Browsing by Subject "Kinetics"
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Item Analysis of Steady-State and Transient Chemical Rates in Molecular Interconversion(2020-07) Foley, BrandonThis dissertation focuses on relating macroscopic observable quantities to the fundamental elementary step reactions that mediate the chemical transformation of reactants to products in composite reactions in the context of steady-state and transient rates of chemical reactions, non-catalytic and catalytic reactions, and product-forming or catalyst-deactivating reactions. With a few notable exceptions, the formation and scission of chemical bonds as a reaction progresses is not directly observable. Instead, chemists rely on making macroscopic measurements of rate as functions of chemical activities (or concentrations) and temperature to elucidate the mechanism of unseen events occurring at the molecular scale. Analytical descriptions that explicitly relate the properties of steady-state and transient reaction rates and steady-state fractional coverages of intermediates to the underlying elementary-step reactions that comprise a reaction mechanism are developed in this research. Specifically, it is demonstrated that all measurable properties of reaction rates (e.g., reaction orders, activation energies, and fractional coverages on catalytic active sites) are explicit functions of sensitivities, which are quantitative measures of the rate-control of elementary steps in composite reactions. These relationships are utilized to elucidate the mechanism of the acid-catalyzed condensation of formaldehyde with benzene to form diphenylmethane (DPM) on HZSM-5 zeolite catalysts, a reaction implicated as causing deactivation during methanol-to-hydrocarbons catalysis. Explicit relationships between deactivation "rate" and the deactivation mechanism in heterogeneously catalyzed reactions are also explored. Metrics to assess the rate, yield, and selectivity of site-loss are defined for catalyzed systems by treating active sites as consumable reactants in deactivation reactions. These metrics enable the elucidation of deactivation mechanisms by measuring site-loss rate, yield, and selectivity as functions of reactant concentrations and temperature in methods analogous to those used to elucidate mechanisms of forming products that are observable in the reactor effluent. These methodologies are demonstrated in the elucidation of formaldehyde-mediated deactivation mechanisms during methanol-to-hydrocarbons catalysis on HZSM-5 zeolite catalysts.Item Biomass to chemicals: process design and kinetic studies for the conversion of sugars into 5-hydroxymethylfurfural(2013-12) Torres Rippa, Ana InesBiomass is an abundant resource that represents a promising, renewable, alternative for the production of fuels and chemicals. In this context, the concept of biorefinery has emerged as the future substitute of the petroleum refinery. Its economic viability will largely depend on integrating the production of biofuels with high-value chemicals. Hence, considerable research effort is devoted to the development of laboratory scale strategies to obtain chemicals from biomass. Systems-type analyses ranging from techno-economic studies to the development of kinetic models are required to evaluate the different process alternatives, and these are the focus of this thesis.In the first part, the production of 5-hydroxymethylfurfural (HMF), a sugar-derived furanic compound that acts as a precursor of building blocks for polymers, is addressed. Two flowsheets for the production of HMF from fructose were developed and evaluated. Rigorous material balances and kinetics, coupled with mathematical optimization were used to calculate the minimum price at which HMF has to be sold in order to balance raw materials (fructose), energy and capital costs. Sensitivity analysis was performed to evaluate the effect of relevant parameters. Based on these, advances that are required to reduce HMF production costs were identified and experimental research directions proposed.The second part of the thesis studies of the isomerization of glucose into fructose using tin containing zeolites (Sn-beta). This step, traditionally done with enzymes, is known to account for a substantial portion of fructose cost, thus alternative processes have the potential to reduce the production costs of sugar-derived molecules. Analysis of preliminary experimental data showed that the conventional kinetic model developed for the enzyme catalyzed reaction breaks down when the reaction is catalyzed by Sn-beta. Motivated by this, a plan that combines design of experiments, modeling and parameter estimation was proposed to elucidate the mechanism. It was found that the catalyst deactivates and that formation of by-products cannot be neglected. A phenomenological