Dottorato in SCIENZE CHIMICHE

Titolo della tesi: Probing the Structure and Reactivity of Metal-organic Frameworks In Operando

This thesis utilizes synchrotron-based X-ray techniques, advanced data analysis methodologies, and theoretical calculations to comprehensively investigate the structural, electronic, and reactive properties of metal-organic frameworks (MOFs) under operando conditions. In the first study, the structural and catalytic properties of MIL-100(Fe) were explored. It was observed that MIL-100(Fe) forms open Fe(II) sites upon thermal activation, which can catalyze the methane-to-methanol (MTM) conversion using molecular oxygen. A combination of X-ray spectroscopy and diffraction techniques was employed to monitor multiple reaction cycles, providing insight into the possible reaction intermediates. DFT calculations were used to propose a reaction mechanism involving O$_2$ and CH$_4$ interactions with Fe(II) sites to form methanol, highlighting stages where methyl radical intermediates might escape, leading to catalyst deactivation. This study indicated that while MIL-100(Fe) shows some catalytic activity, it is limited for industrial MTM processes, prompting the need for more stable trimetallic iron-based MOFs. The second study focused on water adsorption/desorption mechanisms in Mg-MOF-74. A combination of theoretical MD calculations, AP-NEXAFS, and PXRD revealed how water adsorption causes significant structural changes in the MOF's unit cell, specifically expanding the a axis and contracting the c axis. A machine-learning analysis of NEXAFS data showed residual water adsorbed at high temperatures, even when a significant number of open Mg2+ sites are present. Further, a subsequent study highlighted that different metal ions in MOF-74 significantly influence water retention capabilities, with Mg-MOF-74 retaining more water at high temperatures compared to Co-MOF-74 and Ni-MOF-74. Finally, surface defect formation and reactivity in HKUST-1 were investigated using NEXAFS. It was shown that oxidative decarboxylation leads to defective Cu(I)/Cu(II) paddlewheels that can be restored by exposure to CO2. These findings demonstrate how integrating complementary X-ray techniques with theoretical models provides deep insights into MOF properties, enabling the optimization of MOF performance for industrial applications.