Engineering Zeolite Catalysts with Improved Performance for Aromatics Production
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Zeolites are shape-selective microporous crystals that are widely used in refining and (petro)chemical processes owing to their unique porous topologies, high thermal stability, and tunable acidity. This dissertation aims to improve the performance of zeolite catalysts by optimizing some essential design parameters such as framework topology, morphology, and Brønsted and Lewis acidity. We first investigated the synthesis of 2-dimensional (2D) zeolites due to their superior catalytic properties, particularly in applications involving large (bulky) molecules. We selected a common layered zeolite, MCM-22 (MWW framework), to explore methods of preparing 2D nanosheets via a one-pot synthesis in the absence of complex organic templates. Using a combination of high-resolution microscopy and spectroscopy, we show 2D MMW-type layers with an average thickness of 3.5 nm (ca. 1.5 unit cells) can be generated using the surfactant cetyltrimethylammonium (CTA), which operates as a dual organic structure-directing agent (OSDA) and exfoliating agent to affect Al siting and eliminate the need for post-synthesis delamination, respectively. We tested these 2D catalysts using a m odel reaction to assess external (surface) Brønsted acid sites and observed a marked increase in the conversion relative to 3-dimensional MCM-22 and 2D layered MWW (ITQ-2) prepared by post-synthesis exfoliation. To this end, our findings highlight a facile, effective route to directly synthesize 2D MWW-type materials, which may prove to be more broadly applicable to other layered zeolites. Driven by the limited supply of fossil fuels, it is highly desirable to develop high-performance zeolite catalysts with improved selectivity to aromatics, particularly from diversified (non-petroleum) feedstocks. The catalyst ZSM-5 (MFI framework) was chosen to examine more efficient routes to aromatics given its reported shape-selectivity for C6 – C8 products. In this dissertation, we examine ethylene dehydroaromatization (DHA) over metal-exchanged ZSM-5 (both Ag-ZSM-5 and Ga-ZSM-5) and show that extra-framework Lewis acid sites promote DHA reactions with enhanced selectivity to toluene and xylenes. We observe that metal-exchanged ZSM-5 enhances aromatic selectivity approximately 3-fold compared to H-ZSM-5, which tends to produce light olefins and other aliphatic hydrocarbons. Through systematic studies of metal-exchanged ZSM-5 using a combination of experiments and density functional theory (DFT) calculations, we have been able to differentiate the roles of metal species in ethylene activation with enhanced product selectivity to value-added aromatics. Lastly, we have studied the challenges associated with synthesizing ZSM-5 at low temperature (ca. 100°C), which is often necessary to generate small crystals (< 200 nm) with an appreciable quantity of acid sites (i.e., Si/Al < 25). We show that synthesis at low temperature, and most notably in growth mixtures containing high aluminum concentration, results in the incomplete incorporation of Al in ZSM-5 that can be partially removed through post-synthesis mild acid treatment. Our findings suggest that ZSM-5 may be more difficult to synthesize than is commonly perceived, most notably when examining the various types of defects (i.e., extra-framework octahedral- and penta-coordinated Al) that can be incorporated in crystalline products over a wide range of synthesis conditions.