We have applied 109Ag solid-state NMR and optical methods to probe dynamics and local environment of silver cations in a series of silver-exchanged zeolites as a function of hydration. In dehydrated samples of Ag−Rho, a Knight shift produced by the conduction electrons of metallic silver particles with cluster size above 400 Å is observed. Temperature dependent color change upon dehydration in zeolites Ag−X and Ag−A is also present. Although the color change in both zeolites is optically similar, NMR spectra demonstrate that fundamentally different effects lead to the color change. The color change in Ag−X zeolite is accompanied by stronger interaction of silver cations with the zeolite substrate that causes a paramagnetic contribution to the Ag shift as dehydration occurs. In Ag−A zeolite the loss of coordinated water dominates the dynamics of silver cations and leads to a more shielded Ag NMR signal upon dehydration.
A wealth of information about porous materials and their void spaces has been obtained from the chemical shift data in 129Xe NMR spectroscopy during the past decades. In this contribution, the only NMR active, stable krypton isotope 83Kr (spin I = 9/2) is explored as a novel probe for porous materials. It is demonstrated that 83Kr NMR spectroscopy of nanoporous or microporous materials is feasible and straightforward despite the low gyromagnetic ratio and low abundance of the 83Kr isotope. The 83Kr line width in most of the studied cases is quadrupolar dominated and field-strength independent. A significant exception was found in calcium-exchanged zeolites where the field dependence of the line width indicates a distribution of isotropic chemical shifts that may be caused by long-range disorder in the zeolite structure. The 83Kr chemical shifts observed in the investigated materials display a somewhat different behavior than that of their 129Xe counterparts and should provide a great resource for the verification or refinement of current 129Xe chemical shift theory. In contrast to xenon, krypton with its smaller atomic radius has been demonstrated to easily penetrate the porous framework of NaA. Chemical shifts and line widths of 83Kr are moderately dependent on small fluctuations in the krypton loading but differ strongly between some of the studied samples.
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