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

Abstract

17O 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 that framework oxygens in Al-O-Si sites react faster with channel water than 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 limiting step in bulk exchange, at least for temperatures from 150 to 200°C and for exchange times up to 40 h. The observed total amount exchanged agreed well with bulk isotope measurements. The observed oxygen isotope exchange kinetics for Al-O-Si and Si-O-Si sites are not simple exponential. However, they could be approximated with two first-order rate reactions. The mechanism for the oxygen exchange reactions and possible explanations for the kinetics are discussed.