The overall focus of this thesis is on the development and understanding of nanoporous layered silicates and membranes, particularly for potential applications in gas separations. Nanoporous layered materials are a rapidly growing area of interest, and include materials such as layered zeolites, porous layered oxides, layered aluminophosphates, and porous graphenes. They possess unique transport properties that may be advantageous for membrane and thin film applications. These materials also have very different chemistry from 3-D porous materials due to the existence of a large, chemically active, external surface area. This feature also necessitates the development of innovative strategies to process these materials into membranes and thin films with high performance.

Membrane separations are rapidly growing alternatives to traditionally expensive gas separation processes. For natural gas purification, membranes are used to remove carbon dioxide to prevent pipeline corrosion and increase the heating value of the natural gas. The robust chemical and physical properties of polyimide membranes make them ideal for the numerous components and high pressures associated with natural gas production. Typically, the performance of membranes changes over time as a result of physical aging of the polymer. Previous work shows that the thin selective layer of an asymmetric hollow fiber membrane, the morphology of choice for gas separations, ages differently than a thick dense film of the same material. Also, carbon dioxide, which is highly soluble in most polymers, can actively swell and plasticize polymer membranes at higher pressures. In this work, free acid groups present in the model polyimide are covalently crosslinked to stabilize the matrix against plasticization. Physical aging of two different crosslinked derivatives are compared to the free acid polyimide through gas permeation, gas sorption, and refractive index measurements. Thick (~50 m) and thin (~650 nm) films are examined to determine the effects of sample dimension on physical aging. The crosslinking mechanism employs diol substituents to form ester linkages through the free acid group. However, the annealing treatment, above the glass transition temperature, used to "reset" the thermal history of the films is found to form a new crosslinked polymer. Characterization of this new crosslinking mechanism reveals a high-temperature decarboxylation of the free acid creates free-radical phenyl groups which form covalent crosslinks through other portions of the polymer structure. Since ester crosslinks may be vulnerable to hydrolysis in aggressive gas feed streams, this new mechanism of crosslinking may create a more robust membrane for aggressive separations. In addition to the physical aging study, supercritical carbon dioxide conditioning of the two glycol crosslinked polyimides is compared to the free acid polymer. In this case, the free acid polymer is not crosslinked since the esterification crosslinking reaction occurs at much lower temperature than the decarboxylation mechanism. The free acid polymer displays an atypical permeation response under supercritical carbon dioxide conditions which suggests a structural reorganization of the polymer occurs. The crosslinked polymers do not exhibit this type of response. Mixed gas permeation confirms a substantial decrease in the productivity of the free acid polyimide and reveals the enhanced stability of the crosslinked polyimides following the supercritical carbon dioxide conditioning. Finally, examination of structurally similar fluorine-containing polyimides following approximately 18 years of aging allows the study of polymer structure on physical aging. A 6FDA-based polyimide is compared to a BPDA-based polyimide to understand the effects of bulky, CF3 groups on physical aging, and polyimides with diamine isomers reveal the effects of structural symmetry on physical aging.

An evaluation of several commercial adsorbents for use as contacting media in a process for combustion gas desulfurization was performed. Linde Synthetic Molecular Sieves types 13X and AW500 and Davison Silica Gel were the materials studied. The motivation for this investigation was based on the premise that a suitable method for removing sulfur dioxide from combustion gas streams has not been realized. Sulfur dioxide adsorption capacities were obtained for equilibrium (non-flow) and dynamic flow conditions. Sulfur dioxide adsorption on the molecular sieves could be described by a modified Langmuir expression of the form q/q_m = kc^1/n/ 1 + kc^1/n Calculated isosteric heats of adsorption were found to agree with measured data. Small scale dynamic studies consisted of contacting the adsorbents in a packed column with a simulated combustion gas. Evaluation of the adsorbent materials consisted of monitoring the effluent gas concentration after exposure to a step change in sulfur dioxide concentration. Sulfur dioxide loadings were greatest for the type 13X molecular sieve followed by AW500 molecular sieve. The effect of gas throughput was minimal which suggests that mass transfer was adsorbent side controlling. As gas temperature increased, sulfur dioxide adsorption decreased linearly for 100 percent sulfur dioxide concentration and non-linearly for low concentrations (0.003 percent). The effect of water vapor on sulfur dioxide adsorption capacity was determined by monitoring the effluent gas composition for specified sulfur dioxide-water vapor mixtures. Breakthrough time for sulfur dioxide was found to be an inverse function of the inlet water vapor concentration. For a typical combustion gas stream, (8 percent water vapor) the breakthrough time is roughly 10 percent of the water vapor free value. Based on the results obtained, a shallow bed (0.15m, 0.5 ft) of either type 13X or AW500 molecular sieve removed 5 to 3 times that of activated charcoal for a gas temperature of 57.2°C (135°F) and low gas pressure drop 4.6 cm H₂0 (1.85 in. H₂0). Adsorption degradation studies were performed to determine the loss in sulfur dioxide adsorption capacity after adsorbent regeneration. Both the 13X and AW500 molecular sieve could be regenerated, but the loss in adsorption capacity depended on the gas contacting conditions. X-ray spectroscopy was used to determine the homogeneity of the sulfur distribution within the adsorbents. The sulfur dioxide adsorption capacity for subsequent regeneration cycles was found to be a function of the Al₂O₃/SiO₂ ratio of the molecular sieve. Application of the molecular sieve adsorbents in a simulated combustion gas for sulfur dioxide removal was found to be superior to several adsorbents for the temperature range 21-76°C (70-170°F). However, as gas temperature increases, sulfur dioxide adsorption decreases. No adsorption of sulfur dioxide above 148°C (300°F) could be measured. / Ph. D.

