Graphene based adsorbents for CO2 capture

“Graphene is featured with the excellent thermal stability and mechanical properties. Via oxidation treatment, the resultant graphene oxide (GO) possesses a larger surface area, high porosity and abundant O groups (Sali et al., 2019). To improve CO2 adsorption performance, basic groups can be introduced on the surface of GO Bhanja et al. (2016). decorated the GO with imine groups via co-condensation of GO and 3-APTES followed by Schiff base condensation with 2,6-diformyl-4-methylphenol. Owing to the promoted physical and chemical adsorption based on the high surface area and imine N sites, an ultra-high CO2 uptake of 8.1 mmol/g was obtained at 0°C.

Introducing amine groups with polymers is also proven an effective method to strengthen the interaction with CO2. Polyetherimide (PEI) was impregnated onto the GO to form a N-rich surface, promoting the adsorption of CO2 via electron acceptor-donor interaction, which was gradually enhanced with the increase of PEI due to the generation of more chemical linkages in the form of carbamate complex (Sui et al., 2013). The PEI‒GO exhibited a high uptake of CO2 (84 mg/g) and selectivity against N2 (37.13) based on the strong acid‒base interaction (Shin et al., 2016).

The incorporation of amine groups does enhance the adsorption capacity of GO, but the preparation method is relatively complicated and requires extra source of N. In comparison, N-doped GO could be in-situ synthesized by facilely mixing the N-containing polymers and activators followed by carbonization (Alghamdi et al., 2018). In particular, polypyrrole (PPy), C6H5‒SO3‒K and KOH were thermally treated to generate N-GOs. Owing to the large pore size of 50-200 nm and abundant pyridinic-N and pyrrolic-N, a good CO2 adsorption capacity of 1.28 mmol/g and high adsorption heat of 96.04 kJ/mol were achieved with the KOH/polymer ratio of 2:1. Interestingly, despite the twice surface area for N-GO derived from a higher KOH/polymer ratio of 4:1, the adsorption performances were not good as the previous one due to the smaller pore size and limited interaction with the N moieties.

Besides modification with N groups, construction of GO/metal heterostructures is proven feasible to enhance the CO2 adsorption Chen et al. (2014). doped Li and Al onto GO since they were lighter, cheaper and more environmentally friendly than Ti. The metals were anchored on GO surface via epoxy and hydroxyl groups, resulting in a higher binding energy for CO2. Specifically for Li@GO, the much lower binding energy towards O2 enabled a highly effective capture of CO2 even under aerobic conditions, potentially being applied as an excellent candidate for removal of automobile and industrial exhausts. Featured with the strong interaction with CO2, metal oxides hold the potential as a modifier of GO (Mishra and Ramaprabhu, 2014). In the nanocomposite of Fe3O4 and hydrogen exfoliated graphene (HEG), CO2 uptake was enhanced via a physicochemical adsorption mechanism based on the asymmetric stretching of CO2 and bicarbonate formation between CO2 and hydroxyl groups on Fe3O4 surface (Hlaing Oo and McCluskey, 2005Baltrusaitis et al., 2006). Owing to the merits, a high CO2 uptake of 60 mmol/g was obtained at 11 bar and 25°C; even at 100°C, the adsorbent still delivered 24 mmol/g capacity. Another metal oxide MgO is widely applied as a CO2 adsorbent due to the high theoretical capacity (Bian et al., 2010). By coupling with GO, a sandwich-like rGO@MgO/C heterostructure was successfully constructed (Li and Zeng, 2017). The synthesis process was shown in Fig. 10 that the adsorbed Mg ions on the surface O moieties of GO coordinated with ethylene glycol (EG) to form a sheet-like Mg‒EG complex; meanwhile, GO was reduced by EG to produce rGO. Under subsequent thermal treatment, oxidation of Mg into MgO occurred together with the decomposition of organic ligands. The gas products released during the carbonization benefited the pore formation in nanoscale. The resultant MgO crystals exhibited a high dispersion with a particle size of only 3 nm, offering abundant corner- and edge-located O2‒ for CO2 adsorption. Owing to the modified physicochemical properties and hierarchical porous structures, 31.5 wt% CO2 uptake was achieved at 1 bar and 27°C.”

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