HYPERVALENT IODINE METHODS FOR CARBON–NITROGEN AND CARBON–CARBON BOND FORMATION
AuthorSousa e Silva, Felipe Cesar
AdvisorWengryniuk, Sarah E.
Committee memberSieburth, Scott McNeill
Dobereiner, Graham E.
Permanent link to this recordhttp://hdl.handle.net/20.500.12613/2074
MetadataShow full item record
AbstractCarbon-carbon and carbon-nitrogen bond forming events are essential in chemistry. Although numerous stoichiometric/catalytic methods provided elegant and powerful solutions enabling those processes, the use of scarce/toxic reagents and harsh conditions is still ubiquitous in this field. As a result, extensive research has been conducted in the development of environmentally benign and inexpensive reagents for such transformations, however, general solutions remain a challenge. In this context, one of the focuses of our lab is to enable those processes in a more practical and sustainable fashion by using hypervalent iodine reagents. In this dissertation we demonstrate the synthetic applications of λ3-iodane reagents towards the formation of challenging carbon-carbon and carbon-nitrogen bonds in a complementary way to the methods already reported. Chapter 1 of this dissertation outlines the general electronic structure, geometry, synthesis and reactivity of λ3-iodanes as serves and background regarding these reagents. Chapter 2 highlights the applications of λ3-iodanes to access high-oxidation state transition metals until the year of 2017. This literature review provides detailed information about how λ3-iodanes can be applied to access 1st, 2nd and 3rd row high-oxidation complexes, as well as mechanistic details and synthetic utility of high-valent transition metals. Chapter 3 demonstrates our efforts to generate selective carbon-nitrogen and carbon-carbon products from a high-valent nickel complex. This led to important information of this mechanism adopted by the reaction and how the choice of oxidant can impact 1e- versus 2e- oxidative pathways on “hard” nickel pincer scaffolds. Chapter 4 describes our efforts towards the selective formation of α-C(sp2)-C(sp2) bonds at the α-position of enones via a reductive Iodonium-Claisen rearrangement. We demonstrate the utility of β-pyridinium silyl enol ethers as a platform for direct α-arylation, and how the 2-iodo-aryl-α-arylated enones can be used to access diverse heterocyclic structures. Chapter 5 demonstrates our initial efforts towards the selective C2 or C3 carbon-nitrogen bond formation on indoles. By exposing different indoles to (bis)cationic nitrogen-ligated HVI (N-HVI) reagents we found that selective C2 or C3 C-H indole-pyridinium salts can be formed in good to excellent yield. Although, this project is not finished yet, we anticipate the indole-pyridinium salts generated could serve as platform for accessing diverse piperidines, pyridones and primary amines through straightforward procedures. The combined chapters of this dissertation highlight the applications of λ3-iodanes towards transition metals and emphasize the applications of these reagents to enable challenging C–C and C–N bond formation events. More importantly, this dissertation serves as a guide for future development of the hypervalent iodine field.
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Carbon dioxide sequestration by mineral carbonation of iron-bearing mineralsStrongin, Daniel R.; Stanley, Robert J.; Wunder, Stephanie; Schoonen, Martin A. A. (Temple University. Libraries, 2015)Carbon dioxide (CO2) is formed when fossil fuels such as oil, gas and coal are burned in power producing plants. CO2 is naturally found in the atmosphere as part of the carbon cycle, however it becomes a primary greenhouse gas when human activities disturb this natural balanced cycle by increasing levels in the atmosphere. In light of this fact, greenhouse gas mitigation strategies have garnered a lot of attention. Carbon capture, utilization and sequestration (CCUS) has emerged as a possible strategy to limit CO2 emissions into the atmosphere. The technology involves capturing CO2 at the point sources, using it for other markets or transporting to geological formations for safe storage. This thesis aims to understand and probe the chemistry of the reactions between CO2 and iron-bearing sediments to ensure secure storage for millennia. The dissertation work presented here focused on trapping CO2 as a carbonate mineral as a permanent and secure method of CO2 storage. The research also explored the use of iron-bearing minerals found in the geological subsurface as candidates for trapping CO2 and sulfide gas mixtures as siderite (FeCO3) and iron sulfides. Carbon dioxide sequestration via the use of sulfide reductants of the iron oxyhydroxide polymorphs lepidocrocite, goethite and akaganeite with supercritical CO2 (scCO2) was investigated using in