Incorporation of aluminum into silicate anions in aqueous and methanolic solutions of TMA silicates

Abstract

{sup 29}Si and {sup 27}Al NMR spectroscopies were used to characterize dilute, highly alkaline, aqueous and methanolic solutions of tetramethylammonium (TMA) aluminosilicates. The solutions contained 2 mol% SiO{sub 2}, a silicate ratio (R=(SiO{sub 2})/(M{sub 2}O), M{sup +} = TMA{sup +}) of 2, Si/Al molar ratios between {infinity} and 2, and between 0 and 20 mol% methanol (MeOH). TMA aluminosilicate solutions are characterized by a high concentration of double four membered ring (D4R) anions. {sup 29}Si NMR peaks were observed for Si atoms in the dimer, linear trimer, cyclic trimer, branched cyclic trimer, and D4R anions which have one incorporated Al atom. {sup 29}Si NMR peak assignments were based on similar assignments in tetrapropylammonium (TPA) aluminosilicate solutions and confirmed by using the correlation between the partial charge of Si atoms and the {sup 29}Si NMR chemical shift. Comparison between the distribution of silicate and aluminosilicate oligomers in TMA and TPA aluminosilicate solutions indicates that the distribution of Si atoms among anions is very different, but the level of Al incorporation into silicate species is remarkably similar. As a result, the incorporation of Al into silicate anions depends strongly on the solution composition and pH and depends only weakly on the naturemore » of the alkylammonium base. Due to a hydrophobic effects, the addition of methanol to TMA aluminosilicate solutions increases the concentration of D4R and D4R(1Al) anions.« less