J . Phys. Chem. 1992, 96, 2968-2975
2968
*@Siand "Ga NMR Investigations of Alkylammonium Gallosilicate Solutions
R. F. Mortlock, A. T. Bell,* and C . J. Radke Center for Advanced Materials, Lawrence Berkeley Laboratory, and Department of Chemical Engineering, University of California. Berkeley, California 94720 (Received: September 9, 1991)
29Siand "Ga NMR spectroscopies are used to characterize the distribution of species present in highly alkaline solutions of alkylammonium gallosilicates. The solution compositions ranged from 0.1 to 2 mol % SO2,silicate ratios (R= [Si02]/[M20]; M' = TPA' or TMA') from 0.05 to 2, and Si/Ga molar ratios from 2 to a. Based on similar studies of alkylammonium aluminosilicate solutions, tetrapropylammonium (TPA) gallosilicate solutions displayed 29SiNMR peaks assigned to Si atoms in cyclic trimer (S3R), branched cyclic trimer (b-S3R), and double three membered ring (D3R) anions with one incorporated tetrahedra, gallosilicate anions with rather acute Ga atom. Due to the larger size of Ga04S-tetrahedra compared to sio44( 4, one broad exchange-coalesced peak due to monomeric gallate and Ga atoms in gallosilicate anions is observed. Only when Ga incorporates into larger silicate anions does a separate resonance appear. At a Si/Ga = 2, a broad shoulder is resolvable at =190 ppm. The appearance of this broad shoulder corresponds to the rise in the intensity of the Q2( 1Ga) + Q S A ( 1Ga) region in Figure 4e. Therefore, we tentatively assign the signal a t =190 ppm to Ga atoms in gallosilicate anions with two or three siloxane linkages. The slight upfield shift in the intense peak at >215 ppm is a complex function of the degree of ionization of gallosilicate oligomers and the exchange rates between gallate and gallosilicate anions. Figure 7 presents the spectra of a series of gallosilicate solutions for which the [Gal = 0.4 mol % and [TPA20] = 2 mol % and the concentration of Si02 is varied. The narrow single resonance at 222 ppm in Figure 6a is characteristic of monomeric gallate, Ga(OH),-, in aqueous alkaline solution^.^*^^ As the amount of dissolved silica increases, the gallate peak broadens and shifts upfield due to a chemical exchange between gallate anions and gallosilicate anions. Figure 8 plots the fwhm (Hz) and the chemical shift (ppm) versus dissolved silica in solution. The appearance of linear relationships is consistent with the above interpretation that the single resonance in each of the spectra in Figure 7 is an exchange-coalesced peak, due to monomeric gallate anions at 222 ppm and Ga atoms in gallosilicate anions a t
