Reactive A1-O-A1 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 A1O4 tetrahedra do not share corners, i.e., that A1-O-A1 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 I7O triplequantum magic-angle spinning nuclear magnetic resonance (3QMAS NMR). We show that thermodynamically significant concentrations of A1-O-A1 sites can be observed in a naturally occurring zeolite (stilbite), and that these sites react faster with H2O vapor than do Si-O-Si and Si-O-Al sites.