Chemical and Biomolecular Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2751
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Item METHANE VALORIZATION OVER NOVEL CATALYST SYSTEMS VIA DIRECT PATHWAYS(2019) Oh, Su Cheun; Liu, Dongxia; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Methane, when converted to higher hydrocarbons, promises a great future as the substituent for liquid petroleum in petrochemical and fine chemical industries. Methane conversion via direct pathways such as oxidative coupling of methane (OCM) to ethylene and direct non-oxidative methane conversion (DNMC) to C2 (acetylene, ethylene and ethane) and aromatics have attracted much attention given their unique capability in circumventing the intermediate energy-intensive steps found in indirect processes. In the OCM process, the more reactive nature of C2 products leads to the sequential oxidation of C2 to COx (CO or CO2). Selective catalysts that favor C2 formation are desired. The DNMC is challenged by low equilibrium conversion, high endothermicity, and high coke selectivity. Catalysts or reaction systems that concurrently solve these challenges are required. This dissertation aims to develop novel catalyst systems to conquer limitations in OCM and DNMC to realize efficient and effective C2 production. For OCM reaction, hydroxyapatite (HAP), a bioceramic material with the capability of cation and/or anion substitutions, was innovatively employed as a catalyst. The effects of cation and/or anion substitutions in HAP on OCM reaction were studied. The rigorous description of the reaction kinetics of OCM in HAP-based catalysts was conducted. Finally, the selective control of exposed crystalline plane of HAP was realized to further understand the catalytic behaviors of HAP-based catalysts in OCM reactions. It is shown that cation and/or anion substitution can change the physicochemical properties of the HAP catalysts, and as consequences, the OCM catalytic performances. The c-surface (i.e., (002) crystalline plane) of HAP-based catalysts exhibited significant enhancement in areal rate in OCM reaction. The single iron sites confined in the lattice of silica matrix (Fe/SiO2) is an emerging type of methane activation catalyst in DNMC. We innovated a millisecond catalytic wall reactor made of Fe/SiO2 catalyst to enabling stable and high methane conversion, C2+ selectivity, low coke yield, and long-term durability. These effects originate from initiation of DNMC by surface catalysis on reactor wall, and maintenance of the reaction by gas-phase chemistry in reactor compartment. Autothermal operation of the catalytic wall reactor is potentially feasible by coupling and periodical swapping of endothermic DNMC and exothermic oxidative coke removal on opposite side of the reactor. High carbon and thermal efficiencies and low cost in reactor materials are realized for the techno-economic process viability of the DNMC technology. In addition, we created a process of tailoring product selectivity towards to C2 hydrocarbons by employing a mixture of Fe/SiO2 catalyst and mixed ionic-electronic conductive perovskite (SrCe0.8Zr0.2O3−δ) oxide in the presence of hydrogen co-feed in methane stream. The unprecedentedly high C2 yield was realized in DNMC reaction to maximize its potential as a feedstock for ethylene production in chemical industries.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 ENGINEERING HIERARCHICAL MESO-/MICROPOROUS LAMELLAR ZEOLITES WITH VARIABLE TEXTURAL AND CATALYTIC PROPERTIES(2016) EMDADI, LALEH; Liu, Dongxia; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Meso-/microporous zeolites combine the charactersitics of well-defined micropores of zeolite with efficient mass transfer consequences of mesopores to increase the efficiency of the catalysts in reactions involving bulky molecules. Different methods such as demetallation and templating have been explored for the synthesis of meso-/microporous zeolites. However, they all have limitations in production of meso-/microporous zeolites with tunable textural and catalytic properties using few synthesis steps. To address this challenge, a simple one-step dual template synthesis approach has been developed in this work to engineer lamellar meso-/microporous zeolites structures with tunable textural and catalytic properties. First, one-step dual template synthesis of meso-/microporous mordenite framework inverted (MFI) zeolite structures was investigated. Tetrapropyl ammonium hydroxide (TPAOH) and diquaternary ammonium surfactant ([C22H45-N+(CH3)2-C6H12-N+(CH3)2-C6H13]Br2, C22-6-6) were used as templates to produce micropores and mesopores, respectively. The variation in concentration ratios of dual templates and hydrothermal synthesis conditions resulted in production of multi-lamellar MFI and the hybrid lamellar-bulk MFI (HLBM) zeolite structures. The relationship between the morphology, porosity, acidity, and catalytic properties of these catalysts was systematically studied. Then, the validity of the proposed synthesis approach for production of other types of zeolites composites was examined by creating a meso-/microporous bulk polymorph A (BEA)-lamellar MFI (BBLM) composite. The resulted composite samples showed higher catalytic stability compared to their single component zeolites. The studies demonstrated the high potential of the one-step dual template synthesis procedure for engineering the textural and catalytic properties of the synthesized zeolites.