Determination of the real structure of multivariate-MOFs

• Applicant: Dr. Philipp Urban
• Subject Area: Solid State and Surface Chemistry, Material Synthesis
• Term: from 2015 to 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 281085878

The main applications of metal-organic frameworks (MOFs) are gas separation and storage as well as catalysis. In the search for materials, which have specific functions like selective CO2 uptake, so-called multivariate (MTV-) MOFs turned out to be promising candidates, as some of them perform significantly better than mixtures of their univariate congeners. MTV-MOFs are crystalline materials, which are built of inorganic clusters or coordination spheres, which are connected by differently functionalized organic linkers. The average structure of such materials can be elucidated with the evaluation of the Bragg scattering. In addition, diffuse scattering occurs, which is caused by the irregular arrangement of the functional groups in the crystal structure. The interpretation of the diffuse scattering is crucial for an understanding of the linker arrangement and thus the underlying structure-property relationship. The main goal of the proposed research is the description of the real structure of MTV-MOFs by means of the evaluation of the diffuse X-ray scattering. High-quality single crystals of MTV-MOFs (i.e. with an intact framework) can be obtained by one-step solvothermal reactions of metal salts and differently functionalized organic linkers. Work under vacuum may be necessary, as solvent molecules cause additional disorder. Many synthesis routes for single-crystalline MOFs use modulators for crystal growth, ideally these should be avoided for the proposed research because they may be incorporated in the crystal structures and thus interrupt the periodicity, which causes additional diffuse scattering. For the analysis of the real-structure, synchrotron diffraction data collected with detectors providing a high dynamic range are crucial. The structure determination then comprises several steps, namely the determination of the average structure from the Bragg scattering, scrutinizing the average structure for the types of disorder present, transformation of the diffraction pattern from diffractometer space to reciprocal space (reconstruction) and analysis of the diffuse scattering using both, 3D-PDF and Monte Carlo methods. The elucidation of the average structure has to be performed with extreme care, as the subsequent analysis of the diffuse scattering depends on the results of the average-structure determination. Whereas the 3D-PDF is suited to derive qualitative conclusions regarding the types and directions of correlations present in the material investigated, refinement can be performed using both, 3D-PDF as well as Monte Carlo methods. The proposed research will lead to a better understanding and aid the rationalization of the structure-property relationships of multivariate MOFs.

• DFG Programme: Research Fellowships
• International Connection: USA
• Host: Professor Dr. Omar M. Yaghi