The pure silica MFI zeolite, Silicalite s-1, has attracted intensive research attentions because of its superior properties in many applications. It possesses a three-dimensional pore structure consisting of intersecting ten-ring ring channels, whose pore size is appropriate for the separation of organic molecules. This material can be fabricated into membranes with excellent performance in pervaporative separations for isomeric mixtures such as dichlorobenzene (DCB).
However, to fabricate high-performance DCB pervaporative composite membranes, it is necessary to have a suitable stationary phase for the separation of isomers. Many attempts have been made to facilitate this process through the use of metal organic frameworks (MOFs), zeolites, and membranes, but the separation efficiency remains poor due to the low mobility of the isomers.
To improve the separation efficiency, several studies have investigated the incorporation of silicalite s-1 into polymer matrices to create composite membranes. The results from the characterization techniques of these materials have shown that zeolite-polymer composites have superior performance for isomeric DCB separations. However, the preparation of these zeolite-polymer membranes is complicated and time-consuming because of the requirement for a large number of purification and separation steps. In order to simplify the process, a new method to prepare silicalite s-1 zeolite/PDMS composite membranes is proposed in this article.
This work focuses on the effects of key synthesis parameters such as crystallization temperature and seed concentration on the morphology and structure of the zeolite membranes, based on the F-S-1 zeolite. The XRD patterns and SEM images of the prepared samples are systematically analyzed to reveal the relationship between the synthesis conditions and the zeolite structure.
In the present study, a series of F-S-1 zeolite samples were synthesized under different conditions of crystallization temperature and seed concentration for comparison purposes. The reactivity coefficient (RC) of the sample increases with increasing crystallization temperature and seed concentration. This result is in agreement with previous reports that a higher concentration of seeds can significantly accelerate the nucleation rate for MFI zeolite synthesis.
In order to obtain high-quality silicalite s-1 zeolite with a high RC, the following approaches were taken: (1) Ethanolamine (EOA) and tetrabutyl titanate (TBOT) were added to deionized water and fumed silica under stirring in a Teflon autoclave to form solution A. The molar composition of this solution was SiO2: n TiO2: 0.1 TPABr: 0.5 NH2C2H4OH: 30 H2O. The mother liquid produced in this step was kept and used as the starting material for subsequent synthesis. Then, a suspension of nanosized silicalite s-1 seeds was added to the mother liquid and stirred under vigorous conditions to form solution C. The resulting solution was filtered and the filtrate was centrifuged and washed with water to obtain the as-synthesized silicalite s-1 powder. This product was then dried at 120degC overnight and calcined at 550degC for 6 h. The calcined silicalite s-1 was used as the seed for the next batch synthesis. The remaining mother liquid was recirculated and reused. The results show that the morphology of the silicalite s-1 thin film changes with the crystallization time. The surface morphology becomes rougher as the crystallization time increases. The thickness of the silicalite s-1 layer also increases as the synthesis time increases.
Tags:titanium silicalite | zeolite s-1
