Enhancement of CO₂/N₂ selectivity and CO₂ uptake by tuning concentration and chemical structure of imidazolium-based ILs immobilized in mesoporous silica

Imidazolium-based ionic liquids (ILs) with different cation alkyl chain ([i-C₅mim] or [C₄mim]) and inorganic anions ([Cl^-], [Tf₂N^-], [PF₆^-] and [DCA^-]) were synthesized and immobilized in commercial mesoporous silica. The synthesized supported ILs (SILs) were characterized using NMR, FTIR, TGA, BET, SEM and TEM. CO₂ sorption capacity, reusability and CO₂/N₂ selectivity were assessed by the pressure-decay technique. The effects of IL concentration, cation and anion chemical structure in CO₂ sorption capacity and CO₂/N₂ separation performance were evaluated. Tests evidenced that the presence of branching on the cation alkyl side chain increases CO₂/N₂ selectivity. The immobilization of the IL [i-C₅TPIm][Cl] on mesoporous silica in different concentrations (50, 20, 10 and 5 %) revealed that lower IL concentration results in higher CO₂ sorption capacity. Immobilization of ILs containing fluorinated anions at low concentrations in the mesoporous silica support may promote the improvement of the CO₂/N₂ selectivity without interfering on CO₂ sorption capacity of the original support. CO₂ sorption capacity value shown by sample SIL-5 % - [i-C₅TPIm][Tf₂N] (79.50 ± 0.70 mg CO₂ g^-1) was close to the value obtained for the pristine mesoporous silica (81.70 ± 2.20 mg CO₂ g^-1) and the selectivity (4.30 ± 0.70) was more than twice of the one obtained for the support alone (2.32 ± 0.4). Recycle tests demonstrated that the ILs immobilized in mesoporous silica samples are stable, providing a new option to be used in CO₂ capture processes.