CO2 absorption enhancement of fluorinated ionic liquids on nonaqueous biphasic absorbents: Experimental and theoretical study

Nonaqueous biphasic absorbent decreasing the regeneration heat consumption by phase separation (lean-phase and rich-phase) and water evaporation elimination is a promising CO2 capture absorbent, but the low CO2 loading capacities are of great challenges. In this study, ionic liquids (IL) were proposed as catalysts for enhancing the CO2 loading in monoethanolamine (MEA)-1-Butanol nonaqueous biphasic absorbents. Three types of fluorinated IL including 1‑butyl‑3-methylimidazolium tetrafluoroborate ([Bmim][BF4]), 1‑butyl‑3-methylimidazolium hexafluorophosphate ([Bmim][PF6]), and 1‑butyl‑3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([Bmim][TF2N]) under different concentrations (0–10wt%) were detailly analyzed to explore the catalyzation of IL on phase separation behavior and absorption performance. Phase separation study showed that the MEA-1-Butanol-IL system exhibited the wonderful phase separation behavior with a volume ratio (rich-phase) between 30 and 40 %. Three phases were observed in MEA-1-Butanol-[Bmim][BF4] and MEA-1-Butanol-[Bmim][PF6] systems after CO2 absorption and the components of each phase were identified by 13C nuclear magnetic resonance (NMR) spectroscopy. In absorption study, the absorption enhancement of IL was in this order: [Bmim][TF2N] > [Bmim][BF4] > [Bmim][PF6]. MEA-1-Butanol-10wt% [Bmim][TF2N] system could improve 35.9 % CO2 loading and 24.2 % absorption rate compared to basic MEA-1-Butanol. A molecular scale understanding of the three-steps reaction mechanism of MEA-1-Butanol-IL absorbing CO2 was revealed, and the different catalyzation of three types of IL can be attributed to the different affinity for CO2. This study provides new insights into designing nonaqueous biphasic solvents and a promising MEA-1-Butanol-IL system with high CO2 capture performance.