Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
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Item RATIONAL DESIGN OF MULTIFUNCTIONAL ZEOLITES FOR BIOMASS CONVERSION(2021) Zhang, Junyan; Liu, Dongxia; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Biomass conversion to long-chain hydrocarbons is imperative to producing sustainable feedstock for the transportation sector's decarbonization. The complexity of biomass composition requires hybrid conversion technology where the rational design of multifunctional catalysts is indispensable for the efficient downstream process towards platform molecules. Conversion of biomass-derived ethanol or butanediol to C3+ olefins via heterogeneous catalysis is a promising pathway for generating platform hydrocarbon molecules. Zeolite is crystalline microporous material with a framework of silicate tetrahedron (SiO4) and AlO4 tetrahedra. The unique porous structure provides shape selectivity for both reactants and products, and isomorphous heteroatom substitution can form Brønsted or Lewis acidic sites (LAS) with tunable acidity, creating the capability of catalyzing the critical reaction steps in biomass conversion. This dissertation starts with titrating mesopore interconnectivity within pillared MFI and MWW zeolites by atomic layer deposition of titania to get proper candidates with improved diffusion conditions. The results indicate better mesopore connectivity in PMFI than PMWW. Afterward, bifunctional Cu/PMFI was synthesized to catalyze 2,3-butanediol conversion to C3+ olefins, with alkaline-treated mesoporous Cu/ZSM-5 as reference material. It is demonstrated that better mesopore connectivity plays a crucial role in depressing coke formation. For ethanol conversion, the creation of new types of Lewis acidic sites within the BEA zeolite framework was done by incorporating rare-earth and other transition metals. The as-synthesized catalysts can achieve over 80% C3+ olefins yield in one-process-step ethanol conversion. Characterizations and further kinetic study demonstrate the acidity and isolated status of loaded metal species as well as their functionalities. Also, the corresponding reaction network from ethanol to C3+ olefins was revealed. A similar synthesis procedure was applied to other silica supports. However, the as-formed isolated sites showed strongly support-dependent activity. Further improvement focuses on reducing external hydrogen input by utilizing the intrinsically generated H2 in the ethanol dehydrogenation step. A composition of Lewis acid Beta zeolite and single-atom alloy catalyst was studied with a two-bed configuration, which can achieve over 70% C3+ olefins yield without external H2 input. This work provides new types of supports, acid sites, and system designs to overcome the challenges of biomass-derived alcohols upgrading.Item THE UPGRADING OF METHANE TO AROMATICS OVER TRANSITION METAL LOADED HIERARCHICAL ZEOLITES(2017) WU, YIQING; Liu, Dongxia; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)With the boom of shale gas production, the conversion of methane to higher hydrocarbons (MTH) promises a great future as the substituent for hydrocarbon production from crude oil based processes. Among various MTH processes, direct methane aromatization (DMA) is promising since it can achieve one-step methane valorization to aromatics. The molybdenum/zeolite (Mo/MFI or Mo/MWW) has been the most active catalyst for the DMA reaction, which, however, is impeded from industrial practice due to the rapid deactivation by coke deposition. To address this challenge, in this work, transition metal loaded hierarchical 2 dimensional (2D) lamellar MFI and MWW zeolites have been studied as catalysts for the DMA reaction. The effects of micro- and mesoporosity, external and internal Brønsted acid sites, as well as particle size of 2D lamellar zeolites on the DMA reaction have been investigated. Firstly, the spatial distribution of Brønsted acid sites in 2D lamellar MFI and MWW zeolites has been quantified by a combination of organic base titration and methanol dehydration reaction. The unit-cell thick 2D zeolites after Mo loading showed mitigation on deactivation, increase in activity, and comparable aromatics selectivity to the Mo loaded 3D zeolite analogues. A detailed analysis of the DMA reaction over Mo/hierarchical MFI zeolites with variable micro- and mesoporosity (equivalent to variation in particle sizes) showed a balance between dual porosity was essential to modulate the distribution of active sites (Mo and Brønsted acid sites) in the catalysts as well as the consequent reaction and transport events to optimize performance in the DMA reaction. External Brønsted acid sites have been proposed to be the cause of coke deposition on Mo/zeolite catalysts. Deactivation of the external acid sites have been practiced to improve the catalyst performances in the DMA reaction in this work. Atomic layer deposition (ALD) of silica species was conducted on the external surface of 2D lamellar MFI and MWW zeolites to deactivate the external acid sites in Mo/2D lamellar zeolites for the DMA reaction. Another strategy to deactivate external acid sites in Mo/zeolite catalysts was the overgrowth of 2D lamellar silicalite-1 on the microporous zeolites. The as-prepared catalysts showed higher methane conversion and aromatics formation as well as higher selectivity to naphthalene and coke in comparison with Mo loaded microporous analogues.Item PREPARATION OF POLYELECTROLYTE MEMBRANES EMBEDDED WITH ZEOLITE NANOPARTICLES FOR ENHANCED PERFORMANCE IN FORWARD OSMOSIS(2016) Kang, Yan; Mi, Baoxia; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Water scarcity is a global issue that has already affected every continent. Membrane technology is considered as one of the most promising candidates for resolving this worsening issue. Among all the membrane processes, the emerging forward osmosis (FO) membrane process is osmotically-driven and has unique advantages compared with other traditional pressure-driven membrane processes. One of the major challenges to advancing the FO membrane process is the lack of a suitable membrane. Polyelectrolyte thin film prepared via layer-by-layer (LbL) technique has demonstrated its excellent performance in many applications including electronics, optics, sensors, etc. Recent studies have revealed the potential of polyelectrolyte thin films in acting as the active separation layer of FO membranes, but significant efforts are still needed to improve the membrane performance and understand the transport mechanisms. This dissertation introduces a novel approach to prepare a zeolite-embedded polyelectrolyte composite membrane for enhanced FO performance. This membrane takes advantages of the versatile LbL process to unprecedentedly incorporate high loading of zeolite nanoparticles, which are anticipated to facilitate water transport due to the uniquely interconnected structure of zeolites. Major topics discussed in this dissertation include: (1) the synthesis and evaluation of the polyelectrolyte-zeolite composite FO membrane, (2) the examination of the fouling resistance to identify its technical limitations, (3) the demonstration of the membrane regenerability as an effective strategy for membrane fouling control, and (4) the investigation of crosslinking effects on the membrane performance to elucidate the transport mechanisms involved in the zeolite-embedded polyelectrolyte membranes. Comparative studies have been made between polyelectrolyte membranes with and without zeolite incorporation. The findings suggest that the zeolite-embedded membrane, although slightly more susceptible to silica scaling, has demonstrated enhanced water flux and separation capability, good resistance to organic fouling, and complete regenerability for fouling control. Additionally, the embedded zeolite nanoparticles are proved to be able to create fast pathways for water transport. Overall, this work provides a novel strategy to create zeolite-polymer composite membranes with enhanced separation performance and unique fouling mitigation properties.