Innovative Strategies for Scalable Mixed-Matrix Hollow Fiber Membranes with Sub-micron thick Molecular-Sieve-Containing Composite Skin Layers for Tailorable Gas Separations

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The separation of chemically similar gases such as propylene (an olefin) and propane (a paraffin) is a key step in the petroleum-based production of fuels and commodity chemicals. However, the separation of gases with similar molecular sizes and physical properties is extremely challenging, often requiring extensive energy consumption. The use of membranes (i.e., molecular filters) to separate olefins and paraffins is an appealing energy-efficient alternative. However, there are no such commercially available membranes because current membrane manufacturing technologies are prohibitively expensive. The goal of this project is to develop a scalable, cost-effective process for producing membranes capable of olefin/paraffin gas separation. The ability to cost-effectively design and produce membranes capable of high-performance olefin/paraffin separation has the potential to substantially reduce energy usage in refining processes and, subsequently, reduce carbon emissions. Graduate and undergraduate student researchers will be directly engaged in completion of the project, receiving training in chemical synthesis, material characterization methods, and professional skills. The investigator will also host a local high school teacher in the lab each summer so that area K-12 students may indirectly benefit from the project. Videos demonstrating related project content will also be developed and published on YouTube as resources for the public and K-12 educators. The overall goals of this project are to develop scalable approaches for the preparation of zeolitic-imidazolate framework (ZIF)-containing mixed-matrix hollow fiber membranes and modules and to tailor the molecular sieving properties for gas separations of interest. Polycrystalline ZIF-8 membranes have shown promise for energy-efficient propylene/propane separation, but commercial application of such membranes is prohibitively expensive given the complex and slow processing steps required for preparation. Moreover, the discrete apertures of crystalline filler materials like ZIF-8 and zeolites does not allow for sufficient control over selectivity. The investigator hypothesizes that asymmetric mixed-matrix hollow fiber membranes with sub-micron thick selective composite skin layers can be prepared by decoupling the hollow-fiber spinning process from the mixed-matrix formation process using a polymer-modification-enabled in situ metal-organic framework (MOF) formation technique. A second, related hypothesis to be tested is that the molecular sieving properties of such a membrane can be fine-tuned by engineering the structures of the ZIF fillers with multiple linkers and/or metal centers. Three objectives will test the hypotheses: (1) understand how ion-exchange, ion-diffusion, and reaction processes shape the polymer-modification-enabled in situ MOF formation process and fabricate membranes with sub-micron thick selective composite skin layers; (2) engineer the microstructure of the membranes and their modules with tunable molecular sieving properties; and (3) test the performance of the membrane and modules in propylene/propane, ethylene/ethane, and nitrogen/methane separations under industrially-relevant conditions. The outcome of the project will be a strategy to develop efficient gas separation membranes that overcomes many of the limitations of existing polymer-based membrane technologies and the attendant fundamental knowledge necessary for MOF-based mixed-matrix membrane design.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.