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²³Na 2D 3QMAS NMR and ²⁹Si, ²⁷Al MAS NMR investigation of Laponite and synthetic saponites of variable interlayer charge

¹ Bruker S. A. Solid-state NMR Application Lab., F-67166 Wissembourg, France, ² ISTO, UMR 6113, CNRS - Université Orleans, F-45071 Orléans Cedex 2, France, and ³ University of Liege, Institute of Chemistry B6a, COSM, Sart-Tilman, B-4000 Liege, Belgium

^* E-mail: j.grandjean{at}ulg.ac.be

(Received 15 June 2001; revised 5 June 2002)

²⁹Si, ²⁷Al MAS NMR is used to characterize Laponite RD and synthetic saponites of variable interlayer charge. The Si/Al ratios are in good agreement with the calculated charge from chemical analysis except for the lowest-charged saponite. In contrast to the ²⁹Si MAS NMR spectra in which resolved signals are detected, the ²⁷Al MAS NMR spectra show one signal whose linewidth increases with the clay charge. The water content of the clay samples was obtained from ¹H MAS NMR.

The 2D MQMAS NMR technique is required to obtain a high-resolution spectrum of nuclei with strong quadrupolar interaction. This method was applied to the ²³Na nucleus of clay counterions and to the ²⁷Al structural nucleus. One well-defined ²³Na NMR signal is observed for all the clays studied except the highest-charged saponite. Possible explanations for this different behaviour are discussed. The calculated isotropic chemical shift evolves progressively with the clay charge whereas the deduced quadrupolar interaction does not change significantly. The ²⁷Al 2D 3QMAS technique was not able to resolve more than one signal.

KEYWORDS: 2:1 phyllosilicate, saponite, Laponite, ¹H, ²³Na, ²⁷Al and ²⁹Si MAS NMR, 2D 3QMAS NMR

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