ADSORPTION CHARACTERISTICS OF COORDINATIVELY UNSATURATED METAL SITES CONTAINING DOBDC MOFS
6.6 Effect of Metal Cation in the Framework
6.6.2 On Adsorption Isotherms at High Pressures
6.6.2.1 On Isotherms of Non-polar Gases. CH4 isotherms at 294 K on all the four adsorbent samples are compared in Figure 6.29. Due to negligible electrostatic interactions between cus metal sites of these frameworks and non-polar CH4 molecules, the change of metal constituent in the framework has negligible effect on the adsorption characteristics in the low pressure region (ca. < 0.5 bar) (Figures. 6.29a). Thereafter (ca. above 1 bar), loading on these frameworks are related to their pore volumes; while the highest loading was observed for Ni/DOBDC (which has the highest pore volume), lowest capacity is observed for Mg/DOBDC (which has the lowest pore volume. In fact, the saturation uptake capacities vary almost linearly with pore volumes (Figure 6.29b) obtained from N2 isotherm at 77 K. Isotherms for other relatively non-polar gas (Ar, C2H6 and N2) also follow similar trends (Figures 6.30 – 6.32).
Figure 6.29: (a) CH4 isotherms at 294 K on Mg/DOBDC (●), Mn/DOBDC (■), Co/DOBDC (♦) and Ni/DOBDC (▲). (b) Variation of CH4 saturation uptake with pore volume of DOBDC MOFs.
0 5 10 15 20 25
0.01 0.1 1 10 100
N / molecules unit cell-1
f / bar a
19 21 23 25 27
2.9 3 3.1 3.2 3.3 3.4 3.5 3.6
Nmax/ molecules unit cell-1
Pore volume / nm3unit cell-1 b
Ni/DOBDC
Co/DOBDC
Mn/DOBDC
Mg/DOBDC
Figure 6.30: (a) Ar isotherms at 294 K on Mg/DOBDC (●), Mn/DOBDC (■), Co/DOBDC (♦) and Ni/DOBDC (▲). (b) Variation of Ar saturation uptake with pore volume of DOBDC MOFs.
Figure 6.31: (a) C2H6 isotherms at 294 K on Mg/DOBDC (●), Mn/DOBDC (■), Co/DOBDC (♦) and Ni/DOBDC (▲). (b) Variation of C2H6 saturation uptake with pore volume of DOBDC MOFs.
0 4 8 12 16 20
0.1 1 10 100
N / molecules unit cell-1
f / bar a
19 20 21 22 23 24 25
2.9 3 3.1 3.2 3.3 3.4 3.5 3.6
Nmax/ molecules unit cell-1
Pore volume / nm3per unit cell-1 b
Ni/DOBDC
Co/DOBDC
Mn/DOBDC Mg/DOBDC
0 4 8 12 16 20 24
0.001 0.01 0.1 1 10 100
N / molecules unit cell-1
f / bar a
19 20 21 22 23
2.9 3 3.1 3.2 3.3 3.4 3.5 3.6
Nmax/ molecules unit cell-1
Pore volume / nm3per unit cell-1 b
Ni/DOBDC Co/DOBDC
Mn/DOBDC Mg/DOBDC
Figure 6.32: (a) N2 isotherms at 294 K on Mg/DOBDC (●), Mn/DOBDC (■), Co/DOBDC (♦) and Ni/DOBDC (▲). (b) Variation of N2 saturation uptake with pore volume of DOBDC MOFs.
6.6.2.2 On C3H8 Isotherms. C3H8 isotherms at 294 K on M/DOBDC compounds are shown in Figure 6.33. Comparison of C3H8 Henry’s constants for M/DOBDC samples indicates the negligible role of metal constituent in the framework on C3H8 adsorption at low pressure.
However, with increase of pressure, difference arises between adsorption capacity of different frameworks and Co/DOBDC exhibits higher uptake than that by other DOBDC frameworks at saturation. As in case of other gases, the Nmax for all the frameworks are related to their pore volumes; however Co/DOBDC has slightly higher saturation loading (~ 2.7 %) compared to that of Ni/DOBDC. Repeated experiments confirm to these results and we are unable to offer any reasonable explanation for this anomaly at this stage. Molecular simulations may offer insight into this behavior.
