Ionic Liquid-Glycol Mixtures for Direct Air Capture of CO₂: Decreased Viscosity and Mitigation of Evaporation Via Encapsulation

Herein we address the efficiency of the CO₂ sorption of ionic liquids (IL) with hydrogen bond donors (e.g., glycols) added as viscosity modifiers and the impact of encapsulating them to limit sorbent evaporation under conditions for the direct air capture of CO₂. Ethylene glycol, propylene glycol, 1,3-propanediol, and diethylene glycol were added to three different ILs: 1-ethyl-3-methylimidazolium 2-cyanopyrrolide ([EMIM][2-CNpyr]), 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF₄]), and 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF₄]). Incorporation of the glycols decreased viscosity by an average of 51% compared to bulk IL. After encapsulation of the liquid mixtures using a soft template approach, thermogravimetric analysis revealed average reductions in volatility of 36 and 40% compared to the unencapsulated liquid mixtures, based on 1 h isothermal experiments at 25 and 55 °C, respectively. The encapsulated mixtures of [EMIM][2-CNpyr]/1,3-propanediol and [EMIM][2-CNpyr]/diethylene glycol exhibited the lowest volatility (0.0019 and 0.0002 mmol/h at 25 °C, respectively) and were further evaluated as CO₂ absorption/desorption materials. Based on the capacity determined from breakthrough measurements, [EMIM][2-CNpyr]/1,3-propanediol had a lower transport limited absorption rate for CO₂ sorption compared to [EMIM][2-CNpyr]/diethylene glycol with 0.08 and 0.03 mol CO₂/kg sorbent, respectively; however, [EMIM][2-CNpyr]/diethylene glycol capsules exhibited higher absorptions capacity at ∼500 ppm of CO₂ (0.66 compared to 0.47 mol of CO₂/kg sorbent for [EMIM][2-CNpyr]/1,3-propanediol). These results show that glycols can be used to not only reduce IL viscosity while increasing physisorption sites for CO₂ sorption, but also that encapsulation can be utilized to mitigate evaporation of volatile viscosity modifiers.