Studying porous materials with krypton-83 NMR spectroscopy

This report is the first review of Kr-83 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 Xe-129, Kr-83 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 Kr-83 relaxation rate and spectral lineshape. These effects are much more sensitive probes for surfaces than the Xe-129 chemical shielding and provide unique insights into macroporous materials in which the Xe-129 chemical shift is typically of little diagnostic value. The first part of this report reviews the effect of quadrupolar interactions on the Kr-83 linewidth in zeolites and also the Kr-83 chemical shift behavior that is distinct from that of its Xe-129 cousin in some of these materials. The second part reviews hyperpolarized (hp) Kr-83 NMR spectroscopy of macroporous materials in which the longitudinal relaxation is typically too slow to allow sufficient averaging of thermally polarized Kr-83 NMR signals. The quadrupolar-driven T-1 relaxation times of hp Kr-83 in these materials are sensitive to surface chemistry, surface-to-volume ratios, coadsorption of other species on surfaces, and surface temperature. Thus, Kr-83 T-1 relaxation can provide information about surfaces and chemical processes in macroscopic pores and can generate surf ace-sensitive contrast in hp Kr-83 MRI. Copyright (c) 2007 John Wiley & Sons, Ltd.