FIGURE 5 Contour plots obtained for the center of mass (COM) probability distributions of CH4 (a) and N2 (b) in the mixture adsorbed on MIL-120Al at 298 K and 1.0 bar.
Grand canonical Monte Carlo (GCMC) simulations were carried out to give a deeper insight into the separation mechanism of CH4and N2 on MIL-120Al. Figure 5 shows the distribution profiles of CH4/N2 at 298 K and 1.0 bar after reaching equilibrium in the MIL-120Al structure. Obviously, the CH4 molecules prefers to stay in the pore formed between the two aromatic rings. A similar phenomenon has also been reported in the literature,55which has proved that CH4 molecules and the aromatic rings of framework form C-H···π interactions. For N2, the adsorption sites also stay at the middle of the pore, but the density was much lower than that of CH4, as confirmed by the probability distributions of the center of mass. The binding energies of CH4 and N2 were calculated to be 27.11 and 22.26 kJ/mol, respectively, which is consistent with the trend observed for the experimental Qst values (20.87 vs 17.83 kJ/mol). All of the results obtained using GCMC confirmed that the pore centers of MIL-120Al were the most energetically-favorable binding sites for CH4, thus exhibiting a superior recognition ability when separating the CH4/N2mixture.