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

An organic ionic plastic crystal (OIPC), methyl(diethyl)isobutylphosphonium hexafluorophosphate [P122i4][PF6], was investigated for CO2 and N2 absorption using molecular simulations. Ab initio calculations showed that both the cation and anion exhibit larger binding energy for CO2 compared with N2. The CO2 absorption, as calculated from classical molecular dynamics simulations, increased by a factor of 7.5 from 275 K to 325 K, while that of N2 showed low absorption at both temperatures. The simulations suggest that the significant increase in CO2 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 CO2 separations from CO2/N2 mixture.