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[摘要]：Crystallization of aluminum-free zeolite-Beta from viscous gel and incorporation of titanium into its framework were investigated by several spectroscopic techniques. Hydrothermal synthesis was stopped at different times and the gels obtained from the autoclaves were analyzed by liquid-state Si-29 NMR spectroscopy. The gels were subsequently dried and inspected by solid-state Si-29 NMR, Fourier-transform infrared, and X-ray absorption spectroscopy. NMR spectroscopy distinguished signals from oligomers and from two populations of nanoparticles with different typical dimensions. It also provided information about the composition of the oligomers and nanoparticles, in terms of the relative fractions of Q(n) units within them. Investigation showed that the synthesis of zeolite Ti-Beta started with a quick formation of oligomers. After 20 h of the hydrothermal treatment, oligomers with a high fraction of Q(3), units gradually aggregated into primary nanoparticles with a diameter of similar to 2 nm. During first 20 h, primary nanoparticles began to agglomerate into secondary nanoparticles with a diameter of 5-10 nm. These nanoparticles already had a high fraction of Q(4) units and already possessed a zeolitic character. With a further increase in the hydrothermal treatment time, the concentration of secondary nanoparticles increased and they started to gather into zeolitic crystallites. Ti atoms were incorporated into the growing silicate framework at a very early stage of the crystallization process. |
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