421195 Molecular Design of High CO2 Reactivity and Low Viscosity Ionic Liquids for CO2 Separative Facilitated Transport Membrane

Thursday, November 12, 2015: 12:30 PM
155B (Salt Palace Convention Center)
Akihito Otani1,2, Yong Zhang2, Eiji Kamio3, Hideto Matsuyama3 and Edward J. Maginn2, (1)Department of Chemical Science and Engineering, Kobe university, Center for Membrane and Film Technology, Kobe, Japan, (2)Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, IN, (3)Center for Membrane and Film Technology, Department of Chemical Science and Engineering, Kobe University, Kobe, Japan

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 ([P2221o1] +)) 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 Nos-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

=VBT0Pabt0+tPabt0dt

(1)    

where V is the volume of the system, kB is the Boltzmann constant, T is the temperature and Pab 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 Pab (Pxy, Pyz, Pxz, 0.5(Pxx Pyy), 0.5(Pyy Pzz) and 0.5(Pxx Pzz)). 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

=AT0.5expkT-T0

(2)    

where A, k and T0 are fitting parameters. From the calculation, it was found that ionic liquids with P2221o1+ 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. [P2221o1][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, [P2221o1] [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

rxn_pyrr


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