EXPLORING THE ROLE OF METAL VARIABILITY IN MOFS USING SYNCHROTRON RADIATION TECHNIQUES

Metal-Organic Frameworks (MOFs) are a unique class of hybrid materials that combine high porosity, crystallinity, and modularity, making them promising candidates for applications ranging from gas storage to catalysis. This thesis explores the intricate relationship between MOF structure, metal composition, and functional properties, with a particular focus on multi-metal systems and their tunability. In detail, a set of coupled techniques, leveraging the capabilities of synchrotron-based diffraction and spectroscopic techniques, is employed and thoroughly described in each chapter, in order to deeply characterize the multi-metal character of the synthesized systems and to understand its impact over the properties of the system. Significant emphasis is placed on the impact of crystallite size on MOF defectivity, particularly in the prototypical HKUST-1 framework. Using a variety of analytical tech- niques, including in situ ambient pressure near-edge soft X-ray absorption fine structure spectroscopy (AP-NEXAFS) and in situ CO-dosed IR spectroscopy, the role of crystal size in determining the distribution of Cu(I) / Cu(II) species and the kinetics of defect formation under thermal activation is investigated. In following chapters, the presented work shifts toward catalytic applications of MOFs, with a focus on the direct methane-to-methanol (MTM) conversion using molecular oxygen as an oxidant. The performance of MIL-100(Fe) as an heterogeneous catalyst for this process is investigated using advanced operando X-ray-based techniques including XRD, XAS, XES, and RIXS. Together, these techniques confirmed the formation of Fe(II) active sites at elevated temperatures, facilitating MTM conversion. Catalyst deactivation is attributed to CH3• escape, leading to the formation of Fe3O(OMe) and Fe3O(OH) units. Despite this limitation, the system demonstrates catalytic activity over multiple reaction cycles using O2 as oxidant. As a natural continuation of the work on MIL-100(Fe), subsequent work is focused on the synthesis and the structural and spectroscopic characterization of multimetal FeM- PCN-250 (M: Cr(III), Mn(II), Ni(II)) MOFs. The incorporation of secondary metals into the trimeric [Fe3O] cluster is examined using coupled XAS and HR-PXRD. Thanks to the coupled experiments performed using in situ and operando set-ups, it was possible to confirm the robustness of the framework when exposed to reaction conditions and reveal how the metal tuning deeply affects the electronic structure, as well as the catalytic ac- tivity of these MOFs in terms of methane oxidation. Lastly a series of heterobimetallic MOFs, M3−xM’x(OH)2(btca)2 (M: Zn(II), M’: Co(II), x: 0, 1, 1.5, 3), were synthesized to investigate how controlled variations in metal content influence the flexibility of the frameworks and their adsorption behavior, particularly for CO2. The discussion centers on the adaptability of MOFs, citing notable examples from the existing literature, and how changes in the composition of the metal within SBUs affect the dynamics of the framework upon guest adsorption. In particular the adjustment of Zn(II) and Co(II) ion levels at two coordination sites within M(btca) MOFs was found to affect the CO2 ad- sorption behavior of the framework effectively illustrating that the structural response depends on the metal composition. To conclude, the work presented in this PhD thesis offers significant insight into the design principles of multimetal MOFs, illustrating how aspects such as structural flexibility, defect formation, and metal substitution play pivotal roles in dictating their reactivity. The results add to the comprehensive understanding of MOF-based catalysis and establish a foundation for the systematic development of advanced materials aiming at improved performance in future applications.