Héctor RODRÍGUEZ1, Carlos A. PENA1, Ana SOTO1
1CRETUS, Department of Chemical Engineering, Universidade de Santiago de Compostela, Santiago de Compostela, Spain
The development of better processes for the capture of carbon dioxide from anthropogenic sources, substantially improving the current state-of-the-art technology based on the use of aqueous solutions of diluted amines as absorbents, is a key aspect in our toolkit to fight against global warming. Ionic liquids, thanks to their non-volatile nature, their typically good solvation ability, and the possibility of acting directly as non-aqueous solvents (thus avoiding the involvement of water, which causes an important rise in the energy requirements during the solvent regeneration step) have the potential to constitute a more sustainable alternative for new CO2 capture processes. Ionic liquids with an anion exhibiting basicity, such as the acetate anion, have shown a good absorption capacity via chemisorption of the acidic CO2. Ionic liquids such as 1-ethyl-3-methylimidazolium acetate ([C2mim][OAc]) or tetrabutylphosphonium acetate ([P4444][OAc]), with low toxicity and a reliable potential to be manufactured at a competitive cost at industrial scale, have been reported to absorb CO2 chemically in a 1:2 ratio (one mole of CO2 per two moles of ionic liquid). However, the product of the reaction of CO2 with [C2mim][OAc] will be solid at near-ambient temperatures, which would likely result in operational and design difficulties. On the other hand, [P4444][OAc] has a relatively high melting temperature on its own, 58 °C. In looking for a potentially synergistic effect to avoid these issues, in this work an equimolar mixture of [C2mim][OAc] and [P4444][OAc] has been explored for the absorption of CO2, using a gas-liquid equilibrium apparatus equipped with a magnetic suspension balance to develop the corresponding absorption-desorption isotherms in the pressure range 0-15 bar. The absorption isotherm at 25 °C shows an improvement of the CO2 absorption capacity of the equimolar mixture with respect to either of the two ionic liquids individually. However, in the desorption isotherm, a marked hysteresis effect is observed once the concentration corresponding to the 1:2 ratio is achieved, compatible with a CO2 chemisorption almost exclusively on [C2mim][OAc] and with the solid nature of this chemisorption product at the investigated temperature. At 70 °C, the equimolar mixture performs slightly better than either of the ionic liquids individually at low pressures (below ca. 5 bar); whereas the difference in absorption capacity becomes practically negligible as temperature increases. In this case, a slight hysteresis effect is observed, particularly at low pressures, in the absorption-desorption cycle with the equimolar mixture as solvent, with the CO2 concentration being a little higher during the desorption step than during the absorption step at any given pressure.
This work was supported by Xunta de Galicia through project ED431B 2020/021, co-funded by the European Regional Development Fund.