Molecular-Level Understanding of Translational and Rotational Motions of C2H6, C3H8, and n-C4H10 and Their Binary Mixtures with CO2 in ZIF-10

Molecular dynamics simulations have been employed to investigate the translational and rotational motions of C2H6, C3H8, and n-C4H10 and their binary mixtures with CO2 in ZIF-10 at a molecular level. Our simulation results reveal that the translational motions of pure alkanes in ZIF-10 decrease monotonically with the increasing loading partly due to the competition between the enhanced alkane alkane and the weakened alkane-ZIF interactions. Also, the increasing collision frequency with loading can hinder their translational motions considerably. However, their rotational motions are found to be initially accelerated and then decelerated gradually beyond a critical loading for all studied alkanes. The initial acceleration of their rotational motions is due to more alkane molecules not being adsorbed at the internal surfaces but into the free central regions of cages in ZIF-10 as the loading increases. Furthermore, a loading higher than the corresponding critical value can result in a considerable decrease in free spaces in whole ZIF-10 so that more restrictions hinder their rotational motions. On the other hand, the presence of CO2 molecules promotes both the translational and rotational motions of alkanes in ZIF-10 for all studied alkane/CO2 mixtures, most significantly for n-C4H10/CO2. Besides the small size of CO2 molecules, the strong hydrogen bonds with the imidazolate rings can cause CO2 molecules to preferentially occupy the adsorption sites of ZIF-10 compared to alkanes, leading to the weakened interactions between alkanes and ZIF-10. More importantly, such competitive adsorption with CO2 in the mixture cases also results in a noticeable decrease in the collision frequency for alkanes in ZIF-10, which is in turn favorable to the enhanced translational and rotational motions of alkanes in ZIF-10. In addition, the confinement of ZIF-10 is also found to significantly promote the acceleration effect of CO2 molecules on the translational and rotational motions of alkanes.