Humidity-Enhanced Direct Air Capture of Carbon Dioxide Using Amine-Grafted Covalent Organic Frameworks Under Ambient and Sub-ambient Temperatures.

The significant rise in atmospheric CO2 and its impact on accelerating climate change have triggered intense efforts to develop porous sorbents for direct air capture (DAC), a route toward carbon-neutrality. Amine-functionalized covalent organic frameworks (COFs), an emerging class of crystalline porous materials, have recently shown promising potential for DAC at ambient, indoor temperatures (25 °C). However, most of Earth's land area has annual mean temperatures below 25 °C, accompanied by nonzero and variable relative humidity (RH). The performance of amine-grafted COFs under cold, humid conditions remains largely unexplored, even though such climates represent the majority of potential DAC deployment sites. Herein, we report a systematic investigation of a tetrahydroquinoline-linked COF covalently functionalized with diverse amines, evaluating its CO2 adsorption behavior across a broad range of ambient to sub-ambient temperatures (25 °C to -20 °C) and relative humidities (0%-70%). A unique tris-(2-aminoethyl)-amine-functionalized COF (ImCOF-TAEA) achieved a pseudoequilibrium capacity of 0.46 ± 0.02 mmol g-1 under dry conditions, rising ∼137% to 1.09 ± 0.09 mmol g-1 under 70% RH using 400 ppm of CO2 at 25 °C. Upon cooling to 15 °C under 70% RH, the uptake further increased to 1.25 ± 0.02 mmol g-1, showing a 205% enhancement relative to dry conditions. In situ spectroscopic analysis supports the mechanism behind the unusually high enhancement in CO2 adsorption under humid conditions. ImCOF-TAEA also demonstrates excellent recyclability under ambient/sub-ambient conditions and has modest (45 °C-65 °C) requirements for regeneration.