Morphology Control Of Zeolite Silicalite-1 Crystals Using Microwave-Assisted Solvothermal Synthesis

Zeolites as one of the most famous members of the inorganic materials, which have periodic three-dimensional frameworks and well-defined pore structures, have attracted much interest due to their wide applications in catalysis, ion-exchange, chemical separation, adsorption, optics, and membranes. Studies show that the properties of zeolites and related crystalline porous materials are strongly affected by the size and morphology of the crystals. Therefore, it is necessary to develop the synthetic strategies for the control of crystals size and morphology of the inorganic materials and zeolites.In the past decades, the studies on how to control the morphology of the crystals of zeolites have been extensively conducted. Varying the synthetic parameters such as the temperature, reaction time, and alkalinity of the starting gel can affect the size and shape of the crystals of zeolites. Recently, microwave (MW) radiation heating has been widely used in many chemical reactions and the synthesis of materials. Compare to the traditional heating, the rapid heating rate and the special heating mechanism of the microwave heating employed may change the reaction mechanism conducted in a microwave heating system, which may be used to control the morphology of the crystals of zeolites. In this thesis, the development of new synthetic method, strategy, and route on controlling the morphology of crystals of silicalite-1 (Si-MFI) under the microwave assisted hydrothermal/solvothermal synthesis will be discussed.Chapter 1 introduced the state of the art of the zeolites science and morphology control of the crystals of zeolites.In chapter 2, we investigated the morphology control of Si-MFI crystals by employing microwave-assisted solvothermal synthesis in the reaction system TEOS–TPAOH-alcohol using dipolar alcohols as a co-solvent. Isolated, self-stacked or flowered crystals of Si-MFI were obtained in the presence of alcohols with different dielectric constants. Our studies showed that self-stacked Si-MFI crystals can be produced by using alcohols with low dielectric constants as a co-solvent with appropriate aging time and high concentration in the reaction mixture under microwave radiation. It was proposed that the low polar co-solvent may favored the formation of abundant active Si-OH groups at the early stage of the crystallization, which might undergo further condensation to form fibrous crystals due to rapid crystallization under microwave heating condition. Adding the glycerol to the TEOS-TPAOH synthesis system as a co-solvent resulted in the formation of flowered and the overlapped Si-MFI polycrystals.In chapter 3, a new method that can effectively control the aspect ratios of the Si-MFI crystals was discussed. Under microwave assisted solvothermal synthesis condition, the Si-MFI crystals with tunable sizes, shapes, and aspect ratioes were crystallized from a reaction system of TEOS-TPAOH-H2O-Diol. The essential issue of this method is combining the utilization of co-solvent of diols and the microwave heating. Individually applying either one of these two strategies to control the morphology of the Si-MFI crystals obtained under microwave assisted solvothermal synthesis condition was not successful. Under the microwave assisted heating, the type and concentration of the co-solvent of diols can significantly affect the length of the resulting Si-MFI crystals, whereas the width and thickness are less affected. An excellent linear correlation between the aspect ratio of the Si-MFI crystals and the"concentration"of the C/OH in the starting gel was observed. On the basis of this excellent linear relationship, the aspect ratio of the well defined Si-MFI crystals could be effectively tuned.In chapter 4, the influence of the co-solvent of mixed alcohols and diols on the morphology of Si-MFI crystals was investigated. Varying the molar ratios of two mixed alcohols can affect the morphology of the resulting crystals from the self-stacked fibrous shape to prolonged hexagonal thin sheet shape. Two-step microwave heating favored the formation of Si-MFI crystals with prolonged hexagonal thin sheet shape, whereas the tightly self-stacked crystals can be obtained with one step microwave heating.In conclusion, we developed a new method on the basis of microwave assisted solvothermal synthesis and utilization of co-solvent to effectively control the morphology of the Si-MFI crystals.