High-Performance Membrane Materials for Industrial Gas Separations



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Membrane-based gas separation is a potential alternative for expensive and energy intensive conventional separation techniques such as pressure swing adsorption and cryogenic/fractional distillation. Polymer and carbon molecular sieve (CMS) membranes are two commonly studied types of membranes for gas separations. Despite their ease of application and relatively inexpensive nature, polymer membranes suffer from the gas permeability/selectivity trade-off, and they are also highly susceptible to plasticization. On the other hand, physical aging in CMS membranes results in less efficient separations over time, as a result of reduced permeabilities. This dissertation describes approaches to overcome the above problems in polymer and CMS membranes. The first chapter briefly describes current trends and concerns in membrane-based gas separations. This chapter also discusses the importance of membrane-based gas separations as an alternative for conventional gas separation techniques. The second chapter explains the utilization of a thermally crosslinkable polymeric precursor to fabricate aging resistant CMS membranes under simulated industrial conditions. Synthesis of a high molecular weight, thermally crosslinkable polymer, 6FDA-DABA, is reported for the first time. The fabrication technique for aging resistant CMS membranes described is simple and relatively inexpensive compared to previously reported aging prevention techniques. The CMS membranes show excellent propylene/propane separation properties (propylene permeability of 257 Barrer and propylene/propane selectivity of 20) with about 92% permeability retention after 15 days. The third chapter reports the fabrication and characterization of novel carbon-carbon composite membranes (a type of CMS membranes) from a compatibilized immiscible polymer blend, polybenzimidazole (PBI)/6FDA-DAM:DABA (6FDD). Morphology control of the PBI:6FDD immiscible polymer blend is the key to obtaining membranes with higher gas permselectivities, which is an effective way to overcome the gas permeability/selectivity trade-off. In addition, this trade-off can be addressed by converting the morphology controlled immiscible polymer blends into carbon-carbon composite membranes. The synergistic effect of polymer blending and carboncarbon composite membrane fabrication is investigated for H2/CO2 separations. Morphology controlled carbon-carbon composite membranes showed an approximately three-fold increase in both H2 permeability and H2/CO2 selectivity in comparison to the membrane without morphology control. The fourth chapter explains how an immiscible polyimide blend can be converted to a miscible polymer blend using a reactive small molecule compatibilizer. This technique allows the fabrication of CO2/CH4 separation membranes with excellent plasticization resistance and gas permselectivities. Thermal crosslinking of the polymer blend membranes improves the plasticization resistance. Plasticization resistance is greater for the crosslinked miscible polymer blends in comparison to the crosslinked immiscible polymer blends.



Gases -- Separation, Gas research, Molecular sieves, Polymeric membranes