Membrane Gas Separation

C
O
δ
-
δ
+
Figure 10.12 Ion depolarization effect in the presence of CO

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Ionic Liquid Membranes for Carbon Dioxide Separation
197
selectivity associated with an increase in H 
2
solubility. The behaviour of the membrane 
is complicated in this case by the contribution of the amine group to CO 
2
solubility 
through a strong acid – base interaction [29,30] and the impact of CO and H 
2
S on that 
interaction. As shown in Figure 10.8 , when CO and H 
2
S are not present, CO 
2
permeability 
rises rapidly from 310 to 373 K then levels off. This temperature dependence has been 
previously attributed to a change in the rate - determining step from dissociation of the 
acid – base complex to CO 
2
diffusion, as shown in Figure 10.3 . This CO 
2
permeability 
temperature dependence is tied to a peak in CO 
2
/H 
2
selectivity at 348 K. In the presence 
of CO and H 
2
S, no change in slope occurs in CO 
2
permeability and no peak is observed 
in CO 
2
/H 
2
selectivity. As with [hmim][Tf 
2
N] and other physical solvents, CO 
2
permeabil-
ity increases and CO 
2
/H 
2
selectivity decrease in an Arrhenius dependence with increasing 
temperature. 
In the presence of H 
2
S, the behaviour is straightforward to explain. As the stronger 
acid gas, H 
2
S competitively interferes with CO 
2
interaction with the amine site on the 
cation. This interference prevents the increase in CO 
2
solubility which normally results 
from the presence of that amine site and causes the ionic liquid to behave as if the site 
were not present, i.e. as if it were [hmim][Tf 
2
N]. 
In the presence of CO, the rationale for the alteration in behaviour is less clear. Based 
on the similar trends, it would be reasonable to suggest that CO is also interacting with 
the amine site on the cation and interfering with the ability of CO 
2
to do so. There are, 
however, no known strong associations between primary amines and CO. It is possible 
that CO forms such an association with another portion of the ionic liquid cation or with 
the anion. This adduct could then behave as an acid, interacting with the amine site to 
block it from forming complexes with CO 
2
. 
In the simulated fuel gas studies where a gas mixture including approximately 
10 000 ppm CO and 200 ppm H 
2
S was used, the results for supported [hmim][Tf 
2
N] were 
similar to those in the presence of either CO or H 
2
S alone. The result is consistent with 
the argument made earlier that very low concentrations of polar solute gases are suffi cient 
to depolarize the ionic liquid slightly increasing molar volume and H 
2
permeability. The 
membrane performance for supported [H 
2
NC 
3
H 
6
mim][Tf 
2
N] is less affected by the pres-
ence of lower concentrations of both CO and H 
2
S (10 000 ppm CO and 200 ppm H 
2
S, as 
shown in Figures 10.10 and 10.11 ) than by a higher concentration of either contaminant 
alone (50 000 ppm CO and 9000 ppm H 
2
S, respectively, as shown in Figures 10.8 and 
10.9 ), i.e. the slope change in CO 
2
permeability and the CO 
2
/H 
2
selectivity peak are not 
eliminated in the simulated fuel gas studies. The result indicates that the lower concentra-
tions of both contaminants are insuffi cient to competitively interfere with the CO 
2
– amine 
interaction. It also suggests that the direct H 
2
S interference with the interaction is more 
effective than the indirect interference of CO through adduct formation since 9000 ppm 
H 
2
S has a greater effect on the permeability and selectivity trends than the 10 000 ppm 
CO present in the simulated fuel gas.