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NMR studies of silicate & aluminosilicate solutions

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  • Chemistry

Abstract

The work described in this thesis deals with the use of (^29)Si and (^27)Al NMR to obtain information about the chemical structure of aqueous silicate and aluminosilicate solutions. This has extended the knowledge gained in previous studies. A wide range of alkaline and tetraalkylammonium hydroxide silicate and aluminosilicate solutions (mostly also containing sodium) has been examined. Such solutions are shown to contain a large range of anions. The first highly-resolved (^27)Al NMR spectra of alkaline aluminosilicate solutions are presented and discussed. The linewidths and number of resolved lines are shown to depend critically on several factors, especially the pH and Si:Al ratio, as well as the concentration of various components. At least thirteen separate peaks or bands are observed at the optimum conditions of pH~10.35 and Si:Al = 1. Tentative assignments of some bands are presented, based on (^27)Al and (^29)Si shift omparability, (^27)Al linewidths and (^27)Al spin-lattice relaxation measurements. Relative intensities of the various (^27)Al signals are given for the pH ~10.35 solution. A correlation of (^27)A1 and (^29)Si chemical shifts has been established. Dilution effects and dynamic equilibria are considered for the aluminosilicate solutions. Trends in the spectra with changing concentrations of the various components have been identified. Under certain conditions, additional peaks are observed. Two of these are tentatively assigned to aluminium nuclei in "three-membered" rings. Moreover, a few very sharp hues (i.e. with very low electric field gradients), currently of unknown origin, are observed in several spectra. At certain concentrations chemical exchange can be shown to take place. The effects of tetraalkylammonium (TAA) and alkali metal cations on the equilibrium distribution of aluminosilicate oligomers in aqueous alkaline aluminosilicate solutions have been investigated using evolution with time of Al-27 NMR spectra. The results indicate that there are no observable differences in the initial distribution of aluminosilicate species (i.e. immediately following solution mixing) involving a mixture of TAA and Na cations and those involving alkali metal cations alone. However, in the latter case, this distribution, in contrast to those for TAA/Na solutions, is not stable, the species quickly re-equilibrating, giving broad signals of q(^3) and perhaps q(^4) type. The effect of Al and Si concentration on the formation of gel in?aluminosilicate solutions is also investigated with ^^Al NMR spectra. It is shown that the gel time strongly depends on the Al concentration and the temperature. A graph of Al and Si concentrations with gel time has been established.Silicon-29 NMR spectra have been obtained for more than 20 aqueous alkaline silicate solutions containing methanol. A signal assigned to CH(_3)OSi(OH)(_3) or one of its deprotonated congeners is studied in detail for the first time for the solution conditions involved. Its appearance has been monitored as a function of the solution composition. The equilibrium constant for its formation is of the order of 0.65. The effects of alcohol, silicate and TAAOH concentration and of the nature of the alkylammonium base on this reaction have been investigated.

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