Thesis advisor: Shih-Yuan Liu / The first three chapters of this dissertation elaborate on certain facets of the isosteric relationship between different types of boron-nitrogen-containing heterocycles and the corresponding all-carbon compounds. In this vein, Chapter 1 describes selective photoisomerization of aromatic 1,2-dihydro-1,2-azaborines to BN-analogues of bicyclo[2.2.0]hexa-2,5-diene (Dewar benzene). In one instance, the photoisomer product was further derivatized into a series of disubstituted cyclobutanes through manipulations of the boron functionality. Chapter 2 discloses a combined experimental/theoretical mechanistic investigation of preliminary hydrogen release from the amine borane unit in a pair of BN-cycloalkanes. In Chapter 3, the kinetics of complementary dehydrogenation of the alkyl units in a BN-cyclohexene derivative are compared with those of related six-membered carbocycles. Chapter 4 treats with the separate subject of enantioselective silylation of glycerol by a catalytic strategy centered around reversible covalent binding of substrate hydroxyl groups. / Thesis (PhD) — Boston College, 2018. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

As societies depart from current economic models which are built around affordable and easily accessible fossil fuels to energy systems increasingly based on the use of renewable energies, the need grows for a wide-scale clean and sustainable energy vector. Hydrogen fulfils most of the needed equirements, but implementation and large scale penetration, especially for mobile applications, is precluded by technical issues. Among these, arguably the most complex is how to safely, economically and efficiently store hydrogen. Storage in a porous material offers some attractive features, which include fast kinetics, reversibility and moderate energy penalties. A new methodology to analyse hydrogen adsorption isotherms in microporous materials is presented in this thesis. The methodology is applied to hydrogen adsorption in different classes of high-surface area materials but could in principle be used for any supercritical fluid adsorbed onto a microporous material. To illustrate the application of the methodology, high-pressure hydrogen adsorption isotherms of four different materials were analysed, metal-organic frameworks MIL-101 and NOTT-101 and carbons AX-21 and TE7. The analysis extracts important information on the adsorptive capacities of the materials and compares them with conventional storage methods, which include compression, liquefaction and cryogenic compression. The methodology also aids in the calculation of the thermodynamics of adsorption, providing a more accurate calculation method than currently reported techniques, demonstrated with the calculation of the differential isosteric enthalpies for metal-organic framework NOTT-101. NMR and INS are used in a novel way at the same operating conditions of sorption experiments to validate the findings of the analysis. Both methods provide a qualitative validation for the analysis. Remarkably, the INS reveals that the adsorbed hydrogen in TE7 is in a solid-like state. GCMC simulations were also used to compare with the application and findings of the methodology, using silicalite-1 as a test material.

