Chemistry & Biochemistry Theses and Dissertations
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- ItemCorrelating Chemical Activity and Structure in Mesoporous Metal Oxides for Nerve Agent Decomposition(2022) Li, Tianyu; Rodriguez,, Efrain E; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)GB (sarin), a chemical ware fare agent (CWA), due to its extreme fatal toxicity and its involvement in a few terrorist and battle attacks, has become an increasing concern for the national public and military safety. Developing filter materials that can strongly adsorb and effectively decompose GB thus attracts growing research interest. The great diversity of metaloxides and their abundant surface chemistry suggest an opportunity to realize their potential as filter materials. This dissertation outlines our effort to gain a fundamental understanding of the interaction between GB (also its simulant DMMP) and metal oxides. We aim to determine the structural factors that influence the performance of metal oxides on adsorbing and decomposing GB and to ultimately predict the behavior of a given metal oxide. We used two mesoporous metal oxides (TiO2 and CeO2) as two model systems and performed systematic studies on their interaction with GB and its simulant DMMP. We utilized multiple techniques to fully characterize the crystal and surface characters of the mesoporous metal oxides. The interactions between GB/DMMP and metal oxides were explored by different spectroscopic techniques (majorly infrared techniques). Combining the experimental observations and DFT calculations on two different metal oxides, we propose several governing parameters of the metal oxides to impact their reactivity for decomposing GB. We also derive a simplified and qualitative model to predict the reaction behavior and activity of metal oxides when interacting with GB.
- ItemSolution-Processed Clean SWCNTs and Their Use as Templates for One-Dimensional van der Waals Heterostructures(2022) Zhang, Chiyu; Wang, YuHuang; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Single-walled carbon nanotubes (SWCNTs) have shown exceptional electrical, optical, mechanical, and thermal properties. Solution processing is a critical first step to harness these nanomaterials for applications in electronics, biomedicine, and energy technologies. However, dispersion of SWCNTs in solutions requires assistance by surfactants or polymers, which cannot be cleanly removed easily and become unwanted contaminants, resulting in degraded performance of SWCNTs.In this dissertation, I developed strategies to attain clean, solution-processed SWCNTs and further demonstrated their applications as templates for the synthesis of van der Waals heterostructures. We investigated the role of surfactants in dispersing SWCNTs and found that the highest efficiency in dispersing SWCNTs occurs at the critical micelle concentration of surfactants, which is well below the typically required surfactant concentrations. Furthermore, we synthesized a thermally removable surfactant, ammonium deoxycholate (ADC) which can be removed cleanly at a relatively low temperature without damaging the SWCNT structure. Compared to a commonly used surfactant, sodium deoxycholate (DOC), ADC features the same anion, but contains an ammonium (NH4+) cation in place of the metal ion (Na+). ADC exhibits the same high dispersion efficiency for SWCNTs as DOC, but the peak thermal decomposition temperature of ADC is nearly 70 oC lower than that of DOC. A two-step annealing process can remove this new nanotube surfactant while keeping the SWCNTs intact, even with a small diameter of just 0.76 nm. This work also reveals the chemical origin of residues from thermal annealing of surfactant-processed carbon nanomaterials. The clean SWCNTs enable the synthesis of van der Waals heterostructure consisting of pure chiral single-wall carbon nanotubes nested in boron nitride (SWCNT@BN). Transmission electron microscopy and electron energy-loss spectroscopic mapping confirm the successful synthesis of SWCNT@BN from the solution-purified nanotubes. The photoluminescence peak of (7,5)-SWCNT@BN heterostructure is found to redshift by 10 nm relative to that of (7,5)-SWCNT and the Raman G peak of (7,5)-SWCNTs downshift by 10 cm-1 after BN coating.
