FIGURE 6 Experimental breakthrough curves obtained for MIL-120Al during the separation of CH4/N2 (50/50) at 1.0 bar and different temperatures: (a) 273 K, (b) 298 K, and (c) 313 K; (d) a comparison of the CH4 breakthrough uptake of MIL-120Al with some selected water stable benchmark materials reported in the literature. (e) The experimental breakthrough cycles for CH4/N2 (50/50) separation on MIL-120Al at 298 K and 1.0 bar; (f) breakthrough curves obtained upon varying the humidity of a 50/50 CH4/N2 mixture at 298 K and 1.0 bar.
To simultaneously consider the effects of both the thermodynamic and kinetic factors on the separation process, dynamic breakthrough experiments were conducted at 273–313 K using an equimolar CH4/N2 mixture (Figure 6). From the results (Figure 6a–c), N2 first elutes through the column and CH4 elutes after a certain amount of time, indicating that the CH4/N2 mixture can be separated over a wide temperature range from 273 to 313 K using MIL-120Al. In particular, at 298 K and 1 bar, N2 was found to first flow out of the breakthrough column and can be detected at 4.2 min. CH4 shows an obvious breakthrough delay behavior and cannot be the detected until 12.2 min. The calculated retention time for CH4 was three times more than that of N2, which is comparable to that of Al-CDC under similar test conditions.45 Considering the moderate IAST and kinetic selectivity of MIL-120Al at 298 K, we believe that the excellent separation of CH4/N2 on MIL-120Al was controlled by the synergistic effect of the thermodynamic and kinetic factors (Scheme 1). From the calculations using these breakthrough results (Figure 6d and Table S8), the loading of CH4enriched from the mixture was ~19.6 cm3/g, which is comparable to that of the best materials reported in the literature, e.g., NKMOF-8-Me (20.8 cm3/g),50 and higher than that of all previously reported MOFs. For a low concentration of CH4 inlet gas (20/80 and 10/90 CH4/N2), it is also feasible to realize the good separation of CH4 from N2 using MIL-120Al (Figure S12 and S13). In addition, as revealed using multiple breakthrough experiments, the separation time of MIL-120Al can be maintained after 10 cycles, implying its good reusability (Figure 6e). From a practical viewpoint, the effect of H2O in the enrichment process of low-concentration CMM should also be considered,56 thus, breakthrough experiments were performed using the CH4/N2 (50/50) mixture with humidity in the range of 22–81% RH. Figure 6f shows the retention time of N2 and CH4 were similar to those observed in a dry environment even under 81% RH, indicating that the separation performance of this material was not significantly affected by the presence of H2O.