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This thesis deals with a very specific class of functional nanomaterials known as mesoporous zeolites. Zeolites are a class of crystalline aluminosilicate minerals characterized by featuring pores or cavities of molecular dimensions as part of their crystal structure. Mesoporous zeolites are zeolites which in addition to these channels and cavities, i.e. the micropores (less than 2 nm), also feature porosity in the mesopore size region (2-50 nm). The presence and ordered structure of the micropores is of profound influence for different applications of zeolites since they effectively make zeolites behave like molecular sieves capable of separating molecules by their size. This property in combination with acidic properties resulting from hydroxyl groups bridging silicon and aluminum ions in the zeolite framework make zeolites interesting as shapeselective solid acid catalysts. Unfortunately, diffusion in the micropores is inherently slow resulting in poor effective usage of zeolites in catalysis. To the end of improving diffusion in zeolites, several strategies have been pursued including developing widerpore zeolite structures, preparing zeolites in nanocrystalline form, supporting zeolites on carriers, and introducing auxiliary pore systems in each individual zeolite crystal resulting in mesoporous zeolite single crystals. With the exception of the wide-pore zeolites, these materials are termed hierarchically porous zeolites since they feature two (or more) distinct pore systems; the micropores and the meso-/macropores. The main methods for preparing mesoporous zeolite single crystals are by crystallization of the zeolite in the presence of carbon which is subsequently removed by combustion or by subjecting normal purely microporous zeolites to alkaline treatments resulting in mesopore formation by selective extraction of silicon from the framework. It is described how various carbon templates allow for tuning the porosity of mesoporous zeolites and that cheap mesopore templates may be prepared by carbonization of sucrose. It is also described how the two main methods for preparing mesoporous zeolites can be combined so that the porosity of a mesoporous zeolite may be enhanced by subjecting it to alkaline treatment. Finally, it is described how crystallization of synthesis gels containing fluoride lead to new mesoporous zeolite-like materials, namely mesoporous aluminophosphate zeotypes.
|Place of Publication||Kgs. Lyngby, Denmark|
|Publisher||Technical University of Denmark|
|Number of pages||198|
|Publication status||Published - Nov 2008|