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.