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

作者：Takahashi Takafumi^*; Ohkubo Takahiro; Suzuki Kensuke; Ikeda Yasuhisa

来源：Microporous and Mesoporous Materials, 2007, 106(1-3): 284-297.

DOI：10.1016/j.micromeso.2007.03.008

• solid state NMR
• Al-27 3Q MAS
• Al-27/Si-29 MAS
• na-montmorillonite
• alkaline condition
• microscopic change

Dissolution and alteration of Na-montmorillonite (Na-mon) under highly alkaline conditions have been studied by high resolution solid state Al-27/Si-29 MAS and Al-27 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 (Al-VI) and two tetrahedral Al sites (Al-a(IV), Al-b(IV)). The Al-a(IV) and Al-b(IV) sites were assigned to Q(3)(3Si) and Q(4)(4Si) units, respectively. The Al-b(IV) site of altered samples was ascribed to formation of analcime on the basis of powder X-ray diffraction patterns and Al-27 MAS NMR spectra. The average isotropic chemical shifts and the quadrupolar products remained constant before and after the alkaline treatment. The ratio of Al-a(IV)/Al-VI indicated that the tetrahedral Al is more easily dissolved than the octahedral one. a Therefore, we proposed a model that octahedral sites of Na-mon are exposed due to partial removal of external tetrahedral sites. Si-29 MAS NMR spectra showed that tetrahedral sheets of the Na-mon mainly consist of Q(3)(0Al) and Q(3)(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 Si-29 NMR peaks corresponding to Q(3)(0Al) and Q(3)(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.

• 出版日期2007-11-1