Humidity Effects\non Sub-Ambient Direct Air Capture\nof CO<sub>2</sub> with Amine Functionalized Mg-Al LDHs and MMOs

Supported amine-based CO<sub>2</sub> capture materials\nare promising\ndirect air capture (DAC) sorbents due to their high CO<sub>2</sub> uptake capacity, tolerance to varied humidities, and acceptable\nenergy requirements for sorbent regeneration. For the large-scale\ndeployment of supported amine adsorbents for DAC, support materials\nmust be cost-effective and readily available on a large scale. In\nthis study, amine-impregnated Mg<sub><i>x</i></sub>Al-CO<sub>3</sub> layered double hydroxides (LDHs) and Mg<sub><i>x</i></sub>Al-O mixed metal oxides (MMOs) that can be produced with commercially\navailable, earth-abundant chemicals are prepared, and their DAC performance\nis evaluated under a wide range of temperature (−20 to 25 °C)\nand humidity (0–85% RH) conditions. Although the 30 wt % poly(ethylenimine)\n(PEI)-impregnated LDHs and MMOs show moderate 400 ppm CO<sub>2</sub> uptakes (≤1 mmol/g) under dry conditions, impressive adsorption\ncapacities are observed under humid conditions (70% RH) at −20\n(3.2 mmol/g) and 25 °C (2.0 mmol/g). Furthermore, the sorbent\nmaterials demonstrate promising regenerability during 10 humid DAC\ncycles at a 25 °C adsorption temperature with a <10% decrease\nin working capacity. However, a dramatic decrease in working capacity\n(∼40%) is observed after 10 humid DAC cycles at cold temperatures\n(−20 °C) due to reduced CO<sub>2</sub> capture kinetics,\nattributed to amine redistribution. Overall, this study demonstrates\nthe complex behavior that can be observed for an adsorbent over widely\nvarying humidity and temperature conditions, reinforcing the notion\nthat practical adsorbents must be carefully selected for operation\nin specific climatic zones.