Quaternary Ammonium-Functionalized Poly(Arylene Ether Sulfone) Random Copolymers for Direct Air Capture

Direct air capture (DAC) is a promising tool for reducing CO₂ concentrations in the atmosphere and fighting climate change. We synthesized a series of quaternary ammonium (QA)-functionalized poly(arylene ether sulfone) copolymers with varying mole fractions of charge content and demonstrated their potential as sorbents for DAC. Molecular weight analysis determined that a high monomer conversion was achieved in <6 h and relatively high molecular weights for all copolymer compositions. Thermal analysis revealed that all polymers had degradation temperatures >240 °C and a single glass-transition temperature (T_g); T_g decreased with an increase in ion exchange capacity (IEC). Fractional free volume and water uptake percentage increased and water contact angle decreased with the number of QA sites when compared to a neutral, commercially available polysulfone (Udel P-1700 PSU). Next, the novel series of ammonium-functionalized copolymers exhibited a moisture-mediated capture and release of atmospheric CO₂. When exposed to air containing 30% relative humidity, the polymers captured CO₂, and the capacity increased with the IEC. When the relative humidity was increased to 95% the samples released CO₂. This study demonstrates a promising CO₂ DAC sorbent and highlights the possibility for continuous DAC with purpose-built freestanding membranes.