0 5 10 15 20
0.01 0.1 1 10 100
N / molecules unit cell-1
f / bar a
15 17 19 21 23
2.9 3 3.1 3.2 3.3 3.4 3.5 3.6
Nmax/ molecules unit cell-1
Pore volume / nm3unit cell-1 b
Ni/DOBDC
Co/DOBDC
Mn/DOBDC Mg/DOBDC
Figure 6.33: (a) C3H8 isotherms at 294 K on Mg/DOBDC (●), Mn/DOBDC (■), Co/DOBDC (♦) and Ni/DOBDC (▲). (b) Variation of C3H8 saturation uptake with pore volume of DOBDC MOFs.
6.6.2.3 On CO2 Isotherms. CO2 isotherms at 294 K on M/DOBDC MOFs are shown in Figure 6.34a. The CO2 uptake capacities for these frameworks behave differently in two regions. As already discussed, in the Henry’s law region the CO2 uptakes follow the order Mg/DOBDC >
Ni/DOBDC > CoDOBDC > Mn/DOBDC. As the pressure increases, the metal centers are progressively filled; at a loading of ~1 molecule per metal atom (18 molecules per unit cell) cus metal centers are saturated and hence the role of metal atom should be negligible. Accordingly in this region (highlighted region in Figure 6.34a), the difference between adsorption capacity of the four framework is negligible. As the pressure is further increased, the pore volume of the framework governs the adsorption capacity and as in case of non-polar gases, saturation loading on these frameworks varies linearly with pore volume (Figure 6.34b).
0 4 8 12 16 20 24
0.001 0.01 0.1 1 10
N / molecules unit cell-1
f / bar a
16 17 18 19
2.9 3 3.1 3.2 3.3 3.4 3.5 3.6
Nmax/ molecules unit cell-1
Pore volume / nm3per unit cell-1 b
Ni/DOBDC Co/DOBDC
Mn/DOBDC Mg/DOBDC
Figure 6.34: (a) CO2 isotherms at 294 K on Mg/DOBDC (●), Mn/DOBDC (■), Co/DOBDC (♦) and Ni/DOBDC (▲). (b) Variation of CO2 saturation uptake with pore volume of DOBDC MOFs.
6.6.2.4 On CO isotherms. Figure 6.35 reports the CO adsorption isotherms at 294 K on cus metal site containing M/DOBDC frameworks. At low pressure (ca. 0.01 bar), CO adsorption on Ni/DOBDC and Co/DOBDC is considerably higher than that on Mg/DOBDC and Co/DOBDC;
the difference of this magnitude (~100 times) cannot be accounted by the slight difference in the ionic radii of different metals; since this can create only a small difference in electrostatic interactions. In literature, it is reported that CO molecules donate σ-electron lone pair to the Ni2+
[122]. Thus, exceptional CO adsorption uptake on Ni/DOBDC and Co/DOBDC is attributed to the σ-donation of electron lone pair by CO molecules to the cus Ni and Co metal centers. The difference between Ni/DOBDC or Co/DOBDC and the other two frameworks studied in this work are significant even at pressures as high as 10 bar (~ 2 times). Thereafter, as the cus metal centers are occupied, electrostatic interactions get weaker and similar to other gases, CO saturation capacities are found to be linearly related to the pore volume for these frameworks (Figure 6.35b).
29 31 33 35 37
2.9 3 3.1 3.2 3.3 3.4 3.5 3.6
Nmax/ molecules unit cell-1
Pore volume / nm3unit cell-1 b
Mg/DOBDC
Mn/DOBDC Co/DOBDC
Ni/DOBDC
Figure 6.35: (a) CO isotherms at 294 K on Mg/DOBDC (●), Mn/DOBDC (■), Co/DOBDC (♦) and Ni/DOBDC (▲). (b) Variation of CO saturation uptake with pore volume.
The above results indicate that for non-polar gas, the metal atom in the framework plays negligible role; however, it plays an important role for polar gases like CO2 and CO. While Mg/DOBDC exhibits significant high Henry’s constant for CO2, Ni/DOBDC and Co/DOBDC show large value of Henry’s constant for CO due to σ-bonding. As the cus metal cations are filled, the difference between four frameworks becomes negligible for even polar gases. At high pressure pore volume of the framework governs the adsorption and is linearly related to the saturation capacity of the gas.