model that describes the isomerization reaction in the presence of deactivation was developed, and the corresponding kinetic parameters estimated from experimental data. The model thus obtained was used to assess the economics of glucose to HMF processes.Item Describing The Catalytic Role Of Alkaline Earth Metals On The Thermal Activation Of Cellulose(2020-05) Facas, GregoryBiomass fast pyrolysis has considerable potential for the production of renewable fuels and chemicals. Despite pyrolysis being studied for more than a hundred years, only a few commercial pyrolysis processes exist as the optimal feedstock composition and reaction conditions for this process remain unknown. The lack of process optimization can be attributed to the multiscale complexity of the process. During pyrolysis the constituents of biomass are fragmented in a matter of seconds through thousands of chemical reactions, that occur in multiple phases, and are simultaneously competing with various heat and mass transfer processes. All of these fundamental phenomena are understood poorly within pyrolysis literature. Pyrolysis is further complicated by alkali and alkaline earth metals that are naturally present in lignocellulosic biomass. These metals are known to alter pyrolysis chemistry and catalyze the initial breakdown of the polymer constituents of biomass. The main objective of this thesis was to investigate the mechanistic role of alkaline earth metals on the initial fragmentation of cellulose, the main component of biomass. Fundamental knowledge into pyrolysis chemistry has been limited previously due to an inability to obtain intrinsic kinetic, a critical tool used to validate reaction mechanisms. The requirements for proper measurement of high temperature (>400 °C) biomass pyrolysis kinetics are presented. Most importantly, these requirements mandate that for proper measurement of kinetic data, experimental techniques must heat and cool reaction samples sufficiently fast to elucidate the evolution of reaction products with time, while also eliminating substantial reaction during the heating and cooling phases. The ability of the PHASR (Pulse Heated Analysis of Solid Reactions) micro-reactor technique and other common pyrolysis reactor techniques to satisfy these requirements was discussed. PHASR can thoroughly satisfy all the requirements for measuring pyrolysis kinetics unlike other conventional reactor techniques. The PHASR technique was then utilized to study the kinetics of calcium assisted activation of cellulose. Conversion of calcium doped films of α-cyclodextrin, a known cellulose surrogate, was measured over a range of reaction temperatures (370-430 °C) and calcium concentrations (0.1-0.5 mmol Ca2+/g CD). The rate of conversion of α-cyclodextrin was significantly accelerated by the presence of calcium. Activation was shown to have a second order rate dependence on calcium concentration, suggesting the involvement of two calcium ions in the mechanism. First principle density functional theory calculations were performed on calcium catalyzed glycosidic bond cleavage and depict calcium as having two catalytic roles of disrupting hydrogen bonding in the cellulose matrix and stabilizing the transition state. The energetics from experiment and computations agree closely representing the first atomistic mechanism of metal catalyzed activation utilizing both experiments and computations. Kinetics of magnesium assisted activation were then measured with PHASR experiments to discern any effects from the size of the catalytic ion on activation chemistry. PHASR experiments were performed in identical temperature and metal concentrations to the calcium experiments. Magnesium assisted activation exhibited identical behavior to the calcium case with energetics of activation matching within experimental error.Item Development of Activity-Based Probes and Biochemical Methods for the Study of Penicillin-Binding Proteins in Live Bacteria(2021-11) Shirley, JoshBacterial cells are surrounded by a polymeric, mesh-like structure known as the peptidoglycan, and is an essential component of all eubacteria. Multi-protein machinery complexes function to carry out highly orchestrated synthesis and remodeling of the peptidoglycan throughout cell growth and division. One of the key components of these machinery complexes is the class of essential bacterial enzymes known as the penicillin-binding proteins (PBPs). PBPs are a highly conserved class of membrane-associate enzymes that function to carry out the final steps of peptidoglycan biosynthesis. All PBPs have a highly homologous transpeptidation domain, which contains a conserved, catalytic serine, to enable cross-linking of adjacent stem-peptide chains within the peptidoglycan. The catalytic serine residue has been exploited by the β-lactam class of antibiotics