## 4 Solvents for CO\({}_{2}\) Capture
### Chemical Solvents
#### 4.2.1 Single Amine Solvents  
Linear or branched alkanolamines of the general form R\({}_{1}\)R\({}_{2}\)R\({}_{3}\)N, with the different organic groups R\({}_{i}\) defining the nature of the amine, are the most common CO\({}_{2}\) capture solvents. The benchmark solvent monoethanolamine (MEA--R\({}_{1}\) = CH\({}_{3}\)CH\({}_{2}\)OH, R\({}_{2}\) = R\({}_{3}\) = H) has been used for capturing CO\({}_{2}\) from industrial flue gases since the 1950s [46], mainly in chemical processing, making it the most developed solvent for CO\({}_{2}\) capture applications. In general, alkanolamines are compounds with variable (low to moderate) volatility, are relatively cheap, and also achieve fast reaction rates with high selectivity toward CO\({}_{2}\). They are usually classified as primary, secondary, and tertiary depending on the number of carbon atoms bound to the nitrogen atom. The amine group serves as a promoter of the reaction with acid gases, meaning that highly substituted molecules tend to exhibit higher capture capacities, while the hydroxyl group mainly contributes to the reduction in the amine's vapor pressure (and in most cases, also the viscosity) and the subsequent increase in solubility [47]. Primary alkanolamines, such as the widely used MEA, have fast kinetics and acceptable absorption capacity with a thermodynamic maximum in the range of 0.5 moles CO\({}_{2}\) per mole of solvent. The mechanism of CO\({}_{2}\) binding onto the amine molecule follows the dissolution of CO\({}_{2}\) to form carbonate and bicarbonate ions and their reaction with the amine toward a more stable carbamate ion through a fast chemical reaction that follows the zwitterion mechanism. However, they appear to show a highly corrosive behavior in high concentrations; hence, the concentration of the amine in the solution must be always kept to a low value. Moreover, they have high enthalpy of reaction which leads to high energy requirements during the thermal regeneration of the solvent. Another primary amine, diglycolamine (DGA, R\({}_{1}\) = R\({}_{2}\) = CH\({}_{2}\)CH\({}_{2}\)OCH\({}_{2}\)CH\({}_{2}\)OH), shows similar behavior with MEA, although higher concentrations in the liquid phase could be allowed due to the lower vapor pressure, allowing for higher CO\({}_{2}\) loadings in the system [48]. 3-amino-1-propanol (MPA, R\({}_{1}\) = CH\({}_{2}\)CH\({}_{2}\)CH\({}_{2}\)OH) contains one more methylene group compared to the conventional MEA, allowing for denser solutions and equally fast kinetics but lower volatility and thus lower regeneration energy demand by almost 10% [49]. Secondary amines such as diethanolamine (DEA) and diisopropanolamine (DIPA) have two of their hydrogens in the nitrogen atom substituted by ethoxy-groups. This enables them to perform better than primary amines in terms of loading and cyclic capacity. Similar to primary amines, these solvents follow the zwitterion mechanism but they are more resistant to degradation and less corrosive [47]. In addition to DEA, a widely used secondary cyclic amine solvent is piperazine (PZ). PZ has a high absorption rate, good stability, low viscosity, and high capacity [50], but is prone to form solid precipitates [51] and it is therefore rarely used as a standalone solvent. Secondary amines also generally offer the benefit of decreased energy demand for their thermal regeneration than primary amines due to their lower volatility and lower circulation rates. On the other hand, tertiary amines such as triethanolamine (TEA) and methyldiethanolamine (MDEA), which have three carbon chains linked to the nitrogen atom, are more stable molecules with comparable heat of absorption with the secondary amines. However, the increase in carbon atoms in their structure causes an overall increase in the molecular weight which in turn leads to higher viscosities and slower kinetics. In contrast to primary and secondary amines, carbamate ions are not formed with tertiary amines, leading to slower reaction rates due to the kinetically limited hydration of CO\({}_{2}\)[52].