Characterization of dissolved precursors in zeolite synthesis by NMR spectroscopy

This work clarifies the influence of the synthesis magma composition on the structure of the zeolite formed. Attention is focused on the influence of alkalinity and of the choice of alkali metal cation on the structure and reactivity of precursor silicate anions.; Evidence that dissolved precursors control the synthesis of zeolites is reviewed along with recently reported information about the structure and reactions of silicate and aluminosilicate anions in aqueous solution.; Quantitative {dollar}sp{lcub}29{rcub}{dollar}Si NMR spectra of sodium silicate solutions are used to characterize the distribution of soluble silicate species as the solution composition is changed. The concentration of each species depends strongly on the silicate ratio and can be maximized by the selection of that ratio.; Interactions between alkali metal cations and silicate anions in silicate solutions are investigated using NMR spectroscopy of the cations. The concentration of pairs with large anions increases with increasing cation size. In the presence of anions with a range of size, though, the concentration of pairs exhibits a maximum with increasing cation size. These trends are interpreted in terms of the selectivity of ion pairing reactions.; Selective and non-selective inversion recovery {dollar}sp{lcub}29{rcub}{dollar}Si NMR experiments are used to study the rates of exchange of Si between silicate anions as a function of the size of the cation. The maxima are interpreted in terms of the concentration and activity of ion pair intermediates of the Si exchange reaction. The {dollar}sp{lcub}29{rcub}{dollar}Si spin relaxation rates are also studied as a function of the chemical environment produced by each cation in the vicinity of the {dollar}sp{lcub}29{rcub}{dollar}Si nucleus.; The degree of condensation of silicate and aluminate anions was measured using {dollar}sp{lcub}29{rcub}{dollar}Si and {dollar}sp{lcub}27{rcub}{dollar}Al NMR spectroscopy. In this way we deduce the structure of the product of condensation of silicate and aluminate monomers. It is observed that the extent of condensation increases with increasing silicate ratio. This trend is attributed to the redistribution of Si with increasing silicate ratio to more acidic anions. This trend cannot be ascribed to Si redistribution, though, and so it is proposed that it is due to ion-pairing reactions. (Abstract shortened with permission of author.)...