Shielding CO₂-Philic Sites in Trimmed Covalent Organic Framework Pores by Atomic Layer Deposition

Strong adsorptive sites toward unwanted gas molecules in porous framework materials often lead to reversed sorption selectivity, creating tremendous challenges for enhancing the diffusion-driven membrane separations targeted at the weakly adsorbed species in the gas pair. While post-synthetic modification methods have been reported to downsize the pores in covalent organic frameworks (COFs), effective approaches to shield the highly adsorptive sites within the pores are rarely explored. Here, a solvent-less pore modification strategy is developed using atomic layer deposition (ALD). it is shown that controlled amounts of ZnO can be uniformly deposited into the COF pores, offering the ability to fine-tune the pore dimensions. Moreover, the Zn─O moieties grown into the COF pore are found to interact with the CO₂-philic ketoenamine groups, and substantially reduce the CO₂ solubility by 72.4% in the COF membrane. Accordingly, the simultaneously increased diffusion selectivity and sorption selectivity for H₂/CO₂ lead to a 330% improvement of the permselectivity in membrane separation, demonstrating the efficacy of the strategy for pore engineering in COFs.