Engineered zeolitic materials : synthesis and application
|Advisor:||Kleitz, Freddy; Kaliaguine, S.; Arnal, Pablo Maximiliano|
|Abstract:||Zeolites as microporous crystalline materials have shown competence and versatility in a huge number of applications. Their unique properties have persuaded researchers to constantly look for novel pathways to get the most out of these extraordinary substances. Modifying the properties of classical zeolites or combining them synergistically with other materials are found to be two viable techniques to attain efficient zeolitic materials with improved characteristics. In this dissertation, these two approaches were separately used to study, first, the morphological modification of ZSM-12 and second, the formation of mesoporous silica@silicalite-1 core-shell materials. ZSM-12 is a high silica zeolite which has recently attracted much attention owing to its superior performance in adsorption and catalysis. In order to synthesize ZSM-12 with high purity and controlled size and morphology, the crystallization behavior of ZSM-12 zeolite was thoroughly studied by screening different commercially available chemical sources (structure-directing agents, silicon and aluminum source types) and compositions (alkalinity and Na, Al and water contents). The results presented in this study showed that, in contrast to TEAOH organic template, using MTEAOH and Al(o-i-Pr)3 could lead to the formation of mono-sized ZSM-12 single crystals in a shorter time. Alkalinity and Na+ contents were found to be playing the major roles. Following the second approach, zeolitic core-shell composites with a polycrystalline silicalite-1 shell, enclosing a mesoporous silica microsphere core (particle sizes of 1.5, 3 and 20-45 μm) in either pure form or loaded with metal guest species, were synthesized. Seeded growth technique was used as one of the reliable ways for the synthesis of such a material. The quality of the final products in terms of the pore network connectivity and shell integrity, which, together, ensure the shape-selective capability, was studied by varying synthesis parameters, such as core pre-treatments which include surface modification, seeding and calcination steps and the number of secondary hydrothermal crystallization steps. Depending on the core size and the presence of guest species, the quality of the seeding step was found to be influenced by the surface modification technique used, i.e., mesoporous silica microspheres which contain guest species need an additional treatment of chemical functionalization of the external surface with species such as APTES, rather than using a simple surface modification with ionic polymers. It was also shown that depending on the core size, two to four short hydrothermal treatments are required to fully cover the core, with no aggregation and core dissolution. Such materials with a molecular sieve crystalline shell can be used in a wide variety of applications, particularly for shapeselective adsorption and catalysis purposes. Selective adsorption capability of the final product was investigated by conducting a series of batch glycerol adsorption experiments from crude biodiesel with different compositions at different temperatures. Glycerol content of the purified biodiesel by using the core@shell material was compared to those purified by using conventional adsorbents including bare mesoporous silica gel spheres, classical zeolites, e.g., silicalite-1, pure siliceous β-zeolite (Si-BEA) and ZSM-5(H+) crystals as well as a physical mixture of the constitutive materials, i.e., equally mixed silicalite-1 and silica gel spheres. Although mesoporous silica gel spheres showed slightly higher glycerol adsorption capacity, the study revealed that the mesoporous adsorbents tended to trap a significant number of bulkier molecules, such as FAMEs, in their large pore networks (dpore> 6 nm). However, the silicalite-1 shell provided a microporous membrane which hindered the diffusion of FAME into the mesopores while the composite adsorbents benefited from large pore volume of mesoporous silica as core compartment, allowing a multi-layer glycerol adsorption. This feature of the synthesized core@shell material considerably enhanced the dry washing performance in terms of purification yield and adsorption capacity, in comparison to other conventional adsorbents including mesoporous silica gel and classical zeolites.|
|Document Type:||Thèse de doctorat|
|Open Access Date:||24 April 2018|
|Collection:||Thèses et mémoires|
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