Nuclear Magnetic Resonance Studies of Functional Reticular Materials

Metal-organic frameworks (MOFs) are porous, crystalline solids formed from the self-assembly of metal clusters and organic linker molecules in solution. Covalent organic frameworks (COFs) are similar, formed from the combination of two or more organic molecules joined by covalent bonds to make an extended crystalline solid. Both of these materials have seen extensive research and development into both their synthesis and applications. The most common analytical techniques used to study these materials can sometimes fail to paint a full picture of these materials, whose structure and properties can change significantly under perturbations from adsorbates or different thermodynamic conditions. Additionally, these materials may contain disordered, aperiodic structures that may not be easily observed, but are central to their function. In this work, nuclear magnetic resonance (NMR) spectroscopy is used as an element-specific, non-invasive probe to study both the structures of MOFs and COFs, revealing defects, unique chemical sites, and heterogeneous distributions of functional groups, as well as the chemistry of molecules adsorbing and diffusing through these porous materials./nThe strong acid site in sulfated MOF-808 is identified using 1H solid state NMR in conjunction with other characterization techniques. The site is observed as a specific pairing of water and sulfate on the surface of zirconium clusters leading to a hydrogen bonding interaction which results in one of the protons on the water molecule becoming acidic. The structure arises as a specific sequence of molecules on an otherwise disordered surface of the MOF material, revealing powerful functionality arising from disorder./nA 1H NMR pulsed-field gradient (PFG) probe is built and tested, and is shown to be capable of performing diffusion NMR studies of solvents and gases within MOFs. The probe is then used to study the diffusion of solvents in mixed-linker MOF-5. We find that the self-diffusion coefficient of N,N-dimethylformamide (DMF) may be tuned linearly between the diffusion coefficients of MOF-5 and IRMOF-3 as a function of the linker mole fraction. By comparing the diffusion coefficients for a series of solvents at saturation in MOF-5 and IRMOF-3, we conclude that average pore aperture, not solvent-linker chemical interactions, is the primary mechanism by which the linker amino groups tune molecular diffusion./nNMR is applied to the understanding of 2D stacked COF materials. Although these materials are relatively crystalline, domain sizes of the crystallites are incredibly small, rendering structural determination by X-ray diffraction very difficult. Solid state NMR enables observation of chemical sites within these materials, and is applied to the monitoring of chemical conversion of covalent bonds by oxidation in imine COFs. Using isotopic labeling, the conversion is observed to be complete. Additionally, resonances attributable to defect sites are observed and quantified./nCO2 capture in a primary amine-functionalized MOF is explored by using solid state NMR to study materials that have been exposed to 13CO2 at known partial pressures. An apparatus is built to dose activated MOF material in a solid state NMR rotor with 13CO2. It is found that CO2 is adsorbed as a carbamic acid under dry conditions, a relatively uncommon chemical species, and is adsorbed as the more commonly observed ammonium carbamate when adsorbed water is present. These results help to explain the material’s robustness in capturing CO2 selectively in the presence of water.

Advisor: Reimer, Jeffrey A
Yaghi, Omar M