DFT studies for the evaluation of amine functionalized polystyrene adsorbents for selective adsorption of carbon dioxide

The interaction of carbon dioxide with four amine functionalized polystyrene based adsorbents was investigated by means of density functional theory (DFT). The structures of the adsorbents and CO2-adsorbent complexes were optimized using the B3LYP/6-311++G(d, p) method. The interaction energies, equilibrium distances, charge transfer from the functionalized adsorbent to CO2, highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy gap and thermochemical parameters of the complexes were evaluated. Weak intermolecular forces were responsible for the interaction between the active centers of functionalized adsorbents and CO2 as confirmed from experimental studies. The interaction energies and vibrational frequencies confirmed stronger interaction of CO2 towards imidazole functionalized polystyrene (PS-imidazole) followed by Nmethylpiperazine loaded polystyrene (PS-piperazine) and were found to be the least for the bare adsorbent chloromethylated polystyrene (CMPS). Steric hindrance was found to play a major role in the case of dimethylamine (PS-DMA) and diethanolamine (PS-DEA) functionalized polystyrene during their interaction with CO2. On the basis of the HOMO-LUMO energy gap and calculated density of states, a negligible change in the electronic properties of CO2-adsorbent complexes was observed, indicating a physisorption process. The outcomes of the present theoretical studies are in good agreement with the experimental results and provide detailed insights for understanding the interaction of CO2 with the active center of the functionalized adsorbent.