MULTIVARIATE FLEXIBLE METAL ORGANIC FRAMEWORKS: THE ROLE OF FUNCTIONALIZED LINKERS, HETEROGENEITY AND DEFECTS IN ADSORPTION PROCESSES

Metal-Organic Frameworks (MOFs) are a class of synthetic porous crystalline materials based on metal ions connected through spacing ligands. They possess interesting properties such as, high porosity, high concentration of metal centers, flexibility, etc., which are difficult to find in conventional porous materials. As a result of these unique features, MOFs can maintain porous structures upon removal, inclusion, exchange, or reaction of a wide selection of molecular guests, making them useful for applications in selective gas adsorption/separation. Furthermore, a number of MOFs have the ability to respond to specific stimuli such as host-guest interactions, temperature or pressure change, light, etc. To date, most of the applications of these flexible MOFs is related to gas-storage and selective adsorption/separation, but it was soon recognized that the responsive properties of these materials have enormous potential in many areas, a really interesting example are NPs@MOFs composites which can be used to design novel “smart” heterogeneous catalysts capable of reversibly adapt to substrates and/or respond to regulating stimuli (e.g. reversible structural transitions upon adsorption of guest molecules). The work within this thesis is divided into two parts. The first one focuses on the understanding of the host-guest interactions during adsorption processes in highly stable, rigid, porous metal-organic frameworks. of formula Fe2(BDP)3x(BDP-NH2)3(1-x) (x = 0, 0.5, 1) with H2BDP-NH2 = 2-amino[1,4-bis(1Hpyrazol- 4-yl)benzene]. Their investigation was performed through a multi-technique approach exploiting high-resolution powder X-ray diffraction (HR-PXRD), advanced gas adsorption measurements and Monte Carlo simulations. In particular, the host-guest interactions during CO2 adsorption were studied via in situ HR-PXRD dosing CO2 in certain pressure ranges at different temperatures and compared between functionalized and bare systems. Rietveld refinements of the diffraction data collected at each pressure point was used to understand the preferential adsorption sites occupied by the gaseous probe and the host–guest interactions, as well as the breathing behaviour, whose mechanism was not yet completely described in the literature. The metal ion and ligand functionalization modulate the sorption properties of these materials, enhancing their affinity to CO2 molecules. Finally, new flexible, multivariate MOFs of general formula Zn(BDP)0.5(BDP-X)0.5 (X = NH2 or NO2) were synthesized. The deep characterization of the adsorption properties and the structural features of these materials will be crucial to understand the relation between the structural flexibility, functionalization degree and crystal size, with the gas adsorption and separation properties. The second part of this thesis describes a family of pillar-layered MOFs of general formula [M2(fubdc) 2P]n with M2+ = Zn2+, fu-bdc2- = diversely functionalised 1,4-benzenedicarboxylate) and how catalytically inert nanoparticles (silica NPs, 30 nm) can be embedded into the microporous ordered MOFs structures, obtaining hybrid materials where the NPs represent typically a meso-scale “defect” or “heterogeneity” to the system. In particular, the work was focused on the comparison between the plain MOFs and the new composite materials NPs@MOF. The parent [Zn2(bdc)2(dabco)]n is only weakly flexible, whereas the substituted frameworks [Zn2(fu-bdc)2(dabco)]n (with fu = DM, 2,5-bis(methoxy)- 1,4-benzenedicarboxylate or BME, 2,5-bis(2-methoxyethoxy)-1,4-benzenedicarboxylate) contract significantly upon guest removal to a narrow pore (np) form and expand again (lp form) upon adsorption of DMF, EtOH, or CO2, etc. In contrast, N2 is hardly adsorbed and does not open the narrow pore form. These “breathing” dynamics are attributed to the dangling side chains present on the linkers, which interact with mobile guest molecules as well as with themselves and with the framework backbone. The samples were characterized by a variety of techniques: PXRD, FTIR, TGA, DSC, EFTEMEDX, TEM tomography, UV-Vis and PL spectroscopy, standard N2 (77 K) and CO2 (195 K) gas adsorption experiments with the objective of understanding the connection between defects, disorder with flexibility. The in‐depth and in situ structural and spectroscopic characterization carried out, especially gas adsorption and variable temperature PXRD experiments, revealed interesting differences in the flexible behaviour between the parent MOFs and the NPs@MOFs.