3.2.3
| CO2/CH4 and
CO2/N2 adsorption selectivity
Adsorption selectivity of
CO2/CH4and CO2/N2 predicted on the basis of
IAST was employed to evaluate the competitive adsorption potential of
pyrazine-interior-embodied MOF-74. As shown in Figure 6a, the adsorption
selectivity of py-MOF-74a and py-MOF-74b for the equimolar binary
CO2/CH4 is 9.8 and 19.9 at 100 kPa,
respectively, lower than the parent MOF-74. However, further increasing
the pyrazine content is profitable to the improvement of selectivity of
CO2/CH4.
Py-MOF-74c with ultimate pyrazine
insertion displays the highest selectivity (598), which is 17 times of
that of parent MOF-74. It suggests that the molecular sieve effect
dominates the CO2 selectivity in this case. Furthermore,
py-MOF-74c shows overwhelming advantages over other MOFs in adsorptive
CO2/CH4 selectivity, for instance, ZU-66
with selectivity of 136 27, Co-btz-ht with selectivity
of 63 28 and IRH-3 with selectivity of 2729 at 298 K.
Pyrazine-bonded into the MOF-74
framework can also significantly widen the adsorption gaps between
CO2 and N2 as a result of their
differences in boiling point and polarizability besides molecular size
(Table 2). The three modified materials show significantly higher
adsorption selectivity than the original material for
CO2/N2 (Figure 6b and 6c). For 50: 50
CO2/N2 mixture, pyrazine modified
materials show the adsorption selectivity greatly superior to MOF-74,
for example, py-MOF-74c with selectivity of 451 at low pressure (40 kPa)
(Figure 6b and 6c). Unexpectedly, for py-MOF-74b with the moderate
pyrazine loading, the competitive adsorption effect between
CO2 and N2 can be amplified with the
increase of pressure. The
adsorption selectivity of CO2/N2 for
py-MOF-74b reaches 1711 at 100 kPa and 298 K, which is 35 times of that
of MOF-74 and much higher than that of the famous MOFs used for
CO2 selective adsorption, such as an exceptional
candidate PCN-200 (260, at 296 K) 30. Moreover, for
15: 85 CO2/N2 (simulated flue-gas)
mixture, py-MOF-74c also exhibits the optimal adsorption selectivity.
For example, the adsorption selectivity at 100 kPa and 298 K follows the
order of py-MOF-74c (471) > py-MOF-74b (147) >
py-MOF-74a (87) > MOF-74 (49). In brief, the competitive
adsorption potential of py-MOF-74c for CO2 over
N2 is greatly superior to previously reported MOFs at
analogous conditions, such as NJFU-2a (195) 31,
Zn(imPim) (106) 32 and BTU-11 (43)33. These results indicate that
pyrazine-interior-embodied MOF-74 display exceptionally selective
CO2 adsorption properties, which make them good
candidates for MOF membrane materials and nanofillers of mixed matrix
membranes that are implemented in dynamic CO2separations.
Table 2 Physical parameters 34 of
CO2, N2 and CH4