Humidity Effects on Sub-Ambient Direct Air Capture of CO₂ with Amine Functionalized Mg-Al LDHs and MMOs

Supported amine-based CO2 capture materials are promising direct air capture (DAC) sorbents due to their high CO2 uptake capacity, tolerance to varied humidities, and acceptable energy requirements for sorbent regeneration. For the large-scale deployment of supported amine adsorbents for DAC, support materials must be cost-effective and readily available on a large scale. In this study, amine-impregnated MgxAl-CO3 layered double hydroxides (LDHs) and MgxAl-O mixed metal oxides (MMOs) that can be produced with commercially available, earth-abundant chemicals are prepared, and their DAC performance is evaluated under a wide range of temperature (−20 to 25 °C) and humidity (0–85% RH) conditions. Although the 30 wt % poly(ethylenimine) (PEI)-impregnated LDHs and MMOs show moderate 400 ppm CO2 uptakes (≤1 mmol/g) under dry conditions, impressive adsorption capacities are observed under humid conditions (70% RH) at −20 (3.2 mmol/g) and 25 °C (2.0 mmol/g). Furthermore, the sorbent materials demonstrate promising regenerability during 10 humid DAC cycles at a 25 °C adsorption temperature with a <10% decrease in working capacity. However, a dramatic decrease in working capacity (∼40%) is observed after 10 humid DAC cycles at cold temperatures (−20 °C) due to reduced CO2 capture kinetics, attributed to amine redistribution. Overall, this study demonstrates the complex behavior that can be observed for an adsorbent over widely varying humidity and temperature conditions, reinforcing the notion that practical adsorbents must be carefully selected for operation in specific climatic zones.