Quantification of Linker Defects in UiO-Type Metal–Organic Frameworks

Metal–organic frameworks (MOFs) are nanoporous
materials
composed of organic linkers and inorganic nodes. The large variety
of linkers and nodes and the multiple ways to combine them make MOFs
highly tunable materials, which are thoroughly studied for their use
in, e.g., catalysis, gas capture, and separation. The chemistry of
MOFs is further enriched by defects, e.g., missing linker defects,
which provide active sites for catalysis or anchoring sites for introducing
new functionalities. A commonly reported method to quantify linker
defects assumes the presence of one type of linker and the complete
removal of capping agents, solvents, and other impurities upon activation
at high temperature, e.g., 400 °C (M-400). However,
attempts to use this method for MOFs containing different types of
linkers, also called multivariate MOFs (MTV-MOFs), or capping agents
that are not completely removed at 400 °C, give inaccurate results
and hamper comparing results from different publications. In this
work, we have developed a new procedure to compute missing linker
defects in Zr-based MOFs using standard analytical techniques to quantify
the capping agents that remain in the MOF upon activation at 200 °C
(M-200). This method, which has been tested in UiO-66/67
based MOFs, should be applicable to any MOF that (1) has known decomposition
products, (2) has no missing cluster defects, (3) has empty pores
or contain species that can be quantified after activation, and (4)
has a known node composition at 200 °C.

金属有机框架（MOFs）是由有机连接体和无机节点构成的纳米多孔材料。其连接体和节点的多样性和多种组合方式使得MOFs成为高度可调谐的材料，它们在催化、气体捕获和分离等领域的应用得到了广泛的研究。MOFs的化学性质因缺陷（如缺失的连接体缺陷）而进一步丰富，这些缺陷为催化提供了活性位点或为新功能的引入提供了锚定位点。一种常用的量化连接体缺陷的方法是假设存在一种类型的连接体，并在高温（例如400°C，M-400）激活过程中完全去除封端剂、溶剂和其他杂质。然而，尝试将此方法应用于含有不同类型连接体（称为多元MOFs（MTV-MOFs））或封端剂在400°C下未完全去除的MOFs时，会导致不准确的结果，并阻碍不同出版物结果之间的比较。在本研究中，我们开发了一种新方法，用于计算基于Zr的MOFs中的缺失连接体缺陷，该方法利用标准分析技术来量化在200°C（M-200）激活后MOFs中残留的封端剂。这一方法已在UiO-66/67基MOFs中得到验证，并应适用于任何满足以下条件的MOFs：（1）具有已知的分解产物，（2）没有缺失的簇缺陷，（3）具有空孔隙或含有在激活后可进行量化的物种，（4）在200°C时具有已知的节点组成。