(286e) Carbon Molecular Sieve Membranes for Gas Separation Applications

Thermal distillation-based separation processes consume 10 to15% of the global energy production and emit more than 100 million tonnes of CO₂ annually. Membrane technology 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 remains challenging due to the substantially low selectivity of the currently available polymeric materials. Carbon molecular sieve (CMS) membranes – formed by high-temperature pyrolysis of solution-processable polymeric precursors – have shown superior gas separation performance far beyond the polymeric upper bounds for many gas-pairs including CO₂/CH₄ and N₂/CH₄. In this work, we 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 77 but with, however, low ethylene permeability of 0.35 barrer. Furthermore, CMS membranes derived from ladder CANAL-TB-1 – 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. Our work frameworks a facile and effective approach to obtain CMS membranes with exceptional gas separation performance by rational design of the contortion sites of intrinsically microporous ladder polymer-based precursors.