Metal-organic framework (MOF) compounds, which consist of metal-oxide clusters connected by organic linkers are a new class of nanoporous materials with very promising potential applications such as energy storage (H2, CH4, etc.) and gas separation. In order to optimize these materials for practical adsorption--based applications, it is of great importance to understand the fundamental physics of the MOF-gas molecule interaction. Using CH4 molecule as an extreme sensitive probe, we were able to investigate the crystal field of MOF in detail both experimentally and theoretically. The most widely studied MOF material, MOF5, which consists of ZnO4 clusters linked by 1,4-benzenedicarboxylate (BDC)), was chosen as the representing MOF material. Methane molecule is classical in terms of center of mass motion such as phonons but also it acts like a spherical quantum rotor, yielding tunnel splitting. Its hindered rotational spectrum provides rich information about the strength and symmetry of local potentials. Therefore, methane dynamics in many solids have been extensively studied. Here, we first directly determined the CH4 adsorption sites in MOF5 using the difference-Fourier analysis of neutron powder diffraction data. The initial and strongest adsorption site is found to be the position at the center of the three ZnO3 triangular faces (termed the ``cup site''), same as the first adsorption site of hydrogen in MOF5. Remarkably, the orientation of CH4 molecule was clearly revealed, implying strong molecule-host lattice interaction and relatively large energy barriers to rotation. Next, we measured the CH4 phonons and librons, and the rotational tunnel splitting using neutron inelastic scattering (NIS). Finally, the experimental data including absorption site positions, phonon-libron energies and the rotational tunnel-spectrum is used to test widely used phenomenological force fields and potentials obtained from first-principles.

Structure, dynamics and rotational tunneling of methane in metal-organic frameworks

251st National Meeting (2016)

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