CO ₂ Sorption in Moisture Swing Anion Exchange Resins for Direct Air Capture: Experimental Isotherm Determination and Modeling

Moisture swing (MS) sorption is a promising direct air capture (DAC) technology to achieve negative CO₂ emissions and counter global warming. In this study, MS CO₂ sorption in a model MS sorbent, IRA900, was investigated. IRA900 is a macroporous commercial strong-base anion exchange resin (AER) with quaternary ammonium functional groups. A rigorous CO₂ desorption process was developed to desorb all CO₂ from the sample and obtain CO₂ sorption isotherms as a function of relative humidity, CO₂ partial pressure, and temperature. CO₂ sorption increased with decreasing relative humidity, decreasing temperature, and increasing CO₂ partial pressure. Remarkably, the total CO₂ uptake from the sorption isotherms matched the ion exchange capacity (IEC) of the material, suggesting a stoichiometry of one CO₂ molecule reacting per active site. An isotherm model for CO₂ sorption starting with empty sorption sites (i.e., with the AER in the OH^- form) described the experimental data well, supporting the hypothesis that the bicarbonate-loaded AER fully unloaded to the OH^- state following rigorous CO₂ desorption. CO₂ sorption kinetics were studied, and carbon diffusion coefficients were estimated as a function of %RH and CO₂ partial pressure. Overall, this study provides a roadmap for the systematic evaluation of MS CO₂ DAC materials.