Proton environments in hydrous aluminosilicate glasses: A ¹H MAS, ¹H/²⁷Al, and ¹H/²³Na trapdor NMR study

Hydrous aluminosilicate glasses of composition NaAlSi₈O₁₈, NaAlSi₃O₈, NaAlSi₂O₆, and NaAlSiO₄ have been studied with ¹H MAS, and ¹H/²⁷Al, and ¹H/²³Na TRAPDOR NMR spectroscopy. Two different environments for molecular water were identified. Three ¹H OH resonances (1.5, ca. 3.5, and 5-6 ppm) are observed in all the glasses. The resonance at 1.5 ppm shows a large ¹H-²⁷Al dipolar coupling and is assigned an Al(Q³)-OH group; the concentration of this species increases with increasing Al content in the glass. The resonance at ca. 3.5 ppm is assigned to Si(Q³)-OH units, on the basis of its chemical shift, proximity to Na and Al, and the change in the relative intensity of the three resonances, with varying Ai/Si ratio. The resonance at 5-6 ppm is assigned to a Q³-OH unit that is hydrogen bonded to another oxygen atom. These assignments imply that the aluminosilicate framework undergoes depolymerization as an outcome of water dissolution.