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