situ attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The exposure of the different iron oxyhydroxides to aqueous sulfide in contact with scCO2 at ~70-100 ˚C resulted in the partial transformation of the minerals to siderite (FeCO3). The order of mineral reactivity with regard to siderite formation in the scCO2/sulfide environment was goethite < lepidocrocite ≤ akaganéite. Overall, the results suggested that the carbonation of lepidocrocite and akaganéite with a CO2 waste stream containing ~1-5% H2S would sequester both the carbon and sulfide efficiently. Hence, it might be possible to develop a process that could be associated with large CO2 point sources in locations without suitable sedimentary strata for subsurface sequestration. This thesis also investigates the effect of salinity on the reactions between a ferric-bearing oxide phase, aqueous sulfide, and scCO2. ATR-FTIR was again used as an in situ probe to follow product formation in the reaction environment. X-ray diffraction along with Rietveld refinement was used to determine the relative proportion of solid product phases. ATR-FTIR results showed the evolution of siderite (FeCO3) in solutions containing NaCl(aq) concentrations that varied from 0.10 to 4.0 M. The yield of siderite was greatest under solution ionic strength conditions associated with NaCl(aq) concentrations of 0.1-1 M (siderite yield 40% of solid product) and lowest at the highest ionic strength achieved with 4 M NaCl(aq) (20% of solid product). Based partly on thermochemical calculations, it is suggested that a decrease in the concentration of aqueous HCO3- and a corresponding increase in co-ion formation, (i.e., NaHCO3) with increasing NaCl(aq) concentration resulted in the decreasing yield of siderite product. At all the ionic strength conditions used in this study, the most abundant solid phase product present after reaction was hematite (Fe2O3) and pyrite (FeS2). The former product likely formed via dissolution/reprecipitation reactions, whereas the reductive dissolution of ferric iron by the aqueous sulfide likely preceded the formation of pyrite. These in situ experiments allowed the ability to follow the reaction chemistry between the iron oxyhr(oxide), aqueous sulfide and CO2 under conditions relevant to subsurface conditions. Furthermore, very important results from these small-scale experiments show this process can be a potentially superior and operable method for mitigating CO2 emissions.
INVESTIGATION OF ELECTROCATALYTIC ENERGY CONVERSION REACTIONS ON 2D LAYERED MATERIALS: HYDROGEN EVOLUTION ON MoS2 AND CARBON DIOXIDE REDUCTION ON Ti3C2 AND Mo2CStrongin, Daniel R.; Zdilla, Michael J.; Sun, Yugang; Yan, Qimin (Temple University. Libraries, 2019)Anthropogenic release of the greenhouse gas carbon dioxide is believed to be a leading cause in the global rise in temperature. The main source of the carbon dioxide released is from combustion of fossil fuels. Thus, its necessary to mitigate the release of CO2, look for alternatives for fossil fuels and capture and sequester or capture and convert CO2 to other useful fuels and chemicals hence creating carbon neutral or carbon negative energy cycles. This thesis work was primarily focused on design, adapt and understand the chemistry of two-dimensional (2D) layered materials, particularly transition metal dichalcogenide (TMD) molybdenum disulfide and transition metal carbides (MXenes) as catalytic materials for the conversion of renewable energy into fuels and chemicals as an alternative for fossil fuels. This investigation was accomplished by combining electrochemistry, state of the art characterization and density functional theory (DFT) calculations. We hypothesized that it would be possible to improve the electrocatalytic hydrogen evolution reaction (HER) on MoS2 by engineering catalytically active sites on the plane, their edges and their interlayer regions. We also hypothesized 2D MXene sheets would serve as good carbon dioxide reduction reaction (CO2RR) catalysts under aprotic conditions. Conceivably the broad impact of this thesis work utilizing experimental and theoretical studies is the realization of transition metal doped metallic MoS2 as a potential candidate towards HER in alkaline conditions. Initially the interlayer region of MoS2 were investigated for the HER by introducing Na+, Ca2+, Ni2+ and Co2+ cations in the interlayers of metallic phase MoS2. Experimental results show that intercalation of cations (Na+, Ca2+, Ni2+, and Co2+) into the interlayer region of 1T-MoS2 to lower the overpotential for the HER. In acidic media the overpotential to reach 10 mAcm-2 for 1T-MoS2 with intercalated ions is lowered by ~60 mV relative to pristine 1T-MoS2 (~230 mV). DFT calculations suggest that the introduction of states from the intercalated metals whether sp or