Molecular simulation study of CO2 and N2 absorption in a phosphonium based organic ionic plastic crystal

Abstract

An organic ionic plastic crystal (OIPC), methyl(diethyl)isobutylphosphonium hexafluorophosphate [P][PF], was investigated for CO and N absorption using molecular simulations. Ab initio calculations showed that both the cation and anion exhibit larger binding energy for CO compared with N. The CO absorption, as calculated from classical molecular dynamics simulations, increased by a factor of 7.5 from 275 K to 325 K, while that of N showed low absorption at both temperatures. The simulations suggest that the significant increase in CO absorption at 325 K is attributed to a higher degree of disorder and increase in the free volume due to the gas/solid interfaces. While the ab initio calculations were helpful in identifying specific interaction sites on the constituent ions, the classical MD simulations elucidated the importance of interfaces in gas absorption studies in this material. The results show that the OIPC can be a promising material for CO separations from CO/N mixture.

Atomistic Simulation of Gas Uptake and Interface-Induced Disordering in Solid Phases of an Organic Ionic Plastic Crystal

“…The study of OIPCs for gas absorption is still in its infancy, and the fundamental understanding of the gas sorption mechanism and the impact of multiple solid−solid-phase transitions is still lacking. Therefore, in the current study, the adsorption and solubility of CO 28 the initial MD calculations 31 on CO 2 and N 2 absorption in the OIPC demonstrated that the surface effects are crucial in modeling the OIPC solid. In that study, the solid OIPC model with vacuum interface became highly disordered at 325 K, which in turn had a positive effect on CO 2 uptake.…”

Cation effect on small phosphonium based ionic liquid electrolytes with high concentrations of lithium salt

“…Quaternary phosphonium-based ionic liquids (ILs) and their solid state analogs, organic ionic plastic crystals (OIPCs), have been developing rapidly in recent years as a result of many excellent properties being discovered and applied in a number of fields, such as lubricants, phase transfer catalysts, electrochemical media, optical materials, and CO 2 capture, to name a few. [1][2][3][4][5][6][7][8] As a relatively new class of ILs, their application in electrochemical devices, including rechargeable lithium and sodium batteries, has received particular attention of late.…”

Predicting gas selectivity in organic ionic plastic crystals by free energy calculations

“…15,16 The dramatic difference in selectivity between the two gases is attributed to the different interaction strengths between the gas molecules and the OIPC ions. CO 2 interacts with OIPC ions more strongly than N 2 does, as suggested by our previous density functional theory calculation of their binding energy [ 26 Therefore, this facilitates the absorption of CO 2 over N 2 , as supported by the low free energy change of the CO 2 absorption.…”

“…CO 2 interacts with OIPC ions more strongly than N 2 does, as suggested by our previous density functional theory calculation of their binding energy [( E b (P 122i4 –CO 2 ) = −3.64, E b (P 122i4 –N 2 ) = −1.81; E b (PF 6 –CO 2 ) = −3.29, E b (PF 6 –N 2 ) = −1.11, in kcal mol −1 )]. 26 Therefore, this facilitates the absorption of CO 2 over N 2 , as supported by the low free energy change of the CO 2 absorption.…”

“…The MD simulation procedure follows our previous studies with detailed description of the force eld. 16,26 The CHARMM force eld functional was adopted for the OIPC with the parameters described in the work by Chen 32 A cutoff of 1.5 nm was used for both Lennard-Jones and real space Coulomb potentials. The long-range electrostatic energies were treated using a particle-particle particle-mesh technique as implemented in LAMMPS.…”