27Al and 29Si NMR study of sol-gel derived aluminosilicates and sodium aluminosilicates

Solid state ²⁷Al and ²⁹Si NMR was used to examine the structures of aluminosilicates and sodium aluminosilicates prepared by the sol-gel method from metal alkoxides. In contrast to the borosilicate system, where B-O-Si bonds are not formed until heat treatment above 150° C, Al-O-Si formation appears complete upon gelation. Aluminium occupies tetrahedral AlO₄]^– sites in the polymer network and octahedral Al(H₂O)₆]³⁺ (or similar) sites in the intersticies for charge balance. When sodium is added as a counter ion the octahedral aluminium is converted to tetrahedral aluminium in the oxide network. In gels of high aluminium content prepared from (Bu^sO)₂Al-O-Si(OEt)₃, some aluminium in five coordinate environments is also observed. All gels remain amorphous on heating to 800° C.