**2.5. Ionic liquid**

A novel generation of solvents comes recently up as an alternative for traditional amine-based solvents, namely ionic liquids (ILs). These compounds are organic salts with elevated boiling points and thus low vapor pressure, which can selectively absorb acid gases such as CO2 and SO2 , involving relatively low regeneration energy requirements [3]. Recently this topic was reviewed extensively by Zhang et al. [30]

ILs are typically formed with the combination of a large organic cation, that is, imidazolium, pyridinium or phosphonium cation with either an inorganic anion such as Cl<sup>−</sup> , BF4 − and PF<sup>6</sup> −, or an organic anion, that is, RCO2 − and CF<sup>3</sup> SO3 − [31]. Special functional groups are also being under consideration for ILs formulations [30]. In general for conventional ILs, anions have more impact on the solvent performance during the absorption process, being the influence of cations considerably lower. Although ILs provide higher CO2 solubility and selectivity for CCS applications, some authors stated the use of functionalized IL are required in order to make ILs competitive in comparison with amine-based chemical absorption option [30].

Conventional ILs interact with CO2 as a physical solvent. They enhance the CO2 solubility following a Henry's law behavior. Functionalized ILs contain an amino group to improve the CO2 absorption capacity and the kinetics by means of Zwitterion mechanisms as it occurs with primary and secondary amines. Nowadays, researchers are focusing on the development of ILs as a promising CCS approach based on their exceptional properties as a solvent for CO2 capture. In this respect, the key property provided by ILs derived from their extremely high capacity to be synthetized in a large number of configurations. The tuneable solvent characteristic of ILs allows them to offer unique molecular structures specifically designed for each application, in particular for low CO2 concentrated flue gas treatment [30, 32, 33]. Other properties such as their low vapor pressure must be also taking into account due to its impact on the environment. ILs are non-volatile compounds and therefore the presence of ILs in the cleaned exhaust gas after CO2 separation step is negligible. In this sense, solvent losses associated to ILs are assumed to be completely avoided in a CO2 capture process based on these emerging solvents. This property is also related to low energy requirements during the solvent regeneration. Several studies reported up to 15% of reduction of the specific energy consumption of MEA functionalized ILs compared to conventional MEA-based scrubbing [33, 34].

**Solvent Abbreviation Reference**

• Reduction in issues relating to degradation and operation of the solvent caused by corrosion. • Flexibility in the range of amines available to tailor and optimize the composition of the

• High absorption rates observed in single amine solvents can often be maintained in blends

A novel generation of solvents comes recently up as an alternative for traditional amine-based solvents, namely ionic liquids (ILs). These compounds are organic salts with elevated boiling points and thus low vapor pressure, which can selectively absorb acid gases such as CO2

, involving relatively low regeneration energy requirements [3]. Recently this topic was

ILs are typically formed with the combination of a large organic cation, that is, imidazolium,

under consideration for ILs formulations [30]. In general for conventional ILs, anions have more impact on the solvent performance during the absorption process, being the influence

CCS applications, some authors stated the use of functionalized IL are required in order to make ILs competitive in comparison with amine-based chemical absorption option [30].

lowing a Henry's law behavior. Functionalized ILs contain an amino group to improve the

In this respect, the key property provided by ILs derived from their extremely high capacity to be synthetized in a large number of configurations. The tuneable solvent characteristic of ILs

as a promising CCS approach based on their exceptional properties as a solvent for CO2

 absorption capacity and the kinetics by means of Zwitterion mechanisms as it occurs with primary and secondary amines. Nowadays, researchers are focusing on the development of ILs

as a physical solvent. They enhance the CO2

SO3 −

pyridinium or phosphonium cation with either an inorganic anion such as Cl<sup>−</sup>

− and CF<sup>3</sup>

of cations considerably lower. Although ILs provide higher CO2

2-amino-2-methyl-1-propanol and piperazine AMP+ PZ [26] 2-amino-2-methyl-1-propanol and 1,2-ethanediamine AMP+ EDA [27] 3-methylamino propylamine and dimethyl-monoethanolamine MAPA + DMMEA [28]

CO3 [25]

and

−,

, BF4

solubility and selectivity for

[31]. Special functional groups are also being

− and PF<sup>6</sup>

solubility fol-

capture.

Piperazine and potassium carbonate PZ+ K2

**Table 4.** A summary of most relevant solvent blends reported from the literature [14].

Some advantages of blending these amines are listed below [29]:

solvent to achieve the highest absorption efficiency.

• Energy requirement for solvent regeneration can be reduced.

• Improved thermodynamic efficiency.

148 Carbon Dioxide Chemistry, Capture and Oil Recovery

of the individual components.

reviewed extensively by Zhang et al. [30]

or an organic anion, that is, RCO2

Conventional ILs interact with CO2

**2.5. Ionic liquid**

SO2

CO2

Despite their potential for carbon capture, there are some disadvantages that constrain their competitiveness of ILs compared to conventional solvents. ILs show relatively high viscosity, limiting their mass transfer capacities. Indeed, they become excessively viscous once CO2 is absorbed, producing solvent pumping issues as well as mass transfer and operational difficulties during the overall CO2 capture process. According to Luo and Wang [32], the increase of the viscosity after the CO2 absorption is due to the formation of strong and dense hydrogen-bonded networks between the compounds formed by the Zwitterion and dication mechanisms. Authors proposed to promote ILs by the use of non-amine functionalized ILS to avoid the formation of hydrogen bonds [35, 36]. Other proposals such as introducing ether oxygen atoms into the ILs structure and adding particular molecular solvents to provide IL-based solvents were found in the literature. For instance, the use of organic amines instead of water decrease also the viscosity without reducing the CO2 absorption capacity [30]. Finally, it is necessary to point out that they are also relatively expensive in comparison with common amine-based solvents [37]. Other aspect that should be further investigated in order to address the main knowledge gaps are indicated below:


