Effects of Al³⁺ Ions on Formation of Silica Framework and Surface Active Sites for SO₄^2– Ions

Al(3+) ions were introduced into silica framework at 318 K in order to make active Al sites for SO4(2-) by the addition of aqueous sodium silicate solution to aqueous sulfuric acid solution of Al2(SO4)3. The (27)Al and (29)Si NMR spectra of aluminosilicates were measured at 278 K with reaction time. (29)Si NMR spectra were analyzed by the multivariate curve resolution. The addition of Al(3+) ions to aqueous silicate solution promoted gel formation. Small amounts of Al(3+) ions were incorporated as a four-coordinated complex at early stage of polymerization reaction of silicates and during subsequent reaction six-coordinated Al complex increased, suggesting reversible conversion between 4- and 6-coordinated complexes. SO4(2-) ions interact with positive surfaces of aluminosilicates and are specifically adsorbed on the surface sites of 6-coordinated Al(3+) species, which may be stabilized on silicate surfaces as [Al(H2O)5SO4](+).