Reactive Al-O-Al sites in a natural zeolite; triple-quantum oxygen-17 nuclear magnetic resonance

Abstract

In framework aluminosilicate materials (e.g., feldspars, zeolites and many others), it is generally assumed that AlO 4 tetrahedra do not share comers, i.e., that Al-O-Al oxygen sites are avoided when stoichiometry permits. This assumption plays a key role in models of thermodynamic properties and of reaction kinetics. We present a new approach to directly test this assumption, based on 17 O triple­quantum magic-angle spinning nuclear magnetic resonance (3QMAS NMR). We show that thermo-dynamically significant concentrations of Al-O-Al sites can be observed in a naturally occurring zeolite (stilbite), and that these sites react faster with H 2 O vapor than do Si-O-Si and Si-O-Al sites.