(2cu) Understanding Gas Transport in Novel Membranes for Energy-Efficient Gas Separations: Polymers, Carbon Molecular Sieves, and Metal-Organic Frameworks

A significant portion of chemical products used in our day-to-day lives are made of hydrocarbons which are obtained from energy-intensive, thermally driven separation processes. The energy consumed from these separation processes account for 10-15% of the world’s total energy consumption. The pressure-driven membrane separation technology is considered a promising alternative to conventional energy-intensive separation processes. Understanding gas transport in various materials, such as polymers, carbon molecular sieves, and metal organic frameworks is crucial for the development of energy-efficient membranes for gas separation.

First, I investigated the effect of thermal cross-linking polyimide (PI) with ladder-structured polysilsesquioxane (LPSQ) on gas permeability and plasticization resistance. The CO₂ permeability of cross-linked PI/LPSQ membranes increased up to 813% with only an 8.6% decrease in CO₂/CH₄ selectivity due to the formation of larger and/or more interconnected cavities. The hardness and reduced modulus of PI/LPSQ membranes increased after cross-linking, while showing plasticization resistance up to a CO₂ partial pressure of 22 bar under equimolar CO₂/CH₄ feed conditions.

The PI/LPSQ blends are further transformed to N₂-selective carbon molecular sieve (CMS) membranes through pyrolysis. The separation of N₂ from CH₄ in natural gas is challenging due to the small difference in their kinetic diameters (< 5%) and the high critical temperature of CH₄. As a result, most polymeric membranes showed a N₂/CH₄ selectivity of less than 3. CMS PI/LPSQ exhibited enhanced N₂/CH₄ selectivity from increased CH₄ diffusion barrier caused by electron accumulation at SiO_x phases. 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. As a result, 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. However, the collapse of the porous substructure in polymer fibers during thermal treatment impeded the fabrication of thermally treated polymeric fibers or CMS fibers with a high flux. It was discovered that the rigid double-stranded siloxane backbone in LPSQ suppresses substructure collapse by rigidifying PI side chains. Cross-linked polyimide hollow fiber can be prepared by dip-coating a polyamic acid layer on a thermally cross-linked PI/LPSQ support, as well as CMS hollow fiber membranes which were prepared by pyrolyzing a single layer PI/LPSQ hollow fiber membrane. In particular, the CMS fibers exhibited a 546% enhancement in CO₂ flux over the precursor polymeric hollow fiber.

Lastly, I investigated metal-organic framework (MOF) membranes for CO₂/N₂ and CO₂/CH₄ separation. Most previously reported MOF membranes exhibit a low CO₂/N₂ and CO₂/CH₄ due to framework flexibility. In this work, MOF membranes achieved a CO₂/N₂ selectivity of 42 and CO₂/CH₄ selectivity of 95 with an estimated CO₂ permeability of 500 Barrer due to favorable Coulombic interactions.

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.