- ItemDefect Engineering of Supported Metal Catalysts for Selective Hydrogenation(2022) Zhang, Yuan; Liu, Dongxia; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Supported metal catalysts have been used extensively in industry. To construct supported metal catalysts with low cost and high catalytic performance, high dispersion of metal on the support material is greatly favored in recent years. With the downsizing of metal active phase, new challenges in catalyst synthesis and characterization have emerged. The highly dispersed metal active phase is prone to aggregate to decrease surface free energy, which requires innovative synthesis strategy to stabilize the metal species on support. High metal dispersion also created more interfacial sites and bonds between metal and support, therefore the metal-support interaction has more significant effects on the catalytic properties of high dispersion catalysts. Defect engineering has attracted much attention due to its ability to help stabilizing metal species and tune the metal-support interaction.This dissertation focuses on utilizing defect engineering to develop catalysts with high activity and selectivity in hydrogenation reaction. Harsh pH condition was applied in wetness impregnation process to generate cavity sites on TiO2 support surface, which resulted in stronger metal-support interaction between Pt and TiO2. The catalyst synthesized under harsh condition showed higher hydrogenation activity towards -NO2 group. Laser engraving was used as another defect engineering technique to create defects on TiO2 support. The laser engraved support showed distinct electronic and redox properties, which enhanced the electronic metal-support interaction of Pt and TiO2 support. The Pt/TiO2-LE catalyst showed superior activity and selectivity in the hydrogenation of 3-nitrostyrene and furfural alcohol. In addition, an effective method to probe the metal dispersion of Pt by styrene hydrogenation reaction kinetics was developed. This method has the potential to be applied to other catalysts systems and could be used to study the metal-support interaction in catalysts.
- ItemTuning Crystallographic and Magnetic Symmetry in Lithium Transition Metal Phosphates and Thiophosphates(2022) Diethrich, Timothy; Rodriguez, Efrain E.; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Ferroic ordering needs no introduction; ferromagnetic, ferroelectric, and ferroelastic materials have had a significant impact on the materials science community for many years. While these three main types of ferroic ordering are well known, there is a fourth and final, lesser known ferroic ordering known as ferrotoroidicity. A ferrotoroidic material undergoes a spontaneous, physical alignment of toroidal moments under a critical temperature. This study is focused on broadening our current understanding of ferrotoroidics by studying two families of materials: LiMPO4, and Li2MP2S6, where M = Fe, Mn, and Co. While these two materials initially appear to be similar in some regards, many differences can be observed as a deeper dive is taken into their crystallography and magnetic structures. For a toroidal moment to exist, a specific orientation of magnetic moments is required, because of this, only certain magnetic point groups are allowed. For example, LiFePO4 has an “allowed” magnetic point group of m’mm, while it’s delithiated counterpart FePO4 has a “forbidden” magnetic point group of 222. This work has found that by using a new selective oxidation technique, lithium concentration can be controlled in the Li1−xFexMn1−xPO4 solid solution series. Neutron powder diffraction and representational analysis were used to find the magnetic point groups of each member of this series. In the end, each structure was solved and the largest transition temperature to date was reported for a potential ferrotoroidic material. The magnetic exchange interactions can be used to describe the magnetic phase changes that occuracross the Li1−xFexMn1−xPO4 series. The second group of materials in this study is the lithium transition metal thiophosphates of the formula Li2MP2S6, where M = Fe, Co. The structure of Li2FeP2S6 has been previously studied but no magnetic properties of this material have been reported. In addition, neither the structural nor magnetic properties have been reported for the cobalt analog. Single crystalXRD was used to confirm the previously reported crystal structure of Li2FeP2S6 and to find the novel crystal structure of Li2CoP2S6; both crystallize in a trigonal P31m space group. While isostructural in some regard, there are some crucial differences between these materials. The site occupancies are different, resulting in non-trivial charge balances and a unique thiophosphate distortion. Originally, these materials were chosen because their nuclear structure was predicted to host long-range antiferromagnetic order and potentially ferrotoroidic order. Contrary to expectations, magnetic susceptibility and field dependent measurements demonstrated paramagnetic behavior for both the iron and the cobalt sample down to 2 K. This result was further confirmed by a lack of magnetic reflections in the time-of-flight neutron powder diffraction data. While the phosphates and the thiophosphates demonstrated very different structural andmagnetic results, they both remain relevant materials for not only ferrotoroidics, but also magnetoelectrics, spintronics, quantum materials, and much more.
