NMR studies of silicate and aluminosilicate solutions as precursors for zeolites

Abstract

The search for a detailed understanding of the mechanism of zeolite synthesis has, over the past two decades, promoted many investigations into the species present in (alumino)silicate solutions. It is generally accepted that dissolved (alumino)silicate species are involved as precursors in the nucleation of zeolites. Several techniques have been employed to understand the mechanism of formation of these complex structures. NMR spectroscopy has been shown to be a very powerful tool for the detection and characterization of (alumino)silicate species in the solutions in question. This thesis presents a comprehensive study of certain silicate solutions employmg (^29)Si NMR which has extended the knowledge gained in previous studies. The role of structure-direction on the distribution of silicate species in silicate solutions is discussed, the results providing more understanding of this effect. The effects of pH, temperature, Si/cation ratio and silica concentration on the distribution of silicate anions were also studied.Aluminium-27 NMR was applied to investigate the local structure about alummium atoms in a series of alumino silicate solutions with Si:Al mole ratios pertinent to zeolite synthesis. The kinetics of the reaction of aluminate with silicate anions have been studied by investigating the temporal evolutions of (^29)Al NMR spectra and by 2D NMR exchange spectroscopy. Structural analyses of two new silicate crystals were carried out by single-crystal X-ray diffraction. Liquid- and solid-state NMR spectroscopy were employed to characterize the mother liquors, the powdered polycrystalline products, and their melts.The Zeolite SUZ-9 was characterized by application of multinuclear magnetic resonance spectroscopy : (^29)Si, (^27)Al, (^13)C and (^1)H NMR spectra of this zeolite were studied.