ADSORPTION CHARACTERISTICS OF COORDINATIVELY UNSATURATED METAL SITES CONTAINING DOBDC MOFS
6.5 Effect of Physical Properties of the Adsorbate
6.5.1 On Henry’s Constants and Adsorption Enthalpy
The Henry’s constant (β) at 294 K and the adsorption enthalpy at zero coverage (-∆hads,0) of gases studied on M/DOBDC MOFs are shown in Figures 6.22 – 6.25. Both these quantities were obtained from the isotherm model parameters (Tables 6.4 – 6.10). These plots help in understanding the effect of polarity and polarizability on the adsorbate-adsorbent interactions.
Both β and -∆hads,0, vary almost linearly with polarizability for relatively non-polar Ar, CH4, C2H6 and C3H8 gases; this behavior is similar to that for silicalite [99], and MIL-101 [74].
However, for polar gases (CO2, CO, N2), higher values of β and -∆hads,0 are observed, highlighting the role of electrostatic interactions on the adsorption for these gases. The deviation from the trend for polar gases is larger in case of M/DOBDC frameworks than that for MIL-101 [95], indicating that the electrostatic interactions between polar gases and the cus metal sites is more pronounced in M/DOBDC frameworks.
Figure 6.22: Variation of (a) Henry’s constant at 294 K and (b) enthalpy of adsorption at zero occupancy with polarizability of the adsorbate for Mg/DOBDC adsorbent; linear trend lines for non-polar adsorbates are also shown.
y = 0.1281x - 3.3473 R² = 0.994
-2 -1 0 1 2 3 4 5 6
10 20 30 40 50 60 70
ln (β/ mol kg-1bar-1)
Polarizability / (×10-25cm3)
N2 CO2
CO
C3H8
CH4 Ar a
C2H6
y = 0.4238x + 6.1455 R² = 0.9663
5 10 15 20 25 30 35 40 45
10 20 30 40 50 60 70
-∆hads/ kJ mol-1
Polarizability / (×10-25cm3)
N2
CO2
CO
C3H8
CH4 Ar b
C2H6
Figure 6.23: Variation of (a) Henry’s constant at 294 K and (b) enthalpy of adsorption at zero occupancy with polarizability of the adsorbate for Mn/DOBDC adsorbent; linear trend lines for non-polar adsorbates are also shown.
Figure 6.24: Variation of (a) Henry’s constant at 294 K and (b) enthalpy of adsorption at zero occupancy with polarizability of the adsorbate for Co/DOBDC adsorbent; linear trend lines for non-polar adsorbates are also shown.
y = 0.132x - 3.4717 R² = 0.9909
-2 -1 0 1 2 3 4 5 6
10 20 30 40 50 60 70
ln (β/ mol kg-1bar-1)
Polarizability / (×10-25cm3)
N2 CO2
CO
C3H8
CH4 Ar a
C2H6
y = 0.3754x + 6.1297 R² = 0.908
5 10 15 20 25 30 35 40 45
10 20 30 40 50 60 70
-∆hads/ kJ mol-1
Polarizability / (×10-25cm3)
N2
CO2 CO
C3H8
CH4 Ar b
C2H6
y = 0.1367x - 3.3324 R² = 0.9937
-2 -1 0 1 2 3 4 5 6
10 20 30 40 50 60 70
ln (β/ mol kg-1bar-1)
Polarizability / (×10-25cm3)
N2
CO2 CO
C3H8
CH4 Ar a
C2H6
y = 0.4237x + 6.4745 R² = 0.961
5 10 15 20 25 30 35 40 45 50
10 20 30 40 50 60 70
-∆hads/ kJ mol-1
Polarizability / (×10-25cm3)
N2
CO2 CO
C3H8
CH4 Ar b
C2H6
Figure 6.25: Variation of (a) Henry’s constant at 294 K and (b) enthalpy of adsorption at zero occupancy with polarizability of the adsorbate for Ni/DOBDC adsorbent; linear trend lines for non-polar adsorbates are also shown.
Henry’s constant for CO2 at 294 K on the Mg/DOBDC MOF is higher than that on most of the other adsorbents such as CuBTC [76, 216], MIL-53(Al) [42], MIL-101 [74, 94], IRMOF-1, IRMOF-3 [93], MOF-177, silicalite [99], NaX [139], H-mordenite [217] and ZIF-8 [37].
However, Henry’s constant for CH4 at 294 K on M/DOBDC compounds is comparable to that on MOFs such as CuBTC [216] and MIL-101 [74, 94]. In addition, to the best of our knowledge, the Henry’s constant obtained for CO on Ni/DOBDC sample is the highest reported so far (547.2 mol kg-1 bar-1 at 294 K). The reasons for such behavior are discussed in the next section.
Similarly, Henry’s constant for N2 on DOBDC sample is also higher than most of the other solid adsorbents; however, the difference between Henry’s constant of DOBDC MOFs and other adsorbents for N2 is not as pronounced as that for polar gases such as CO2 or CO.
