3.2 Kinetic and equilibrium adsorption
Single-component CH4 and N2 adsorption
isotherms of MIL-120Al were measured at 273–313 K and up to 1 bar
pressure, as shown in Figure 3a. MIL-120Al shows an obviously
preferential adsorption of CH4 over N2at all temperatures studied, especially at low pressure (273 K),
indicating its high affinity toward CH4. Specifically,
at 298 K and 1.0 bar, the CH4 uptake capacity of
MIL-120Al was up to 33.7 cm3/g, which is comparable to
the benchmark Al-based MOF Al-CDC (32.0
cm3/g)45 and far exceeds other
porous materials, such as CAU-21-BPDC (22.2
cm3/g),46 Co-MA-BPY (20.6
cm3/g),47 SB-MOF-1 (20.6
cm3/g),48 and
Cu-(INA)2 (18.6
cm3/g),49 which demonstrates its
extremely high performance for CH4 capture (Figure 3c).
A more comprehensive comparison is given in Table S7 (Supporting
information). The corresponding CH4 volume adsorption
uptake of MIL-120Al based on the framework density was calculated to be
52.91 cm3/cm3, which also exceeds
other previously reported materials (Figure S6 and Table S3) with the
exception of NKMOF-8-Me (54.11
cm3/cm3).50 In
contrast, MIL-120Al only adsorbs a small amount of N2(10.5 cm3/g) under the same conditions. The
repeatability of the adsorption performance was also tested using a
cycling experiment. The adsorption of CH4 over five
cycles was maintained at ~33 cm3/g,
proving its excellent reproducibility (Figure S5). To further explore
the CH4/N2 separation properties of
MIL-120Al, the adsorption selectivity’s for 50/50
CH4/N2 mixtures were predicted using
ideal adsorbed solution theory (IAST)
(Figure 3b). As shown in Figure
3b, the CH4/N2 selectivity for MIL-120Al
can reach 6.0 at 298 K and 1 bar, which is higher than other most
previously reported MOFs (Table S7 and Figure 3c).51-53