CNTs based materials for CO2 capture

By introducing amine groups into CNTs, additional adsorption sites will be provided for CO₂ adsorption (Su et al., 2014) Lu et al. (2008). modified CNT and AC with 3-aminopropyltriethoxysilane (3-APTES) and compared the CO₂ adsorption properties of pristine CNT and AC as well as the modified ones. Compared with (modified) AC, the adsorption performance of modified CNTs was greater than that of AC in both physical (73.9 vs. 71.5 mg/g) and chemical (22.4 vs. 8 mg/g) adsorption (Table 1) (Lu et al. 2008). Due to the abundant large pores in the CNTs, more space was provided for CO₂ uptake at high pressures. In addition, surface amine groups reacted with the adsorbed CO₂ under dry conditions to form carbamate ions (RNHCO₂^‒), which could further reacted with CO₂ as electron-donating groups (Fig. 9a) (Lu et al. 2008). Notably, the micropore volume and surface area decreased in the modified CNTs, which might be the result of pore blockage by the primary amine groups inside and outside the CNTs (Su et al., 2014).”

“Introduction of diamine onto the CNT surface enables an effective adsorption of CO₂ and carbaminate anion production. When phenylenediamine (PDA) was added to functionalize the carboxylated multi-wall carbon nantubes (MWCNTs) via covalent bonding (Hu et al., 2017), an enhanced capacity of 0.6 mmol/g was obtained at 200 kPa and 30°C, outperforming that of pristine MWCNTs (0.17 mmol/g). The electrostatic force and hydrogen bonding formed between the abundant primary/secondary amines and CO₂ molecules were responsible for the better performance. The detailed mechanism was elucidated in another study (Fig. 9b) that the reaction of CO₂ and amine-N generated CO₂‒amine zwitterion, which was deprotonated into carbamate ions (Khalili et al., 2013).

Besides diamine species, tetraethylenepentamine (TEPA) as another N source was used as a modifier of the CNTs (Irani et al., 2017). With the increase of TEPA contents (50‒75%), more amine groups were decorated on the surface, benefiting the CO₂ adsorption. However, a further increase of TEPA up to 80% would adversely affect the capacity due to the agglomeration of TEPA. On the other hand, KOH activation was exerted to improve the pore structure by forming defects in the wall of CNTs (Raymundo-Pinero et al., 2005). Without KOH activation, the optimal TEPA loading was only 50% and the corresponding capacity was 2.5 mmol/g, half of that obtained in KOH-activated and TEPA-modified carbon. More interestingly, excessive amount of humidity would lower the CO₂ uptake resulting from the competitive adsorption of steam on the active sites (Mohammad and Gasem, 2012). Based on the results, the optimal moisture concentration was 1%; at this feed ratio, the CO₂ adsorption capacity was twice of that obtained in the absence of water (He et al., 2012). In addition, a low activation energy for desorption of CO₂ was approached of 39.9 kJ/mol, suggesting a reduced cost of CO₂ capture using this TEPA-modified carbon (Irani et al., 2017).

Apart from small molecules containing amines, polyaspartamide (PAA) derived from the biodegradable polysuccinimide (PSI) grafted with ethylenediamine (EDA) interacted with CO₂ through carbamate formation. To address the reduced surface area of PAA compared with PSI, MWCNTs were incorporated to produce a PAA-MWCNT adsorbent, which enhanced the surface area and pore volume by 31 and 41 times. In turn, due to the coverage of PAA on the MWCNTs, carbon-containing defects were generated while the graphitized structures were decreased. This composite structure enabled a higher uptake of CO₂ (70 mg/g) at 25°C (Ngoy et al., 2014).

Although the CO₂ adsorption capacity can be enhanced by modification, higher adsorption energy suggests more energy input for desorption, that is, the regeneration energy consumption is high (Álvarez-Gutiérrez et al., 2016). To pursue adsorbents with high recovery rate and low regeneration energy consumption, Jiao et al. (2014) explored the electrocatalytic reversible capture of CO₂ on pyridine-N incorporated CNTs. With the introduction of electrons, the distance between CO₂ and N atoms was gradually shortened while the adsorption energy increased up to ‒80.72 kJ/mol when the fourth electron was injected, much greater than that before the addition of electrons (‒26.17 kJ/mol). Due to the metallicity of carbon nanotubes, the delocalized electrons over the whole structure rendered the N atoms with a stronger electronegativity and higher electron cloud density, adsorbing CO₂ in the form of C‒N bonds; since N₂ gas was only physically adsorbed on the N-doped CNTs, a high selectivity of CO₂ against N₂ was obtained. Moreover, the C‒N bond would break upon removal of electrons, which left the CO₂ only physically interacted with the surface, facilitating the desorption. By changing the state of charge-carrying, both the adsorption capacity and energy efficiency were greatly enhanced.”