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

沸石是一类由角共用四面体三维骨架组成的晶态无机材料。沸石和具有沸石型拓扑结构(沸石类型)的相关材料凭借其固有的孔隙率在各种大规模应用中得到应用，例如在气液分离、催化和离子交换中。理想沸石相当于由氧原子连接的四面体配位原子(T原子)的完美骨架。然而，有许多实际的沸石结构的例子，其中一些T原子的配位数(Cn)大于4，因为这些位置上有额外的非桥物种。本项目通过在色散校正密度泛函理论(DFT)的框架下通过电子结构计算来探索这类具有更高配位的T原子的沸石类型材料。重点将放在两类材料上，即(1)含氟全硅沸石和(2)水合磷酸铝(ALPO)。在前者中，氟离子共价键合到骨架硅原子上，形成三角-双锥体结构的SiO4F-单元。在水合Alpos中，水与骨架铝原子的配位导致形成五配位或六配位的铝。虽然对于这两个基团来说，较高配位的T原子的存在是众所周知的，但在很大程度上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，将使用密度泛函计算来阐明决定结构中哪些T原子最容易出现配位数超过4的晶体化学因素。这些结构研究将得到基于密度泛函理论的振动性质预测的补充，以获得对主客体相互作用的进一步了解。此外，这些计算将用于识别指纹模式，这些模式表明振动光谱中存在更高配位的T原子。对于含氟全硅沸石，将使用额外的分子动力学计算来研究氟离子无序。最后，由于一些关于GeO4F-单元在这些体系中的存在的相互矛盾的观察结果，我们将研究在含Ge的分子筛类型中氟的成键和动力学。本项目的主要目的是在基础水平上加深对具有较高配位T原子的沸石类型体系的理解。然而，可以预见的是，这些发现也将对应用程序具有一定的相关性。例如，对氟离子结构导向性质的新见解可能有助于合理开发新的合成路线，而更好地从原子水平了解水合AlPO中的骨架-水相互作用有助于解释这些材料不同程度的水稳定性，这是各种应用中的关键性质。

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.