d, to lower the Gibbs free energy for H-adsorption (ΔGH) relative to intercalant-free 1T-MoS2. The DFT calculations suggest that Na+ intercalation results in ΔGH closest to zero, which is consistent with our experiments where the lowest overpotential for the HER is observed with Na+ intercalation. In order to explore the activity of the edge sites of MoS2 and the effect of a conductive support we used a microwave-assisted growth technique to synthesize interlayer expanded MoS2 with a vertically orientation on conductive two-dimensional Ti3C2 MXene nanosheets (MoS2⊥Ti3C2). Judicious choice of reaction temperature allows a control over the density of the edges obtained. Compared to pure MoS2 this unique inorganic hybrid structure allows an increased exposure of catalytically active edge sites of MoS2. The produced materials were investigated as electrocatalysts for the hydrogen evolution reaction (HER) in acidic conditions. The MoS2⊥Ti3C2 catalyst synthesized at 240 0C exhibited a low onset potential (-95 mV vs RHE) for the HER and a low Tafel slope (~40 mV dec-1). The decrease in the overpotential is linked to decrease in the charge transfer resistance of the materials with the electrode and the increased edge site density. In a third study the basal plane of metallic MoS2 was engineered by doping with transition metals Co and Ni to be evaluated as a catalyst for the alkaline HER. Due to a lack of oxygen evolution catalysts that can oxidize water at the anode under acidic conditions, there is an urgency to realize HER catalysts that can efficiently reduce water to hydrogen gas under alkaline conditions. Though metallic MoS2 has an optimum H binding free energy for the HER, the sluggish water dissociation step under alkaline conditions has made the implementation of MoS2 as a catalyst at higher pHs harder. We hypothesized that doping transition metals in the basal plane of metallic MoS2 that can efficiently catalyze the water dissociation step in alkaline conditions would help to reduce the overpotential required for the HER under alkaline conditions. Ni and Co were doped in orthorhombic MoO3 which was then converted metallic MoS2 under hydrothermal conditions. The polarization plots obtained in 1.0 M KOH solution shows a low onset overpotential of -75 mV vs RHE for the 10% Ni doped metallic MoS2 with an overpotential of -145 mV to reach a current density of 10 mA/cm2. Pure metallic MoS2 reaches the same current density at an overpotential of -238 mV vs RHE while samples doped with 10% Co atoms reached 10 mA/cm2 at -165 mV. This improvement in the doped samples is attributed to the improved kinetics of the water dissociation step under the alkaline reaction conditions. 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Nano structured precious metals like silver and gold have shown promise as cathode materials for the conversion of CO2 to CO. In this thesis work we evaluate the electrocatalytic properties of Mo2C and Ti3C2 MXenes towards the electrochemical CO2 reduction reaction (CO2RR) as cheaper alternatives for precious metals. Though there have been theoretical predictions of the ability of MXenes with certain composition to have the ability to reduce CO2 to hydrocarbons, there are no experimental findings to support these calculations. In this study we observe very high faradaic efficiencies, ~90% for the CO2 reduction to CO at low overpotentials ~250 mV in acetonitrile/ionic liquid electrolytes on Mo2C MXene while Ti3C2 shows ~65% FE at an overpotential of ~600 mV for the cathodic half reaction. 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How does land-use and climate change affect soil organic carbon stocks and processes in temperate grasslands?Toran, Laura (Temple University. Libraries, 2014)Climate and land use changes have significant consequences on the global carbon (C) cycle. Changes to the C cycle in soils of temperate grasslands are important to consider because they often comprise regions of human agriculture and they may significantly alter global C cycles for hundreds to thousands of years. Experiments in temperate grasslands around Western Europe, Northern China, and the United States have shown either slight increases in soil organic carbon (SOC) stocks or no net change over the 20th century, possibly due to increased CO2 stimulating plant growth, in turn offsetting increased SOC loss via soil respiration. There is little consensus in the scientific community over what causes these effects. Experiments testing hypothesis on some SOC factors, such as soil microbial communities, have yielded inconclusive or conflicting results. Uncertainties and lack of inclusion of certain SOC dynamics and experimental variables may explain why model simulations show widely varying predictions of future global and regional SOC stocks and dynamics.