Zeolites are well known for their ordered microporous networks, good hydrothermal stability, large surface area, high acidity and selectivity. These excellent properties make zeolites extremely useful for petrochemical processes and refining. However, the sole presence of microporous channels also restricts the diffusion of reactants and products into and out of the microporous networks, especially limiting zeolite applications involving bulky molecules. The importance of developing hierarchical zeolites has attracted great attention in recent years due to the prospect of increased accessibility for bulky molecules. Introducing additional mesoporosity, and even macroporosity, into conventional zeolites produces a combination of three different size scales of porosity. It expands the original zeolite hierarchical structure and greatly enhances the mass transport of molecules while maintaining the intrinsic size, shape and transition state selectivity of zeolite. The promising applications of this new zeolite architecture have prompted a multitude of efforts to develop a variety of different synthesis strategies. Apart from hierarchical modification, core-shell structure also gives zeolites great advantages from both core and shell zeolites, such as ion exchange property for deposition of metal particles on inner zeolite, meanwhile a hydrophobic zeolite shell can protect the core complex from inhibition by water. The thesis herein focused on the development of zeolites with hierarchical and core-shell structures.

We designed a zeolite stacking structure by using hydraulic pressing and programmed temperature calcination synthesis procedures. ZSM-5 type zeolites with particle sizes of approximately 100 nm, 1 μm and 2 μm were used to synthesize stacking ZSM-5 with a size ranging from 45 to 63 μm. The prepared ZSM-5 zeolite stacking structure was used as a support to deposit palladium. The performance of the palladium/stacking ZSM-5 was investigated on Sonogashira coupling reactions. Stacking samples with micro-sized units (1 μm and 2 μm) showed a 200-300% higher turnover number (TON) than their unit counterparts. However, stacking samples with nano-sized units (100 nm) showed decreased TON conversion compared with that of their unit counterparts, probably due to partial destruction of the nano-sized ZSM-5 structure during the stacking synthesis process at high temperature. The palladium/stacking ZSM-5 (micro-sized units) also showed better conversion on different bromides and alkynes than that of traditional homogenous catalysts. Moreover, the stacking composites showed good durability by recycling 4 runs without losing significant catalytic activity. The design of the stacking MFI structure exhibited improved catalytic activity, sustainability and hierarchical-resemblance properties.

We also investigated the structure-performance relationships in different zeolite-solvent systems that are suitable for microwave-assisted dehydration of fructose to 5-hydroxymethylfurfural (HMF). Different types of zeolites (MFI, BEA, and Y) were examined as acid catalysts. The results showed that the HMF yield is independent of particle size for MFI zeolite in water. The secondary porosities improved the HMF yield, while byproducts formation (via rehydration or polymerization) was also increased due to the enlarged channels in zeolites. All tested zeolites showed higher fructose conversion, HMF yield, and HMF selectivity in organic-water solvent systems than in water. The synergistic effect of the substrates, catalysts, and solvent-product interactions in the hydrophobic Y zeolite/DMSO system yielded the highest fructose conversion (72.4%) and HMF yield (49.2%). This study supports the understanding of the requirement for microwave-assisted biomass conversion in biorefinery.

Next, we developed a core-shell zeolite structure comprised of palladium-deposited ZSM-5 core and silicalite-1 (S-1) shell which favors selectivity towards light olefin in hydrogenation via increased diffusion length. A well designed S-1/Pd/ZSM-5 core-shell structure was prepared via secondary crystallization of S-1 layer on the Pd/ZSM-5 core. The catalytic and selectivity performance of the S-1/Pd/ZSM-5 composite was evaluated in catalytic hydrogenation of alkenes in liquid phase. The synthesized S-1/Pd/ZSM-5 gives a much higher selectivity towards 1-hexene (87%) over cyclohexene (13%) even though both reactants are able to enter the 10-membered ring channels of the core-shell structure. The zeolitic core-shell composite also showed an increased selectivity towards 1-hexene over 1-heptene as the S-1 layers built up, even though both are linear alkenes with similar kinetic diameters that are accessible to the MFI framework. In this work, we demonstrated a strong correlation between the thickness of the S-1 shell layer and the selectivity towards light olefins due to faster mass transfer rate. The design of the core-shell MFI structure is a new example of how selectivity in a zeolite-catalyzed reaction can be changed and enhanced without relying on typical molecular size exclusion process.

Lastly, we synthesized a core-shell zeolite structure comprised of palladium-deposited nano-sized ZSM-5 core and silicalite-1 (S-1) shell, which favors ethylene adsorption and shows improved humidity tolerance. A well designed S-1/Pd/ZSM-5 core-shell structure was prepared via secondary crystallization of S-1 layer on the nano-sized Pd/ZSM-5 core. The ethylene removing ability of the S-1/Pd/ZSM-5 in both dry and humid conditions was evaluated by pressure drop method. The synthesized S-1/Pd/ZSM-5 gives a much higher ethylene uptake up to 290% than plain ZSM-5. The S-1 coated samples also shows strong humidity tolerance by only losing 7.6% of the ethylene uptake under 5 h of humidity pre-treatment, while the Pd/ZSM-5 lost 20% of ethylene uptake under the same condition. The design of the core-shell structure may offer the advantages of long-term economic benefit and sustainability for the fresh products market.