On the move: High intracrystalline mobilities and fast exchange between the crystal and the surrounding gas phase were found in NMR studies on the diffusion of methane, ethane, n‐hexane, and benzene in large crystals of the metal‐organic framework MOF‐5 (see SEM image). The results support future use of metal‐organic framework materials as tailorable sorbents for fast gas processing and gas‐storage materials in industry.
The pore architecture of MCM-41 was investigated by pulsed field gradient nuclear magnetic
resonance (PFG NMR) self-diffusion measurements of adsorbed water. Over diffusion length scales which
exceed the radius of the one-dimensional channels of the MCM-41 by 2 orders of magnitude but which are
smaller than the size of the individual MCM-41 particles, the molecular propagation of the guest molecules
was found to be highly anisotropic. The PFG NMR experimental data are best represented by an axisymmetrical
diffusion tensor rather than by pure one-dimensional diffusion along the channels. This suggests that the pore
walls of the one-dimensional channels are permeable for water or that over the probed diffusion length the
channels are disordered and bent. Both interpretations provide information on the pore structure of the nanoporous
material which is not available from X-ray diffraction data.
The presented work aims at exploring the influence of the mobility of the sorbent framework on both the equilibrium and the kinetic properties of the sorbed phase by means of molecular dynamics computer experiments under isochoricÀisothermal and isobaricÀisothermal statistical ensembles for several host model options, combined by Widom averaging along the entire trajectory of the hostÀguest system toward rigorously obtained sorbate isotherms within a fully flexible lattice. The methodology is adapted to the study of the self-diffusivity and the collective (MaxwellÀStefan and transport) diffusivities of carbon dioxide (CO 2 ) and methane (CH 4 ) within the zeolite imidazolate framework-8 (ZIF-8). The simulation predictions are compared with measurements from pulsed-field gradient nuclear magnetic resonance (PFG NMR), as well as with recently conducted infrared microscopy (IRM) experiments elaborated on the basis of the current modeling in the flexible ZIF-8. The modeling results reveal a significant influence on sorbate transport exerted by the 2-methilimidazolate ligands surrounding the cage-to-cage entrances, whose apertures are commensurate with the guest molecular dimensions. Moreover, calculations of the singlet probability density distribution of the sorbate molecules at selected regions within the imidazolate framework provide a plausible explanation of the transport diffusivity as a function of sorbate occupancy, measured via IRM.
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