Use of oxygen-17 isotope in the characterization of zeolites and related structures by nuclear magnetic resonance spectroscopy

Solid-state nuclear magnetic resonance spectroscopy (NMR) is a powerful tool in the structure determination of zeolites. In this literature review the current status of the use of 170 NMR in the characterization of zeolites and related structures has been surveyed. All the major components of zeolites contain an NMR active isotope (29Si, 27Al and 170). There has been only few NMR studies of the third major component, 170. The natural abundance of 170 iS low (0,037 %), and thus it requires enrichment of the sample with 170 isotope. Samples enriched with 170 can be prepared with quite simple methods, either by synthesizing from starting materials, which contain ~70 or by exchanging with 170 isotope already synthesized materials. Latter method is preferential if commercial zeolite catalysts are to be studied, although there are several possible disadvantages when using this method. The isotopes may not be exchanged uniformly throughout the crystallites, the treatment may be time consuming and it can also cause some unintended modifications in the sample. Treatment with 170 enriched oxygen gas seems to be a quite reliable method, however. The high cost of 170 isotope is one of the reasons for limited use of 170 NMR in the characterization of zeolites. 170 iS a quadrupolar nucleus and the quadrupolar interaction broadens and shifts the NMR signal. Thus in order to get higher resolution a high static magnetic field (Bo) and MAS (magic angle spinning) technique with high spinning frequency should be used. Recently, DAS (dynamic angle spinning) and DOR (double rotation) techniques have also been shown to be able to significantly improve the resolution of 170 NMR spectra. Lineshape simulations are necessary to determine the 170 NMR parameters of quadrupolar interaction, which give valuable additional information of the structure.