# Analysis of Technologies for Carbon Dioxide Capture from the Air
## 2 DAC Technologies
### Adsorption
#### 2.2.2 Chemisorbents
At 308 K and 400 ppm, they measured an adsorption capacity of 2.5 molCO\({}_{2}\)/kg, using temperature swing adsorption for regeneration. With a sensitivity study, it was found that regeneration temperatures depend on CO\({}_{2}\) purity and that for the capture of CO\({}_{2}\) from the air a regeneration temperature above 125 \({}^{\circ}\)C is needed in order to maintain a reasonable cyclic capacity (above 1.5 molCO\({}_{2}\)/kg). An amine mixture consisting of PEI and poly(ethylene glycol) (PEG) impregnated onto a fumed silica support has been reported in [PERSON] et al. [71] as allowing good CO\({}_{2}\) uptake and at higher temperatures. The experimental results showed that the outside surface area of the support was more important than the inside porosity, offering a more effective surface area and morphology for the distribution of the amine for CO\({}_{2}\) capture without the negative side-effects of agglomeration and stickiness. Better efficiencies were reported for an amine mixture of PEI and PEG200 loaded onto SBA-15 by [PERSON] et al. [73]; the CO\({}_{2}\) uptake was 0.79 molCO\({}_{2}\)/kg. The authors found that the mixture of the two amines provided a way of tuning the sorbent performance and reducing capital costs (via increased amine efficiency), though with increased operating costs. Generally, silica materials, such as commercial silica, fumed silica, SBA-15, silica fiber and mesocellular foam (MCF), have been widely investigated due to their large surface areas and high pore volumes [72]. In addition, non-silica supports, such as mesoporous carbon and \(\gamma\)-alumina, have been studied, and other interesting supports have been taken into account. Fer example, PEI was loaded onto Mg-Al-CO\({}_{3}\) layered double hydroxide-derived mixed metal oxides (MMOs) in [PERSON] et al. [72]. At the same amine loading (67% \(w/w\)), the proposed PEI/MMO sorbent showed a higher CO\({}_{2}\) uptake (2.27 molCO\({}_{2}\)/kg) than that obtained with PEI/SBA-15 (1.92 molCO\({}_{2}\)/kg) due to the ideal morphologies and nanostructures of their supports. Kinetics and stability were also better for the former, making this sorbent attractive for ultra-dilute CO\({}_{2}\) capture. Another comparison between two different supports with PEI amines has been reported in [PERSON] et al. [68], where SBA-15/PEI and \(\gamma\)-alumina/PEI sorbents were evaluated. The results showed that, under dry conditions at 298 K, the second provided higher CO\({}_{2}\) adsorption capacities and amine efficiencies, making alumina-supported amine materials promising for direct air capture technology. Moreover, the alumina-supported amine sorbent was more stable in short multicycle temperature swing tests and was more robust upon direct contact with steam. The sorbent SBA-15/PEI has also been investigated by [PERSON] et al. [70], who reported a CO\({}_{2}\) uptake of 0.77 molCO\({}_{2}\)/kg at 298 K. However, the best adsorption capacity was obtained at 348 K, with a breakthrough and saturation capacity of 63.1 and 66.7 mg CO\({}_{2}\)/g, respectively. The sorbent can be easily and completely regenerated under mild conditions (50-110 \({}^{\circ}\)C) and is stable in cyclic operations for at least 20 cycles. Class I sorbents have poor stability over repeated adsorption/desorption cycles. In order to improve the stability of basic PEIs and CO\({}_{2}\) adsorption kinetics, modified PEIs have been analyzed in [PERSON] et al. [65], who considered 3-aminopropyltrimethoxysilane (A-PEI/silica) and tetraethyl orthotianate (T-PEI/silica) sorbents in addition to the traditional one, for which there is a stronger bond with silica surfaces. The authors found that this allowed higher adsorption capacity, stability and kinetics until after four temperature swing cycles. In the first cycle, CO\({}_{2}\) uptake was 2.36 molCO\({}_{2}\)/kg, 2.26 molCO\({}_{2}\)/kg and 2.19 molCO\({}_{2}\)/kg for the traditional PEI/silica, A-PEI/silica and T-PEI/silica sorbent, respectively.