# Analysis of Technologies for Carbon Dioxide Capture from the Air
## 2 DAC Technologies
### Adsorption
#### 2.2.2 Chemisorbents
However, after four cycles, the values for this parameter were, respectively, 1.65 molCO\({}_{2}\)/kg, 2.05 molCO\({}_{2}\)/kg and 2.16 molCO\({}_{2}\)/kg. In addition to PEI, other amines have been studied. In [PERSON] et al. [81], a branched poly(propyleneimine) (PPI) was synthesized via cationic ring opening polymerization of azetidine using various acid initiators (HBr, HClO\({}_{4}\), HCl and CH\({}_{3}\)SO\({}_{3}\)H). The results showed that a CO\({}_{2}\) uptake of 0.31, 0.25, 0.15 and 0.17 molCO\({}_{2}\)/kg was present for the sorbents prepared using HClO\({}_{4}\), HBr, HCl and CH\({}_{3}\)SO\({}_{3}\)H, respectively. However, HBr and HClO\({}_{4}\)-initiated composites also showed slightly higher oxidative resistances than those initiated with HCl or CH\({}_{3}\)SO\({}_{3}\)H.  
[PERSON] et al. [87] showed that PPI in a linear and dendritic form and supported in silica has better performance (in terms of CO\({}_{2}\) adsorption capacity) than PEI-based sorbents, a higher resistance to oxidative degradation and that it is less sensitive to oxygen during regeneration, allowing a longer life. The increased adsorption capacity is due to the increased basicity of the amines in PPI and decreased nearest-neighbor interactions. On the other hand, secondary amines linked by propylene spacers still have much of their CO\({}_{2}\) capture capability even after exposure to oxidation conditions relevant to carbon capture, making them less sensitive to regeneration conditions. [PERSON] et al. [82] studied TEPA loaded onto an SBA-15 support, an amine-modified mesoporous silicate, and found an adsorption capacity of 3.6 molCO\({}_{2}\)/kg. However, the sorbent has problems in terms of degradation over repeated cycling and amine deactivation in the presence of NO\({}_{x}\), SO\({}_{x}\), O\({}_{2}\) and CO.  
N,N\({}^{\prime}\)-dimethylethylenediamine (mmen) was impregnated onto a MOF, M\({}_{2}\)(dobpdc) in the work of [PERSON] et al. [84], showing an excellent CO\({}_{2}\) capture capacity at low concentrations. A CO\({}_{2}\) uptake of 2 molCO\({}_{2}\)/kg was measured at 0.39 mbar in dry conditions. Dynamic gas adsorption/desorption cycling experiments demonstrated that mmen-Mg2(dobpdc) can be regenerated upon repeated exposures to simulated air, with cycling capacities of 1.05 mmol/g after 1 h of exposure. Poly(allylamine) (PAA) is an alternative to PEI due to the high number of nitrogen groups that are present. In particular, compared to PEI, PAA has a higher density of primary amines and an extra carbon atom in the building unit, which may offer better thermal stability for CO\({}_{2}\) capture. [PERSON] et al. [66] compared a sorbent made of silica mesocellular foam (MCF) impregnated with PAA and two other sorbents with branched and linear PEI. The results showed that at a low amine loading, PAA and branched PEI had an adsorption capacity higher than that of linear PEI in the simulated ambient air due to the very small primary amine loading in linear PEI. For the sorbent with PAA, branched PEI and linear PEI, CO\({}_{2}\) uptakes were, respectively, 0.63 molCO\({}_{2}\)/kg, 0.61 molCO\({}_{2}\)/kg and 0.44 molCO\({}_{2}\)/kg. The authors also found that at higher loadings of PAA the amine sites did not seem to be as accessible, such that CO\({}_{2}\) capacities and amine efficiencies decreased. Many studies have considered the effect of moisture on adsorption capacity. In [PERSON] et al. [69], adsorption capacities of 1.7 molCO\({}_{2}\)/kg (dry condition) and 1.41 molCO\({}_{2}\)/kg (humid condition) were measured at 298 K with a PEI/fumed silica sorbent. They considered that the adsorbed water blocked access to amine groups, thereby reducing CO\({}_{2}\) capacity. However, an opposite trend was registered for a lower amine loading, achieving a CO\({}_{2}\) uptake of 1.77 molCO\({}_{2}\)/kg at 67% relative humidity. The different behavior was due to gas diffusion into the adsorbents.