for ~ 100 years. Despite the success of the β-lactams as the most clinically used class of antibiotics, significant gaps in knowledge regarding the PBPs remain. The specific roles and regulations of individual PBP homologs is poorly understood and this can be attributed to a lack of appropriate tools to enable these studies. Our group has undertaken a chemical biology approach to addressing this gap, through the development of activity-based probes and biochemical methods that enable the visualization of PBP activities in native environments. The work presented in this thesis is focused on the expansion of available tools and methods that we have at our disposable to study the PBPs within live cells of both Gram-negative and Gram-positive bacteria. Efforts focused on the expansion of a β-lactone library of activity-based probes demonstrated that bioorthogonal probes retained the same activity as previously synthesized fluorophore-conjugated molecules but have increased utility in protein pull-down experiments to investigate the protein-protein interactions of specific PBP homologs. Next, the development of a live-cell kinetics assay in Streptococcus pneumoniae has provided a novel means to determine the potency values of inhibitors against the entire complement of an organism’s PBPs in one assay. The data that will be generated from future work will enable quantitative structure-activity-relationship studies to be performed, which in turn will inform us on the rational design of future PBP-selective molecules. Finally, the development of a live-cell method to study inhibitors of the PBPs in non-hypersusceptible Gram-negative species provides a means to identify molecules that are selective for the PBPs in these species and enable the development of activity-based probes for PBP homologs in understudied bacteria. In sum, we present new tools and methods that when combined with existing strategies will provide a more complete understanding of how individual PBPs function within live cells, ultimately enabling us to identify targets for the next generations of antibiotics.Item Enzyme catalyzed perhydrolysis, molecular basis and application(2011-10) Yin, Delu (Tyler)Enzyme catalyzed perhydrolysis converts a carboxylic acid or ester to a peracid. In the former reaction, the amount of peracid generated is thermodynamically controlled (Keq = 3) – while in the latter, the reaction is kinetically controlled, thus a higher concentration of peracid can be generated. Enzymes that catalyze perhydrolysis of carboxylic acids share high sequence similarity and are thought to use an esterase-like mechanism. Alternatively, carboxylic acids can also use a non-covalent mechanism, such as those used by hydroxynitrile lyases. To test whether carboxylic acid perhydrolases use an esterase-like mechanism, we identify a key covalent intermediate by mass spectrometry that can be attributed to an esterase-like mechanism but not a non-covalent mechanism. We also find that carboxylic acid perhydrolases are good catalysts for hydrolysis of peracetic acid, suggesting that their natural role is to degrade peracids generated as by-products in a living organism. Next, we determine how perhydrolases increase the rate of perhydrolysis. Carboxylic acid perhydrolases increase the rate of perhydrolysis by either increasing the selectivity for hydrogen peroxide or lower the activation barrier towards acylenzyme formation. We measure the selectivity of hydrogen peroxide using wild-type Pseudomonas fluorescens esterase (PFE) and L29P PFE (a model carboxylic acid perhydrolase). The L29P PFE variant is less selective for hydrogen peroxide than the wild-type despite having higher perhydrolysis activity. We measure the rate of acyl-enzyme formation using isotope exchange of acetic acid in H218O/H216O. The L29P PFE variant catalyzes the isotope exchange rate faster than the wild-type. Thus, carboxylic acid perhydrolases favor the formation of acyl-enzyme from carboxylic acids. We find that carboxylic acid perhydrolase (L29P PFE) does not catalyze ester perhydrolysis for accumulating high concentrations of peracetic acid. Instead, wild-type PFE and a new variant, F162L PFE accumulate up to 130 mM of peracetic acid. We measure kinetic parameters and show that hydrolysis of peracetic acid limits maximum accumulation. The F162L PFE variant minimizes hydrolysis of peracetic acid by lower ing the Km and increasing the kcat for ethyl acetate hydrolysis. The kinetic parameters are also used to predict the maximum amount of peracetic acid that can be accumulated. The F162L PFE variant is used to improve the efficiency of lignocellulose pretreatment from a previously published result using wild-type PFE. Enzymatically generated peracetic acid reacts converts lignin into smaller and more soluble lignin pieces. The chemoenzymatic