Synergistic Polar Sites in Hydroxyl-Functionalized Covalent Organic Frameworks Enable Efficient Ammonia Adsorption...

In designing covalent organic frameworks (COFs) for ammonia adsorption, adding polar functional groups is a common strategy to improve uptake. However, the interplay between functionalization, stability, and crystallinity often limits performance, making it difficult to rationally design effective COF adsorbents. So far, clear guidelines for tailoring COFs for ammonia adsorption are still lacking. Herein, a series of COF-xOH (x = 0, 1, 2, 3) bearing halogen and hydrazide groups were systematically engineered with varying numbers of hydroxyl sites and evaluated for their ammonia adsorption performance. Remarkably, the ammonia uptakes of COF-xOH increase progressively to 5.18, 7.83, 9.24, and 12.50 mmol g^-1, exhibiting a linear correlation with the number of hydroxyl sites, while crystallinity exerts only a minor influence. COF-3OH shows superior ammonia adsorption at 25.0°C and 1.0 bar, outperforming most reported COFs, and exhibits high selectivity in separating ammonia from low-concentration gas mixtures, demonstrating its potential as an efficient adsorbent for ammonia-containing tail gas in Haber-Bosch processes. The adsorption mechanism is primarily governed by synergistic interactions among multiple polar functional sites. This work not only presents a top-tier COF-based ammonia adsorbent but also establishes a general strategy for the functionalization-driven design of porous materials for gas capture and separation.