651
Toxicity: Even though ILs are regarded as green solvent, they are not actually ecofriendly because of their ecotoxicity [45]. Researchers even have shown the
acute toxicity of ILs [46]. The toxicity of ILs depends on the type of anion and
cation [47].
Biodegradability: Another cause of concern is the biodegradability of ILs. The
most of ionic liquids are not biodegradable and have serious concerns [48]. The
biodegradability depends mostly on the type of cation and the functional group
attached [49].
3.2 CO 2 Capture with Ionic Liquids
The following properties of ILs make them advantageous for CO 2 capture as compared to conventional solvents [61–64]:
• The lesser energy requirement for their regeneration compared to conventional
solvents due to the physical absorption mechanism.
• Low vapor pressure provides little or no loss of solvent into the gas stream.
• High thermal and chemical stability.
• Favorable physicochemical properties like heat capacities, thermal decomposition temperature, non-corrosiveness, and non-toxicity make ionic liquids as
green solvent.
3.2.1 Room Temperature Ionic Liquids (RTILs)
The ability of ionic liquids for CO 2 uptake was first reported in 1-butyl-3methylimidazolium hexafluorophosphate ([bmim][PF 6 ]). The ([bmim][PF 6 ]) has
ability to absorb the CO 2 , and at 8 MPa, the mole fraction of CO 2 solubility can reach
up to 0.6 [50]. After this, Blanchard et al. investigated several ionic liquids 1-n-octyl3-methylimidazolium tetrafluoroborate ([C 8 mim][BF 4 ]), 1-n-octyl-3-methylimidazolium hexafluorophosphate([C 8 mim][PF 6 ]), 1-ethyl-3- methylimidazolium ethyl
sulfate ([C 2 mim][EtSO 4 ]), 1-n-butyl-3-methylimidazolium nitrate([C 4 mim][NO 3 ]),
and N-butylpyridinium tetrafluoroborate ([N-bupy][BF 4 ]) at high pressure and concluded that these ionic liquids have varying degree of CO 2 capture ability [51]. The
maximum CO 2 uptake (~0.3 mol of CO 2 /mol of ILs) was found in bmim[PF 6 ] among
all the above listed ILs. The CO 2 absorption in different imidazolium-based ionic
liquid was explored, and it was observed that CO 2 loadings increase by increasing
the imidazolium alkyl side chain length. Also, CO 2 loadings in ionic liquid having
Tf 2 N anion were higher compared to ILs having PF 6 anions. The impact of cation
fluorination on the CO 2 was also investigated, and it was deduced that the CO 2 solubility increases as the number of fluorine atoms in the alkyl chain increases [52]. The
similar study was carried out by several other researches, and it was observed that
with increasing fluorine atom number in the anion increases the affinity for CO 2 .
Potential Application of Ionic Liquids and Deep Eutectic Solvents in Reduction…
Toxicity: Even though ILs are regarded as green solvent, they are not actually ecofriendly because of their ecotoxicity [45]. Researchers even have shown the
acute toxicity of ILs [46]. The toxicity of ILs depends on the type of anion and
cation [47].
Biodegradability: Another cause of concern is the biodegradability of ILs. The
most of ionic liquids are not biodegradable and have serious concerns [48]. The
biodegradability depends mostly on the type of cation and the functional group
attached [49].
3.2 CO 2 Capture with Ionic Liquids
The following properties of ILs make them advantageous for CO 2 capture as compared to conventional solvents [61–64]:
• The lesser energy requirement for their regeneration compared to conventional
solvents due to the physical absorption mechanism.
• Low vapor pressure provides little or no loss of solvent into the gas stream.
• High thermal and chemical stability.
• Favorable physicochemical properties like heat capacities, thermal decomposition temperature, non-corrosiveness, and non-toxicity make ionic liquids as
green solvent.
3.2.1 Room Temperature Ionic Liquids (RTILs)
The ability of ionic liquids for CO 2 uptake was first reported in 1-butyl-3methylimidazolium hexafluorophosphate ([bmim][PF 6 ]). The ([bmim][PF 6 ]) has
ability to absorb the CO 2 , and at 8 MPa, the mole fraction of CO 2 solubility can reach
up to 0.6 [50]. After this, Blanchard et al. investigated several ionic liquids 1-n-octyl3-methylimidazolium tetrafluoroborate ([C 8 mim][BF 4 ]), 1-n-octyl-3-methylimidazolium hexafluorophosphate([C 8 mim][PF 6 ]), 1-ethyl-3- methylimidazolium ethyl
sulfate ([C 2 mim][EtSO 4 ]), 1-n-butyl-3-methylimidazolium nitrate([C 4 mim][NO 3 ]),
and N-butylpyridinium tetrafluoroborate ([N-bupy][BF 4 ]) at high pressure and concluded that these ionic liquids have varying degree of CO 2 capture ability [51]. The
maximum CO 2 uptake (~0.3 mol of CO 2 /mol of ILs) was found in bmim[PF 6 ] among
all the above listed ILs. The CO 2 absorption in different imidazolium-based ionic
liquid was explored, and it was observed that CO 2 loadings increase by increasing
the imidazolium alkyl side chain length. Also, CO 2 loadings in ionic liquid having
Tf 2 N anion were higher compared to ILs having PF 6 anions. The impact of cation
fluorination on the CO 2 was also investigated, and it was deduced that the CO 2 solubility increases as the number of fluorine atoms in the alkyl chain increases [52]. The
similar study was carried out by several other researches, and it was observed that
with increasing fluorine atom number in the anion increases the affinity for CO 2 .
Potential Application of Ionic Liquids and Deep Eutectic Solvents in Reduction…