Enthalpy of adsorption at zero loading (-∆hads,0) of CO2 on the studied M/DOBDC frameworks (~28 – 42 kJ mol-1) are higher than that on MOF-177 (~15 kJ mol-1) [68]. -∆hads,0 of CO2 on Mg/DOBDC (~42 kJ mol-1) obtained in this work is similar to that reported in earlier literature [68]. It is also interesting to note that enthalpy of adsorption at zero coverage (-∆h ) for CO
y = 0.1392x - 3.4728 R² = 0.9959
-2 -1 0 1 2 3 4 5 6 7
10 20 30 40 50 60 70
ln (β/ mol kg-1bar-1)
Polarizability / (×10-25cm3)
N2
CO2 CO
C3H8
CH4 Ar a
C2H6
y = 0.5652x + 3.2255 R² = 0.9932
5 10 15 20 25 30 35 40 45 50
10 20 30 40 50 60 70
-∆hads/ kJ mol-1
Polarizability / (×10-25cm3)
N2
CO2 CO
C3H8
CH4 Ar b
C2H6
on Mn/DOBDC (28 kJ mol-1) is considerably lower than that on isostructural Mg/DOBDC (42 kJ mol-1) indicating smaller energy penalty for regeneration, but at the same time loading is also significantly lower (2.6 mol kg-1 on Mn/DOBDC as opposed to 4.95 mol kg-1 on Mg/DOBDC at 0.15 bar and 294 K). On the other hand, at about the same conditions, a saturated metal site containing MOF like Ni/DABCO has a loading of 0.28 mol kg-1 and enthalpy of ~20 kJ mol-1. Enthalpy of adsorption at zero coverage (-∆hads,0) of CO on Ni/DOBDC (~50 kJ mol-1) is higher than that on most of the other MOFs such as MIL-101 (~45 kJ mol-1) [95] and CuBTC (~24 kJ mol-1) [95] . In case of CH4 N2, Ar, C2H6 and C3H8 adsorption on M/DOBDC samples, -∆hads,0 is comparable to that on MIL-101 [95], CuBTC [95].
Variation of Henry’s constant with temperature is shown in Figure 6.26. As already mentioned, Henry’s constant for CO2 is larger on Mg/DOBDC at all the temperatures. Ni/DOBDC and Mn/DOBDC exhibit the highest Henry’s constants for CO and C3H8 respectively at all the temperatures. This comparison indicates that the electrostatic interactions are least dominant in case Mn/DOBDC and is to be expected because of larger ionic radii of Mn2+ (Table 6.2). In case of Co/DOBDC, Henry’s constant for CO was higher on than that for C3H8 at 294 and 315 K. At 352 K, Henry’s constant of C3H8 surpassed Henry’s constant of CO due to decrease in the electrostatic interactions between cus metal site and CO molecules. For all four DOBDC adsorbent considered, Henry’s constant is lowest for Ar due to its non-polarity and low polarizability at all the three temperature studied in this work. With the increase in the temperature, the decrease in Henry’s constant of polar gases (CO2 and CO) is more pronounced than that of non-polar gases (Ar, CH4). This can easily be attributed to the reduction in the electrostatic interactions between cus metal centers and polar adsorbates as temperature increases.
Figure 6.26: Variation of Henry’s constant with temperature for CO2 (■), CO (♦), CH4 (●), N2
(), Ar (×), C2H6 (+) and C3H8 (‒) on DOBDC MOFs. Lines are drawn as a guide to the eye.
It is easier to understand the effect of cus site availability on adsorption properties by studying the variation in adsorption enthalpy with loading. The enthalpies of adsorption on all the four frameworks for CO2, CO, CH4 N2, Ar, C2H6 and C3H8 were calculated from respective isotherm models (Table 6.4 – 6.10). The variation with loading is shown in Figure 6.27.
-3 -2 -1 0 1 2 3 4 5 6 7
2.8 3 3.2 3.4 3.6
ln (β/ mol kg-1 bar-1 )
1000/T, K-1 Mg/DOBDC
-3 -2 -1 0 1 2 3 4 5
2.8 3 3.2 3.4 3.6
ln (β/ mol kg-1 bar-1 )
1000/T, K-1 Mn/DOBDC
-3 -2 -1 0 1 2 3 4 5 6 7
2.8 3 3.2 3.4 3.6
ln (β/ mol kg-1 bar-1 )
1000/T, K-1 Co/DOBDC
-3 -2 -1 0 1 2 3 4 5 6 7
2.8 3 3.2 3.4 3.6
ln (β/ mol kg-1 bar-1 )
1000/T, K-1 Ni/DOBDC
Figure 6.27: Variation of adsorption enthalpy with loading for CO2 (■), CO (♦), CH4 (●), N2
(), Ar (×), C2H6 (+) and C3H8 (‒) on DOBDC MOFs. Lines are drawn as a guide to the eye.
In case of CO2 and CO, the adsorption enthalpy is initially high due to electrostatic interactions between cus metal centers and these gases. After certain loading (~18 molecules per unit cell or 1 molecule per metal atom) the adsorption enthalpy decreases sharply as the cus metal centers are progressively filled and attains a steady value. On other hand, with increase in loading, a slight increase in adsorption enthalpy is observed for Ar, N2, CH4, C2H6 and C3H8 and is attributed to the increase in lateral interactions between adsorbate molecules. It should be noticed here that the increase in adsorption enthalpy becomes more prominent as polarizability of adsorbates increases. For example, the increase in enthalpy with loading for C3H8 is much more pronounced than that for CH4, N2 or Ar.
0 5 10 15 20 25 30 35 40 45 50
0 2 4 6 8 10
-∆hads/ kJ mol-1
N / mol kg-1
Mg/DOBDC
0 5 10 15 20 25 30 35 40 45 50
0 2 4 6 8 10
-∆hads/ kJ mol-1
N / mol kg-1
Mn/DOBDC
0 5 10 15 20 25 30 35 40 45 50
0 2 4 6 8 10
-∆hads/ kJ mol-1
N / mol kg-1
Co/DOBDC
0 5 10 15 20 25 30 35 40 45 50
0 2 4 6 8 10
-∆hads/ kJ mol-1
N / mol kg-1
Ni/DOBDC