# Analysis of Technologies for Carbon Dioxide Capture from the Air
## 2 DAC Technologies
### Adsorption
#### 2.2.2 Chemisorbents
With a lower amine loading, the amine was better dispersed on the support, allowing easier access to amino groups for the incoming gases. On the other hand, at a higher amine loading, the amino group might not be as accessible due to a poorer dispersion on the support's surface and agglomeration of the coated particles. The positive effect of humidity on CO\({}_{2}\) capture from the air for other sorbents has been reported in several other works. [PERSON] et al. [76] considered a mesoporous carbon support with polyethylenimine in a fixed bed column and found that the CO\({}_{2}\) adsorption capacity was higher with moisture and that an excellent stability was present at a low CO\({}_{2}\) concentration under temperature swing adsorption/desorption cycles, measuring only a 2-5% drop in the sorption capacity after 10 cycles. In fact, 2.25 molCO\({}_{2}\)/kg was obtained at an ambient temperature and in dry conditions, while 2.58 molCO\({}_{2}\)/kg was measured at 80% relative humidity. On the other hand, after reducing the CO\({}_{2}\) concentration to 5000 ppm, in dry conditions, the adsorption capacity increased to 3.34 molCO\({}_{2}\)/kg. The authors developed, also, a first-order kinetic deactivation model based on experimental breakthrough curves in order to study the influence of CO\({}_{2}\) diffusion on the adsorption kinetics. The positive effect of moisture on adsorption capacity has also been reported in the work of [PERSON] et al. [75], who considered a PEI-functionalized hierarchical bimodal meso/microporous silica support. At 323 K, they measured 2.6 molCO\({}_{2}\)/kg in dry conditions and 3.36 molCO\({}_{2}\)/kg at 303 K with 19% relative humidity. This adsorption capacity value is the highest CO\({}_{2}\) uptake value recorded for class I sorbents, to the best of the authors' knowledge, and shows how the presence of water vapor mitigates the kinetic limitations of aminopolymer deposits. The same effect was found by [PERSON] et al. [83]: the CO\({}_{2}\) adsorption capacity increased from 0.5 molCO\({}_{2}\)/kg to 2.2 molCO\({}_{2}\)/kg for air with 20% and 80% relative humidity, respectively, for a sorbent composed of oxidized nanofibrillated cellulose (NFC) and a high-molar mass PEI. The authors underlined that the presence of water is also advantageous, as it stabilizes the PEI-based sorbent by inhibiting the formation of urea species. The effect of steam exposure time was studied by [PERSON] and [PERSON] [77], who examined a PEI-impregnated mesoporous \(\gamma\)-alumina sorbent for which an adsorption capacity of 1.71 molCO\({}_{2}\)/kg was measured at 303 K and 50% relative humidity. The authors found that CO\({}_{2}\) adsorption can be reduced by increasing the time of steam exposure due to PEI leaching; after 24 h of steam exposure, a capture reduction of 61.5% was obtained. However, the formation of boehmite on the sorbent surface was not significant with respect to amine efficiency. Adsorption capacities can be improved by tuning the support, as in [PERSON] et al. [67], where Zr was incorporated onto SBA-15 to this end, changing the acid-base properties of the support. This study showed that Zr also improves regenerability and stability over continued recycling in capturing CO\({}_{2}\) from fine gas and air. The authors reported that an adsorption capacity of 0.85 molCO\({}_{2}\)/kg was obtained and verified that the acid-base properties of the support have a critical, previously unrecognized role in creating more efficient adsorbents, improving thermal stability and adsorbent longevity. An overview of class II amine-functionalized sorbents is shown in Table 3[88; 89; 90; 100]. Among class II sorbents, [PERSON] et al. [97] considered a new synthesis to transform an alkyl halide-functionalized SBA-15 silica containing Br, Cl and I into an amine-functionalized SBA-15 silica through a procedure involving gas-phase post-grafting, using NH\({}_{3}\) at high pressure.