process is further improved by forming peracetic acid in a biphasic layer which allows the reuse of enzyme. The pretreatment reaction conditions were also optimized by increasing the temperature to 60 °C and reducing the reaction time to 6 hours.Item Experimental discovery of surgical guidelines for cervical disc augmentation(2012-05) Mehta, Hitesh PrathivirajIntervertebral disc degeneration of the cervical spine affects over one half of all individuals over the age of 40 years and the last decade has seen an alarming increase in cervical disc degenerative disease related surgeries. In spite of newer technological advancements in devices for disc degeneration disease, spinal disc replacement and fusion, revision surgery rates have remained unchanged. 90% of the disc replacement revisions and 50% of fusion related revision can be attributed to improper device selection. Therefore, the objective of this research is to evaluate disc arthroplasty (replacement) and arthrodesis (fusion) devices and identify optimal implant size (height) selection criteria for biomechanical competence in force transmission, motion, and neurologic tissue protection. Eleven osteo-ligamentous human cadaver cervical spines were biomechanically evaluated after surgical augmentation with different sized implants for both arthroplasty and arthrodesis. The biomechanical outcomes measured were range of motion, neutral zone, stiffness, articular pillar strains, facet forces and intervertebral foramen area. Increased disc distraction was found to increase lordosis of the spine, increase compressive strains in articular pillars and increase in intervertebral foramen area. The kinematics outcomes were surgery type and implant size dependent where fusion lead to decreased range of motion, while arthroplasty maintained the range of motion with differential outcomes based upon the size of the implant. The integration of these biomechanical data demonstrate an implant size /spacer height relationship with direct clinical importance and the ability to guide clinical decision making so as to reduce revision surgery due to deviant biomechanical function.Item Fate and impact of antibiotics in slow-rate biofiltration processes.(2010-12) Wunder, David BarnesAntibiotics have been detected in surface waters worldwide at concentrations up to 1.9 micrograms/L, but are typically detected at low nanogram/L concentrations. The potential health effects of exposure to low levels of these compounds via tap water are not known, but there is significant concern among water consumers regarding the occurrence of antibiotics and other pharmaceutical compounds in water supplies. Thus, a significant amount of research has been performed recently to investigate the removal of pharmaceuticals via conventional and advanced water treatment processes. While conventional treatment processes (i.e., coagulation, flocculation, sedimentation, and filtration) are generally not effective, oxidation processes (e.g., chlorination, ozonation) and granular activated carbon exhibit some effectiveness at removing pharmaceuticals. As expected, removals are highly dependent on compound structure. Furthermore, some oxidants, such as chloramines, are not effective at oxidizing pharmaceuticals. Slow-rate biofiltration processes (SRBF), such as slow sand filtration (SSF) and riverbank filtration (RBF), are drinking water treatmeant systems comprised of two stages in sequence: 1) a relatively shallow biotic region where media (i.e., filter sand or aquifer material) is colonized by biofilm bacteria, followed by 2) an deeper abiotic filtration zone. These processes are extensively used in Europe and developing global regions and are seeing increased usage in the United States. There is evidence in the literature that SRBFs can remove a wide variety of trace organic pollutants including: pesticides, disinfection byproducts, and some pharmaceuticals. Little is known regarding the ability of SRBF processes to remove antibiotics from water supplies nor has any work been done to investigate the potential adverse effects of antibiotics on the biofilm bacteria that are critical to SRBF system performance. Thus, this research was performed to determine the extent and mechanisms (i.e., sorption versus biodegradation) of antibiotic removal in SRBF processes and the effects of antibiotics on biofilm bacteria (i.e., activity and community composition). The effect of antibiotics on bacterial activity and community structure was investigated by growing biofilm in the presence and absence of a mixture of antibiotics in a continuous-flow rotating annular bioreactor (CFRAB) with acetate as substrate. Three representative compounds were selected for use in this research: sulfamethoxazole (SMX), erythromycin (ERY), and ciprofloxacin (CIP). These antibiotics were selected because