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Controlling the physical characteristics of heterogeneous catalysts is crucial to effectively tune their material properties for different applications. Zeolites prove to be a desirable material for a broad range of these applications as numerous techniques exist to modify their crystalline structure and characteristics. This dissertation focuses on improving the understanding of synthesis-structure-reactivity relationships within zeolites to design well-defined catalysts by developing novel synthesis procedures and investigating how material properties affect catalyst performance.One novel technique developed in this work allows for the synthesis of nano-sized Lewis acid zeolites. While the small pores of zeolites are beneficial for size selectivity, they can also hinder mass transfer, limiting the use of zeolites for converting bulky reactants. This is observed in the conventional synthesis of Sn containing zeolite Beta that produces large micron sized particles that can experience significant diffusion limitations. Here, a procedure is proposed to synthesize nano-Sn-Beta that has a particle size of 100 nm. This catalyst demonstrates superior performance for the epoxide ring opening reaction with large reactants of epoxyoctane and ethanol, demonstrating the ability to overcome diffusion limitations.Whereas nano-Sn-Beta has increased activity over the conventional Sn-Beta for bulky reactants, it has lower performance for small reactants when mass transfer limitations are not a factor. It is observed that this is likely caused by the inherent hydrophilicity of the nano material that originates from the synthesis procedure. To decouple the two resultant material properties, particle size and material hydrophobicity, a post synthetic procedure is developed to treat the nano-Sn-Beta with fluoride to partially recrystallize the structure and increase its hydrophobicity. After fluoride treatment, nano-Sn-Beta has increased hydrophobicity as observed through water adsorption measurements. This change is explained from the detection of fewer silanol defects throughout the framework via FTIR and 29Si NMR measurements. A corresponding increase in the performance of the catalyst is seen for the epoxide ring opening reaction for both small and large reactants, corroborating the hypothesis that a hydrophobic material is beneficial for this reaction.Lastly, defects within zeolites that are detrimental to catalytic activity are re-imagined as beneficial targets for material modification. These defects, which are modeled as a nest of silanols, can serve as a location for new heteroatoms to be incorporated into the purely siliceous framework. Current defect materials are created by stripping aluminum from quickly synthesized Al-Beta, thereby creating sites where Sn or other heteroatoms can be post synthetically incorporated. This technique has the advantage of a significantly reduced synthesis time as compared to hydrothermal synthesis. However, materials synthesized in this manner often exhibit negative characteristics, largely caused by the harsh conditions to de-aluminate the parent material. To overcome the harsh de-alumination process, a synthesis procedure is proposed where alkyl silanes are used to create controlled defects within the zeolite. Additional work expands on this approach to use alkyl Sn precursors to create unique catalytic environments within the zeolite.Overall, through the investigation of synthesis-structure-reactivity relationships in zeolites, this dissertation develops a variety of tools to tune material properties to create functional catalysts for a number of industrially important reactions. |
