Fundamental Study of Free Volume Characteristics on Membrane H2/CO2 separation
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As one of the basic chemicals and green energy sources, hydrogen (H2) is mainly produced from fossil fuels via steam reforming or gasification with carbon dioxide (CO2) as a byproduct. The CO2 must be removed for the H2 to be used, and with a low-cost and energy-efficient H2/CO2 separation technology, the CO2 can be captured for sequestration or utilization.1-3 with inherently high energy-efficiency, membrane technology has been extensively investigated for H2/CO2 separation. The key to the success of this technology is membrane materials with high H2 permeability and H2/CO2 selectivity (preferably above 15) at syngas processing temperatures of 100 – 300 oC . Solubility and diffusivity can be independently tuned to design high-performance membrane materials . The under-valued solubility and solubility selectivity are powerful tools for designing materials with superior separation properties. Chapter 2 highlights recent achievements of sorption-enhanced materials with superior gas separation performance, including fluorinated polymers for He/gas and gas/CH4 separations, and polar polymers and mixed-matrix materials comprising metal-organic frameworks for olefin/paraffin separations.On the other hand, Diffusivity selectivity is governed by the relative molecular size of penetrants to be separated and the free volume of membrane materials and it has received great attention, yielding a wealth of size-sieving materials reported in the literature. Cross-linking has been extensively utilized to improve Diffusivity selectivity of materials and enhance the separation properties for important industrial gas separations such as H2/CO2, CO2/CH4, and C3H6/C3H8 separations. However, it remains unexplored how the cross-linking influences the physical properties of the polyimides such as Tg, FFV and gas permeability. Chapter 3 critically reviews XI the strategies utilized to chemically cross-link polyimides (including the reaction via imide rings or DABA functional groups) and the gas separation properties in the resulted polymer networks. In chapter 4 we looking into the cross-linking of P84 for the H2/CO2 separation application. The effects of crosslinking on the P84 polyimide chain packing and segmental mobility have a profound impact on the membrane transport properties. These effects are analyzed through permeation, sorption, fractional free volume, and X-ray diffraction characterization.In chapter 5, we demonstrate, for the first time, that carbonization of PBI, a leading material for the H2/CO2 separation, at optimal conditions can lead to the desired glasshour structure that is suitable for H2/CO2 separation properties at 100 – 200 oC. The effect of pyrolysis temperature on gas permeation and sorption were analyzed to provide insight into the factors governing separation properties in PBI-CMS membrane for H2/CO2 separation. Finally, the general conclusion of the current work and a perspective outlook on future research directions have been addressed.