has some known deficiencies. 82 As a consequence, the E zg values calculated with this method cannot be considered ''accurate'' in a quantitative sense, but they can still serve as reference values to gauge whether the force field calculations give reasonable trends.…”

Local Distortions in a Prototypical Zeolite Framework Containing Double Four-Ring Cages: The Role of Framework Composition and Organic Guests

“…Therefore, it was assumed that all TMA molecules have the same orientation, preserving the body-centering of the structure and leading to 4 ̅ symmetry (Figure 1). 65 The situation is different for the QNU cation, which has a dipole moment. An essentially random orientation of the QNU molecules would require the use of a large supercell, and an identical orientation of all molecules (which would preserve the body-centering) would lead to an overall polarisation, which appears unlikely.…”

“…57,58,[62][63][64] In a previous investigation by one of us, DFT structure optimisations and DFT-based ab-initio molecular dynamics (AIMD) calculations were used to study the local environment and fluoride dynamics in AST-type germanosilicates. 65 It was found that the fluoride anions reside at the cage centre in the pure end members SiO2-AST and GeO2-AST, but that they are displaced towards one Ge atom, or towards Ge-O-Ge edges, in mixed (Si,Ge)O2 systems, often forming directional Ge-F bonds with a length of 2.2 to 2.4 Å. The formation of these bonds leads to a reduction in the freedom of motion that is clearly visible in the root mean square displacements (RMSDs) of fluoride, and in the F-Ge radial distribution functions (RDFs).…”