(750e) Facile Infusion of Size-Sieving Macrocyclic 3D Cavities into Polymers of Intrinsic Microporosity for Ultra-Efficient Gas Separation and Enhanced Stability Against Aging

Efficient membrane-based gas separation typically requires strong molecular-sieving characteristic in the constituting materials, but achieving excellent size-sieving effect in polymeric membranes without sacrificing their favorable processability, scalability or mechanical stability has remained a challenge. As a new and facile approach to enhance the molecular-sieving nature of polymeric membranes for gas separation, 4-sulfocalix[4]arene (SCA4), an organic macrocycle which possesses uniquely sized 3-dimensional (3D) open cavities, is for the first time infused into amidoxime-functionalized polymers of intrinsic micro-porosity (AOPIM) to create an interconnected porous network coupled with additional 3D channels that execute ultra-effective size discrimination against gas molecules. Owing to its susceptibility for hydrogen-bonding and electrostatic interactions, SCA4 is able to be steadily and evenly distributed within the AOPIM-1 polymers via a simple and scalable solvent exchange process. In addition, the organic nature of SCA4 allows it to possess naturally strong affinity for organic polymers, which enables the as-fabricated AOPIM-SCA4 membranes to completely preserve or even improve on their mechanical strength and stability. The unique size-sieving 3D cavities of SCA4 also contribute to a drastic enhancement on both the H₂/N₂ and H₂/CH₄ selectivity of the AOPIM-SCA4 membranes, while the H₂ permeability is only minimally compromised. After being infused with only 3 mol.% of SCA4, the AOPIM membrane demonstrates 4.6 and 10.6 times higher H₂/N₂ and H₂/CH₄ selectivity respectively than the pristine sample, with merely a 15.6% decrease in H₂ permeability. As a result, the overall H₂/N₂ and H₂/CH₄ separation performance of the AOPIM-SCA4 membranes can approach or even surpass the proposed 2015 upper bounds, and is comparable to or even better than many of the state-of-the-art PIM membranes and carbon molecular sieve (CMS) membranes. The separation performances of H₂/CO₂, O₂/N₂ and CO₂/CH₄ are also greatly improved after SCA4 infusion, and are all able to surpass the 2008 upper bounds. Meanwhile, the size-sieving effect of the infused SCA4 cavities becomes increasingly prominent as a function of time because the collapse of excess free volume during physical aging reduces the amount of less selective regions in the membrane. For an AOPIM membrane infused with 5 mol.% of SCA4 that has been aged for two months, there is a 76% and 39% increase in H₂/N₂ and H₂/CH₄ selectivity respectively, whereas the H₂ permeability drops only around 18.4%. As such, comparing with the fresh samples, the aged AOPIM-SCA4 membranes exhibit completely uncompromised or even further improved overall H₂/N₂ and H₂/CH₄ separation efficiency, demonstrating good performance stability with time. This simple yet effective approach offers an exciting new platform for enhancing the molecular-sieving nature of polymeric membranes, especially towards hydrogen purification applications, and to also draw more research attention to the utilization of organic macrocycles with gas-favorable cavity size in membrane-based gas separation.