Polar groups, charge density, and backbone flexibility as molecular determinants of CO2 capture efficiency and absorption kinetics of polymerized ionic liquids.

This study investigates the synthesis, characterization, and CO2 capture efficacy of eight structurally varied polymeric ionic liquids (PILs) across four cationic head-group families-ammonium, phosphonium, imidazolium, and pyridinium-each comprised of distinctive functional groups to investigate structure-property correlations in CO2 capture. The synthesized PILs are: poly(allyl tris(2-hydroxyethyl)ammonium chloride) [P[AyTHEA][Cl]], poly(4-vinylbenzyl trimethylammonium chloride) [P[4-VBTMA][Cl]], poly(4-vinylbenzyl triethylammonium chloride). [P[4-VBTEA]]Poly[2-(methacryloyloxy)ethyl trimethylammonium chloride] [P[MOETMA][Cl]], poly(4-vinylbenzyl)bis(dimethylamino)phosphino-trimethylammonium chloride [P[4-VBPDMA-TMA][Cl]], poly[(4-methylbenzyl)tris(dimethylamino)phosphonium chloride] [P[4-MBP(DMA)3][Cl]], poly(1-allyl-3-propanenitrile imidazolium chloride) [P[AyPrCNIm][Cl]], and poly(allyl pyridinium chloride) [P[AyPyr][Cl]]. Structural verification was achieved using 1H NMR spectroscopy and elemental (CHNS) analysis, while water content was determined via coulometric Karl Fischer titration. CO2 solubility was investigated gravimetrically using magnetic suspension balance (MSB) at temperatures ranging from 298.15 to 358.15 K and at pressures of 1 to 20 bar. The time-dependent uptake was analyzed using a first-order saturation model, resulting in equilibrium mole fractions (x eq), first-order rate constants (k), and half-saturation periods (t½) for each PIL. The CO2 sorption isotherms exhibited nonlinearity and were characterized by the effective Henry's Law constants (K eff H) which were obtained from the dual-mode sorption (DMS) model, with the Henry dissolving coefficient (k D) fit to zero for all PILs, hence confirming mostly glassy-state, Langmuir-controlled sorption. The bifunctional PIL P[4-VBPDMA-TMA][Cl] exhibited the maximum CO2 uptake (x CO2 = 0.6156 at 20 bar, 298.15 K), while P[AyPyr][Cl] had the lowest performance. The calculated enthalpy values for the prepared PILs indicated predominantly physical sorption and advantageous regeneration economics. K eff H was used to estimate the standard thermodynamic solvation functions-Gibbs free energy (ΔG 0 sol), enthalpy (ΔH 0 sol), and entropy (ΔS 0 sol)-over the whole temperature range. The findings highlight distinct structure-activity relationships linking cation architecture, functional-group identity, and polymer backbone flexibility to CO2 capture capacity and kinetics in chloride-based PIL systems, offering rational design principles for next-generation PIL sorbents with enhanced performance for CO2 capture applications.