As it was mentioned previously, numerous ILs formulae have been developed for last 20 years. Pure ILs configurations, typically referred as room-temperature ionic liquid (RTILs), retain CO<sup>2</sup> by physisorption mechanism showing an unfeasible CO2 absorption performance in comparison with conventional amine-based solvents. The next generation of ILs was defined from the combination of conventional ILs with a functionalized amine group, preferable. Based on this configuration, amine-functionalized IL, also called task-specific IL, reacts with CO<sup>2</sup> by chemisorption showing further improvements on the CO2 capture process derived from physisorption-based ILs. The development of this type of ILs enhanced the performance of IL in both biogas/natural gas treatment and CCS. **Table 5** summarizes the best performance IL reported from the literature.


of the CO2

*2.6.1. Water-free solvents*

issues along the CO2

*2.6.1.1. Aminosilicones*

backbone and a CO2

73–96% (**Table 6**).

balls represent Si atoms.

amines with superbase [41].

ties that its chemical nature offers.

*2.6.1.2. Non-aqueous organic blends*

in order to avoid the above-mentioned issues.

capture process. Two main research lines about new generation solvents will be dis-

chemical absorption plant, its presence in solvent formulation increases

reactive amino group (**Figure 4**) [42, 43]. The absorption capacity of these

capture due to the possibili-

Solvents for Carbon Dioxide Capture http://dx.doi.org/10.5772/intechopen.71443 151


Despite the benefits of using water as a diluent in order to reduce the corrosion and viscosity

the energy requirements in the regeneration stage. In this sense, several novel water-free solvents are being formulated such as non-aqueous organic amine blends, aminosilicones or

Aminosilicones are one of the most relevant solvents currently under investigation. Besides the absence of water in its formulation, the hybrid nature of this type of solvents (physisorb-

compounds is higher than the theoretical of the selected amino group due to the physisorption phenomenon that occurs in this type of blends. However, the possibility of solid formation and the increase of viscosity during the absorption process make necessary to use cosolvents

This type of solvents has been studied by some research groups including, for example, Kim et al. In this work, sterically hindered amines 2-[(1,1-dimethylethyl)amino]ethanol (TBAE) and 1-[(1,1-dimethylethyl)amino]-2-propanol (TBAP) were tested using organic compounds as solvents such as methanol and ethylene glycol [44–47]. The efficiency of this type of solvents is also revealed by Mani et al. In this work, AMP mixed with different alkanolamines (DEA, MDEA, MMEA and DIPA) and using organic solvents were analyzed [48, 49]. The tests concluded that, among other considerations, the absorption efficiency at equilibrium ranged

**Figure 4.** GAP-0 (on the left) and GAP-1 aminosilicones (on the right). Note that gray balls represent C atoms; white balls represent H atoms; red/dark gray balls represent O atoms; dark blue/black balls represent N atoms; black small

cussed in this section, namely water-free solvents and biphasic solvents.

ing and chemisorbing) provides a potential improvement in CO2

Perry et al. developed GAP-0 and GAP-1 aminosilicones formulated as a CO2

**Table 5.** Summary of the best performance IL applied as solvents for CO2 separation.

The basic of the minimum energy requirement for CO2 release from ILs consist of the weak interaction between both species. In general, some studies indicated that CO2 -IL interaction strongly depend on van der Waals forces in case small and symmetric molecular structures are provided, whereas electrostatic interactions dominates as large and asymmetric molecular structures are used. Besides the acid-base interaction also plays a key role as a mechanism for CO2 -IL interaction. It should be noted that some authors state the relevant impact that the structure of IL has on the CO2 solubility. The amount of free space provided by means, that is, the length chain, the presence of species such as F− increase the CO<sup>2</sup> capacity of IL. The higher free volume available, the higher CO2 solubility of the IL [30].

Most recent applications of IL involve the use of membranes for CO2 separation. Research on this field demonstrate the combination of IL with membrane significantly reduces the viscosity during the CO2 absorption process and also ensures further improvements of the gas separation performance in terms of CO2 permeability and selectivity [30]. The supported IL membranes (SILM), the poly(ionic liquid)-ionic liquid composite membranes, the combination of facilitated transport membranes with IL and the incorporation of task-specific IL into mixed matrix membranes have shown a high potential as a CO2 separation approach. Based on the literature, two main mechanisms are identified for IL-based membranes, namely solution-diffusion and facilitated transport mechanism [30]. The new pathway discovered regarding IL and its use as a CO2 separation approach requires further investigation.

#### **2.6. New generation solvents for carbon dioxide capture**

New generation solvents proposed are focused on energetic consumption reduction in order to make CO2 chemical absorption a cost-competitive technology to be deployment at CCS industrial scale. It is well-known that most of the energy consumption takes place in the regeneration step of the CO2 capture process. Two main research lines about new generation solvents will be discussed in this section, namely water-free solvents and biphasic solvents.
