High resolution solid-state NMR studies on dissolution and alteration of Na-montmorillonite under highly alkaline conditions

Dissolution and alteration of Na-montmorillonite (Na-mon) under highly alkaline conditions have been studied by high resolution solid state 27Al/29Si MAS and 27Al 3Q MAS NMR. Six altered samples were prepared by treating Na-mon in a highly alkaline Ca(OH)2 solution (0.01 M: M = mol dm−3) from 1 week to 3 months. The 3Q MAS NMR spectra of non-altered and altered samples indicated the existence of one octahedral Al site (AlVI) and two tetrahedral Al sites (AlaIV,AlbIV). The AlaIV and AlbIV sites were assigned to Q3(3Si) and Q4(4Si) units, respectively. The AlbIV site of altered samples was ascribed to formation of analcime on the basis of powder X-ray diffraction patterns and 27Al MAS NMR spectra. The average isotropic chemical shifts and the quadrupolar products remained constant before and after the alkaline treatment. The ratio of AlaIV/AlVI indicated that the tetrahedral Al is more easily dissolved than the octahedral one. Therefore, we proposed a model that octahedral sites of Na-mon are exposed due to partial removal of external tetrahedral sites. 29Si MAS NMR spectra showed that tetrahedral sheets of the Na-mon mainly consist of Q3(0Al) and Q3(1Al) units, and that the estimated bond angles (Si–O–Si) and bond lengths (Si–Si) are not affected by alkaline alteration. Furthermore, the full widths at half-maximum (FWHMs) of 29Si NMR peaks corresponding to Q3(0Al) and Q3(1Al) sites were constant before and after the highly alkaline treatment. These results suggest that the coordination structure around tetrahedral Si is not largely changed by the alkaline treatment.