they: 1) represent three prominent classes of antibiotics with differing mechanisms of action against bacteria, 2) have been detected in surface water, 3) exhibit different chemical characteristics, and 4) have differing levels of biodegradability. Areal acetate utilization rates for a constant feed of antibiotics were similar to the control experiments, and utilization rates did not change during an antibiotic shock loading experiment. Attached biomass levels were greater for experiments involving a high CIP concentration (3.33 micrograms/L), however, yielding comparatively lower steady-state biomass-normalized substrate utilization rates. Microbial community analyses via automated ribosomal intergenic spacer analysis (ARISA) revealed shifts in community structure for the high dose CIP experiments. A CFRAB was also used to investigate antibiotic sorption to bacterial biofilm. The extent of sorption, as indicated by the organic carbon partition coefficient (Koc), was 15 to 23 times greater for CIP compared to ERY and SMX. The Koc values did not correlate with experimentally-determined Kow values, suggesting that the sorption of relatively hydrophilic (i.e. Kow < 1.7) and charged antibiotics to typically negatively charged biofilm is driven by ionic interactions (i.e. ion exchange) rather than hydrophobic interactions. The attenuation and impact of antibiotics in SRBF systems was investigated by conducting bench-scale filter column experiments with mixtures of SMX, ERY, and CIP at high (3.33 micrograms/L, each) and low (0.33 microgram/L, each) antibiotic feed conditions. Consistent with the CFRAB experiments, antibiotic breakthrough times were greatest for CIP, with very little uptake of SMX or ERY. Biodegradation was not observed for any antibiotic during 6-weeks of filter column operation or in complementary batch experiments. A one-dimensional advection-dispersion equation (with linear sorption) model was validated against experimental results and used to compare antibiotic retardation in SSF, RBF, and rapid gravity biofiltration (RGBF) systems. Of the modeled systems, antibiotic retardation was greatest in RBF, with little antibiotic removal expected for SSF. Based on analysis of ARISA data, the community structure of bacterial biofilm was not affected in filters exposed to antibiotics at low concentrations (i.e. 0.33 microgram/L, each) similar to those found in surface waters, with a few species impacted under high concentration conditions (3.33 microgram/L, each). The results of this work will help those interested in understanding and predicting antibiotic fate in engineered and natural systems where biofilm is present. The results indicate that antibiotic removal in SRBF processes will be dictated by compound properties such as charge and hydrophobicity, and that limited removal of antibiotics in SRBF processes can be expected. Finally, the results suggest that that mixtures of antibiotics at concentrations typically observed in surface waters are unlikely to adversely affect SRBF biofilm bacteria or process performance.Item Gas-phase synthesis of gold- and silica-coated nanoparticles.(2011-01) Boies, Adam MeyerComposite nanoparticles consisting of separate core-shell materials are of interest for a variety of biomedical and industrial applications. By combining different materials at the nanoscale, particles can exhibit enhanced or multi-functional behavior such as plasmon resonance combined with superparamagnetism. Gas-phase nanoparticle synthesis processes are promising because they can continuously produce particles with high mass-yield rates. In this dissertation, new methods are investigated for producing gas-phase coatings of nanoparticles in an "assembly-line" fashion. Separate processes are developed to create coatings from silica and gold that can be used with a variety of core-particle chemistries. A photoinduced chemical vapor deposition (photo-CVD) method is used to produce silica coatings from tetraethyl orthosilicate (TEOS) on the surface of nanoparticles (diameter ~5-70 nm). Tandem differential mobility analysis (TDMA) of the process demonstrates that particle coatings can be produced with controllable thicknesses (~1-10 nm) by varying system parameters such as precursor flow rate. Electron microscopy and infrared spectroscopy confirm that the photo-CVD films uniformly coat the particles and that the coatings are silica. In order to describe the coating process a chemical mechanism is proposed that includes gas-phase, surface and photochemical reactions. A chemical kinetics model of the mechanism indicates that photo-CVD coating proceeds primarily through the photodecomposition of TEOS which removes ethyl groups, thus creating activated TEOS species. The activated TEOS then adsorbs onto the surface of the particle where a series of subsequent reactions