Few-layered ultrathin covalent organic framework membranes for gas separation: a computational study

Ultrathin films have the intrinsic feature to show high flux, which may become ideal membranes if they are fabricated to also have high selectivity. 2D covalent organic frameworks (COFs) are a class of crystalline materials with well-defined layered porous structures. Utilizing this feature, 2D-COF nanosheets can be stacked into few-layered ultrathin membranes, and thus the newly formed interlayer flow passages can be regulated to tune their separation properties, making them potential candidates for high-performance membranes. In this work, a series of few-layered 2D-COF membranes were constructed to explore their capability for gas separation as well as the underlying gas transport mechanisms by taking CO2/N-2 separation as an example. The results showed that various few-layered 2D-COF membranes can be fabricated to show very different separation performances, from nonselective to highly selective, and even to the molecular sieving level. Furthermore, it was revealed that the energetic microenvironment around the narrow interlayer passages plays a crucial role, and introduction of interacting surfaces to generate van der Waals potential sites near these passages by tuning stacking modes can achieve few-layered membranes with both high CO2 flux and high CO2/N-2 selectivity, leading to their separation performance far above the Robeson's upper bound. The mechanisms revealed and the design strategies proposed in this work may provide useful guidance for preparing ultrathin membranes with outstanding performance for gas separation.