Local Environment and Dynamic Behavior of Fluoride Anions in Silicogermanate Zeolites: A Computational Study of the AST Framework
Local Environment and Dynamic Behavior of Fluoride Anions in Silicogermanate Zeolites: A Computational Study of the AST Framework
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DOI：
10.1021/acs.jpcc.8b10770
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发表时间：
2019-01-24
影响因子：
3.7
通讯作者：
Fischer, Michael
中科院分区：
文献类型：
--
作者：
Fischer, Michael
在含双四环(D4R)构筑单元的硅锗分子筛中，Ge原子优先占据立方结构单元的角落，但由于缺乏长程有序，无法用结晶学方法确定Si和Ge原子的择优排列。如果在合成过程中存在氟离子，它们就会被结合到这些笼子中。由于F-19化学位移对当地环境的敏感性，核磁共振实验可以间接了解主要的(Si，Ge)排列。然而，关于Ge-O-Ge键的偏好或避免，以及关于氟化物在笼子内的平衡位置，报道了相互矛盾的解释，其中氟化物可能占据笼子中心或参与部分共价的Ge-F键。为了阐明能量优先的局域排列，对AST骨架进行了周期性色散校正密度泛函理论(DFT)计算，该骨架可在(Si1-n，Ge-n)O-2(0)范围内合成。
In silicogermanate zeolites containing double four-ring (d4r) building units, the germanium atoms preferentially occupy the corners of these cube-like units, but the absence of long-range order precludes the determination of the preferred arrangements of Si and Ge atoms by means of crystallographic methods. If fluoride anions are present during the synthesis, they are incorporated into these cages. Because of the sensitivity of the F-19 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 fluoride inside the cage, where fluoride might either occupy the cage center or participate in a partly covalent Ge-F bond. In order to shed light on the energetically preferred local arrangements, periodic dispersion-corrected density functional theory (DFT) calculations were performed for the AST framework, which is synthetically accessible across the range of (Si1-n,Ge-n)O-2 compositions (0