The present work reports a study of the effects of the formation of a nanostructure induced by high-energy ball milling, compositions, and various catalytic additives on the hydrogen storage properties of LiNH2-LiH and LiNH2-MgH2 systems. The mixtures are systematically investigated using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and a Sieverts-type apparatus. The results indicate that microstructural refinement (particle and grain size) induced by ball milling affects the hydrogen storage properties of LiNH2-LiH and LiNH2-MgH2 systems. Moreover, the molar ratios of the starting constituents can also affect the dehydrogenation/hydrogenation properties. In the LiNH2-LiH system, high-energy ball milling is applied to the mixtures of LiNH2 and LiH with molar ratios of 1:1, 1:1.2 and 1:1.4 LiH. The lowest apparent activation energy is observed for the mixture of LiNH2-LiH (1:1.2) milled for 25 h. The major impediment in the LiNH2-LiH system is the hydrolysis and oxidation of LiH, which causes a fraction of LiH to be inactive in the intermediate reaction of NH3+LiH→LiNH2+H2. Therefore, the LiNH2-LiH system always releases NH3, as long as a part of LiH becomes inactive, due to hydrolysis/oxidation, and does not take part in the intermediate reaction. To prevent LiH from undergoing hydrolysis/oxidation during desorption/absorption, 5 wt. % graphite is incorporated in the (LiNH2+1.2LiH) system. The DSC curve of the mixture does not show a melting peak of retained LiNH2, indicating that graphite can prevent or at least substantially reduce the oxidation/hydrolysis of LiH. Moreover, compared to the mixture without graphite, the mixture with graphite shows more hydrogen capacity, thus this mixture desorbs ~5 wt.% H2, which is close to the theoretical capacity. This system is fully reversible in the following reaction: LiNH2+LiH→Li2NH+H2. However, the equilibrium temperature at the atmospheric pressure of hydrogen (0.1 MPa H2) is 256.8°C for (LiNH2+1.2LiH) mixture, which is too high for use in onboard applications. To overcome the thermodynamic barrier associated with the LiNH2/LiH system, LiH is substituted by MgH2; therefore, the (LiNH2+nMgH2) (n=0.55, 0.6 and 0.7) system is investigated first. These mixtures are partially converted to Mg(NH2)2 and LiH by the metathesis reaction upon ball milling. In this system, hydrogen is desorbed in a two-step reaction: [0.5xMg(NH2)2+xLiH]+[(1-x)LiNH2+(0.5-0.5x)MgH2]→0.5Li2Mg(NH)2+1.0H2 and 0.5Li2Mg(NH)2+MgH2→0.5Mg3N2+LiH+H2. Moreover, this system is fully reversible in the following reaction: Li2Mg(NH)2+2H2→ Mg(NH2)2+2LiH. Step-wise desorption tests show that the enthalpy and entropy change of the first reaction is -46.7 kJ/molH2 and 136.1 J/(molK), respectively. The equilibrium temperature at 0.1 bar H2 is 70.1°C, which indicates that this system has excellent potential for onboard applications. The lowest apparent activation energy of 71.7 kJ/mol is observed for the molar ratio of 1:0.7MgH2 milled for 25 h. This energy further decreases to 65.0 kJ/mol when 5 wt.% of n-Ni is incorporated in the system. Furthermore, the molar ratio of MgH2/LiNH2 is increased to 1.0 and 1.5 to increase the limited hydrogen storage capacity of the (LiNH2+0.7MgH2) mixture. It has been reported that the composition changes can enhance the hydrogen storage capacity by changing the dehydrogenation/hydrogenation reaction pathways. However, theoretically predicted LiMgN is not observed, even after dehydrogenation at 400°C. Instead of this phase, Li2Mg(NH)2 and Mg3N2 are obtained by dehydrogenation at low and high temperatures, respectively, regardless of the milling mode and the molar ratio of MgH2/LiNH2. The only finding is that the molar ratio of MgH2/LiNH2 can significantly affect mechano-chemical reactions during ball milling, which results in different reaction pathways of hydrogen desorption in subsequent heating processes; however, the reaction’s product is the same regardless of the milling mode, the milling duration and their composition. Therefore, the (LiNH2+0.7MgH2) mixture has the greatest potential for onboard applications among Li-Mg-N-H systems due to its high reversible capacity and good kinetic properties.

