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²⁹Si, ²⁷Al and ²³Na MAS NMR study of the natural zeolite barrerite and its dehydrated phases

¹ Dipartimento di Scienze della Terra, Università degli studi di Modena e Reggio Emilia, Largo S.Eufemia 19, I-41100 Modena, Italy
² Laboratoire de Matériaux Minéraux, CNRS URA 428, Ecole Nationale Supérieure de Chimie de Mulhouse, 3, rue Alfred Werner, F-68093, Mulhouse Cedex, France
³ Groupe Sécurité et Ecologie Chimiques, Ecole Nationale Supérieure de Chimie de Mulhouse, 3, rue Alfred Werner, F-68093, Mulhouse Cedex, France
⁴ Istituto di Chimica Quantistica ed Energetica Molecolare, CNR, via Alfieri 1, I-56010 Ghezzano (Pisa), Italy

E-mail: sani{at}esrf.fr

Barrerite from Alaska was investigated by ²⁹Si, ²⁷Al and ²³Na MAS NMR both in its untreated (as-found) and heated forms. The chemical shift of the ²⁹Si NMR lines of the natural phase revealed the presence of three different Si(nAl) configurations. The ²⁷Al NMR signal of the natural phase, positioned in the region of tetrahedrally coordinated Al, becomes more shielded with increasing temperature, in agreement with the changes of the Al-O-Si bond angles caused by the distortion of the framework observed by means of X-ray diffraction (Alberti & Vezzalini, 1978). Changes in the ²⁷Al NMR line-width were also correlated to dehydration at high temperature. ²⁷Al 2D triple-quantum MAS NMR spectra were recorded on natural and dehydrated barrerite. In neither case the five tetrahedral sites could be differentiated by the MQMAS technique. ²³Na MAS spectra were preliminarily collected on the untreated barrerite and on the sample heated at 450°C and the shift of the resonance line towards higher fields in the heated phase was interpreted as a consequence of the heating process and of the increased coordination number.

Key-words: barrerite, dehydration, MAS NMR, MQMAS.