3.2.1 | Adsorption isotherms
Gas adsorption of CO2, CH4 and
N2 executed on MOF-74 and py-MOF-74 at 298 K are
depicted in Figure 4. The isotherms were fitted by the dual-site
Langmuir-Freundlich model (Table S1-S4). As shown in Figure 4a, the
uptake capacity of CO2 amounts to 6.65 mmol
g-1 (29.3 wt.%) at 100 kPa, whereas it adsorbs
relatively low quantity of CH4 (1.25 mmol
g-1, 5.50 wt %) and N2 (0.65 mmol
g-1, 2.86 wt.%). The adsorption properties of
pyrazine-interior-embodied MOF-74 were further examined. As shown in
Figure 4b-4d, the uptake capacity
of CO2 at 298 K
is
2.32 mmol g-1(10.21
wt.%)
and 1.35 mmol g-1 (5.95 wt.%) for py-MOF-74a and
py-MOF-74b, respectively, presenting a proportional correlation with the
BET surface area of materials (as shown in Figure 5). Nevertheless, the
saturation uptake of py-MOF-74c at 100 kPa reaches 1.07 mmol
g-1 (4.71 wt.%), far beyond the theoretically
anticipated value, which indicates that the inserted pyrazine with one
dangling N atom can be available as a compensation site for trapping
CO2 by terms of Lewis acid-base interactions. This value
is also comparable to the saturation uptake of some well-known MOF
materials at 100 kPa and 298 K, such as ZIF-8 (3.30 wt.%)19, ZIF-95 (3.87 wt.%) and ZIF-100 (4.21 wt.%)26. Besides, py-MOF-74c exhibits a steep adsorption
behavior below 10 kPa over py-MOF-74a and py-MOF-74b, which further
proves the special affinity of this composite to CO2.
Owing to the pyrazine blocking effect, the adsorption capacity of larger
molecules CH4 and N2 on modified MOF-74
also significantly reduces.