This thesis investigated a potential improvement to hydrogen storage for fuel cells using a thermally efficient hydrogen storage method. The efficiency of the storage system was improved using a metal hydride system to act as a thermal control unit for an exothermic chemical hydrogen storage system. A cylindrical shaped “hybrid” reactor was created to allow hydrogen production from a sodium borohydride packed bed reactor and the metal hydride. Additionally, a custom built pressure-composition-temperature apparatus was built to record the amount of hydrogen desorption from the metal hydride while isolating the metal from potential poisons such as oxygen. Before using the chemical hydride packed bed, heat transfer through the reactor was studied using circulating water. The water experiments showed that an increase in heat flux to the reactor led to a faster desorption rate of hydrogen from the metal hydride resulting in a larger temperature drop throughout the reactor. After the operating characteristics of the hybrid reactor were studied, a 10 wt% solution of sodium borohydride was created and pumped through the packed bed to produce enough hydrogen for a 300 W fuel cell. The amount of heat produced from the packed bed portion of the reactor was significant, but temperatures levelled to around 80 °C. As expected, temperature control was directly proportional to the rate of hydrogen release from the metal hydride. On average, approximately 10% of the available heat energy was transferred to the metal hydride, and the hybrid reactor operated with gravimetric and volumetric energy densities of 0.27 kWh·kg-1 and 1.29 kWh·L-1 respectively. If the hybrid reactor is used solely to control peak temperatures, the amount of metal hydride necessary for thermal control could be decreased. Additionally, improvements in heat transfer as well as the hydrogen storage materials themselves would increase the energy density values further. When compared to other energy storage devices, the hybrid reactor without improvements is competitive as a backup power generator due to its silent operation and large volumetric energy density. Since the hybrid reactor can provide quiet and cool energy storage in a relatively small volume, it may become an effective and efficient means for hydrogen storage with limited improvements. / Thesis (Master, Mining Engineering) -- Queen's University, 2007-12-06 14:45:56.551 / AUTO21

No description available.

Current physical supports used in cryogenic storage vessels, in which liquid hydrogen is stored, conduct heat from the environment to the liquid hydrogen which causes the hydrogen temperature to rise and ultimately leads to hydrogen losses due to boil-off. The focus of this study is to investigate magnetic levitation as a possible suspension mechanism, eliminating the use of current physical supports and so doing reducing hydrogen losses due to boil-off. A conceptual design of a container which makes use of magnetic suspension is presented in this study. The concept is validated on the basis of the forces obtainable between a paramagnetic aluminium plate and an electromagnet, as well as the forces obtainable between a neodymium magnet and a bulk Yttrium-Barium-Copper-Oxide superconductor. The forces between the paramagnetic aluminium plate and electromagnet were determined mathematically and tested experimentally. The forces between the magnet and superconductor were determined mathematically and by finite element modelling and simulations using ANSYS Multiphysics. The results obtained in the mathematical- and finite element studies were then validated experimentally. It was found that the forces obtained experimentally between the aluminium plate and electromagnets are inadequate for magnetic suspension of the inner vessel given in the conceptual design. It was also found that the forces obtained experimentally and in the simulation studies for the magnet and superconductor of this study were inadequate due to shortcomings in the magnet and superconductor obtained for experimental tests. The conclusion of this study is that electromagnetic levitation should not be used as a magnetic suspension mechanism for storage of liquid hydrogen. It is also concluded that superconducting levitation can not be used as a suspension mechanism for the concept presented in this study, unless the methods suggested to increase the levitation forces between the neodymium magnet and superconductor are executed. / Thesis (MIng (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2013

Current physical supports used in cryogenic storage vessels, in which liquid hydrogen is stored, conduct heat from the environment to the liquid hydrogen which causes the hydrogen temperature to rise and ultimately leads to hydrogen losses due to boil-off. The focus of this study is to investigate magnetic levitation as a possible suspension mechanism, eliminating the use of current physical supports and so doing reducing hydrogen losses due to boil-off. A conceptual design of a container which makes use of magnetic suspension is presented in this study. The concept is validated on the basis of the forces obtainable between a paramagnetic aluminium plate and an electromagnet, as well as the forces obtainable between a neodymium magnet and a bulk Yttrium-Barium-Copper-Oxide superconductor. The forces between the paramagnetic aluminium plate and electromagnet were determined mathematically and tested experimentally. The forces between the magnet and superconductor were determined mathematically and by finite element modelling and simulations using ANSYS Multiphysics. The results obtained in the mathematical- and finite element studies were then validated experimentally. It was found that the forces obtained experimentally between the aluminium plate and electromagnets are inadequate for magnetic suspension of the inner vessel given in the conceptual design. It was also found that the forces obtained experimentally and in the simulation studies for the magnet and superconductor of this study were inadequate due to shortcomings in the magnet and superconductor obtained for experimental tests. The conclusion of this study is that electromagnetic levitation should not be used as a magnetic suspension mechanism for storage of liquid hydrogen. It is also concluded that superconducting levitation can not be used as a suspension mechanism for the concept presented in this study, unless the methods suggested to increase the levitation forces between the neodymium magnet and superconductor are executed. / Thesis (MIng (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2013