remove the remaining ethyl groups to produce a silica film with an open site for further attachment. The model results show good agreement with the experimentally measured coating trends, where increased TEOS flow increases coating thickness and increased nitrogen flow decreases coating thickness. Gold decoration of nanoparticles is accomplished by evaporation of solid gold in the presence of an aerosol flow. A hot-wire generation method is developed where gold particles are produced from a composite gold-platinum wire. Investigations of the hot-wire generator show that it can produce particles with a range of sizes and that more uniform, non-agglomerated particles are produced when using smaller diameter tubes where gas velocities across the wire are higher and recirculation zones are diminished. When gold is evaporated in the presence of silica nanoparticles, the silica aerosol is decorated by gold through either homogeneous gold nucleation and subsequent scavenging by the silica nanoparticles, or by heterogeneous nucleation on the silica surface in which the gold "balls up" due to the high surface tension of gold on silica. In both cases the resulting particles exhibit a plasmon absorbance resonance typical of gold nanoparticles (λ~550 nm). Finally, the silica coating and gold decoration processes are combined with a thermal plasma technique for synthesizing iron-oxide to produce tri-layer nanoparticles.Item Kinetic and spectroscopic studies of cobalt- and manganese-substituted extradiol-cleaving homoprotocatechuate 2,3-dioxygenases(2013-02) Fielding, Andrew JayHomoprotocatechuate (HPCA) 2,3-dioxygenase (HPCD) is an Fe(II)-dependent extradiol-cleaving dioxygenase, which oxidatively cleaves the aromatic C(2)-C(3) bond of its catecholic substrate. Here we compare the reactivity of Fe-HPCD with its Mn(II)- and Co(II)-substituted analogues. While Mn-HPCD exhibits steady-state kinetic parameters comparable to those of Fe-HPCD, Co-HPCD exhibits significantly higher KMO2 and kcat values. The high activity of Co-HPCD is surprising, given that cobalt has the highest standard M(III/II) redox potential of the three metals. These kinetic differences and the spectroscopic properties of Co-HPCD have proven to be useful in further exploring the unique O2 activation mechanism associated with the extradiol dioxygenase family. Employing the electron-poor substrate analogue 4-nitrocatechol (4NC), which is expected to slow down the rate of catechol oxidation, we were able to trap and characterize the initial O2-adduct in the single-turnover reaction of 4-nitrocatechol by Co-HPCD. This intermediate exhibits an S = 1/2 EPR signal typical of low-spin Co(III)−superoxide complexes. Both the formation and decay of the low-spin Co(III)−superoxide intermediate are slow compared to the analogous steps for turnover of 4NC by native high-spin Fe(II)-HPCD, which is likely to remain high-spin upon O2 binding. Possible effects of the observed spin-state transition upon the rate of O2 binding and catechol oxidation are discussed. Two transient intermediates were detected in the reaction of the [M-HPCD(4XC)] enzyme-substrate complexes (M = Mn or Co, and 4XC = 4-halocatechols, where X = F, Cl, and Br) with O2. The first intermediate (Co4XlCInt1) exhibited an S = 1/2 EPR signal associated with an organic radical species. Based on the UV-Vis and EPR data, Co4XCInt1 was assigned to a unique low-spin [Co(III)(4XSQ*)(hydro)peroxo] species where the semiquinone radical is localized onto C4 of the ring. M4XCInt2 was observed to have a high-spin metal(II) center by EPR and exhibit intense chromophores similar to the independently synthesized halogenated quinones (4XQ). Based on the UV-Vis and EPR data, M4XCInt2 is assigned to a [M(II)(4XQ)(hydro)peroxo] species. The M4XCInt2 species were further characterized by resonance Raman spectroscopy. Resonance enhanced vibrations between 1350-1450 cm-1 suggest that M4XCInt2 is a metal-semiquinone species, conflicting with the initial assignment of these intermediates as a quinone species. Based on the EPR and resonance Raman data, M4XCInt2 might be assigned to a [M(II)(SQ*)O2*-] diradical species.Item The oxidation of zinc vapor and non-stoichiometric ceria by water and carbon dioxide to produce hydrogen and carbon monoxide.