Synthesis water-free zeolite

Figure 12.11 Scheme for the water-free synthesis of zeolites [199]. 

Blasco et al. (12,13) developed a novel method for the synthesis of Al-free Ti-beta zeolite in a fluoride medium. The Ti-beta zeolite thus obtained (Ti-beta(F)) was free of connectivity defects and was hydrophobic. The typical unseeded synthesis of Al-free Ti-beta zeolite (Ti-beta(F)) involves hydrolysis of TEOS in aqueous solutions of TEAOH (35%) and H202, followed by hydrolysis of TEOT and evaporation of ethanol and water. The water lost in the evaporation and... 

Jia/Noble and coworkers [87,88] reported the successful synthesis of silicalite membranes on y-alumina composite supports using an interesting modification of the in situ crystallisation method. The support consisted of a short a-alumina tube coated on the inside with a 5 pm thick y-alumina film with an average pore diameter of 5 nm, commercially available from US Filter. The precursor solution was put into the support tube after plugging both ends with teflon and the filled tube was then placed in a teflon-lined autoclave. Hydrothermal treatment was carried out at 180°C for 12 h. After removal from the autoclave and washing the formed zeolite layer with water, the procedure was repeated with the tube inverted from its previous orientation to obtain a uniform coating. As reported by Vroon et al. [82,84,98], Jia/Noble [88] also concluded that at least two synthesis steps are necessary to obtain defect-free membranes. 

One of the most fundamental basis of the hydrothermal synthesis of zeolites is the mineralizing role of water, which is greatly cissisted by the free OH concentration in the solution / hydrogel. Apart from this basic requirement of mineralizability, other factors like. Si / A1 molar ratio, pH of the gel, aging at lower temperature, crystallization temperature and time etc., influence the type and quality of the crystalline material in rather specific ways [1]. It is clear that the enhancement of the crystallisation rate is not much dependent of the choice of counter cation (H, Na or K) of a particular oxyanion promoter, at least for high silica... 

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