CO2 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.