# Analysis of Technologies for Carbon Dioxide Capture from the Air
## 2 DAC Technologies
### Adsorption
#### 2.2.2 Chemisorbents
[98] and a CO\({}_{2}\) uptake in the range of 0.15-0.75 molCO\({}_{2}\)/kg, according to the amine loading, was measured. On the other hand, good results were obtained in the work of [PERSON] et al. [96], whose sorbent has great potential to be used in direct air capture technology. The sorbent was PPN-6-CH\({}_{2}\)DETA, with a CO\({}_{2}\) uptake of 1.04 molCO\({}_{2}\)/kg and an extraordinarily high CO\({}_{2}\) selectivity (3.6 \(\times\) 10\({}^{10}\)). The authors affirmed that low costs could be achieved with this kind of sorbent due to its particular properties. High CO\({}_{2}\) adsorption capacities were also obtained in the work of [PERSON] et al. [88], where a TRI-PE-MCM-41 sorbent was suggested (MCM-41 silica with the surface functionalized by a triaminesilane (TRI-PE)). CO\({}_{2}\) was removed from dry and humid air at different CO\({}_{2}\) concentrations. At 298 K, at 400 ppm and 300 ppm of CO\({}_{2}\), adsorption capacities were, respectively, 0.98 molCO\({}_{2}\)/kg and 0.9 molCO\({}_{2}\)/kg. Moisture had a positive effect on capture. In fact, for the last concentration, a relative humidity of 26% and 67% allowed uptakes of 1.19 molCO\({}_{2}\)/kg and 1.4 molCO\({}_{2}\)/kg, respectively. The same support functionalized by a diethylenediamine organosilanes (DT) amine was used in [PERSON] et al. [89], allowing, for the simulated air, capacities up to 0.61 molCO\({}_{2}\)/kg at 303 K, while lower temperatures ensured an uptake of 1.16 molCO\({}_{2}\)/kg. Moreover, in their experiments, the authors found that a decrease in the sorbent's capacity was due to the formation of urea groups, while reactions with atmospheric trace gases or oxygen were neglected. MOF supports, highly crystalline materials with large surface areas and diverse structures, have also been investigated. In [PERSON] et al. [99], Mg\({}_{2}\)(dobpdc) functionalized with ethylenediamine achieved a CO\({}_{2}\) uptake of 2.83 molCO\({}_{2}\)/kg. A good stability was obtained during adsorption/desorption cycles; this sorbent is promising for CO\({}_{2}\) capture, with an ultrafast CO\({}_{2}\) adsorption rate when compared with porous materials. The results showed that the adsorption of CO\({}_{2}\) onto this amine led to the formation of carbamic acid and not carbamate. In [PERSON] et al. [100], the previous amine was replaced by hydrazine (H\({}_{2}\)N\({}_{4}\)), increasing the adsorption capacity to 3.89 molCO\({}_{2}\)/kg. This high CO\({}_{2}\) adsorption capacity, even at low pressure, was due to the ultrahigh concentration of amine groups and the chemisorption of CO\({}_{2}\) associated with carbamic acid formation. Moreover, these good performances could be conserved under mixed gas kinetic conditions and with humidity. Another MOF, Mg/DOBDC (magnesium dioxybenzenedicarboxylate), modified with ethylenediamine (ED), has been analyzed in [PERSON] et al. [102], ensuring a CO\({}_{2}\) uptake of 1.5 molCO\({}_{2}\)/kg after four regeneration cycles. This value was constant after adsorption/desorption cycles. In [PERSON] et al. [86], a MIL-101(Cr) material functionalized with tris (2-amino ethyl) (TREN) has been evaluated to capture CO\({}_{2}\) from simulated air. The results showed a CO\({}_{2}\) uptake of 0.35 molCO\({}_{2}\)/kg; the sorbent is not suitable for direct air capture systems. Similar results were obtained by [PERSON] et al. (2014) using MIL-101(Cr) loaded with smaller amine molecules, such as DETA and DADPA. The authors found that these sorbents are more suitable for CO\({}_{2}\) capture from fine gas or for CO\({}_{2}\)/N\({}_{2}\) separation, thanks to the mild regeneration energy. In order to improve performances, [PERSON] et al. [101] developed a new method based on the Bronsted acid-base reaction to tether alkylamines into Cr-MIL-101-SO\({}_{3}\)H for CO\({}_{2}\) capture. The new sorbent was functionalized with tris(2-aminoethyl)amine (TAEA), allowing a CO\({}_{2}\) uptake of 1.12 molCO\({}_{2}\)/kg. An interesting study has been carried out by [PERSON] et al.