|Title: ||Zeolite catalysts through new synthesis concepts and post-synthesis modification|
|Other Titles: ||Zeolietkatalysatoren door nieuwe syntheseconcepten en post-synthese modificatie|
|Authors: ||De Baerdemaeker, Trees|
|Issue Date: ||22-May-2015 |
|Abstract: ||Zeolites – crystalline, microporous aluminosilicates consisting of three-dimensional networks of corner sharing tetrahedra - have been used in industry as catalysts, adsorbents and ion-exchangers for decades. The catalytic behavior of a zeolite is strongly influenced by its framework connectivity (topology), composition, morphology and texture. These properties depend on the synthesis conditions, but they can also be altered through post-synthesis modifications. In this dissertation, the effect of new synthesis routes and post-synthesis modification on the adsorption and catalytic properties are investigated.|
In the first part of the dissertation, well-known zeolite topologies are discussed. A perspective is provided on the potential impact of a molecular trapdoor mechanism in the zeolite chabazite on the field of gas separations. Additionally the implications of the discovery of Ti-containing zeolite Beta more than twenty years ago on the field of oxidation catalysis and heteroatom containing zeolite catalysts are discussed. Zeolite Beta was also synthesized using an organic-free, seed-directed synthesis route. Normally, this framework topology is synthesized in the presence of organic molecules, such as tetraethylammonium hydroxide, that end up in the zeolite pores after synthesis and need to be removed via high-temperature calcination. The products of these organic-free synthesis procedures possess a high Al-content and they were also found to contain a high density of strong acid sites. As a result, the activities and selectivities observed in the alkylation of benzene with ethylene were higher than those of the zeolite Beta obtained in presence of organics. However, post-synthesis dealumination was required to facilitate the desorption of the polar products in acylation reactions. In the hydroconversion of n-decane, the dealumination also resulted in an improved balance between the acid sites of the zeolite and the hydrogenation/dehydrogenation function introduced through Pt impregnation.
The second part demonstrates the use of layered zeolite precursors as building blocks for new catalysts. These layered zeolite precursors consist of two-dimensional (alumino)-silicate sheets, which condense into a three-dimensional zeolite framework upon calcination. Using an interlayer expansion treatment, an additional T-atom is inserted between the layers, expanding the pore size of the resulting zeolitic product by two T-atoms. Typically, a silylating agent, such as dichlorodimethylsilane or diethoxydimethylsilane is used as a source for this additional T-atom, resulting in interlayer galleries lined with methyl groups. Instead of methyl groups, sulfonic acid groups were introduced by using an alternative silylating agent, resulting in shape-selective Brønsted acid catalysts. Alternatively, an interlayer expanded structure was also obtained in presence of an Fe-salt without further addition of a silylating agent. The obtained catalysts contained isolated Fe-species with Lewis acid properties. Because these zeolites derived from layered precursors typically consist of plate-like crystals with micropore channels running parallel to the crystal plates, the center of the crystal is only accessible from the crystal edge. To improve the accessibility, alkaline treatments were performed to introduce an additional mesopore system through desilication. Remarkably, not only mesoporosity was generated, but the alkaline treatment of interlayer expanded and calcined materials could also be used to recombine the zeolite layers in a different stacking order, resulting in a different topology than the topology that would be obtained from topotactic condensation of the original layered zeolite precursor.
|Publication status: ||published|
|KU Leuven publication type: ||TH|
|Appears in Collections:||Centre for Surface Chemistry and Catalysis|