Abstract
The objective of this thesis was the synthesis of high silica zeolite with modified pore structure and crystal size. Zeolites are a large group of materials, including crystalline hydrated aluminosilicate compounds that contain alkali and alkaline earth metals. Among the various types of zeolites, the high silica synthetic zeolite ZSM-5 (Mobil Synthetic Zeolite-5) is considered a powerful catalyst because of its unique pore structure which has an excellent shape selectivity. It is a medium pore zeolite formed by ten-membered rings which possesses a pore dimension of 0.54-0.56 nm.Nevertheless, the small microporous channels of ZSM-5 catalyst are subjected to diffusion limitations and this results in reducing the access of molecules to be catalyzed in its internal surfaces, thus not taking full advantage of its catalytic activity.In recent years, many researchers have given particular emphasis in the direction of (i) reducing the synthetic cost, by using alternative cheap silica sources ...
The objective of this thesis was the synthesis of high silica zeolite with modified pore structure and crystal size. Zeolites are a large group of materials, including crystalline hydrated aluminosilicate compounds that contain alkali and alkaline earth metals. Among the various types of zeolites, the high silica synthetic zeolite ZSM-5 (Mobil Synthetic Zeolite-5) is considered a powerful catalyst because of its unique pore structure which has an excellent shape selectivity. It is a medium pore zeolite formed by ten-membered rings which possesses a pore dimension of 0.54-0.56 nm.Nevertheless, the small microporous channels of ZSM-5 catalyst are subjected to diffusion limitations and this results in reducing the access of molecules to be catalyzed in its internal surfaces, thus not taking full advantage of its catalytic activity.In recent years, many researchers have given particular emphasis in the direction of (i) reducing the synthetic cost, by using alternative cheap silica sources for producing zeolites and (ii) improving the catalytic activity of various zeolites including ZSM-5, by trying to improve the access of molecules to be catalyzed, within the zeolite framework.In this direction, two different approaches have been adopted for improving the catalytic activity of zeolitic materials.In the first approach, investigations have been focused on reducing the zeolite crystal’s size and thus increasing the external surface area of the catalyst and thereby reducing the diffusion path in its internal surfaces.In the second approach, researches have been followed for introducing mesopores within the zeolite crystal’s network, thereby interfacing imported mesopores to zeolite’s micropores and thus, increasing the accessibility of the molecules to the interior surface of the catalyst.The addressed issues in this thesis can be divided into two sections. In the first part, high-silica zeolite was synthesized using industrial by-product-rice husk ash- as alternative inexpensive sources of silica. At first, the burning conditions of rice husk were studied for obtaining crystalline and amorphous ash. Subsequently, an extent study was followed on the synthetic conditions and their influence on the reaction time and morphology of the final products. For this purpose, three different methods were performed: (i) synthetic reaction under mild conditions with low temperature and atmospheric pressure, (ii) hydrothermal treatment using an autoclave under high temperature and autogenous pressure, and (iii) utilization of microwave irradiation using a microwave oven. In addition, the combination of using microwave irradiation along with synthesis under mild conditions and separately with autoclave was studied in order to explore the influence that this has in the whole process and morphology of the obtained materials.The second section of the thesis was intended to synthesize ZSM-5 zeolite using commercial sources of silica in order to (i) reduce the zeolite crystal’s size, via the confined space synthetic method, by using mesoporous carbon as a templating agent and (ii) introduce mesopores in the zeolite lattice by using carbon nanotubes during the synthetic procedure, followed by burning after completion of the reaction for its removal. Finally, additional experiments were performed for synthesizing ZSM-5 zeolite in the presence of activated carbon in order to reduce the available synthesis space of zeolite crystals.From this study, it can be concluded that rice husk ash can be used as an alternative source of silica for synthesizing` ZSM-5 both under mild synthetic conditions and via hydrothermal treatment. Microwave assisted synthesis leads to a significant decrease of the reaction time. Inert carbon matrices were successfully used in the synthesis of high silica zeolites both for the introduction of mesopores in the zeolite lattice as well for reducing the crystal size.
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