Preferential Location of Germanium in the UTL and IPC-2a Zeolites

Odoh, Samuel O.; Deem, Michael W.; Gagliardi, Laura

doi:10.1021/jp510495w

Cited by 24 publications

(51 citation statements)

References 41 publications

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“…As discussed above,the ability of the Ge atom to form d4r and/or d3r units is the major driving force for the synthesis of new extra-large-pore zeolites.Although precise experimental information about the Ge distribution in germanosilicates is usually unavailable,c omputational studies have been performed at various levels of theory.S astre et al [27] explained the Ge preference for d4r in BEA, BEC, ISV, AST and ASV structures with smaller T-O-T angles based on atomistic force field calculations.U sing the DFT and the periodic model of the octadecasil zeolite,the 19 FNMR signals for F À inside the d4r unit were interpreted, showing that chemical shift increases with the number of Ge atoms in d4r. [28] As trong preference of Ge for sites in d4r was also reported for UTL by Odoh et al [29] based on DFT calculations;replacing Si by Ge in d4r is energetically more favourable than its replacement in any other site.F urthermore,a tl east 3G ea toms can be located in the d4r unit before other sites start to be noticeably occupied. TheG ed istribution in AST germanosilicates was thoroughly investigated at the DFT level, focusing on the equilibrium locations of fluoride and its dynamic behaviour.…”

Section: Insight From Computational Investigationsupporting

confidence: 65%

“…Using the DFT and the periodic model of the octadecasil zeolite, the 19 F NMR signals for F − inside the d4r unit were interpreted, showing that chemical shift increases with the number of Ge atoms in d4r . A strong preference of Ge for sites in d4r was also reported for UTL by Odoh et al . based on DFT calculations; replacing Si by Ge in d4r is energetically more favourable than its replacement in any other site.…”

Section: Structure‐directing Role Of Gementioning

confidence: 55%

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Synthesis and Post‐Synthesis Transformation of Germanosilicate Zeolites

et al. 2020

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Zeolites are one of the most important heterogeneous catalysts, with a high number of large‐scale industrial applications. While the synthesis of new zeolites remain rather limited, introduction of germanium has substantially increased our ability to not only direct the synthesis of zeolites but also to convert them into new materials post‐synthetically. The smaller Ge‐O‐Ge angles (vs. Si‐O‐Si) and lability of the Ge−O bonds in aqueous solutions account for this behaviour. This Minireview discusses critical aspects of germanosilicate synthesis and their post‐synthesis transformations to porous materials.

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Section: Insight From Computational Investigationsupporting

confidence: 65%

Section: Structure‐directing Role Of Gementioning

confidence: 55%

Synthesis and Post‐Synthesis Transformation of Germanosilicate Zeolites

et al. 2020

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“…The Ge atoms preferentially occupy T-sites in d4r/d3r units, as shown by structure refinement, [106] indirectly evidenced by NMR studies [107] and computational modeling using periodic DFT. [108] Although stable in as-synthesized forms (with the organic structure-directing molecule occluded), Ge-rich d4d/d3d-containing zeolites usually suffer from hydrolytic instability after the removal of organic molecules from the pores ( Table 1). A facile degradation of such zeolites by liquid water or even by ambient moisture may originate from a combination of factors, including: i) Larger void volumes available for water molecules, the collective action of which may facilitate hydrolysis of framework GeOSi bonds, as it was recently shown for dealumination of aluminosilicates [109] ii) Decreasing framework thermodynamic stability (enthalpy of zeolite formation) when Ge substitutes Si, as shown by theoretical calculations [108b] and calorimetric studies [110] iii) Variable coordination numbers of Ge ranging from 4 to 6, which makes framework Ge atoms susceptible for nucleophilic substitution reactions iv) The instability of GeO bonds against water attack.…”

Section: Hydrolysis Of Germanosilicatesmentioning

confidence: 99%

Zeolite (In)Stability under Aqueous or Steaming Conditions

et al. 2020

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zeolites, modification of zeolite acidity for use as fluid catalytic cracking (FCC) catalysts or even for the preparation of novel zeolite frameworks. [1] Considering the significance of zeolite applications at the industrial level, a thorough understanding of zeolite (in)stability under aqueous conditions has been identified as a very important problem in catalysis and zeolite science. Zeolite (in)stability in water or under steaming conditions has been investigated by a number of experimental techniques, in particular magic angle spinning (MAS) NMR, IR, powder X-ray diffraction (PXRD), calorimetry and adsorption. Experimental studies have often been augmented by computational modeling and a large number of relevant theoretical papers can be found in the literature. While the main focus of this review is on reactive interactions of water with zeolites, it is also important to review the most important observations obtained for nonreactive water adsorption in zeolites. It is not surprising that all-silica zeolites, which exhibit negligible water uptake are stable even under conditions where Alcontaining zeolites lose crystallinity: the effective framework hydrolysis can only take place when sufficient water is present in the zeolite channel system. The amount of water inside the zeolite channels depends critically on the concentration of heteroatoms and/or framework defects, such as silanol nests, which show much higher affinity toward water than hydrophobic SiOSi bridges. Zeolite hydrophobicity increases with increasing Si/Al ratio and incomplete pore filling by water is commonly observed for zeolites with high Si/Al at standard pressure. It has been suggested already in the 1950s by Young that water can only adsorb on surface silanol groups, while the parts of the silica surfaces formed by SiOSi bridges are hydrophobic. [2] Both water adsorption in zeolites and zeolite (in)stability under aqueous conditions depend on the zeolite composition (Si/Al ratio, charge-compensating cations, and defect concentration in particular). Our primary goal is to review the most critical aspects of zeolite (in)stability for a broad range of temperature and partial water pressure (p 0), focusing on the framework zeolite composition (Si/Al ratio and presence of Ge, Sn, and Ti heteroatoms) and defect concentration. Zeolites (in)stability in water is discussed for increasing extent of framework degradation, starting from the nonreactive interaction with water, reversible hydrolysis of TO bonds, mild zeolite demetallation, mesopore formation, and total amorphization (Figure 1). Zeolites are among the most environmentally friendly materials produced industrially at the Megaton scale. They find numerous commercial applications, particularly in catalysis, adsorption, and separation. Under ambient conditions aluminosilicate zeolites are stable when exposed to water or water vapor. However, at extreme conditions as high temperature, high water vapor pressure or increased acidity/basicity, their crystalline framework can be destroyed....

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“…17 Besides the experimental work, a preference of Ge for the d4r unit has also been found in computational modelling studies using a variety of techniques, comprising force field calculations, 9,11,12 Hartree-Fock calculations for small cluster models, 14 and periodic density functional theory (DFT) calculations. [18][19][20] In systems with a mixed occupation of the vertices of a d4r unit by Si and Ge, the distribution of the two elements exhibits no long-range ordering, precluding a determination of preferred arrangements by means of crystallographic methods. Some information on the local ordering can be inferred from 29 Si-NMR spectroscopy.…”

Section: Introductionmentioning

confidence: 99%

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

Fischer

2018

J. Phys. Chem. C

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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.

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Preferential Location of Germanium in the UTL and IPC-2a Zeolites

Cited by 24 publications

References 41 publications

Synthesis and Post‐Synthesis Transformation of Germanosilicate Zeolites

Synthesis and Post‐Synthesis Transformation of Germanosilicate Zeolites

Zeolite (In)Stability under Aqueous or Steaming Conditions

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

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