The research presented in this thesis aims to determine the effectiveness of the uptake of toxic gases by several MOFs for future use in gas-mask cartridges, and to attempt to compensate for any deficiencies they show in “real-world” conditions. The main findings of this thesis confirm that MOFs are suitable candidates for the use in respirator cartridge materials and provide high capacity for adsorption of toxic gases like ammonia and STAM-1 in particular showed an impressive improvement in humid conditions, which normally decrease the performance of MOFs made from the same materials, such as HKUST-1. STAM-1's improved performance in humid conditions is attributed to the structural shift it displays upon dehydration and rehydration and this was shown to be the case in a structural analogue, CuEtOip, which was synthesised in the author's research group. This analogue was analysed using a combination of single crystal XRD and solid state MAS-NMR, both of which showed the structural change occurring and displays similar gas sorption behaviours, suggesting that this mechanism is the source of STAM-1's improved performance in humid conditions. This thesis also examines the “Armoured MOF” process and investigates the transferability of the process of deposition of mesoporous silica onto MOFs with vastly different properties and synthetic methods compared to those published in the original publication. Alongside this, attempts to protect MOFs using mesoporous silicates were investigated for their viability.

Over the past decade, membrane based gas separations have gained traction in industry as an attractive alternative to traditional thermally based separations due to their potential to offer lower operational and capital expenditures, greater ease of operation and lower environmental impact. As membrane research evolves, new state-of-the-art membrane materials as well as processes utilizing membranes will likely be developed. Therefore, their incorporation into existing thermally based units as a debottlenecking step or as a stand-alone separation unit is expected to become increasingly more common. Specifically for natural gas, utilization of smaller, more remote natural gas wells will require the use of less equipment intensive and more flexible separation technologies, which precludes the use of traditional, more capital and equipment intensive thermally based units. The use of membranes is, however, not without challenges. Perhaps the most important hurdle to overcome in membrane development for natural gas purification is the ability to maintain high efficiency in the presence of harsh feed components such as CO₂ and H₂S, both of which can swell and plasticize polymer membranes. Additionally, as this project demonstrates, achievement of similarly high selectivity for both CO₂ and H₂S is challenged by the different governing factors that control their transport through polymeric membranes. However, as others have suggested and shown, as well as what is demonstrated in this project, when CO₂ is the primary contaminant of interest, maintaining high CO₂/CH₄ efficiency appears to be more important in relation to product loss in the downstream. This work focuses on a class of fluorinated, glassy polyamide-imides which show high plasticization resistance without the need for covalent crosslinking. Membranes formed from various polyamide-imide materials show high mixed gas selectivities with adequate productivities when subjected to feed conditions that more closely resemble those that may be encountered in a real natural gas well. The results of this project highlight the polyamide-imide family as a promising platform for future membrane material development for materials aimed at aggressive natural gas purifications due to their ability to maintain high selectivities under aggressive feed conditions without the need for extensive stabilization methods.

