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.