Separation of CO2 and H2 Using Acetate and Amino Acid Ionic Liquids

We will show theoretical and experimental results related to the use of acetate and amino acid ionic liquids to separate mixtures of H₂ and CO₂.  Simulations show that H₂ solubility in [emim][CH₃COO] is very low, with Henry’s law constant of about 10⁴ bar and permeabilities in the range of 29-79 barrer at 313-373 K.  Computational analysis also shows that CO₂ exhibits two absorption states in [emim][CH₃COO].  In State I, CO₂ molecules interact very strongly with the [CH₃COO]^-1 anion through strong complexation interactions leading to high CO₂ solubility.  The C atom of CO₂ interacts with the O atoms of the [CH3COO]^-1 anion with an average distance of about 1.61 Å.  CO₂ molecules in State I are significantly altered from isolated linear CO₂ molecules, with bond angles of about 138 degrees and C-O bond distances of 1.22 Å.  Significant charge transfer occurs from the [CH₃COO]^-1 anion to CO₂ in State I, leading to 30-40% charge decrease on the [CH₃COO]^-1 anion.  In State II, CO₂ interacts with [emim][CH₃COO] through simple van der Waals and electrostatic interactions, similar to CO₂ absorption in [emim][PF₆].  The predicted high CO₂ permeability and permeability selectivity for CO₂ over H₂ in [emim][CH₃COO] are verified by experiments.  The experimental CO₂ permeability in [emim][CH₃COO] is in the range 1325-3701 barrer, and experimental CO₂/H₂ permeability selectivities of 21-37 at 313-373 K are observed.  Simulations further demonstrate that [emim]⁺¹ cations compete with CO₂ to interact with the [CH₃COO]^-1 anion.  Based on this observation, several possible cations are proposed to replace the [emim]⁺¹ cation and improve CO₂ solubility and permeability.  The H₂ permeability in one of the proposed ILs is computed to be very low, suggesting that the proposed IL will exhibit high CO₂/H₂ separation performance.

Additionally, we have theoretically investigated CO₂ absorption mechanism in amino acid—based ionic liquids. One ionic liquid has been identified which is expected to show high CO₂ permeability and high permeability selectivity of CO₂ over H₂.