Solid state NMR study of oxygen site exchange and Al-O-Al site concentration in analcime

Oxygen isotope exchange between natural analcime and water vapor at low pre ssure is studied by O-17 NMR, which can distinguish Si-O-Si and Si-O-Al sit es and provide information on site specific exchange kinetics and mechanism s. Si-O-Al sites in analcime are found to exchange faster than Si-O-Si site s at lower temperature (400 degrees C). At higher temperature (500 degrees C), the exchange rates for the two types of site are similar, suggesting th at the exchange process for Si-O-Si has a higher activation energy, as pred icted by published theoretical calculations. The overall exchange reaction appears to be controlled largely by reaction at sites, not diffusion. Stati c O-17 spectra show an anomolously high ratio of Si-O-Si to Si-O-Al sites, suggesting the presence of a few percent of Al-O-Al sites, which are indeed resolved and observed in triple quantum magic-angle spinning (3QMAS) spect ra.