Effects of materials, processing, and operating conditions on the morphology and gas transport properties of mixed matrix membranes
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Gas separation membranes are currently based on polymers, which are limited by a trade-off between permeability (productivity) and selectivity. Zeolites offer significantly higher selectivities than polymers; however their properties make them prohibitively expensive to process into membranes. Organic-inorganic, or “mixed matrix”, materials may provide the basis for the next generation of economical, high performance membranes. The topic of this research is mixed matrix materials comprising a dispersion of zeolites in a polymer matrix. A major limitation of mixed matrix technology is the inability to prepare membranes from selected polymers and sieves with properties approaching the theoretical predictions. This difficulty is related largely to undesirable properties of the polymer- sieve interface, and this work seeks to understand and control these interfacial properties. First, an understanding of membrane formation is presented to explain how nonideal interfacial morphologies form. Factors affecting this process include: polymer flexibility, polymer – sieve affinity, and membrane preparation conditions. Membrane preparation conditions affect the propensity for stress to accumulate at the polymer- sieve interface, and depending on the severity of the stress, the likelihood that the polymer – sieve interface will fail. The next part of this work details experiments undertaken to better understand the factors affecting membrane morphology and transport properties. Factors ranging from material selection (e.g. silane coupling agent selection), dope formulation (e.g. polymer “priming”, sieve settling), and membrane preparation conditions (e.g. casting surface, temperature) were investigated. The final part of this work considers additional effects on mixed matrix properties caused by contaminants and minor feed components. A framework has been developed to account for the effects of potential impurities in the feed gas on the polymer, zeolite, and mixed matrix membrane. Based on this framework and results with model impurities, it appears that strongly sorbing components selectively displace the desired gases from the zeolite, preventing improved selectivity in mixed matrix membranes. This work has developed a better understanding of the factors that affect mixed matrix membrane performance and identified new ones that require additional study. After further development, this technology should allow for the increased application of membranes for the separation of gases and possibly also vapors and liquids.