Beyond tetrahedral coordination in zeolite-type materials - A computational approach

沸石是一类由共享角四面体的三维框架组成的结晶无机材料。由于其固有的多孔性，沸石和具有沸石型拓扑结构（zeotypes）的相关材料在各种大规模应用中得到了应用，例如在气液分离、催化和离子交换中。理想的沸石对应于由氧原子连接的四面体配位原子（T原子）的完美框架。然而，在实际的沸石结构中有许多例子，其中一些T原子的配位数（CN）大于4，因为额外的非桥接物质与这些位点结合。本项目通过色散校正密度泛函理论（DFT）框架下的电子结构计算，探索了这种具有“高配位”T原子的沸石型材料。重点将放在两组材料上，即(1)含氟全硅沸石和(2)水合磷酸铝（AlPOs）。在前一组中，氟离子与框架Si原子共价结合，形成三角双锥体SiO4F-单元。在水合AlPOs中，水与框架Al原子的配位导致五或六配位铝的形成。虽然高配位T原子的存在是众所周知的，但仍然不清楚为什么它们的形成优先发生在某些位置。为了解决这个问题，将使用DFT计算来阐明晶体化学因素，这些因素决定了结构中哪些T原子最容易假设配位数超过4。这些结构研究将辅以基于dft的振动特性预测，以进一步了解主客体相互作用。此外，这些计算将用于识别指纹模式，表明在振动光谱中存在高配位T原子。对于含氟的全硅分子筛，额外的分子动力学计算将用于研究氟阴离子紊乱。最后，氟化物在含锗分子式中的键合和动力学将被研究，因为在这些体系中存在一些相互矛盾的关于GeO4F-单元的观察结果。该项目的主要目的是在基础水平上进一步了解具有高配位T原子的沸石型系统。然而，可以预见的是，研究结果也将与应用有一定的相关性。例如，对氟化物阴离子的结构导向特性的新见解可能有助于合理开发新的合成路线，并且对水合AlPOs中框架-水相互作用的更好的原子水平理解可以帮助解释这些材料不同程度的水稳定性，这是各种应用的关键特性。

Zeolites are a class of crystalline inorganic materials consisting of a three-dimensional framework of corner-sharing tetrahedra. By virtue of their intrinsic porosity, zeolites and related materials with zeolite-type topologies (zeotypes) find use in various large-scale applications, e.g. in gas and liquid separation, catalysis, and ion exchange. Ideal zeolites correspond to a perfect framework of tetrahedrally coordinated atoms (T atoms) linked by oxygen atoms. However, there are many examples of actual zeolite structures where some T atoms have a coordination number (CN) that is larger than 4 because additional non-bridging species are bonded to these sites.The present project explores such zeolite-type materials with 'higher-coordinated' T atoms by means of electronic structure calculations in the framework of dispersion-corrected density-functional theory (DFT). The focus will be on two groups of materials, namely (1) fluoride-containing all-silica zeolites and (2) hydrated aluminophosphates (AlPOs). In the former group, fluoride anions are covalently bonded to framework Si atoms, forming trigonal-bipyramidal SiO4F- units. In hydrated AlPOs, the coordination of water to framework Al atoms leads to the formation of five- or six-coordinated aluminium. While the existence of higher-coordinated T atoms is well-known for both groups, it remains largely unclear why their formation occurs preferentially at certain positions. To address this, DFT calculations will be employed to elucidate the crystal-chemical factors that determine which T atoms in a structure are most susceptible to assume a coordination number beyond 4. These structural investigations will be complemented by a DFT-based prediction of the vibrational properties to gain further insights into the host-guest interactions. Furthermore, these calculations will serve to identify fingerprint modes that indicate the presence of higher-coordinated T atoms in the vibrational spectra. For fluoride-containing all-silica zeolites, additional Molecular Dynamics calculations will be used to study fluoride anion disorder. Finally, the bonding and dynamics of fluoride in germanium-containing zeotypes will be investigated, as there are some conflicting observations regarding the existence of GeO4F- units in these systems.It is the primary aim of the project to further the understanding of zeolite-type systems with higher-coordinated T atoms on a fundamental level. Nevertheless, it can be anticipated that the findings will also have a certain relevance to applications. For example, new insights into the structure-directing properties of fluoride anions may aid the rational development of new synthesis routes, and a better atomic-level understanding of the framework-water interaction in hydrated AlPOs can help to explain the different degree of water stability of these materials, which is a crucial property for various applications.