Anion Size-Dependent Carbon Dioxide Adsorption Capacity in High-Purity Diallyldimethylammonium-Based Poly(ionic liquid)s

Abstract

Poly(ionic liquid)s (PILs) are solid materials composed of cationic or anionic polymers combined with counter ions. PILs are investigated and applied as CO₂ adsorbents because they possess both the high CO₂ affinity of ionic liquids and the excellent stability and processability of polymeric materials. This work explored the correlation between the type of anion in PILs and their CO₂ adsorption capacity via anion-exchange. As the skeleton of PILs, we focused on poly (diallyldimethylammonium chloride) (P[DADMA][Cl]). Among the quaternary ammonium cations known to exhibit high CO₂ adsorption capacity, P[DADMA][Cl] contains the highest density of cationic sites, indicating the potential for further enhanced CO₂ adsorption capacity. However, conventional anion-exchange reactions often yield residual by-products in the resulting PILs, making it difficult to reveal the inherent correlation between counter anions and CO₂ adsorption capacity. In this work, high-purity PILs (P[DADMA][AcO], P[DADMA][TFMS], and P[DADMA][SCN]) without by-products were successfully prepared for the first time through careful dialysis. The CO₂ adsorption capacity of these PILs increased in proportion to the size of the counter anion, and P[DADMA][TFMS] exhibited a seven-fold increase in the adsorption capacity compared to that of P[DADMA][Cl]. This work truly demonstrates that CO₂ adsorption capacity can be enhanced by designing the counter anions in PILs.