A Magnetic Resonance Perspective of Adsorbates Motion in Metal Organic Frameworks

One of the most promising novel materials being developed is a type of nanoporousmedia referred to as Metal Organic Frameworks (MOFS). MOFs can beenvisioned as Tinker Toys(TM), with inorganic paddle wheels and organic connectingsticks. MOFs are a novel type of porous materials that have shown unprecedentedstorage capacities, separation selectivity, and chemical reactivity as solid catalysts.These attributes are further enriched by the modularity of MOF structures, includingthe ability to tune geometry (pore size), topology (pore shape(s)), chemical anity(linker design), and reactivity (metal coordination chemistry). This diverse array ofapplications requires a thorough understanding of MOF-adsorbate interactions ina wide range of thermodynamically diverse operating conditions. Although thereare many macroscopic techniques available to researchers, very few techniques canprobe the relevant times (ns) and length scales (nm) of the molecular interactionsthat contribute to the overall performance MOF materials. I have chosen to useNuclear Magnetic Resonance (NMR) to probe these interactions because NMR isa non-destructive tool that can access in-situ molecular motion and energetics atthese time and length scales, while minimizing the amount MOF needed (mg). TheNMR observables (i.e. trends in longitudinal relaxation, transverse relaxations, selfdiusion coecients, and NMR lineshapes and crystal orientation studies) at relevantoperating conditions provide information that when complimented by molecularsimulations can be extended to macroscopic material attributes for separations. Inpursuit of these systematic investigations this dissertation includes the constructionNMR instrumentation to observe the behavior of both liquid and gaseous adsorbents.Characterization of the translational motion of xylene molecules in MOF-5 throughobservation of the self-diusion coecients to develop understanding of motion ofxylene molecules in isotropic connement. The investigation of these structurepropertyrelationships via NMR in IRMOFs (iso-reticular MOFs) systems has leadto understanding the influence of the open-metal site pore chemistry on moleculartransport of methane (CH4) in an IRMOF, M2(dobdc), where M is either the (Mg,Ni, or Zn) metal. NMR relaxometry conducted on CH4 in this systems suggest alarge dierence in local density causes increase rotational correlations times. Diusivediraction behavior was observed at certain pressures in this family of materials anda kinetic Monte Carlo program was written in order to interpret the data. Finally, aninternational collaboration with the Prof. Blmich at RWTH Aachen was executed inorder to expand the applicability of magnetic resonance methods for characterizingtransport on more prototypical MOFs. These gradients must be considered forinterpretation of relaxometry data