(696a) 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 CO₂on 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

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

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