Carbon aerogel-based adsorbents

“Benefiting from the uniform pore dispersion, small-sized molecules and narrow microporous structures, carbon aerogels (CAs) are widely utilized as gas adsorbents. Further modification with heteroatoms/metals enables an improved adsorption property (Heo et al., 2019). CAs are usually prepared through supercritical or freeze drying routes; however, the operation cost and complexity become a concern for large-scale applications. To address this issue, Robertson and Mokaya (2013) mixed resorcinol, formaldehyde and glacial acetic acid as raw materials, and obtained CAs by carbonizing the precursors at 1050°C under N2 atmosphere. Compared with supercritical drying, this preparation method benefited the generation of micropores, which occupied 80% of the total surface area. Through a subsequent activation of KOH, the textural properties of CA were further improved (1980 m2/g and 2.03 cm3/g), together with a better anti-corrosion ability than ACs especially at higher temperatures. As a result, a high CO2 uptake of 3 mmol/g was approached at ambient conditions. In another study, pre-oxidation was found effective in reducing carbon contents and creating extra defects, thus enhancing the adsorption capacity (Pruna et al., 2019).

In addition to the preparation methods, the selection of precursors also determines the performance of CAs. Renewable and cheap biomass materials are promising precursors for preparing CAs Geng et al. (2020). synthesized CA with a hierarchical porous architecture from oxidized cellulose nanofibers and Kraft lignin powder via ice-templating followed by carbonization. By adjusting the precursor composition, formation of micro- and macropores was controlled, thus realizing a high surface area of 806 m2/g and CO2 adsorption capacity of 3.39 mmol/g at 0°C. By removing the sodium salt embedded in the pores (Wei et al., 2019), the surface area and capacity were significantly enhanced up to 1101 m2/g and 5.23 mmol/g.

Apart from the synthesis processes and precursors, the introduction of N moieties improves the surface basicity and benefits the selective CO2 adsorption (Kamran and Park, 2021). For example, via a hydrothermal treatment, ethylenediamine (EDA) and GO were converted to N-doped CA, which possessed pyridine N atoms and graphitic defects, delivering an enhanced CO2 capture capacity (Pruna et al., 2019). In another scenario where tetraethylenepentamine (TEPA) was added to modify the CA surface with N functionalities, a high CO2 uptake of 4.1 mmol/g was obtained due to the large mesopores (24.7 nm) and dispersed N moieties (Chen et al., 2019).

Despite the increased contents of N groups, the use of low molecular weight amine compounds potentially blocks the pores and reduces the lifespan of the adsorbent, especially at a high concentration (Wang et al., 2017Zhu et al., 2010). In comparison, natural polymer chitosan (CS) rich in amine groups can be used as the nitrogen dopant for the preparation of CAs. To overcome the drawbacks of CS (e.g., low surface area and poor pore structures) (Kumar et al., 2016Alhwaige et al. 2013), grafting onto high specific surface area materials is a promising strategy. In the GO-based aerogels (Hsan et al., 2019), there was a strong interaction between CS and GO. Besides the formation of hydrogen bonds, the basal plane of GO was modified by epoxy groups and the edges were decorated by carboxyl groups. The ethylenediamine interacted with the surface epoxy group via nucleophilic substitution reaction, and combined with the carboxyl group through amidation reaction at the edge of GO. Owing to this strengthened GO‒CS contact, a synergy was generated that the N atoms and charged groups brought by CS to graphene oxide enhanced the capacity and selectivity of CO2 chemical adsorption; in turn, GO improved the pore volume and surface area, which were crucial for physical adsorption. Similarly, Alhwaige et al. (2013) prepared a series of CAs with CS and GO. The introduction of calcination step further increased the porosity of aerogels apart from the GO contribution. Meanwhile, CS‒GO aerogels were more stable than pure CS aerogels, suggesting the improvement effect of the nano-filler on the stability of the polymer. It was well known that water-soluble quaternized chitosan (QCS) might not be suitable for adsorption (Fernandes et al., 2010). To improve the applicability, polyvinyl alcohol (PVA) was coupled with QCS to generate a hybrid aerogel adsorbent (Song et al., 2018). This aerogel enjoyed a higher surface area and more ordered nanopore structure than chitosan aerogels. More interestingly, the CO2 adsorption performance showed negligible degradation after 9 adsorption/desorption cycles by humidity swing. This anti-humidity adsorption performance was explained by the following mechanism that carbonate ions were derived from the bicarbonate ions and CO2 was released in the presence of high humidity; when the humidity was lower, the adsorbed CO2 reacted with hydroxide ions to produce hydrogen carbonate ions. The internal transformation of carbonates and bicarbonates enabled a highly reversible performance (Shi et al., 2016a2016b).”

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