# Analysis of Technologies for Carbon Dioxide Capture from the Air
## 2 DAC Technologies
### Ion-Exchange Resin  
Ion-exchange resin technology based on moisture swing adsorption was proposed by [PERSON] [107] to capture CO\({}_{2}\) from the air, providing a solution to the high regeneration costs of amine-functionalized sorbents and replacing the thermal and pressure swing processes. This technique is also known as \"artificial trees\" because it simulates the process of CO\({}_{2}\) capture from the air as trees do in nature [3]. Essentially, [PERSON] presented an amine-based exchange resin dispersed on a flat sheet of polypropylene that is able to capture CO\({}_{2}\) when it is dry and release CO\({}_{2}\) when it is wet. Solid resins have the advantages of low degradation rates and fast kinetics, receiving attention for CO\({}_{2}\) capture [108]. Many moisture-swing sorbents are strongly basic ion-exchange resins with quaternary ammonium ions (NR\({}_{4}\)+) acting as a strong base, analogous to Na\({}^{+}\) ions in aqueous solutions. Cations are balanced by anions present in the matrix, such as carbonate (CO\({}_{3}\)\({}^{2-}\)), bicarbonate (HCO\({}_{3}\)-) and hydroxide ions (OH\({}^{-}\)). The relative amounts of these anions depend on the amount of CO\({}_{2}\) and moisture, as shown in Figure 7.  
In the empty-wet condition, the resin is wet but without CO\({}_{2}\) loading. In this case, positive quaternary ammonium ions are balanced by carbonate ions that are stabilized by the presence of water. In the following step, during the drying, the water content is reduced so that carbonate ions are unstable and cause the splitting of the remaining water to form bicarbonate ions and OH\({}^{-}\) ions (empty dry state). OH\({}^{-}\) has a strong affinity with CO\({}_{2}\), so that by increasing the loading of CO\({}_{2}\), ions of HCO\({}_{3}\)\({}^{-}\) are obtained, allowing the capture of CO\({}_{2}\) from the air. In this condition, the resin is completely bicarbonate (fully dry state). After that, by injecting moisture, the resin is wetted; therefore, two HCO\({}_{3}\)\({}^{-}\) ions react to produce CO\({}_{3}\)\({}^{2-}\), H\({}_{2}\)O and CO\({}_{2}\). This state is called full-wet and after the reaction CO\({}_{2}\) is released (desorbed), resulting in the empty-wet state. The cycle can restart again. During these cycles, the moisture can be provided as liquid water or water vapor in air, and it is estimated that 12-37 mole of water are needed per mole of CO\({}_{2}\)[109; 110]. The main reactions involved in this capture technique are the following [111] (see Equations (19)-(22)):  
\[H_{2}O\ \leftrightarrow H^{+}+OH^{-} \tag{19}\]  
\[CO_{3}^{2-}+H^{+}\ \leftrightarrow HCO_{3}^{-} \tag{20}\]  
\[OH^{-}+CO_{2}\ \leftrightarrow\ HCO_{3}^{-} \tag{21}\]  
\[HCO_{3}^{-}+HCO_{3}^{-}\ \leftrightarrow\ CO_{3}^{2-}+CO_{2}+H_{2}O \tag{22}\]  
There are several advantages to moisture swing adsorption, such as the following:  
* The use of water as an energy source for the adsorption and desorption stages reduces energy costs due to a low energy consumption of about 50 kJ/molCO\({}_{2}\)[107];
* Absorbed water and CO\({}_{2}\) on the sorbent moves in opposite directions: energy penalties for the absorption and desorption of water are completely eliminated (for other solvents, water binds more strongly than CO\({}_{2}\), and, considering that it is present at concentrations 10-50 times higher than those of CO\({}_{2}\), there is a high energy penalty for desorbing water and CO\({}_{2}\));
* Heating and cooling of sorbents are avoided;
* Higher efficiencies can be obtained by introducing temperature and pressure variations. However, one disadvantage is that performances are highly weather- and geography-dependent [3]. Moreover, the used water must be relatively clean to avoid contamination  
Figure 7: Reaction scheme of CO\({}_{2}\) adsorption/desorption for an anion-exchange resin. Adapted from [109]. with the resin (contaminations can be minimized by using vapor water instead of liquid water).