New Structural Model of Hydrous Sodium Aluminosilicate Gels and the Role of Charge-Balancing Extra-Framework Al

A new structural model of hydrous alkali aluminosilicate gel (N-A-S-H) frameworks is proposed, in which charge-balancing extra-framework Al species are observed in N-A-S-H gels for the first time. This model describes the key nanostructural features of these gels, which are identified through the application of 17O, 23Na, and 27Al triple quantum magic angle spinning solid-state nuclear magnetic resonance spectroscopy to synthetic 17O-enriched gels of differing Si/Al ratios. The alkali aluminosilicate gel predominantly comprises Q4(4Al), Q4(3Al), Q4(2Al), and Q4(1Al) Si units charge-balanced by Na+ ions that are coordinated by either 3 or 4 framework oxygen atoms. A significant proportion of Al3+ in tetrahedral coordination exist in sites of lower symmetry, where some of the charge-balancing capacity is provided by extra-framework Al species which have not previously been observed in these materials. The mean SiIV–O–AlIV bond angles for each type of AlIV environments are highly consistent, with composition...