SI-29 NMR OBSERVATION OF AN AMORPHOUS MAGNESIUM-SILICATE FORMED DURING IMPREGNATION OF SILICA WITH MG(II) IN AQUEOUS-SOLUTION

{H-1-Si-29} CP/MAS NMR allowed detection of an amorphous silicate form ed during contact of solvated Mg2+ cations with silica at room tempera ture and moderately basic pH. At constant contact time between the two spin systems the sample without magnesium showed three CP/MAS NMR res onances at -90, -100, and -110 ppm, while two new features at -84 and -92.5 ppm appeared and increased concurrently with the magnesium loadi ng. Fitting of the polarization growth provided for spin dynamics info rmation (TSI-H) as well as for a mean to relatively quantify each sili con species (Mo). The resonances could be clearly separated into two g roups according to their cross-polarization dynamics: the resonances a t -84, -90, and -100 ppm with T-Si-H in the range of milliseconds, on the one hand, and the resonances at -92.5 and -110 ppm, on the other h and, with much longer cross-polarization time in the order of tens of milliseconds. The resonances at -90, -100, and -110 ppm are attributed to geminal silanols, simple silanols, and siloxane silicons. The dyna mic parameters were consistent with this attribution and in line with what has been reported by other investigators on silica. The resonance at -92.5 ppm was attributed to a Q(3)(Mg) environment for the followi ng reasons. First, the chemical shift indicated a Q(3) coordination; s econd, the spin dynamics (T-Si-H) was not compatible with a Q(3)(OH) e nvironment; and, third, the magnetization limit (M(0)) was proportiona l to the Mg(II) loading. The -84 ppm resonance was attributed to termi nal. Q(2)(Mg, OH). FT-IR analysis confirmed the quantitative formation of a magnesium silicate. Therefore, a high-surface area proto-phyllos ilicate gel phase on the surface of the silica was responsible for the fixation of Mg(II). More generally, careful quantitative analysis of CP-MAS NMR was established here as a powerful technique for the quanti tative detection of surface amorphous hydrous silicates of ill-defined structures. The constitution of a Si-O-Mg bond during impregnation at room temperature was clearly evidenced, demonstrating the formation o f a secondary phase including both the cation constitutive of the supp ort (Si) and the dispersed species (Mg), and thereby recusing a surfac e-assisted magnesium hydroxide precipitation model.