Membrane Gas Separation

10.4
Discussion 
In understanding the similar effects of CO and H 
2
S on the macroscopic membrane per-
formance parameters of supported [hmim][Tf 
2
N], it is necessary to consider the molecu-
1000T
-1
/K
-1
2.0
423 K
373 K
348 K
323 K
2.2
2.4
2.6
2.8
3.0
3.2
3.4
1
CO
2
/H
2
Selectivity
10
310 K
100
Figure 10.9 Carbon dioxide selectivity of [H 
2
 NC 
3
 H 
6
 mim][Tf 
2
 N] on cross - linked nylon; 
 
䊏

→
without contaminants;
䊐

→
5% CO;  
Δ

→
0.9 % H 
2
 S

---

Ionic Liquid Membranes for Carbon Dioxide Separation
195
CO
2
Permeability/m
2
s
-1
Pa
-1
1000T
-1
/K
-1
1.00E-14
1.00E-16
2.2
2.4
2.6
2.8
3.0
3.2
3.4
423
373 K
348
323
310
Figure 10.10 Carbon dioxide permeability of [hmim][Tf 
2
 N]
䊏
and [H 
2
 NC 
3
 H 
6
 mim][Tf 
2
 N]
䊐

on cross - linked nylon with simulated fuel gas
1
10
2.2
2.4
2.6
2.8
3.0
3.2
3.4
CO
2
/H
2
Selectivity
423 K
373 K
348 K
323 K
310 K
1000T
-1
/K
-1
Figure 10.11 Carbon dioxide selectivity of [hmim][Tf 
2
 N]
䊏
and [H 
2
 NC 
3
 H 
6
 mim][Tf 
2
 N]
䊐

on cross - linked nylon with simulated fuel gas

---

196
Membrane Gas Separation
lar - level interactions of the gases with the liquid ions. The results are initially puzzling 
since it appears that both CO and H 
2
S have very little effect on the CO 
2
permeability, but 
a substantial effect is observable on CO 
2
/H 
2
selectivity owing to an increase in H 
2
perme-
ability. H 
2
permeability is a combination of diffusivity and solubility. Gas diffusivities 
are closely tied to viscosity in ionic liquids [22 – 24] . If the [hmim][Tf 
2
N] viscosity were 
being substantially affected by the presence of CO and H 
2
S, it is likely that there would 
be an observable change in CO 
2
permeability as well. Since this is not the case, it is 
reasonable to believe that CO and H 
2
S are impacting H 
2
solubility. Also puzzling is the 
lack of substantial differences in the magnitude of the H 
2
solubility effect at different CO 
and H 
2
S concentrations. It appears that 10 ppm of CO or H 
2
S is a suffi cient concentration 
to saturate the effect. 
CO solubilities in ionic liquids were reported by Laurenczy et al. [25] . The researchers 
noted that the CO solubility is more dependent upon the nature of the ionic liquid than 
H 
2
and attributed this to CO ’ s strong dipole moment and greater polarizability. Also 
reported was the greater solubility of CO in ionic liquids made up of the 1 - butyl - 3 -
methylimidazolium cation and the [Tf 
2
N] anion than ionic liquids having the same cation 
and other anions. This result is consistent with CO having relatively strong interactions, 
with the [Tf 
2
N] anion. That anion is also known to have strong interactions with CO 
2
associated with weak acid – base interactions [13,26] . Since H 
2
S is also an acid gas, simi-
larly strong interactions are expected between that gas and [Tf 
2
N]. 
Given the strong associations of the polar CO and H 
2
S molecules with the ionic liquid, 
one explanation for the increase in H 
2
solubility may be a net depolarization effect. By 
closely associating with the anion (and potentially also with the cation), even low con-
centrations of the relatively small, polar gas molecules may decrease the net strength of 
the interactions among the much larger anions and cations, increasing the molar volume 
of the ionic liquid [7] , represented in Figure 10.12 . The weak interaction of H 
2
with the 
ionic liquid causes its solubility to be strongly tied to molar volume [27,28] . The increase 
in ionic liquid molar volume associated with the presence of CO and H 
2
S could result in 
an increased H 
2
solubility and permeability which would in turn decrease the CO 
2
/H 
2
selectivity.
A similar effect is observed on the membrane performance parameters of supported 
[H 
2
NC 
3
H 
6
mim][Tf 
2
N] in the presence of CO and H 
2
S. A decrease occurs in CO 
2
/H 
2