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