Measuring and Modeling Water and Carbon Dioxide Adsorption on Amine Functionalized Alumina under Direct Air Capture Conditions

Water vapor is an unavoidable component of ambient air that sorbents designed for atmospheric CO₂ capture must contend with. Amine functionalized sorbents often exhibit an enhancement of CO₂ uptake in the presence of moisture through a variety of mechanisms, and in this work, we investigate the coadsorption of water and CO₂ on amine functionalized alumina. Sorbent performance is examined under varying levels of humidity and temperature using three common measurement techniques: gravimetric, volumetric, and breakthrough methods. Our findings show that water increasingly enhances CO₂ adsorption up to the monolayer saturation point of water, above which no further enhancement is observed. Competitive adsorption is observed primarily at low relative humidities, and a novel dual-site isotherm model is developed that successfully describes these behaviors. Additionally, this study highlights the unique advantages of each measurement technique for accurately characterizing sorbent performance under direct air capture (DAC) conditions. These insights contribute to the understanding and optimization of amine-based sorbents in DAC applications.