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Joan Brennecke1

1, University of Texas at Austin, Austin, Texas, United States

Ionic liquids (ILs) present intriguing possibilities for removal of carbon dioxide from a wide variety of different gas mixtures, including post-combustion flue gas, pre-combustion gases, air, and raw natural gas streams. Even by physical absorption, many ILs provide sufficient selectivity over N2, O2, CH4 and other gases. However, when CO2 partial pressures are low, the incorporation of functional groups to chemically react with the CO2 can dramatically increase capacity, while maintaining or even enhancing selectivity. Previously, we have shown how the reaction stoichiometry can be doubled over conventional aqueous amine solutions to reach one mole of CO2 per mole of IL by incorporating the amine on the anion, how we can virtually eliminate any viscosity increase upon complexation of the IL with CO2, by using aprotic heterocyclic anions (AHA ILs) that eliminate the pervasive hydrogen bonding and salt bridge formation that is the origin of the viscosity increase, and how the process energy can be further reduced by using ‘phase change’ ionic liquids, which are AHA ILs whose melting points when reacted with CO2 are more than 100 °C below the melting point of the unreacted material. Here, we show how mass transfer challenges of the relatively viscous ILs can be overcome by increasing the gas/liquid contact area by encapsulation of the ILs in silicone based shells. In particular, we show that the polymeric shells do not provide significant resistance to transport of CO2 and the capacity of the AHA ILs is maintained in the shells. The absorbent material in the shells can be cycled repeatedly without loss of capacity. Moreover, we show that the reaction of CO2 with the AHA ILs in the presence of water, which involves some reprotonation of the anion and formation of bicarbonate, is completely reversible and can be cycled for both the neat material and when it is encapsulated in the shells. Results on both the equilibrium and the rates of these reactions will be presented. Finally, we will show how the encapsulated phase change IL material works effectively in a fluidized bed reactor.

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