- ItemNon Traditional Solvent Effect On Protein Behavior(2022) Lee, Pei-Yin; Matysiak, Silvina; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Protein preservation has been a long lasting research topic due to its importance in many bio-pharmaceutical applications. A ”cold chain” is a commonplace solution to protein preservation, which stores biochemical products at a refrigerated temperature. A big advantage of cold chain is that the storing process is straightforward, without many further processes before the use of stored bio-products. However, it can also experience malfunction of the cooling system and results in economic lost and health care crisis. Ionic liquids (ILs), as a type of non traditional solvents, consist only of ions and are reported to be a potential candidate to replace the use of cold chain. The advantages of ILs include low flammability, high conductivity and less toxicity compared to some organic solvents. The most interesting feature of ILs is their extremely large number of cation-anion combinations, that can be tailored for specific use according to different needs. This thesis aims to investigate specific mechanism behind how ILs modulate protein behavior, specifically, how ILs affect protein stability, activity, and aggregation. We approach the research questions through the lens of molecular dynamics (MD) simulations and complement with experimental findings. In the first part of the thesis we first investigate the effects of two imidazolium based ILs (1-ethyl-3-methylimidazolium ethylsulfate, [EMIM]+[EtSO4]− and 1-ethyl-3-methylimidazolium diethylphosphate, [EMIM]+[Et2PO4]−) on lysozyme stability and activity. We collaborate with an experiment group at the University of Massachusetts (Bermudez lab) to complement our simulation results. Both ILs are found to destabilize lysozyme stability. In addition, both the cation and anions lower the stability of lysozyme, but in a different fashion. [EMIM]+ interacts with an Arg-Trp-Arg bridge that is critical in lysozyme stability through π–π and cation–π interactions, leading to a local induced destabilization. On the other hand, both anions interact with the whole protein surface through short-range electrostatic interactions, with [Et2PO4]− having a stronger effect than [EtSO4]−. Lysozyme activity is also reduced by the presence of the two ILs, but can be recovered after rehydration. It is found that the protein-ligand complex is less stable in the presence of ILs. In addition, a dense cloud of [EMIM]+ is found in the vicinity of the lysozyme active site residues, possibly leading to a competition with the sugar ligand. A fast leaving of these [EMIM]+ is observed after rehydration, which explains the reappearance of the active site and the recover of lysozyme activity. Although classical all-atom MD simulations can provide us with a great deal of microscopic information, they are often limited by the temporal-spatial scale of the simulated systems. For example, systems with high viscosity solvents or systems involving large number of atoms will be difficult to reach convergence for all-atom MD. In this case, coarse grained (CG) MD can come into play to achieve the desired time- and length- scales. The faster sampling obtained from CG MD is achieved by reducing the degree of freedom of the system and by removing local energetic barriers. In CG MD, similar atoms are grouped to functional groups and thus the free energy landscape is smoothen. We develop a novel CG MD named ”Protein Model with Polarizability and Transferability (ProMPT)”. The novelty of this model is the inclusion of the charged dummies that can result in change of dipoles. These dipoles can reflect the change of environments and thus allow the model to respond to different environmental stimulus. We validate ProMPT with several benchmark proteins: Trp-cage, Trpzip4, villin, ww-domain, and β-α-β. ProMPT is able to simulate folding-unfolding and secondary structure transformation with minimal constraints, which is not feasible with previous CG models. In addition, ProMPT can also reproduce the experimental results for the dimerization of glycophorin A (GpA) with different point mutations. Here we demonstrate the ability of the model to capture the change of conformational space caused by point mutation. In the last part of this thesis, we combine ProMPT and an in-house CG IL model to study the effects of [TEA]+[Ms]− on amyloid beta 16-22 (Aβ16−22) aggregation. Aβ16−22 is the hydrophobic core region and is the smallest fragment of Aβ that can fibrilize. Aβ has been extensively linked to the pathogenesis of the Alzheimer’s disease. [TEA]+[Ms]− is reported to suppress the formation of β-sheets and induce helices at high concentration. From our results, both β-sheet content and the aggregate size decrease with the increase of IL concentration, which are in agreement with experiments. Aggregates can form in both water and IL, but with different morphologies. In water, a nice hydrophobic core involving Phe-Phe interactions can form as well as intact β-sheet contacts. In addition, a cross β-sandwich structure is also observed, as seen from previous literature. However, the same hydrophobic core can not persist in the presence of IL. Aggregate structures in IL are not stable over time due to the [TEA]+-Phe interaction. Helicity is also computed for Aβ16−22 in water and in IL at different concentrations and a positive correlation is found. The increase in helicity at high [TEA]+[Ms]− concentration can be explained by the reduction of the inter-peptide contacts, which then increases the opportunity for the peptides to form helical structures. Single peptide studies also reveal that [TEA]+[Ms]− increases the helicity, possibly through cation-induced dipole enhancement. In this thesis, a series of detailed investigations on the effects of ILs on protein behavior is performed. Specific interactions between IL functional groups and protein local/global structures are examined. The mechanisms we studied here will help constructing a holistic view for the design of IL-protein pair applications. The construction of the new CG protein/IL model provides another tool for the scientific community to study secondary structure transformation, folding- unfolding, and other biochemical processes that are sensitive to the environment with CG MD.