Industrial separations – primarily dominated by thermally driven distillation-based processes – consume 10-15% of the global energy production and emit more than 100 million tonnes of CO2 annually. Membrane technology, a 90% thermodynamically more energy-efficient than distillation processes, could be a desirable alternative with potentially lower energy consumption and lower carbon footprint. Industrial implementation of membrane technology, particularly for olefin/paraffin separations and hydrogen purification from syngas, remains challenging due to the substantially low mixed-gas selectivity of the currently available polymeric materials. Carbon molecular sieve (CMS) membranes – formed by high-temperature pyrolysis of solution-processable polymeric-based precursors at an oxygen-free atmosphere – have shown superior gas separation performance far beyond the polymeric upper bounds for many gas-pairs (e.g., CO2/CH4, N2/CH4). The ultimate goal of the research reported in this dissertation was to develop highly performing CMS membranes for industrially important but challenging gas separation applications (e.g., C2H4/C2H6, H2/CO2, etc.).
This work successfully introduced a promising approach to fine-tune the pore size distribution of CMS membranes through a systematic modification of the contortion sites of highly aromatic ladder polymer of intrinsic microporosity (PIM) precursors. CMS membranes derived from Trip(Me2)-TB – a precursor with large and thermally stable triptycene units – demonstrated unprecedented pure- and-mixed C2H4/C2H6 selectivities of 96 and 57, respectively, with relatively higher ethylene permeability than other CMS membranes. Similarly, CMS membranes derived from an alternative ladder PIM-based precursor, EA(Me2)-TB, also showed an outstanding performance for C2H4/C2H6 with a pure-gas selectivity up to 89 but with, however, low ethylene permeability of 0.35 barrer. Furthermore, CMS membranes derived from ladder CANAL-TB – a precursor with the smallest contortion site – exhibited superior pure- and-mixed H2/CO2 selectivities of 248 and 174, respectively, due to their tightly packed structure enabled by the lack of any shape-persistence unit such as triptycene.
CMS membranes fabricated in this work also showed promising gas separation performance for many other important energy-intensive industrial applications, including CO2/CH4, O2/N2, N2/CH4, H2/CH4, etc. In summary, this dissertation frameworks a facile and effective approach to obtaining CMS membranes with exceptional gas separation performance by rational design of the contortion sites of intrinsically microporous ladder polymer-based precursors.
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