The main theme of this dissertation is to engineer nanoporous materials and nanostructured surfaces for applications in gas separation membranes. Tunable methods have been developed to create inorganic hydroxide nanostructures on zeolite surfaces, and used to control the inorganic/polymer interfacial morphology in zeolite/polymer composite membranes. The study of the structure-property relationships in this material system showed that appropriate tuning of the surface modification methods leads to quite promising structural and permeation properties of the membranes made with the modified zeolites. First, a facile solvothermal deposition process was developed to prepare roughened inorganic nanostructures on zeolite pure silica MFI crystal surfaces. The functionalized zeolite crystals resulted in high-quality ̒mixed matrix̕ membranes, wherein the zeolite crystals were well-adhered to the polymeric matrix. Substantially enhanced gas separation characteristics were observed in mixed matrix membranes containing solvothermally modified MFI crystals. Gas permeation measurements on membranes containing nonporous uncalcined MFI revealed that the performance enhancements were due to significantly enhanced MFI-polymer adhesion and distribution of the MFI crystals. Solvothermal deposition of inorganic nanostructures was successfully applied to aluminosilicate LTA surfaces. Solvothermal treatment of LTA was tuned to deposit smaller/finer Mg(OH)₂ nanostructures, resulting in a more highly roughened zeolite surface. Characterization of particles and mixed matrix membranes revealed that the solvothermally surface-treated LTA particles were promising for application in mixed matrix membranes. Zeolite LTA materials with highly roughened surfaces were also successfully prepared by a new method: the ion-exchange-induced growth of Mg(OH)₂ nanostructures using the zeolite as the source of the Mg²⁺ ions. The size/shape of the inorganic nanostructures was tuned by adjusting several parameters such as the pH of the reagent solution and the amount of magnesium in the substrates and systematic modification of reaction conditions allowed generation of a good candidate for application in mixed matrix membranes. Zeolite/polymer adhesion properties in mixed matrix membranes were improved after the surface treatment compared to the untreated bare LTA. Surface modified zeolite 5A/6FDA-DAM mixed matrix membranes showed significant enhancement in CO₂ permeability with slight increases in CO₂/CH₄ selectivity as compared to the pure polymer membrane. The CO₂/CH₄ selectivity of the membrane containing surface treated zeolite 5A was much higher than that of membrane with untreated zeolite 5A. In addition, the use of metal organic framework (MOF) materials has been explored in mixed matrix membrane applications. ZIF-90 crystals with submicron and 2-μm sizes were successfully synthesized by a nonsolvent induced crystallization technique. Structural investigation revealed that the ZIF-90 particles synthesized by this method had high crystallinity, microporosity and thermal stability. The ZIF-90 particles showed good adhesion with polymers in mixed matrix membranes without any compatibilization. A significant increase in CO₂ permeability was observed without sacrificing CO₂/CH₄ selectivity when Ultem® and Matrimd® were used as the polymer matrix. In contrast, mixed matrix membranes with the highly permeable polymer 6FDA-DAM showed substantial enhancement in both permeability and selectivity, as the transport properties of the two phases were more closely matched.

Mixed matrix hollow fiber membranes maintain the ease of processing polymers while enhancing the separation performance of the pure polymer due to inclusion of molecular sieve filler particles. This work shows the development process of high loading mixed matrix hollow fiber membranes for butane isomer separation, from material selection and engineering of polymer-sieve interfacial adhesion to mixed matrix hollow fiber spinning. The matching of gas transport properties in polymer and zeolite is critical for forming successful mixed matrix membranes. The nC4 permeability in glassy commercial polymers such as Ultem® and Matrimid® is too low (< 0.1 Barrer) for commercial application. A group of fluorinated (6FDA) polyimides, with high nC4 permeability and nC4/iC4 selectivity, are selected as the polymer matrix. No glassy polymers can possibly match the high permeable MFI to make mixed matrix membranes with selectivity enhancement for C4s separation. Zeolite 5A, which has a nC4 permeability (~3 Barrer) and nC4/iC4 selectivity (essentially ∞), matches well with the 6FDA polymers. A 24% nC4/iC4 selectivity enhancement was achieved in mixed matrix membranes containing 6FDA-DAM and 25 wt% treated 5A particles. A more promising mixed matrix membrane contains 6FDA-DAM-DABA matrix and 5A, because of a better match of gas transport properties in polymer and zeolite. Dual layer hollow fibers, with cellulose acetate core layer and sheath layers of 6FDA polyimides, were successfully fabricated. Successive engineering of the 6FDA sheath layer and the dense skin is needed for the challenging C4s separation, which is extremely sensitive to the integrity of the dense skin layer. The delamination-free, macrovoid-free dual layer hollow fiber membranes provide the solution for the expensive 6FDA polyimides spinning. Mixed matrix hollow fiber membranes are spun base on the platform of 6FDA/Cellulose acetate dual layer hollow fibers. Preliminary results suggest that high loading mixed matrix hollow fiber membranes for C4s is feasible. Following research is needed on the fiber spinning with well treated zeolite 5A nanoparticles. The key aspect of this research is elucidating the three-step (sol-gel-precipitation) mechanism of sol-gel-Grignard treatment, based on which further controlling of Mg(OH)2 whisker morphologies is possible. A Mg(OH)2 nucleation process promoted by acid species is proposed to explain the heterogeneous Mg(OH)2 growing process. Different acid species were tried: 1) HCl solution, 2) AlClx species generated by dealumination process and 3) AlCl3 supported on zeolite surfaces. Acids introduced through HCl solution and dealumination are effective on commercial 5A particles to generate Mg(OH)2 whiskers in the sol-gel-Grignard treatment. Supported AlCl3 is effective on both commercial and synthesized 5A particles (150 nm-1 µm) during the sol-gel-Grignard treatment, in terms of promoting heterogeneous Mg(OH)2 whiskers formation. But the byproduct of Al(OH)3 layer separates the Mg(OH)2 whiskers from zeolite surface, and leads to undesirable morphologies for polymer-zeolite interfacial adhesion. The elucidation of sol-gel-Grignard mechanism and importance of zeolite surface acidity on Mg(OH)2 formation, builds a solid foundation for future development towards ''universal'' method of growing Mg(OH)2 whiskers on zeolite surfaces.