Studying porous materials with krypton-83 NMR spectroscopy

This report is the first review of ⁸³Kr nuclear magnetic resonance as a new and promising technique for exploring the surfaces of solid materials. In contrast to the spin I = 1/2 nucleus of ¹²⁹Xe, ⁸³Kr has a nuclear spin of I = 9/2 and therefore possesses a nuclear electric quadrupole moment. Interactions of the quadrupole moment with the electronic environment are modulated by surface adsorption processes and therefore affect the ⁸³Kr relaxation rate and spectral lineshape. These effects are much more sensitive probes for surfaces than the ¹²⁹Xe chemical shielding and provide unique insights into macroporous materials in which the ¹²⁹Xe chemical shift is typically of little diagnostic value. The first part of this report reviews the effect of quadrupolar interactions on the ⁸³Kr linewidth in zeolites and also the ⁸³Kr chemical shift behavior that is distinct from that of its ¹²⁹Xe cousin in some of these materials. The second part reviews hyperpolarized (hp) ⁸³Kr NMR spectroscopy of macroporous materials in which the longitudinal relaxation is typically too slow to allow sufficient averaging of thermally polarized ⁸³Kr NMR signals. The quadrupolar-driven T₁ relaxation times of hp ⁸³Kr in these materials are sensitive to surface chemistry, surface-to-volume ratios, coadsorption of other species on surfaces, and surface temperature. Thus, ⁸³Kr T₁ relaxation can provide information about surfaces and chemical processes in macroscopic pores and can generate surface-sensitive contrast in hp ⁸³Kr MRI.