Depolymerization effect of water in aluminosilicate glasses: Direct evidence from ¹H-²⁷Al heteronuclear correlation NMR

We have applied one-dimensional (ID)¹H MAS NMR, ²⁷Al → ¹H CP MAS NMR, as well as 2D²⁷ Al triple-quantum (3Q) MAS NMR, ²⁷Al → ¹H heteronuclear correlation (HETCOR) and high-resolution 3QMAS/HETCOR NMR techniques to KAlSi₃O₈ (Or), NaAlSi₃O₈ (Ab) and NaAlSiO₄ (Nc) glasses containing 0-2 wt% H₂O to shed light on the dissolution mechanisms of water in aluminosilicate melts (glasses). An Al Q³-OH group, characterized by ¹H chemical shifts of 1.3-1.9 ppm, was identified for all hydrous glasses. Its abundance increases with bulk Al/Si ratio. The ²⁷Al chemical shifts (δ₁^Al) of this species are 64-68 ppm, larger than those of Al Q⁴by 3-6 ppm. Despite this difference, it is only through ²⁷Al → ¹H HETCOR and 3QMAS/HETCOR, but not ²⁷Al MAS or 3QMAS NMR that the peaks are resolved. This study has demonstrated that depolymerization and formation of AIOH/SiOH is a general water dissolution mechanism for polymerized aluminosilicate melts (glasses), and HETCOR NMR experiments involving ¹H are the key to its revelation.