Molecular Design of High CO2 Reactivity and Low Viscosity Ionic Liquids for CO2 Separative Facilitated Transport Membrane

A facilitated CO₂ transport membrane is a membrane containing CO₂ carriers which react with CO₂ on the feed side, diffuse across the membrane and release CO₂ on the permeate side. This unique active transport by the CO₂ carriers allows it to show significantly higher CO₂ permselectivity compared to polymeric membranes. Ionic liquids, which are molten salts that exist as liquid around room temperature, have unique properties such as negligible vapor pressure, high thermal stability and easiness of designing the physical and chemical properties. In our group, facilitated transport membranes, which contain various CO₂ reactable ionic liquids, working as carriers and diffusion medium at the same time, have been studied and found to show promising performance. Although an ionic liquid is designable and there are many kinds of ionic liquid, however, it is challenging to find out the most suitable structure as a CO₂ carrier of facilitated transport membrane.

In this work, 10 ionic liquids composed of various combinations of anions (2-cyanopyrrolide ([2-CNpyr] ^-), pyrrolide ([pyrr] ^-), pyrazolide ([pyra] ^-), 2-methoxypyrrolide ([2-Mtpyr] ^-) and 3-methoxypyrrolide ([3-Mtpyr] ^-)) and cations (tetraethylphosphonium ([P₂₂₂₂]⁺) and triethyl(methoxymethyl)phosphonium ([P_2221o1] ⁺)) were designed using molecular simulations as carrier candidates. Their properties before and after the reaction with CO₂ were calculated in order to find out the most suitable candidate for CO₂ separative facilitated transport membrane.

Molecular dynamics simulations were performed with LAMMPS in the isothermal-isobaric (NpT) and canonical (NVT) ensembles. Temperature was maintained at 350, 400, 450 and 500 K, respectively, by using the Nosé-Hoover thermostat. NpT simulations were performed for 2 ns to estimate the densities, after energy minimization. Subsequently, NVT simulations were conducted for 20 ns to calculate the diffusivities of ILs. In addition, 20 independent 3 ns NVT simulations (with initial configurations taken arbitrary from 20 ns NVT simulations) were run for viscosity calculation. In this study, 5 compositions of unreacted and reacted ionic liquids (unreacted, 25%, 50%, 75% and 100% reacted) were generated to express the extents of reaction in order to investigate the viscosity change after CO₂ absorption.

The viscosities at each temperature were calculated using the Green-Kubo relation

ƒÅ=V‚‹BT0�‡Pabt0+t∙Pabt0dt

(1)

where V is the volume of the system, k_B is the Boltzmann constant, T is the temperature and P_ab represents the off-diagonal components of the stress tensor. The angle brackets indicate the equilibrium average obtained by averaging over all the time origin t₀ and 6 components of P_ab (P_xy, P_yz, P_xz, 0.5(P_xx – P_yy), 0.5(P_yy – P_zz) and 0.5(P_xx – P_zz)). The viscosities at each time step were averaged over 20 simulation results to get converged integration of eq. (1). Then, viscosities between calculated temperatures were estimated using the Vogel-Fulcher-Tammann (VFT) equation

ā=AT0.5expkT-T0

(2)

where A, k and T₀ are fitting parameters. From the calculation, it was found that ionic liquids with P_2221o1⁺ cation showed lower viscosity than ionic liquids with P₂₂₂₂⁺, due to weaker electric interaction and larger free volume derived from electro donating and flexible methoxy group^[1], allowing it to show higher mobility. [P_2221o1][pyrr] showed the lowest viscosity in this temperature range even after CO₂ absorption.

Furthermore, CO₂ reaction enthalpies of anions were calculated by using Gaussian09 (B3LYP/6-311+g(d,p)) after geometry optimization. The reaction enthalpies of 2-CNpyr^-,pyrr^-, pyra^-, 2-Mtpyr^- and 3-Mtpyr^- were -34.5, -98.8, -74.2, -62.7 and -93.8 kJ/mol, respectively. As the result, pyrr^- was expected to have the highest reactivity, followed by 3-Mtpyr^-.

As a conclusion, [P_2221o1] [pyrr] can be expected as the most suitable ionic liquid for CO₂ separative facilitated transport membrane because of their high reactivity and low viscosity after CO₂ absorption.

Reference:

[1] K. Tsunashima and M. Sugiya, Electrochem. Commun., 9, (2007), 2353-2358

Graphical Abstract