Low-temperature (4.2 K) 23Na and 27Al swept-field nuclear magnetic resonance spectroscopy of zeolites: observation of framework and non-framework aluminum sites

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A new method for acquiring 23Na and 27Al nuclear magnetic resonance (NMR) spectra of zeolites is introduced. The key features are operation at low temperature, 4.2 K, and the ability to scan across large spectral regions. In this method, the magnetic field is slowly swept through the resonance condition as the line shape is recorded by a lowpower single pulse or spin echo pulse sequence. A single crystal and powder sample of andalusite are studied to verify the spectral response features of the pulse sequences. Then, four hydrogen faujasites are studied; the samples have different amounts of non-framework aluminum. By acquiring the spectra at 4.2 K, the motion of ions and water molecules is suppressed and the NMR spectra are obtained in the static limit. We compare these spectra to simulations based on 27Al quadrupole coupling constants previously obtained in room temperature studies. There is partial agreement, but the 4.2 K spectra appear to show more 27Al sites than are observed at room temperature. © 1995.

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Microporous Materials

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