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Metal-organic frameworks are a class of often crystalline, potentially porous materials constructed from metal ions connected through organic linkers. The materials excel in modularity, metal concentration and well-defined porosity rendering them suitable candidates for various applications like catalysis, gas sorption and separation. The often limited stability of MOFs with low-valent cations drove the focus towards M4+-based materials, such as Zr and Ti MOFs, resulting in a large number of Zr MOFs. Ti MOFs feature additional redox-and photoactivity, but only a few structures are reported due to their challenging synthesis procedures: Ti salts are prone to uncontrolled hydrolysis leading to ill-defined oxohydroxides instead of the desired MOF structures. In this PhD thesis, we reported the synthesis of a new layered Ti MOF, COK-47, from the hydrolytically more stable titanocene dichloride precursor. The material could be synthesized as an inherently defective, nanoparticulate MOF, denoted COK-47S, and is the first Ti MOF with known defects. Bridging methoxides were observed on the missing-linker defects, which could be converted to open sites with neighboring terminal methoxides upon activation. The open sites imbue the material with catalytic activity, as demonstrated by the oxidative desulfurization of thiophenes. Furthermore, the photoactivity of COK-47 was demonstrated by the degradation of Rhodamine 6G. In the second part of this thesis, another type of redox-active M4+-MOFs was studied: Ce MOFs. First, we tackled the challenging synthesis of Ce MOFs with reactive tetracarboxylate linkers, which could not be performed conventionally due to the highly oxidative nature of the Ce4+ salt. A widely applicable synthesis method was developed by adding a preformed molecular Ce6 cluster to the synthesis mixture as redox-stable Ce precursor, thereby circumventing any reactivity issues. The redox activity of the benchmark Ce MOF, Ce-UiO-66, was first demonstrated by the TEMPO-mediated alcohol oxidation, but the exact role of the metal sites remained unclear. We therefore performed an X-ray absorption spectroscopy investigation, revealing that only one Ce4+ ion per cluster could be reduced, which explained the need for a redox mediator. Ce MOFs would thus be more suitable for one-electron oxidations, which was tested on the industrially relevant selective catalytic reduction (SCR) of NO by NH3.Ce-UiO-66 outperformed CeO2 as SCR catalyst by virtue of its larger Ce accessibility, but both catalysts suffered from a limited acidity and consequently low ammonia adsorption and activity. Therefore, a new Cex/Zr-CAU-24 series was developed because the CAU-24 materials exhibit a larger number of accessible sites and the stronger Zr acid could improve the NH3 adsorption. Ce10/Zr-CAU-24 contains open Ce sites surrounded by two Zr ions that can adsorb NH3, which resulted in a much higher SCR activity.