2975equilibria. For example, in contrast to the results presented above for increases in Ga concentration, increases in the A1 concentration achieved by adding pure Al wire to aluminosilicate solutions leads to a depolymerization of oligomeric anion^.^'-^^ The dissolution of 1 mol of pure A1 consumes 1 mol of hydroxide ions to produce 1 mol of aluminate anions, but the addition of 1 mol of Ga3+ ions consumes 4 mol of hydroxide ions to produce 1 mol of gallate anions. Thus, the addition of Ga(N03)3 to silicate solutions causes a greater reduction in the pH of the solution than the addition of pure Al. The lower pH results in an increase in the extent of solution polymerization. It is also noted by 29Si NMR peak widths that the exchange rates between gallate anions and small silicate anions (i.e., dimer and linear trimer anions) is faster than those observed for aluminosilicate solutions. Faster exchange rates result in broad 29Si N M R peaks which are indistinguishable from the background noise. Finally, Ga prefers to incorporate into silicate anions where the SiO-Ga-OSi bond angle is rather acute, Le., in S3R(lGa) and D3R(lGa) anions, due to the larger size of Ga atoms compared to A1 or Si atoms. On the other hand, A1 incorporates into a wide variety of silicate anions. The overall size of the anion might limit the stabilization of gallosilicate anions larger than the D3R( 1Ga) or D4R anions due to the effect of cation crowding.48 This explains why D4R( 1Ga) and doubly Ga substituted anions are not observed. The smaller size of A1 allows for stabilization of significant concentrations of D4R( 1Al) anions and aluminosilicate anions exhibiting Q3(2Al), Q2(2A1), and QjA(2A1) connectivities. ConclusionsThe present investigation demonstrates the usefulness of 29Si and 71Ga N M R spectroscopies for studying tetralkylammonium gallosilicate solutions. Gallosilicate anions with Si atoms of QzA(lGa), Qz( lGa), and Q3A(lGa) connectivities are observed in TPA gallosilicate solutions. Specific 29Si N M R resonances assigned to Si atoms in S3R( lGa), b S 3 R ( lGa), and D3R( 1Ga) anions were identified. S3R( 1Ga) anions are especially stable and become the dominant species as the solution temperature is raised to near synthesis conditions. Ga atoms undergo a fast chemical exchange with the dimer and linear trimer silicate anions, causing the 29Si peaks for Si atoms in dimer(1Ga) and linear trimer(1Ga) anions to broaden into the background noise. 'IGa N M R experiments provide evidence for the chemical exchange of Ga atoms between gallate anions and gallosilicate anions, but 71Ga N M R spectra cannot generally be resolved to give peaks for Ga atoms associated with specific siloxane bond connectivities. Only at silicate ratios of greater than 2 can 71Ga spectra be resolved into two peaks, one for gallate anions and one for Ga atoms in gallosilicate oligomers. A simple set of chemical equilibria explains the changes in the distribution of solution species with changes in the Si02, Ga, and TPA20 concentrations. Gallate anions appe...
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