OXYGEN SITE EXCHANGE KINETICS OBSERVED WITH SOLID-STATE NMR IN A NATURAL ZEOLITE

O-17 solid state nuclear magnetic resonance (NMR) was applied to study the oxygen exchange kinetics between framework oxygens and oxygens in the channel waters in the zeolite mineral stilbite. We have shown tha t framework oxygens in Al-O-Si sites react faster with channel water t han oxygens in Si-O-Si sites and that water self-diffusion in stilbite is fast and thus the oxygen site exchange reaction is the rate limiti ng step in bulk exchange, at least for temperatures from 150 to 200 de grees C and for exchange times up to 40 h. The observed total amount e xchanged agreed well with bulk isotope measurements. The observed oxyg en isotope exchange kinetics for Al-O-Si and Si-O-Si sites are not sim ple exponential. However, they could be approximated with two first-or der rate reactions. The mechanism for the oxygen exchange reactions an d possible explanations for the kinetics are discussed. Copyright (C) 1998 Elsevier Science Ltd.