(2012-06) Venstrom, Luke J.Experimental studies of two pathways for solar thermochemical metal oxide cycles to split water and carbon dioxide are presented. The heterogeneous oxidation of Zn(g) is investigated in Part I, and the oxidation of porous ceria is investigated in Part II. The heterogeneous oxidation of Zn(g) is proposed as an improved approach for rapid and complete oxidation of Zn. Reaction rates are measured gravimetrically in a quartz tube flow reactor at atmospheric pressure for conditions in which Zn is the limiting reactant, at temperatures between 800 and 1150 K, and for Zn(g), H2O(g), and CO2 partial pressures between 10-5 and 0.25 atm. The rate of Zn(g) oxidation by CO2 is between 0.3×10-8 and 6.5×10-6 mol cm-2 s-1, permitting conversions of Zn to ZnO greater than 84% in one second. The rate of Zn(g) oxidation by H2O is between 0.8×10-7 and 1.5×10-5-2 s-1 permitting conversions greater than ~80% in one second. A finite volume based numerical model decouples mass transfer and surface kinetics from the reaction rate data. The CO2-splitting kinetics are second-order, proportional to the Zn(g) and CO2 concentrations. The kinetic parameter is expressed in Arrhenius form, and the activation energy and pre-exponential factor are 44±3 kJ mol-1 and 92±6 mol m-2 s-1 atm-2, respectively. When expressed in second-order form, the apparent activation energy and pre-exponential factor of H2O-splitting are -110 kJ mol-1 and 1.8×10-5 mol m-2 s-1 atm-2 between 800 and 1050 K. At 1100 K, the activation energy becomes positive. A precursor mechanism, where the apparent activation energy is the sum of the heat of adsorption of H2O and the activation energy of the rate-limiting kinetic step is postulated to explain this behavior. The benefit of completely converting Zn via the heterogeneous oxidation of Zn(g) is an increase in the Zn/ZnO cycle efficiency from ~6% for polydisperse aerosol reactors, which have been limited to Zn conversions of 20% for reaction times on the order of a minute, to 27% and 31% for H2O- and CO2-splitting, respectively. In Part II, the effect of material morphology on the reduction and oxidation of ceria is investigated. The oxidation by H2O and CO2 of three-dimensionally ordered macroporous ceria (3DOM CeO2), which features an interconnected, ordered pore network, solid feature sizes between 80 and 200 nm, and a moderate specific surface area of 10 m2 g-1, is compared to the oxidation of non-ordered mesoporous ceria and sintered, low porosity ceria at 1100 K in 6 isothermal chemical cycles. The 3DOM CeO2 increases the maximum H2 and CO production rates over the low porosity CeO2 by 125 and 260%, and increases the maximum H2 and CO production rates over the non-ordered mesoporous CeO2 by 75 and 175%. 3DOM CeO2, non-ordered macroporous ceria (NOM CeO2), and aggregates of ceria nanoparticles are also cyclically reduced at ~1500 K under pO2 = 10-5 atm and oxidized at ~1100 K by 25 mol% CO2. The 3DOM and NOM CeO2 retain an interconnected, disordered pore network and achieve maximum CO production rates of 6.4 and 4.0 mL min-1 g-1, respectively, an order of magnitude increase over the ~0.1 mL min-1 g-1 rate of CO production of the sintered ceria nanoparticles and low porosity ceria. The present study demonstrates the importance of engineering ceria with interconnected porosity and solid feature sizes on the order of hundreds of nm.Item Spectroscopic And Kinetic Investigation Of N-Oxygenation By Cmli, A Diiron-Cluster-Containing Oxygenase Involved In Antibiotic Biosynthesis(2017-04) Komor, AnnaDiiron-cluster-containing oxygenases catalyze a wide range of biological reactions. The chemical and biological breadth of the field is still being defined by discovery of new enzymes. The N-oxygenase CmlI, which plays a role in the biosynthesis of the broad-spectrum antibiotic chloramphenicol, is a new entry that brings two significant characteristics to this enzyme class. First, the active oxidant of CmlI is a diferric peroxo species (P), in contrast to the high-valent intermediates that often serve as oxidants in the cycles of nonheme iron oxygenases. P has unique spectroscopic features and a long lifetime in the absence of substrate (t1/2 ~ 3 h at 4 °C, pH 9), which facilitated the discovery of its novel η1- η1 or η1- η2 diferric peroxo geometry that distinguishes it from the common cis-μ-1,2 geometry and may account for its unique reactivity. Characterization of the structure and function of P has led to a new understanding of the role of peroxo species in oxygen activation and insertion chemistry. The second novel aspect of CmlI is the chemistry that it performs, the six-electron conversion of an aryl-amine precursor to the aryl-nitro group of the active antibiotic. Utilization of P in single turnover reactions allowed spectroscopic and kinetic characterization of each step of this conversion. The six-electron transformation begins when P converts the aryl-amine substrate into an aryl-hydroxylamine, which acts as a mid-cycle reductant to re-reduce and prime CmlI to regenerate P by reaction with O2 while itself being converted to the aryl-nitroso species. The regenerated P species then performs the last oxidation to convert the aryl-nitroso into the aryl-nitro group of chloramphenicol. Transient kinetic studies show that the substrate is likely to stay in a single active site during the entire, multistep