(2fg) Understanding Fundamental Gas Transport in Next Generation Membranes for Energy-Efficient Gas Separations: Carbon Molecular Sieve and Metal Organic Framework Membranes

Most chemical products seen daily are made of hydrocarbons which are obtained mostly by thermally driven, energy-intensive separation processes. The chemical separation processes consume a significant amount of energy, which accounts for 10-15% of the world’s total energy consumption. Membrane separation technology, a pressure-driven separation process, is considered a promising alternative to the conventional energy-intensive separation processes. The understanding of gas transport in various materials, including polymer, carbon molecular sieves, and metal organic frameworks is vitally important in developing next generation membranes for energy-efficient gas separations.

Both permeability and plasticization resistance of conventional polyimide (PI) membranes can be significantly improved by thermally crosslinking polyimide and ladder-structured polysilsesquioxane (LPSQ) [1]. The cross-linked PI/LPSQ membrane show up to 813% increase in CO₂ permeability with only 8.6% decrease in CO₂/CH₄ selectivity due to the formation of larger and/or more interconnected cavities during thermal cross-linking. Cross-linked PI/LPSQ membranes increase both hardness and reduced modulus compared to the as-cast membrane, with plasticization resistance up to a CO₂ partial pressure of 22 bar under equimolar CO₂/CH₄ feed mixture.

The separation of N₂ from CH₄ in natural gas is especially challenging due to the small difference in their kinetic diameters (< 5%) and the high critical temperature of CH₄. As a result, most polymeric membranes show a N₂/CH₄ selectivity of less than 3 [2]. The PI/LPSQ blends can be transformed to carbon molecular sieve membranes (CMSMs), referred to as the next generation gas separation membrane by pyrolysis. CMS membranes derived from PI/LPSQ show enhanced N₂/CH₄ selectivity from increased CH₄ diffusion barrier caused by electron accumulation at SiO_x phases [3]. We found that increasing the soaking temperature during pyrolysis enhances the N₂/CH₄ solubility selectivity due to strong repulsive interaction between the newly formed ultramicropores with CH₄. The CMS membranes exhibit an excellent single gas and N₂/CH₄/C₂H₆ (20/76/4) mixed gas N₂/CH₄ selectivity (28 and 16, respectively).

Membrane modules in the hollow fiber configuration are the most commercially viable due to their large surface area per volume. Especially, fabrication of a thin selective layer is crucial for developing hollow fiber membranes with high gas flux. Previously, thermally-induced substructure collapse of the precursor membrane during pyrolysis inhibited the preparation of CMS hollow fibers with a thin selective layer. CMS hollow fiber membranes with a thin, defect-free selective layer can be fabricated by pyrolyzing a homogeneous fluorinated-PI/LPSQ blend [4]. The rigid double-stranded siloxane backbone suppresses thermal stress-induced sub-structure collapse by rigidifying polyimide side chains, and the resulting CMS fibers exhibits a 546% enhancement in CO₂ flux over the precursor polymeric hollow fiber.

Lastly, I explored the use of CO₂-selective metal-organic framework (MOF) membranes for CO₂/N₂ and CO₂/CH₄ separation [4]. MOF are microporous and crystalline materials constructed by the coordination of metal nodes with organic linkers. Most previously reported MOF membranes exhibit a low CO₂/N₂ and CO₂/CH₄ due to the flexibility of the framework. By using frameworks which engage in favorable Coulombic interactions with CO₂, however, it is possible to achieve a CO₂/N₂ selectivity of 42 and CO₂/CH₄ selectivity of 95 with an estimated CO₂ permeability of 500 Barrer.

Teaching Interests

I look forward to being an instructor because sharing knowledge that I have obtained first-hand is exciting. Not only that, but I also gain new insight from the questions of students, which helps my research. I am interested in teaching undergraduate or graduate courses in heat and mass transport, engineering mathematics. I would also like to teach an advanced course that focuses on topics such as polymer engineering and separation processes.

I was a teaching assistant at Sogang University in an engineering mathematics course, and an experimental course focused on unit operations and analytical chemistry where I lectured students on the theory behind experiments. I was also the teaching assistant for an undergraduate research course where I enjoyed the opportunity to introduce students to topics such as polymer synthesis, membrane fabrication, and characterization and the related experimental processes. I believe that my past experiences in teaching and communicating with students has prepared me for the role of a teacher.

References

[1] H. J. Yu, C-H Chan, S. Y. Nam, S-J Kim, J. S. Yoo, J. S. Lee, Thermally cross-linked ultra-robust membranes for plasticization resistance and permeation enhancement – A combined theoretical and experimental study, J. Membr. Sci. 646 (2022), 120250.

[2] L.M. Robeson, The upper bound revisited, J. Membr. Sci. 320 (2008), 390-400.

[3] H.J. Yu, J.H. Shin, A.S. Lee, S.S. Hwang, J-H Kim, S. Back, J.S. Lee, Tailoring selective pores of carbon molecular sieve membranes towards enhanced N2/CH4 separation efficiency, J. Membr. Sci. 620 (2021), 118814.

[4] J.H. Shin, H.J. Yu, H. An, A.S. Lee, S.S. Hwang, S.Y. Lee, J.S. Lee, Rigid double-stranded siloxane-induced high-flux carbon molecular sieve hollow fiber membranes for CO₂/CH₄ separation, J. Membr. Sci. 570-571 (2019), 540-512.

[5] D-S Chiou, H.J. Yu, T-H Hung, Q. Lyu, C-K Chang, J.S. Lee, L-C Lin, D-Y Kang, Highly CO2 selective metal-organic framework membranes with favorable Coulombic effect, Adv. Fun. Mater., 31 (2021), 2006924.

Keywords

Membrane-Based Separations