Chapter 9: Porous Materials Explored by Hyperpolarized Xenon NMR

Hyperpolarized (HP) ¹²⁹Xe NMR experiments, in one and two dimensions, have been applied to explore a variety of materials that exhibit the important property of being porous and absorptive with respect to gases. In the present review, we addressed crystalline and amorphous substances which display permanent porosity or permeability to gases, with a particular attention paid to organic and hybrid substances. A few families of materials were chosen as case studies: they include porous molecular crystals, intercalated clays, metal–organic frameworks, periodic mesoporous organo-silicas and polymers with a porous morphology. Continuous production of HP xenon ensures the exposure of the sample to a flow of freshly polarized gas atoms, which collect information on pore accessibility, pore shape, space available in the cavities, interactions with matrix walls and with other xenon atoms as a function of partial pressure and temperature, diffusion rate and exchange time between confined and free gas. Particular attention was paid to the generation of chemical shift anisotropy from the directional constriction of the crystal structure, describing for instance the axial symmetry of the channels and their absolute orientation in single crystals. A number of further phenomena and properties could be highlighted, such as the single-file diffusion along narrow channels, selective permeation of galleries by size-exclusion and the glass transition of the material walls.