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

Hydrous aluminosilicate glasses of composition NaAlSiO18 NaAlSi3O8, NaAlSi2O6, and NaAlSiO4 have been studied with H-1 MAS, and H-1/Al-27 and H-1/Na-23 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 H-1-Al-27 dipolar coupling and is assigned an Al(Q(3))-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 Al/Si ratio. The resonance at 5-6 ppm is assigned to a Q(3)-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.