ADSORPTION CHARACTERISTICS OF COORDINATIVELY UNSATURATED METAL SITES CONTAINING DOBDC MOFS
6.7 Prediction of Binary Selectivity using IAST
example, in case of Mn/DOBDC the decrease in CO2/N2 selectivity with pressure is smaller (as compared to that in case of Mg/DOBDC), since the electrostatic interactions of CO2 with this framework are weaker.
Figure 6.38: Variation of CO2 selectivity over (a) CH4, (b) N2 and (c) Ar at 294 K with pressure on Mg/DOBDC (●), Mn/DOBDC (■), Co/DOBDC (♦) and Ni/DOBDC (▲). CO2 mole fraction in all binary mixtures is 20%. Symbols are calculated values; lines are drawn as a guide to the eyes.
The variation in CO2 selectivity over CO with pressure from a 20% CO2 in the binary mixture at 294 K is shown in Figure 6.39. Several interesting observations can be highlighted in this figure.
0 50 100 150 200 250 300
0 1 2 3 4 5
Selectivity(CO2/CH4)
f / bar a
0 50 100 150 200 250 300 350
0 1 2 3 4 5
Selectivity(CO2/N2)
f / bar b
0 200 400 600 800 1000 1200
0 0.5 1 1.5 2 2.5
Selectivity(CO2/Ar)
f / bar c
In this case both CO2 and CO molecules experience electrostatic interactions with cus metal centers of M/DOBDC frameworks and thus CO2 selectivity for CO2/CO binary mixture is considerably lower than for CO2/CH4, CO2/N2 and CO2/Ar binary mixtures. Moreover, CO2
selectivity over CO in the Mg/DOBDC framework does not change appreciably with pressure, since the decrease in electrostatic interactions occurs for both the gases as the cus sites are occupied. Mg/DOBDC and Mn/DOBDC frameworks are selective for CO2 throughout the entire pressure range. In fact, CO2 selectivity at zero coverage and 294 K on Mg/DOBDC (24) is higher than that on many MOFs such as Zn/DABCO (7) [121] and CuBTC (7) [121, 158]. But the selectivity of CO2/CO mixture is lower than 1 at low pressures for Ni/DOBDC and Co/DOBDC samples, indicating that these samples are more selective to CO. In fact at zero loading the CO/CO2 selectivity on these samples are as high as 7.5 (ratio of Henry’s constants on these samples) and the samples remain selective to CO even up to 10 bar. However, as in the other cases, as the cus metal centers are filled with increasing pressure, the CO/CO2 selectivity gradually decreases. At pressures higher than 16 bar, the samples have preferential selectivity for CO2 over CO (since the saturation capacity of CO2 is higher).
Figure 6.39: Variation of CO2 selectivity over CO at 294 K with pressure on Mg/DOBDC (●), Mn/DOBDC (■), Co/DOBDC (♦) and Ni/DOBDC (▲). CO2 mole fraction is 20%. Symbols are calculated values; lines are drawn as a guide to the eyes.
The DOBDC compounds are found to be selective for CO over CH4 and N2 (Figures 6.40) due to presence of electrostatic interactions in case of CO adsorption.
Figure 6.40: Variation of CO selectivity over (a) CH4 and (b) N2 at 294 K with pressure on Mg/DOBDC (●), Mn/DOBDC (■), Co/DOBDC (♦) and Ni/DOBDC (▲). CO mole fraction in all binary mixtures is 20%. Symbols are calculated values; lines are drawn as a guide to the eyes.
Both Ni and Co containing DOBDC frameworks, exhibit higher selectivity for CO over CH4 and N2 (Figures 6.40a – 6.40b) as compared to that by Mg/DOBDC and Mn/DOBDC. CO selectivity over CH4 obtained in this work on Ni/DOBDC at zero loading and 294 K is 441; this value is much higher than that on MIL-101 (30) [95] and CuBTC (~1) [71]. CO selectivity over N2 on Ni/DOBDC at zero loading and 294 K is even higher (677). As in case of CO2, the selectivity of CO over CH4 and N2 decreases with pressure due to reduced electrostatic interactions. Decrease in CO selectivity over N2 is lesser pronounced than that over CH4, because of the quadrupole moment of N2. For Mg/DOBDC and Mn/DOBDC samples, decrease in CO selectivity over CH4 and N2 with pressure, is less pronounced than in case of Ni/DOBDC and Co/DOBDC (which show exceptional Henry’s constants for CO adsorption).
1 10 100 1000
0 0.5 1 1.5 2 2.5 3 3.5
Selectivity (CO/CH4)
f / bar
a
1 10 100 1000
0 0.5 1 1.5 2 2.5
Selectivity (CO/N2)
f / bar
b
The effect of temperature on CO2 selectivity over N2 is shown in Figure 6.41a. CO2 selectivity for a binary gas mixture (20% CO2 at 1 bar) decreases by ~2.4 to 3 times when the temperature is increased from 294 to 352 K. This decrease can be readily attributed to the larger decrease in the electrostatic interactions for CO2 molecules than that for N2 molecules by the increase of temperature. Similar behavior is also observed for CO2 selectivity over CH4, Ar (Figure 6.41b – 6.41c) and for CO selectivity over CH4, N2 (Figure 6.42a – 6.42b). On the other hand, CO2
selectivity over CO (Figure 6.41d) does not change appreciably since both of the gases have considerable polarity; thus the decrease in selectivity with (increasing) temperature will be smaller in case of a binary mixture containing two polar species than in case of a binary mixture containing one polar and one non-polar adsorbate.
Figure 6.41: Effect of temperature on CO2 selectivity (for 20% molar composition of CO2) over (a) N2, (b) CH4 , (c) Ar and (d) CO at 1 bar total pressure for Mg/DOBDC (●), Mn/DOBDC (■), Co/DOBDC (♦) and Ni/DOBDC (▲). Symbols are calculated values; lines are drawn as a guide to the eyes.
0 50 100 150 200 250
294 314 334 354
Selectivity (CO2/N2)
Temperature / K
a
0 20 40 60 80 100 120 140
294 314 334 354
Selectivity (CO2/CH4)
Temperature / K
b
0 100 200 300 400 500 600
294 314 334 354
Selectivity (CO2/Ar)
Temperature / K
c
0.1 1 10 100
294 314 334 354
Selectivity (CO2/CO)
Temperature / K
d
Figure 6.42: Effect of temperature on CO selectivity (for 20% molar composition of CO) over (a) N2 and (b) CH4 at 1 bar total pressure for Mg/DOBDC (●), Mn/DOBDC (■), Co/DOBDC (♦) and Ni/DOBDC (▲). Symbols are calculated values; lines are drawn as a guide to the eyes.