The present work reports a study of the effects of the formation of a nanostructure induced by high-energy ball milling, compositions, and various catalytic additives on the hydrogen storage properties of LiNH2-LiH and LiNH2-MgH2 systems. The mixtures are systematically investigated using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and a Sieverts-type apparatus. The results indicate that microstructural refinement (particle and grain size) induced by ball milling affects the hydrogen storage properties of LiNH2-LiH and LiNH2-MgH2 systems. Moreover, the molar ratios of the starting constituents can also affect the dehydrogenation/hydrogenation properties. In the LiNH2-LiH system, high-energy ball milling is applied to the mixtures of LiNH2 and LiH with molar ratios of 1:1, 1:1.2 and 1:1.4 LiH. The lowest apparent activation energy is observed for the mixture of LiNH2-LiH (1:1.2) milled for 25 h. The major impediment in the LiNH2-LiH system is the hydrolysis and oxidation of LiH, which causes a fraction of LiH to be inactive in the intermediate reaction of NH3+LiH→LiNH2+H2. Therefore, the LiNH2-LiH system always releases NH3, as long as a part of LiH becomes inactive, due to hydrolysis/oxidation, and does not take part in the intermediate reaction. To prevent LiH from undergoing hydrolysis/oxidation during desorption/absorption, 5 wt. % graphite is incorporated in the (LiNH2+1.2LiH) system. The DSC curve of the mixture does not show a melting peak of retained LiNH2, indicating that graphite can prevent or at least substantially reduce the oxidation/hydrolysis of LiH. Moreover, compared to the mixture without graphite, the mixture with graphite shows more hydrogen capacity, thus this mixture desorbs ~5 wt.% H2, which is close to the theoretical capacity. This system is fully reversible in the following reaction: LiNH2+LiH→Li2NH+H2. However, the equilibrium temperature at the atmospheric pressure of hydrogen (0.1 MPa H2) is 256.8°C for (LiNH2+1.2LiH) mixture, which is too high for use in onboard applications. To overcome the thermodynamic barrier associated with the LiNH2/LiH system, LiH is substituted by MgH2; therefore, the (LiNH2+nMgH2) (n=0.55, 0.6 and 0.7) system is investigated first. These mixtures are partially converted to Mg(NH2)2 and LiH by the metathesis reaction upon ball milling. In this system, hydrogen is desorbed in a two-step reaction: [0.5xMg(NH2)2+xLiH]+[(1-x)LiNH2+(0.5-0.5x)MgH2]→0.5Li2Mg(NH)2+1.0H2 and 0.5Li2Mg(NH)2+MgH2→0.5Mg3N2+LiH+H2. Moreover, this system is fully reversible in the following reaction: Li2Mg(NH)2+2H2→ Mg(NH2)2+2LiH. Step-wise desorption tests show that the enthalpy and entropy change of the first reaction is -46.7 kJ/molH2 and 136.1 J/(molK), respectively. The equilibrium temperature at 0.1 bar H2 is 70.1°C, which indicates that this system has excellent potential for onboard applications. The lowest apparent activation energy of 71.7 kJ/mol is observed for the molar ratio of 1:0.7MgH2 milled for 25 h. This energy further decreases to 65.0 kJ/mol when 5 wt.% of n-Ni is incorporated in the system. Furthermore, the molar ratio of MgH2/LiNH2 is increased to 1.0 and 1.5 to increase the limited hydrogen storage capacity of the (LiNH2+0.7MgH2) mixture. It has been reported that the composition changes can enhance the hydrogen storage capacity by changing the dehydrogenation/hydrogenation reaction pathways. However, theoretically predicted LiMgN is not observed, even after dehydrogenation at 400°C. Instead of this phase, Li2Mg(NH)2 and Mg3N2 are obtained by dehydrogenation at low and high temperatures, respectively, regardless of the milling mode and the molar ratio of MgH2/LiNH2. The only finding is that the molar ratio of MgH2/LiNH2 can significantly affect mechano-chemical reactions during ball milling, which results in different reaction pathways of hydrogen desorption in subsequent heating processes; however, the reaction’s product is the same regardless of the milling mode, the milling duration and their composition. Therefore, the (LiNH2+0.7MgH2) mixture has the greatest potential for onboard applications among Li-Mg-N-H systems due to its high reversible capacity and good kinetic properties.

