| Abstract |
Gas storage and separation processes are closely linked to various aspects of industrial and social activity and evolution, such as environmental protection, industrial processes and manufacturing as well as energy production/consumption. Climate change that has been unequivocally observed is considered to occur mainly due to the continuously increasing emission of harmful gas by-products of fossil fuels combustion into the atmosphere. CO2 represents almost 75 % of the of the so called green-house gases. Thus, controlling and subsequently reducing its atmospheric concentration level, constitutes a vital environmental priority globally. Therefore, selective capture of CO2 from its emission points before it is released into the atmosphere, and its safe storage represent the most pressing and immediate course of action. The most common CO2 capture technology is the use of amine solutions. CO2 reacts with the amino group creating a chemical bond in a process showing 98% yield, nevertheless the process has a major drawback related to the high energy cost of regeneration. Moreover, due to the finite, constantly decreasing reserves of fossil fuels, strong emphasis has been given to the use of alternative energy sources that can be as efficient as conventional fuels and at the same time more environmentally friendly. Hydrogen is considered to be the ideal fuel of the future as it is abundant on the planet and its combustion has zero carbon footprint. However, existing technologies are still far from widespread use, due to the significant technical difficulties related to its whole supply-use chain. Natural gas, consisting mainly of methane (CO2 can be as high as 30%), is considered as an intermediate solution, since it carries sufficient energy density per unit mass and has the highest hydrogen-to-carbon ratio of all hydrocarbons, addressing some major environmental concerns. Nonetheless, similarly to hydrogen, there are still serious technological obstacles, the most important of which is its safe storage and efficient upgrade/purification. During the last decades, the use of solid adsorbents, which can retain significant amounts of gases on their surface through physical adsorption, has attracted great interest. Physical adsorption occurs due to weak van der Waals-type electrostatic forces between the surface of the solid and the gas, which leads to a fast, reversible phenomenon that is usually not connected with a significant energy penalty for regeneration and reuse of the adsorbent. Therefore, the main objective for an efficient adsorption-based process is to maximize the gas-solid interaction. Materials possessing extensive nanoporous networks exhibit high specific surface areas (and thus extended gas-solid interfaces) and total pore volume values. Furthermore, it has been shown that in nanosized pores, i.e., comparable to the molecular diameter of the adsorbate gases, increased interactions are observed due to the overlap of potential energies of the neighboring atoms. Various materials have been extensively investigated as solid adsorbents for gas storage and separation processes, such as zeolites, activated carbons, etc. Metal-organic frameworks (MOFs), a particular group of crystalline nanoporous materials have attracted significant interest due to their remarkable properties and intrinsic structural features. They are usually produced through one-step reactions, from the assembly of organic ligands with metal ions or metal clusters, resulting to the creation of three-dimensional crystalline nanoporous networks presenting very high values of specific surface area and total pore volume. However, their most intriguing feature is the possibility to control their structure and properties through "reticular chemistry" syntheses, which allows to pre-design sizes, shapes and functional groups of their pore surface, thus incorporating characteristics tailored to the specific requirements of each process. This thesis focused on the study of two existing, however relatively new bi-metallic MOF structures for CH4 storage and CO2 separations that have been prepared via an innovative synthetic concept, i.e. by using a palladated organic linker. By following this strategy, it was attempted to increase the electron density of the MOF crystal lattice in order to enhance the gas-surface interactions. In this context, two novel microporous structures have been successfully obtained: (a) Cu-Pd-nbo, utilizing the dinuclear Cu-paddlewheel cluster linked with the palladated linker, and (b) In-Pd-soc produced by the association of the In(III)-based trimeric oxo-centered cluster with the same ligand, constituting the first indium-based MOF with soc topology. In the first part of the thesis, the possibility of activating the new materials was investigated. Cu-Pd-nbo was easily activated by means of mild heating under high vacuum. However, this was not the case for In-Pd-soc and a special activation methodology had to be developed. For this reason, a supercritical carbon dioxide flow experimental device was designed and built, with the aim of removing the trapped (during synthesis) solvent molecules and other impurities from the material’s porous network, while leaving the framework intact. In addition, the chemical and thermal stability as well as the morphology of the materials were investigated by means of a combination of experimental techniques (XRD, IR, TGA, SEM). Moreover, the structural features of the materials’ porous network were fully characterized by measuring nitrogen/argon adsorption isotherms at 77 and 87 K respectively. In a next step, the adsorption properties of the materials were systematically evaluated. Using specialized volumetric and gravimetric experimental methods, CO2, N2 and CH4 adsorption isotherms were measured in a wide range of temperatures (100 – 300 K) and pressures (0 – 20 bar). From the analysis of the experimental results, critical thermodynamic and kinetic parameters such as, total capacities, isosteric heat of adsorption, diffusion time constants, etc., were determined. By fitting the adsorption isotherms with appropriate theoretical equations, it was possible to deduce some first estimates for the separation ability of the materials with respect to CO2/N2 and CO2/CH4 containing mixtures, by calculating the respective selectivities. Cu-Pd-nbo exhibited superior structural features and sorption capacities for all gases. More specifically, its excess CO2 adsorption uptake was calculated to be 8.5 mmol/g at 273 K and 1 bar, a value which ranks it among the top performing CO2 adsorbents within the whole family of MOFs, essentially verifying the novel synthetic approach. For this reason, Cu-Pd-nbo was selected to be further investigated for its CH 4 storage capacity. More specifically, high pressure CH4 adsorption isotherms were performed up to 100 bar at 273, 288 and 298 K. Although CuPd-nbo exhibited rather modest gravimetric storage capacities, it showed remarkable total volumetric uptake, approaching 80% of the DOE target. This is attributed to the relatively high crystallographic density of the material, as a result of the Pd presence in the organic linker. The latter constitutes an interesting choice for increasing the volumetric capacity, which is an important metric for CH4 storage. In the last part of the thesis the gas mixture separation properties of Cu-Pd-nbo were investigated under conditions simulating those of a real process. More specifically, an experimental rig was constructed for performing dynamic column breakthrough experiments on the aforementioned mixtures (CO 2/CH 4, CO 2/N 2) at different ratios, (10:90, 50:50), and pressures from 1 to 5 bar at room temperature. Complete separation was successfully achieved in every case. Mean retention times, and actual selectivities were calculated from the resulting breakthrough curves while the derived experimental data will be used to build a theoretical model that will fully describe the separation process through selective adsorption of CO2 over the other components.
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