Highly Selective Capture of CO₂ by Ether-Functionalized Pyridinium Ionic Liquids with Low Viscosity

In this work, three kinds of ether-functionalized pyridinium-based ILs [E<i>_n</i>Py]­[NTf₂] with low viscosity were designed and synthesized and used for highly selective separation of CO₂ from CH₄. It was found that the ether groups play an important role on physicochemical properties and CO₂/CH₄ selectivity in these three ILs. Compared with the nonfunctionalized analogues [C<i>_m</i>Py]­[NTf₂], the viscosities of [E_<i>n</i>Py]­[NTf₂] are lower and obviously decrease with the increasing number of ether oxygen atoms. The presence of ether groups on the cation has weak impacts on CO₂ solubility of the ILs, but it contributes to a much lower CH₄ solubility, which leads to the great increase of CO₂/CH₄ selectivity using [E_<i>n</i>Py]­[NTf₂]. Moreover, CO₂/CH₄ selectivity in all investigated ILs greatly decreases with the increasing temperature due to the weaker temperature dependence of CH₄ solubility. In addition, the thermodynamic properties including the Gibbs free energy, enthalpy, and entropy of CO₂ and CH₄ in these ILs were also obtained, and the CO₂ and CH₄ dissolution mechanisms were further analyzed. The results demonstrated that the gas–IL interaction plays a dominate role in CH₄ solubility in the investigated ILs, but CO₂ dissolution in ILs is determined by both the IL–gas interaction and free volume of ILs. This work will offer new insights into designing more competitive ILs for selective separation of CO₂ from CH<sub>4