EFFECT OF SILICATE RATIO ON THE DISTRIBUTION OF SILICATE AND ALUMINOSILICATE ANIONS IN TPA ALUMINOSILICATE SOLUTIONS

Si-29 and Al-27 spectroscopies were used to characterize dilute, highly alkaline tetrapropylammonium (TPA) aluminosilicate solutions. The solution compositions ranged from 0.1 to 2 mol % SiO2, silicate ratios (R = [SiO2]/[(TPA)2O]) from 0.05 to 1, and Si/Al molar ratios from 0.25 to 5. Si-29 NMR resonances were observed for Al incorporation into the dimer, linear trimer, cyclic trimer, branched cyclic trimer, double three-membered ring (D3R), and double four-membered ring (D4R) anions. In aluminosilicate solutions with 2 mol % SiO2, silicate ratios of less than 1.5 lead to the stabilization of D3R(1Al) anions. The addition of DMSO to TPA aluminosilicate solutions provides the solutions with a high concentration of D3R(1Al) and D4R(1Al) anions. Semiempirical quantum mechanical calculations were performed to calculate the partial charges on Si atoms in silicate and aluminosilicate anionic structures. A correlation between the Si atom partial charge and its experimentally observed chemical shift assists in the assignment of Si-29 NMR peaks due to soluble aluminosilicates.