Theoretical and Experimental Studies of CO₂ and H₂ Separation Using the 1-Ethyl-3-methylimidazolium Acetate ([emim][CH₃COO]) Ionic Liquid

Abstract

The performance of [emim][CH(3)COO] ionic liquid (IL) to separate mixtures of CO(2) and H(2) is studied using both classical and ab initio simulation methods and experiments. Simulations show that H(2) solubility and permeability in [emim][CH(3)COO] are quite low with Henry's law constants about 1 × 10(4) bar and permeabilities in the range 29-79 barrer at 313-373 K. In the case of CO(2) absorption in [emim][CH(3)COO], ab initio molecular dynamics simulations predict two types of CO(2) absorption states. In type I state, CO(2) molecules interact with the [CH(3)COO](-) anion through strong complexation leading to high CO(2) solubility. The C atom of CO(2) is located close to the O atoms of the [CH(3)COO](-) anion with an average distance of about 1.61 Å. The CO(2) bond angle (θ(OCO)) is about 138°, significantly perturbed from that of an isolated linear CO(2). In type II state, the CO(2) molecule maintains a linear configuration and is located at larger separations (>2.2 Å) from the [CH(3)COO](-) anion. The weaker interaction of CO(2) with the [CH(3)COO](-) anion in type II state is similar to the one observed when CO(2) absorbs in [bmim][PF(6)]. Simulations further demonstrate that the [emim](+) cation competes with CO(2) to interact with the [CH(3)COO](-) anion. The predicted high CO(2) permeability and low H(2) permeability in [emim][CH(3)COO] are also verified by our experiments. The experimental CO(2) permeability in [emim][CH(3)COO] is in the range of 1325-3701 barrer, and high experimental CO(2)/H(2) permeability selectivities of 21-37 at 313-373 K are observed. We propose that by replacing [emim](+) cation with 1-butyl-1-methylpyrrolidinium ([PY(14)](+)) further enhancement of CO(2) solubility in [PY(14)][CH(3)COO] IL will be obtained as well as good performance to separate CO(2) and H(2).