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

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