Zhang Wanli, Chen Shoushun, Terskikh Victor V, Lucier Bryan E G, Huang Yining
Department of Chemistry, The University of Western Ontario, London, Ontario, N6A 5B7, Canada.
Frontiers Science Center for Rare Isotopes, Lanzhou Magnetic Resonance Center, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, 730000, China.
Solid State Nucl Magn Reson. 2022 Jun;119:101793. doi: 10.1016/j.ssnmr.2022.101793. Epub 2022 Mar 21.
Metal-organic frameworks (MOFs) are emerging materials with many current and potential applications due to their unique properties. One critical feature is that the physical and chemical properties of MOFs are tunable. One of the methods for tuning MOF properties is to introduce defects by design for desired applications. Characterization of MOF defects is important, but very challenging due to the local nature and short-range ordering. In this work, we have introduced the ordered vacancies (the defects) in the form of the coordinatively unsaturated sites (CUSs) into the framework of MOF MIL-120(Al). The creation of ordered vacancies is achieved by replacing one quarter of the BTEC (1,2,4,5-benzenetetracarboxylate) with BDC (benzene-1,4-dicarboxylate) linkers. Both parent and defective MOFs were characterized by multinuclear solid-state NMR spectroscopy. H MAS NMR is used to characterize the hydrogen bonding in these MOFs, whereas C CP MAS NMR confirms unambiguously that the BDC is incorporated into the framework. One-dimensional Al MAS NMR provides direct evidence of the coordinatively unsaturated Al sites (the defects). Furthermore, Al 3QMAS experiments at 21.1 T allow direct identification of one penta-coordinated and three chemically inequivalent octahedral Al sites in the defective MIL-120(Al). Two of the above-mentioned octahedral Al sites are in the domain which appears defect-free. The third octahedral Al site is near the defective site. This work clearly demonstrates the power of solid-state NMR spectroscopy for characterization of defective MOFs.
金属有机框架材料（MOFs）因其独特的性质而成为具有众多当前和潜在应用的新兴材料。一个关键特性是MOFs的物理和化学性质是可调的。调节MOF性质的方法之一是通过设计引入缺陷以实现所需的应用。MOF缺陷的表征很重要，但由于其局部性质和短程有序性，极具挑战性。在这项工作中，我们以配位不饱和位点（CUSs）的形式将有序空位（缺陷）引入到MOF MIL-120(Al)的框架中。通过用BDC（对苯二甲酸）连接体取代四分之一的BTEC（1,2,4,5-苯四甲酸）来实现有序空位的产生。母体和缺陷型MOFs均通过多核固态核磁共振光谱进行表征。H MAS NMR用于表征这些MOFs中的氢键，而C CP MAS NMR明确证实BDC已掺入框架中。一维Al MAS NMR提供了配位不饱和Al位点（缺陷）的直接证据。此外，在21.1 T下进行的Al 3QMAS实验能够直接识别缺陷型MIL-120(Al)中的一个五配位和三个化学不等价的八面体Al位点。上述八面体Al位点中的两个位于看似无缺陷的区域。第三个八面体Al位点靠近缺陷位点。这项工作清楚地证明了固态核磁共振光谱在表征缺陷型MOFs方面的强大功能。
