Density functional theory-based analyses on selective gas separation by β-PVDF-supported ionic liquid membranes

Finding proper candidates for polymer-supported ionic liquid (IL)-based gas separating membranes is a challenge. The current article elucidates the quantum chemical perspective of the selective gas adsorption efficiency, from a mixture of CO₂, CO, CH₄, and H₂, of α- and β-polyvinylidene fluoride (PVDF)-supported imidazolium- and pyridinium-based six ionic liquid membranes. Although IL-based membrane efficiency mainly depends on the gas solubility of ILs, IL/support binding and gas adsorption on the support material are also studied to describe the overall gas adsorption properties of the PVDF/IL complexes. β-PVDF exhibits better binding with the ILs, and better gas affinity, thus, qualified as a more suitable membrane component as compared to α-PVDF. Dispersion-corrected density functional calculations are performed to provide a detailed insight into the energetic interactions, nonbonding intermolecular interactions based on symmetry adapted perturbation theory (SAPT), natural bond orbitals (NBO), Bader's quantum theory of atoms in molecules (QTAIM), reduced density gradient (RDG), frontier orbital interactions, density of states (DOS), and thermochemical analyses of the gas-adsorbed systems. Gas molecules interact with the membrane components through weak hydrogen bonds and exhibit low interaction energies, indicating physisorption of the gases. Gas adsorption energies are more negative than the mutual interaction energies of the gas molecules, ensuring effective gas adsorption by the membrane components. All the β-PVDF/IL systems have shown the highest and lowest affinity for CO2 and H2, respectively, leading to effective separation of CO₂ and H₂ from the other gases.