654
High viscosity and non-volatility of ionic liquid prevent the membrane solvent
from flowing out of the porous membrane [74]. Four water stable anions
bis(trifluoromethanesulfonyl)amide [Tf 2 N]
−
, trifluoromethanesulfone [CF 3 SO 3 ]
−
,
dicyanamide [DCA]
−
, and chloride [Cl]
−
, supported on the hydrophilic polyethersulfone (PES), were studied for CO 2 absorption [75]. Results showed that higher
permeability along with greater selectivity of CO 2 can be achieved. These permeability and selectivity were even better than the polymeric membranes. Nerves
et al. found that the nature of ionic liquid affects the CO 2 absorption behavior
[76]. The hydrophobic ionic liquids are more stable and have high capacity for
CO 2 absorption as compared to hydrophilic ionic liquids. Presence of water vapor
in the gas stream increases the permeability but results in the lower CO 2 selectivity due to the water cluster formation. It was also investigated that increasing the
temperature increases the permeability of these membranes which in turn
decreases the CO 2 selectivity. The selectivity and permeability of SILM largely
depend on the nature of porous material. The CO 2 absorption in SILM can be
increased by increasing the van der Waals intermolecular interaction of CO 2 and
ionic liquid [77].
3.2.4 CO 2 Capture with Polymeric Ionic Liquid
The polymerized ionic liquids (PILs) have also shown the capability to absorb the
CO 2 . Tang et al. first made the ionic liquid into polymeric form [78]. These polymerized ionic liquids have greater CO 2 absorption capacity than the conventional
ionic liquids. Tetraalkylammonium-based polymeric ionic liquid has 7.6 times
larger CO 2 solubility as compared to conventional ionic liquid [C 4 mim][BF 4 ] [79].
CO 2 absorption capacity in PILs depends on the cation, and the highest CO 2 absorption was observed in ammonium-based PILs. Imidazolium-based PILs show relatively lower CO 2 solubility [80, 81]. Similarly, anions of the PILs significantly affect
the CO 2 solubility and follow the trend as BF 4
−
> PF 6 > Tf 2 N.
3.3 Ionic Liquid Blends
The viscosity of conventional ionic liquid is approximately 40 times higher than
that of aqueous alkanol amine (30 wt%) which causes huge difference in the CO 2
solubility [82]. Amine-functionalized ionic liquid showed better CO 2 solubility
than the conventional ionic liquids. However, the functionalized ionic liquids synthesis requires complex procedure and subsequent purification steps [83].
Therefore, in addition to modifying the structural and functional group, mixing
with other solvent is an alternative way to improve the CO 2 absorption ability of
these ionic liquids.
M. B. Haider et al.
High viscosity and non-volatility of ionic liquid prevent the membrane solvent
from flowing out of the porous membrane [74]. Four water stable anions
bis(trifluoromethanesulfonyl)amide [Tf 2 N]
−
, trifluoromethanesulfone [CF 3 SO 3 ]
−
,
dicyanamide [DCA]
−
, and chloride [Cl]
−
, supported on the hydrophilic polyethersulfone (PES), were studied for CO 2 absorption [75]. Results showed that higher
permeability along with greater selectivity of CO 2 can be achieved. These permeability and selectivity were even better than the polymeric membranes. Nerves
et al. found that the nature of ionic liquid affects the CO 2 absorption behavior
[76]. The hydrophobic ionic liquids are more stable and have high capacity for
CO 2 absorption as compared to hydrophilic ionic liquids. Presence of water vapor
in the gas stream increases the permeability but results in the lower CO 2 selectivity due to the water cluster formation. It was also investigated that increasing the
temperature increases the permeability of these membranes which in turn
decreases the CO 2 selectivity. The selectivity and permeability of SILM largely
depend on the nature of porous material. The CO 2 absorption in SILM can be
increased by increasing the van der Waals intermolecular interaction of CO 2 and
ionic liquid [77].
3.2.4 CO 2 Capture with Polymeric Ionic Liquid
The polymerized ionic liquids (PILs) have also shown the capability to absorb the
CO 2 . Tang et al. first made the ionic liquid into polymeric form [78]. These polymerized ionic liquids have greater CO 2 absorption capacity than the conventional
ionic liquids. Tetraalkylammonium-based polymeric ionic liquid has 7.6 times
larger CO 2 solubility as compared to conventional ionic liquid [C 4 mim][BF 4 ] [79].
CO 2 absorption capacity in PILs depends on the cation, and the highest CO 2 absorption was observed in ammonium-based PILs. Imidazolium-based PILs show relatively lower CO 2 solubility [80, 81]. Similarly, anions of the PILs significantly affect
the CO 2 solubility and follow the trend as BF 4
−
> PF 6 > Tf 2 N.
3.3 Ionic Liquid Blends
The viscosity of conventional ionic liquid is approximately 40 times higher than
that of aqueous alkanol amine (30 wt%) which causes huge difference in the CO 2
solubility [82]. Amine-functionalized ionic liquid showed better CO 2 solubility
than the conventional ionic liquids. However, the functionalized ionic liquids synthesis requires complex procedure and subsequent purification steps [83].
Therefore, in addition to modifying the structural and functional group, mixing
with other solvent is an alternative way to improve the CO 2 absorption ability of
these ionic liquids.
M. B. Haider et al.