OXYGEN SITES IN THE ZEOLITE STILBITE - A COMPARISON OF STATIC, MAS, VAS, DAS AND TRIPLE QUANTUM MAS NMR TECHNIQUES

Static, magic angle spinning (MAS), variable angle spinning (VAS), dyn amic angle spinning (DAS) and triple quantum magic angle spinning (3QM AS) NMR techniques were applied to separate and quantify oxygen signal s from Al-O-Si and Si-O-Si sites of O-17-enriched samples of the miner al stilbite, a natural zeolite. DAS experiments showed that there was a distribution of quadrupolar coupling constants, asymmetry parameters and isotropic chemical shifts. Two methods were used to study the qua ntification problem of DAS and 3QMAS. Our results showed that DAS was quantitative. In 3QMAS, signal intensity from sites with larger quadru polar coupling constants was reduced because of less efficient excitat ion. All techniques have shown a clear difference in rates of exchange between the two types of sites with interchannel H2O molecules. (C) 1 998 Elsevier Science B.V. All rights reserved.