Wanja Gschwind: Of MOFs and Modifications: Exploring New Depths in Single-Crystalline UiO Materials

Abstract

The climate is rapidly changing for the worse, driven by our excessive consumption of planetary resources. Our best chance at mitigating the effects of climate change is the transition to sustainable resources. The Sun represents an undepletable energy source that could easily cover our growing energy requirements. Energy is best stored in fuels, which is achieved by converting low-energy precursors to high-energy products using catalysts. To be relevant on a global scale, these catalysts have to fulfill a number of requirements, which may be met by combining heterogeneous and molecular catalysis.

Metal-organic frameworks (MOFs) are a newly emerged type of crystalline material, displaying great potential for this crucial task. MOFs are customizable, chemically well-defined and offer large internal surface areas. They can be used as heterogeneous supports for catalysts. This can be achieved by functionalizing the framework to host introduced catalysts. The distribution of the catalysts within the material strongly affects catalysis; however, assessing catalyst distribution is challenging and therefore often neglected. Also, many catalytic MOFs are limited by stability issues. This lack of fundamental understanding hinders the optimization of catalytic MOFs.

In this work, we investigate the synthesis and stability of UiO-67 (UiO = Universitetet i Oslo) thin films. We reveal how humidity affects MOF growth and present mitigation strategies to make the materials more stable and their synthesis more reproducible.

We also present the development of a Rutherford backscattering spectrometry (RBS) method that enables the elemental depth profiling of UiO single crystals. We demonstrate the potential of the method to assess the distribution of post-synthetically introduced functional groups into MOFs. The method was then improved to include lighter metals and to investigate concentration gradients. With this, we investigated the effect of the linker ratio in mixed-linker UiO-67-bpy on the diffusion and distribution of post-synthetically introduced Ni2+ cations. We show that high ratios of metal-binding linkers slow down diffusion and result in inhomogeneous cation distributions.

RBS also revealed cavities at the core of UiO-67-bpy crystals under certain synthesis conditions. We rationalized the synthesis of such hollow MOFs based on emerging literature challenging some prevalent ideas about coordination equilibria in MOFs.

The work presented in this thesis provides foundational knowledge that can be used for the design of future MOF catalysts, accelerating research within the field and hopefully contributing to the much-needed transition to sustainable energy production.