# Analysis of Technologies for Carbon Dioxide Capture from the Air
## 2 DAC Technologies
### Adsorption
#### 2.2.2 Chemisorbents
The results showed that this route could increase amine loading in aminosilica sorbents without sacrificing amine efficiency and ensuring a CO\({}_{2}\) adsorption capacity of 0.1 molCO\({}_{2}\)/kg at 303 K.  
The same support was grafted with low amounts of 3-aminopropyltrimethoxysilane in [PERSON] and [PERSON] [94], achieving an adsorption capacity of 0.14 molCO\({}_{2}\)/kg. Compared with other zeolite sorbents, the authors found that this amine-functionalized sorbent was able to work in wet conditions but with a lower space velocity of 1500 1/h due to slower uptake rates. A novel amine-based nanofibrillated cellulose (NFC) sorbent was investigated by [PERSON] et al. [91]. This support was functionalized by 3-aminopropylmethyldiethoxysilane (APDES), allowing a high CO\({}_{2}\) uptake (2.13 molCO\({}_{2}\)/kg) with humidity. The same support was impregnated with N-(2-aminoethyl)-3-aminopropylmethyldimethoxysilane (AEAPDMS) in [PERSON] et al. [92], where an adsorption capacity of 1.39 molCO\({}_{2}\)/kg was measured at 298 K with 40% relative humidity and the kinetics of CO\({}_{2}\) and H\({}_{2}\)O co-adsorption were analyzed. The system showed a good stability over 100 cycles, with a reduction in CO\({}_{2}\) uptake of less than 5%. The positive effect of moisture has been verified, as well, in [PERSON] et al. [90], who demonstrated experimentally the stability of temperature vacuum swing adsorption/desorption cycles in a packed bed with diamine-functionalized silica gel sorbents. The CO\({}_{2}\) uptake at 40% relative humidity was 0.44 molCO\({}_{2}\)/kg, while in dry conditions it was 0.4 molCO\({}_{2}\)/kg. Higher efficiencies were measured, also, in the work of [PERSON] et al. [93]. Three sorbent materials were obtained by grafting a primary aminosilane (3-aminopropyltrimethoxysilane, APS) to silica SBA-15, varying the degree of surface coverage: SBA-APS-low (a submonolayer surface coverage of amines), SBA-APS-medium (an approximately monolayer coverage of amine) and SBA-APS-high (a dense, multilayer array of amines) [93]. The results showed that the CO\({}_{2}\) adsorption capacity could be increased by increasing the partial pressure of CO\({}_{2}\) and the coverage of amines. At 400 ppm, CO\({}_{2}\) uptakes were, respectively, 0.12, 0.24 and 0.8 molCO\({}_{2}\)/kg in dry conditions. However, at low relative humidity, the authors found that the formation of bicarbonate species with low surface amine coverage allowed an increase in amine efficiency. This was the first time that bicarbonate formation, which is known to occur in liquid aqueous amine solutions, was observed for sorbents capturing CO\({}_{2}\) from the air. Other supports have been studied. In [PERSON] et al. [95], in particular, a comparison between a primary (3-aminopropylsilyl, APS), a secondary (N-methyl-3-aminopropylsilyl, MAPS) and a third (N,N-dimethyl-3-aminopropylsilyl, DMAPS) amine-functionalized mesocellular foam (MCF) has been reported. The results showed that the primary amine had the greatest potential for CO\({}_{2}\) capture, which at low pressure (100 ppm) was 1 molCO\({}_{2}\)/kg. In fact, primary amines possess both the highest amine efficiency for CO\({}_{2}\) adsorption as well as enhanced water affinity (which improves CO\({}_{2}\) adsorption from the air) compared to other amine types. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Support** & **Anline** & **Conditions** & **Con. CO.(ppm)** & **Adapting Capacity** & **CO\_N** & **Method** & **Reference** \\ \hline MCM-41 & Triminesilane (TB) & 298 K & 400 & 0.98 & 0.12 & TCA & [88] \\ \hline MCM-41 & Triminesilane (TB) & 298 K & 300 & 0.9 & 0.11 & PB & [88] \\ \hline MCM-41 & Triminesilane (TB) & 298 K, zwitter structure humidity & 300 & 1.19 & 0.15 & PB & [88] \\ \hline MCM-41 & Triminesilane (TB) & 298 K, zwitter structure humidity & 300 & 1.4 & 0.18 & PB & [88] \\ \hline FP-MCM-41 & Derby/Determining organizers (DT) & 303 K  
A mesoporous alumina support functionalized by APS has been analyzed in [PERSON] et al.