Fluoride Anions in All-Silica Zeolites: Studying Preferred Fluoride Sites and Dynamic Disorder with Density Functional Theory Calculations
Fluoride Anions in All-Silica Zeolites: Studying Preferred Fluoride Sites and Dynamic Disorder with Density Functional Theory Calculations
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DOI：
10.1021/acs.jpcc.1c01440
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发表时间：
2021-04-15
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
在通过“氟化物路线”合成的全硅沸石中，氟阴离子通常被引入小笼中，形成[SiO 4F]（-）三角双锥。虽然衍射和NMR实验可以阐明氟化物的位置和动态无序的发生/不发生，但它们对决定平衡位置和动态行为的因素提供了有限的见解。为了更深入地理解，在色散校正密度泛函理论（DFT）的框架下对五种全硅沸石（NON、STF、IFR、STT和CHA框架）进行了电子结构计算。DFT为基础的预测的积极首选的氟化物位置在一个给定的笼子里，大多是非常符合实验。除了已知的氟阴离子倾向于靠近小环，没有容易推广的晶体化学规则来预测最可能的氟化物位点。基于DFT的分子动力学计算预测和解释的氟阴离子的动力学行为，这在不同的框架显着不同。模拟结果表明，从氟阴离子到下一个最近的相邻Si原子的距离是决定动态无序是否发生的关键。虽然与有机结构导向剂的长程相互作用往往起不太决定性的作用，但在某些情况下，它们可以导致对动态无序的抑制。除了提供合成的全硅沸石中氟阴离子的行为的详细了解，本工作的结果可能有助于进一步阐明其在沸石合成过程中的结构导向作用。
In all-silica zeolites synthesized via the "fluoride route", the fluoride anions are typically incorporated in small cages, forming [SiO4F](-) trigonal bipyramids. While diffraction and NMR experiments can elucidate the fluoride location(s) and the occurrence/absence of dynamic disorder, they provide limited insights into the factors that determine equilibrium position and dynamic behavior. To develop a more thorough understanding, electronic structure calculations in the framework of dispersion-corrected density functional theory (DFT) were performed for five all-silica zeolites (NON, STF, IFR, STT, and CHA frameworks). DFT-based predictions of the energetically preferred fluoride location within a given cage were mostly in excellent agreement with experiment. Apart from the known tendency of fluoride anions to be located close to small rings, there are no easily generalizable crystal-chemical rules to predict the most probable fluoride sites. DFT-based molecular dynamics calculations were employed to predict and explain the dynamic behavior of the fluoride anions, which differs markedly among the different frameworks. The simulations showed that the distances from the fluoride anion to the next-nearest neighboring Si atoms are key to determining whether dynamic disorder can occur or not. Although longer-range interactions with the organic structure-directing agents tend to play a less decisive role, they can lead to a suppression of dynamic disorder in some cases. In addition to providing detailed understanding of the behavior of fluoride anions in as-synthesized all-silica zeolites, the findings of the present work could contribute to a further elucidation of their structure-directing role during zeolite synthesis.