FIGURE 3 (a) CH4 and N2adsorption isotherms obtained for MIL-120Al at different temperatures; (b) IAST selectivity’s of MIL-120Al for CH4/N2 at 273, 298, and 313 K; (c) a comparison of the IAST selectivity of CH4/N2 and CH4 uptake of MIL-120Al versus those of previously reported water stable benchmark MOFs (orange represents the Al-based MOFs reported in this work); kinetic adsorption profiles obtained for CH4 and N2 for MIL-120Al at 273 K (d), 298 K (e), and 313 K (f).
Furthermore, due to the comparable kinetic diameters of the gas molecules and pore dimensions, we also investigated the time-dependent adsorption kinetics profiles of MIL-120Al at 263–353 K from 0 to 1.0 bar (Figures 3d–f and S7). Figures 3d–f show that MIL-120Al exhibits a considerably faster uptake of CH4 than N2 at the temperatures studied and increasing the temperature gradually increases the difference in the equilibration time between CH4 and N2. At 313 K, the time to reach equilibrium for CH4 was only 9.3 min, while N2 required 11.1 min. We speculate that the faster diffusion rate of CH4 than N2 can be attributed to the non-polar porous walls of MIL-120Al formed by the aromatic rings. It is noteworthy that the faster diffusion rate of CH4 than N2 has been observed for the first time in MOFs. This phenomenon is contrary to that of conventional MOF adsorbents, such as NKMOF-8-Me and Al-CDC,50 in which N2 always preferentially reaches equilibrium. To quantify the kinetic selectivity of MIL-120Al, the kinetic selectivity [Dꞌ(CH4)/Dꞌ(N2)] was obtained, and the value was up to 1.8 at 298 K (Table S2). It is reasonable to believe that the kinetic adsorption properties of MIL-120Al will plays a crucial role in CH4/N2 separation for practical applications.