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A Magnetic Resonance Perspective of Adsorbates Motion in Metal Organic Frameworks


One of the most promising novel materials being developed is a type of nanoporous

media referred to as Metal Organic Frameworks (MOFS). MOFs can be

envisioned as Tinker Toys(TM), with inorganic paddle wheels and organic connecting

sticks. MOFs are a novel type of porous materials that have shown unprecedented

storage capacities, separation selectivity, and chemical reactivity as solid catalysts.

These attributes are further enriched by the modularity of MOF structures, including

the ability to tune geometry (pore size), topology (pore shape(s)), chemical anity

(linker design), and reactivity (metal coordination chemistry). This diverse array of

applications requires a thorough understanding of MOF-adsorbate interactions in

a wide range of thermodynamically diverse operating conditions. Although there

are many macroscopic techniques available to researchers, very few techniques can

probe the relevant times (ns) and length scales (nm) of the molecular interactions

that contribute to the overall performance MOF materials. I have chosen to use

Nuclear Magnetic Resonance (NMR) to probe these interactions because NMR is

a non-destructive tool that can access in-situ molecular motion and energetics at

these time and length scales, while minimizing the amount MOF needed (mg). The

NMR observables (i.e. trends in longitudinal relaxation, transverse relaxations, selfdi

usion coecients, and NMR lineshapes and crystal orientation studies) at relevant

operating conditions provide information that when complimented by molecular

simulations can be extended to macroscopic material attributes for separations. In

pursuit of these systematic investigations this dissertation includes the construction

NMR instrumentation to observe the behavior of both liquid and gaseous adsorbents.

Characterization of the translational motion of xylene molecules in MOF-5 through

observation of the self-diusion coecients to develop understanding of motion of

xylene molecules in isotropic connement. The investigation of these structureproperty

relationships via NMR in IRMOFs (iso-reticular MOFs) systems has lead

to understanding the influence of the open-metal site pore chemistry on molecular

transport 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 a

large dierence in local density causes increase rotational correlations times. Diusive

diraction behavior was observed at certain pressures in this family of materials and

a kinetic Monte Carlo program was written in order to interpret the data. Finally, an

international collaboration with the Prof. Blmich at RWTH Aachen was executed in

order to expand the applicability of magnetic resonance methods for characterizing

transport on more prototypical MOFs. These gradients must be considered for

interpretation of relaxometry data.

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