The role of non-covalent interaction for the adsorption of CO2 and hydrocarbons with per-hydroxylated pillararene: a computational study†
A systematic study has been performed with DFT calculations for the physisorption of CO2, CH4, and n-butane gases by pillararene (PA) in gas phase. The DFT(B3LYP)-D3 calculations showed that CO2 and n-butane could be adsorbed more efficiently inside the cavity of PA compared to the CH4 molecule. The order of the binding energies of the adsorbed gases by PA is n-butane > CO2 > CH4 at 1 atm and 298 K. The hydroquinone units of PA play an important role in the adsorption of the gas molecules. The strong cooperative binding of n-butane compared to CO2 and CH4 inside the cavity of PA facilitates adsorption of n-butane inside the PA cavity. The structural analysis of the gas-adsorbed PA shows that the carbon atom of CO2 is in close proximity to the aromatic hydroquinone ring of PA, and the oxygen atom of CO2 is in close contact to the hydrogen atom of the hydroxyl group of the hydroquinone unit of PA. Similarly, the hydrogen atoms of the hydrocarbon (methane and n-butane) closely interact with the aromatic Pi-electron walls of the hydroquinone ring of PA, and the electronegative oxygen (O) atoms of the hydroxyl group (–OH) belong to the hydroquinone unit of PA. The calculated results show that four CO2, four CH4, and two n-butane molecules can reside inside the cavity of PA. The atoms in a molecule (AIM) analyses performed with adsorbed CO2, CH4 and n-butane inside the cavity of PA reveal the strong ‘closed shell’ type interactions for n-butane to be held inside the PA cavity. In addition to adsorption, the desorption of CO2, CH4, and n-butane from PA was accounted with the desorption enthalpies (ΔHDE) calculated per gas molecule, indicating that both adsorption and desorption are feasible in nature. The DFT studies of PA with CO2, CH4, and n-butane gases may help to understand the development of new design materials that can efficiently capture and separate such gases. The (B3LYP-D3) computed results corroborate the experimental observations that n-butane can adsorb better with PA compared to CH4 gas molecules. The associative butane–butane interactions seem to be superior over the CO2–CO2 interactions inside the PA cavity that promotes the adsorption of hydrocarbons.