Understanding and utilizing linker mobility in metal-organic frameworks

Porous and crystalline metal-organic frameworks (MOFs) are a class of crystalline porous solids consisting of metal ion nodes held together by polytopic organic linkers. This class of materials has become one of the most intensely researched topics in materials chemistry: MOFs have the highest specific surface area of any crystalline porous material and therefore promise exciting capabilities in a number of fields (e.g. gas sorption and separation, catalysis, energy storage). Now that a large number of MOF structures have been reported, research attention shifts towards a more fundamental understanding of these materials, with the goal of fine-tuning their properties through crystal engineering. Remarkably, even some of the most stable MOFs can undergo post-synthetic modifications such as ligand exchange, resulting in mixed-linker MOFs that often can be considered 'solid solutions'. While these methodologies have now become commonplace, a clear and complete understanding of the process and its key parameters is still absent. In this work, the mechanisms behind post-synthetic linker exchange are unraveled, and the role of solvent and crystal defects are identified. Furthermore, a solvent-free approach is introduced in which linker exchange occurs through solid-vapor reactions. Finally, a new characterization method is introduced as a means to streamline porous media characterization in general and MOFs in particular. This approach combines nuclear magnetic resonance with conventional physisorption and complements established techniques, thus offering new insights into adsorption mechanisms, energetics, and structural flexibility of the MOF adsorbents.

Publication year:2020

Accessibility:Open