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.