reaction. Herein is described a new and highly efficient diiron-cluster-mediated N-oxygenase mechanism.Item Synthesis and reactivity of high-valent copper complexes and the design of copper monooxygenase model complexes(2022-02) Bouchey, CaitlinCopper plays a vital role in various enzymatic and catalytic transformations. Specifically, copper-oxygen and high-valent copper species are implicated as intermediates in oxidations by metalloenzymes and catalysts. In order to study the nature and the role of copper in these transformations, copper model complexes have been sought after and investigated for their properties and reactivities. This thesis describes several such copper model complexes. Chapter 1 outlines the biological precedence of copper-oxygen complexes in a monooxygenase enzyme and a class of copper complexes that mimic the monooxygenase active site. Additionally, the literature relevant to high-valent copper complexes discussed herein is reviewed. In chapter 2, the development of two biomimetic, monoanionic ligands and their copper complexes is discussed. The characterization of the ligands and complexes and efforts to access copper-oxygen complexes bearing the monoanionic ligands are shown. Chapter 3 details the generation of a new high-valent copper-nitrite complex and its oxidative proton-coupled electron transfer (PCET) and anaerobic phenol nitration reactivity. Mechanistic considerations for the unusual anaerobic phenol nitration are made. Lastly, chapter 4 describes the synthesis and characterization of two copper-amidate complexes and the generation of their high-valent counterparts. The PCET reactivity of the high-valent copper-amidate complexes are contrasted with each other and previous high-valent copper-oxygen complexes. The results from the projects described herein provide insights into copper coordination chemistry, electronic structure, and reactivity, which helps augment the knowledge of copper enzymes and catalysts.Item Thermal and chemical inactivation of ricin and Shiga toxins in orange juice.(2010-01) Wang, NaThe potential use of ricin and Shiga toxins (Stxs) as bioterror weapons in the food supply is a major concern for homeland security. Denaturation effects of thermal and chemical treatments are expected to reduce the toxicity of ricin and Shiga toxins in water solutions, but their effectiveness and stabilities in food matrices are largely unknown. The objective of this project was the identification of heat and chemical treatments capable of inactivating ricin and Shiga toxins in orange juice so that large quantities can be safely disposed in the event of an intentional attack. Diluted ricin was mixed with orange juice for inactivation studies. Thermal stability was determined in capillary tubes using a water bath at high temperatures typical of pasteurization. For chemical inactivation, sodium hypochlorite (NaOCl), sodium hydroxide (NaOH) and peracetic acid (PA) were added alone or in combination to samples with or without thermal treatment. The ricin concentration in samples was determined using an ELISA. The Arrhenius model was used to evaluate temperature dependence. Enterohemorrhagic Escherichia coli strains were used to produce Shiga toxin 1 (Stx1) and Shiga toxin 2 (Stx2). Shiga toxins were added into phosphate buffered saline (PBS) or orange juice to study the inactivation effects. The same inactivation method was also used for heat treatment of Stxs. The concentration of Stxs was determined by an ELISA and a cytotoxicity assay was conducted to confirm the inactivation. Kinetics studies were done to evaluate inactivation parameters. Heat inactivation of ricin followed first-order kinetics. The half-life (t1/2) of ricin at 72, 80, 85 and 90°C were 72.6, 9.0, 2.0 and 0.5 min, respectively. The Z value was 8.8°C indicating high temperature sensitivity. When the concentration of each chemical was increased to a sufficient amount, the detection limit of the ELISA kit was reached when measuring ricin inactivated within 5 s at room temperature. A significant synergism between NaOCl and NaOH and considerable efficacy with treatment with PA alone were observed. The heat inactivation of Stxs in PBS and orange juice also followed first-order reaction kinetics. Both Shiga toxins in PBS and orange juice would reach the concentration that was not detectable with ELISA within 30 s at 90°C and 120 s at 85°C. The Z values for Stx1 and Stx2 were 6.7 and 7.2°C in PBS as well as 8.7 and 6.9°C in orange juice, respectively. This study delivered the first series of time/temperature/concentration conditions that would serve as the basis for recommendations for treating orange juice subjected to intentional adulteration with ricin or Shiga toxins in an orange juice plant with typical pasteurization equipment so it can be safely disposed into the environment.