Probing the location and distribution of paramagnetic centers in alkali metal-loaded zeolites through (7)Li MAS NMR.

The nature and surroundings of lithium cations in lithium-exchanged X and A zeolites following loading with the alkali metals Na, K, Rb, and Cs have been studied through (7)Li solid-state NMR spectroscopy. It is demonstrated that the lithium in these zeolites is stable with respect to reduction by the other alkali metals. Even though the lithium cations are not directly involved in chemical interactions with the excess electrons introduced in the doping process, the corresponding (7)Li NMR spectra are extremely sensitive to paramagnetic species that are located inside the zeolite cavities. This sensitivity makes (7)Li NMR a useful probe to study the formation, distribution, and transformation of such species

Cs-133 NMR and ESR Studies of Cesium-Loaded LiX and LiA Zeolites

The species generated when cesium metal was loaded into zeolites LiA, LiX, and LiLSX by vapor deposition were systematically investigated by 133Cs NMR and ESR as a function of loading level. The primary 133Cs NMR signal in Cs-loaded LiA at low loading was assigned to Cs+ in the eight-ring SII sites and showed axial anisotropy of the chemical shift. The primary 133Cs NMR signal in Cs-loaded LiX and LiLSX was isotropic and was assigned to Cs+ dynamically exchanging among the SIII sites. This dynamics was found to be frozen out at 173 K. At higher loadings, additional broader 133Cs NMR signals, some with large shifts, indicated the presence of diamagnetic species influenced by neighboring paramagnetic species. The ESR results were characteristic of interacting paramagnetic species, and the spin counts showed that a significant fraction of the introduced spins became spin-paired, consistent with the presence of both paramagnetic and diamagnetic clusters. The existence of caeside Cs- ion in any of these materials is still an open question. © 2008 American Chemical Society

Alkali metal loaded zeolite LiA: evidence for highly symmetrical Rb(-) and K(-).

Peer reviewed: YesNRC publication: Ye

Characterization of zn-containing metal-organic frameworks by solid-state ⁶⁷Zn NMR spectroscopy and computational modeling

Metal-organic frameworks (MOFs) are an extremely important class of porous materials with many applications. The metal centers in many important MOFs are zinc cations. However, their Zn environments have not been characterized directly by 67Zn solid-state NMR (SSNMR) spectroscopy. This is because 67Zn (I=5/2) is unreceptive with many unfavorable NMR characteristics, leading to very low sensitivity. In this work, we report, for the first time, a 67Zn natural abundance SSNMR spectroscopic study of several representative zeolitic imidazolate frameworks (ZIFs) and MOFs at an ultrahigh magnetic field of 21.1-T. Our work demonstrates that 67Zn magic-angle spinning (MAS) NMR spectra are highly sensitive to the local Zn environment and can differentiate non-equivalent Zn sites. The 67Zn-NMR parameters can be predicted by theoretical calculations. Through the study of MOF-5 desolvation, we show that with the aid of computational modeling, 67Zn-NMR spectroscopy can provide valuable structural information on the MOF systems with structures that are not well described. Using ZIF-8 as an example, we further demonstrate that 67Zn-NMR spectroscopy is highly sensitive to the guest molecules present inside the cavities. Our work also shows that a combination of 67Zn-NMR data and molecular dynamics simulation can reveal detailed information on the distribution and the dynamics of the guest species. The present work establishes 67Zn-SSNMR spectroscopy as a new tool complementary to X-ray diffraction for solving outstanding structural problems and for determining the structures of many new MOFs yet to come. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim