Abstract
{sup 29}Si and {sup 27}Al spectroscopies were used to characterize dilute, highly alkaline tetrapropylammonium (TPA) aluminosilicate solutions. The solution compositions ranged from 0.1 to 2 mol % SiO{sub 2}, silicate ratios (R = (SiO{sub 2})/((TPA){sub 2}O)) from 0.05 to 1, and Si/Al molar ratios from 0.25 to 5. {sup 29}Si 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 % SiO{sub 2}, 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 {sup 29}Si NMR peaks due to soluble aluminosilicates.
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