Tunable Water and CO₂ Sorption Properties in Isostructural Azine-Based Covalent Organic Frameworks through Polarity Engineering

The use of covalent organic frameworks (COFs) in environmental settings such as atmospheric water capture or CO₂ separation under realistic pre- and post-combustion conditions is largely unexplored to date. Herein, we present two isostructural azine-linked COFs based on 1,3,5-triformyl benzene (AB-COF) and 1,3,5-triformylphloroglucinol (ATFG-COF) and hydrazine building units, respectively, whose sorption characteristics are precisely tunable by the rational design of the chemical nature of the pore walls. This effect is particularly pronounced for atmospheric water harvesting, which is explored for the first time using COFs as adsorbents. We demonstrate that the less polar AB-COF acts as a reversible water capture and release reservoir, featuring among the highest water vapor uptake capacity at low pressures reported to date (28 wt % at <0.3 p p₀^-1). Furthermore, we show tailored CO₂ sorption characteristics of the COFs through polarity engineering, demonstrating high CO₂ uptake at low pressures (<1 bar) under equilibrium (sorption isotherm) and kinetic conditions (flow TGA, breakthrough) for the more polar ATFG-COF, and very high CO₂ over N₂ (IAST: 88) selectivity for the apolar AB-COF. In addition, the pore walls of both COFs were modified by doping with metal salts (lithium and zinc acetate), revealing an extremely high CO₂ uptake of 4.68 mmol g^-1 at 273 K for the zinc-doped AB-COF.