Encapsulation of functional solvents for improved thermal stability in CO2 capture applications
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abstract
Herein we address the efficiency of 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. 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 on average 51% compared to bulk IL. After encapsulation of the liquid mixtures using a soft template approach, thermogravimetric analysis revealed reductions in volatility of 36 and 40 % on average compared to the unencapsulated liquid mixtures, based on isotherms 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 capacity determined from breakthrough measurements, [EMIM][2-CNpyr]:1,3-propanediol had 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 CO2 (0.66 compared to 0.47 mol CO2/kg sorbent for [EMIM][2-CNpyr]:1,3-propanediol). These results show that multiple glycols can be used to reduce IL viscosity while increasing physisorption sites for CO2 sorption and encapsulation can be utilized to mitigate evaporation of volatile viscosity modifiers.