Thesis (Ph. D.)--Physics, Georgia Institute of Technology, 2008. / Committee Chair: Mei-Yin Chou; Committee Member: Erbil, Ahmet; Committee Member: First, Phillip; Committee Member: Landman, Uzi; Committee Member: Wang, Xiao-Qian.

Hydrogen has long been touted as an alternative fuel which could form the basis of a sustainable energy system: the hydrogen economy. This thesis advances the application of light metal amide materials in the realisation of this transformative potential. One of the most vexing technical challenges to the widespread adoption of hydrogen in transportation applications is its low volumetric energy density, which makes the storage of a sufficient amount of hydrogen in a vehicle very difficult. In their conventional application, light metal amides (<b>M(NH₂)_x</b>),where M is a Group I or II metal) have been promoted as a means of storing large quantities hydrogen in the solid state, significantly increasing this energy density. This thesis highlights the impressive characteristics of amide-based materials, primarily the facile nature of the reversibility of the hydrogen storage reaction, as a model for the development and optimisation of solid-state hydrogen stores. The study of the relationship between the crystal structures of the relevant materials and their hydrogen storage properties through in situ X-ray and neutron powder diffraction measurements is reported for the lithium amide - lithium hydride (Li-N-H) hydrogen store. These investigations provide strong evidence for ionic mobility as the basis of reversible hydrogen storage in the Li-N-H system. The hydrogen storage and release reactions are seen to progress through a continuum of non-stoichiometric states, a transformation which is facilitated by its topotactic nature. The structural and energetic properties of these non-stoichiometric phases are reported, showing that they are intrinsically disordered and thermodynamically unstable relative to their parent structures. The study of the behaviour of the Li-N-H system is extended to many tens of hydrogenation-dehydrogenation cycles to examine practical performance, confirming the mechanism of capacity loss through the formation of parasitic lithium hydride, and showing that the addition of nitrogen improves the cycling lifetime of the system. An unexplored aspect of light metal amide chemistry is also presented, where the hydrogen storage and release reactions of sodium amide are performed simultaneously. Together, these reactions effect the chemical decomposition of ammonia. Ammonia is a high energy density liquid hydrogen carrier which has been largely overlooked, partly due to the difficulty extracting its stored hydrogen. This work demonstrates a new method of ammonia decomposition which gives comparable performance to the expensive rare-metal catalysts which are currently used for the productions of high-purity hydrogen. A survey of the ammonia decomposition efficiency of a number of light metal amides and imides is presented, showing that it is not only amides which decompose into their constituent elements (such as sodium amide) which are active in ammonia decomposition, but also imide-forming amides. Indeed, imides and imide-forming amides are shown to be advantageous from the perspective of containing the catalyst material. Neutron diffraction and isotope exchange measurements provide some initial insights into the mechanism of reaction, identifying clear avenues for development of these systems, and inviting further discussion of the potential of ammonia as a sustainable energy vector.