Salt treated biochar for CO2 capture

https://doi.org/10.1016/j.ccst.2022.100039

“Apart from acids and alkalis, various salts (e.g., KCl, ZnCl₂, FeCl₃, CaCl₂ and KMnO₄) exhibit great potentials as an activator of biomass carbons to modify the pore structures and surface chemistry (Singh et al., 2019a; Abuelnoor et al., 2021). In particular, FeCl₃ has been proven effective to generate pores with small diameters due to the small radius of Fe³⁺ ions, resulting in an admirable micropore volume of 0.468 cm³/g and surface area of 780 m²/g (Theydan and Ahmed, 2012). Different from FeCl₃, another metal chloride ZnCl₂ is featured with its strong dehydration effect, which is usually mixed with the carbon precursors directly before activation (Singh et al., 2019a). The ZnCl₂ activation underwent through three stages with the increase of temperatures: first, large biomass molecules were decomposed into smaller intermediates and volatiles; second, the molten ZnCl₂ continued to react with the intermediates to produce char, tar and volatiles; third, the resultant char was consumed by ZnCl₂ to generate pores (Singh et al., 2019a). Compared with the strong basic KOH, ZnCl₂ is a more reliable structural modifier to produce a highly hierarchical porous structure (especially mesopores and macropores) since it maintains a molten state and homogeneously penetrates into the carbon lattice at a lower temperature (even at ambient conditions) (Kumar and Mohan Jena, 2015), facilitating the transformation of the micropores derived from the dehydration into wider and larger pores (Chang et al., 2019). Moreover, the production of tar during the biomass activation can also be inhibited owing to the adjusted pathways of decomposition (Singh et al., 2019a). In addition to the pore structures, the presence of ZnCl₂ enables a stabilized N species on the surface of AC. At 600°C, N6 pyridinic nitrogen was greatly favored, which produced the C‒O‒N bonds and protected the active sites from being oxidized (Boyjoo et al., 2017; Chen et al., 2013). Together with the much smaller particle size, a superior CO₂ uptake of 8.43 mmol/g was obtained (Balou et al., 2020). Based on the strong interaction with N species, Ca²⁺ in the form of CaCl₂ exhibited a similar stabilizing effect on the pyridinic N functionalities (Burrow et al., 2020; Harder, 2010). With an excessive amount of Ca²⁺, the strengthened interaction with organic species could be reflected from the generation of NCCN and Ca(CN₂), which inhibited the loss of pyridinic species at high temperatures. On the other hand, as a microporogen of carbon, CaCl₂ was advantageous over KOH in terms of the non-corrosiveness and low cost (Harder, 2010; Wang et al., 2018). Benefiting from the high concentration of N doping and hierarchically nanoporous structure, an excellent CO₂ adsorption capacity (1.9 mmol/g) and outstanding selectivity against N₂ (S_IAST=105) were achieved with an enhanced heat of adsorption (> 35 kJ/mol) (Burrow et al., 2020). Different from the dehydration or stabilization effects of metal chlorides, KMnO₄ as a strong oxidizing agent could open the inaccessible pores and widen the micropores upon oxidation, thus exhibiting abundant mesopores based on the N₂-adsorption/desorption isotherms (Li et al., 2014). However, the pore wall collapse or coverage by Mn oxides would reduce the surface area of the AC (Zeng et al., 2021). Compared with KMnO₄, some potassium salts (e.g., K₂C₂O₄, KHCO₃ and potassium citrate) have been studied as a less corrosive alternative to the conventional activators (Wang et al., 2017; Deng et al., 2015; Cui et al., 2021a). For example, treated by the mild activator K₂C₂O₄, the prepared AC was co-doped with O, S and N atoms, and possessed a high surface area of 1418 m²/g, exhibiting a good CO₂ adsorption capacity of 3.82 mmol/g at 25°C (Guo et al., 2021). In another scenario where potassium citrate was adopted as the in-situ activator, a superior adsorption performance of CO₂ was approached (7.35 mmol/g) at 1 bar and 0°C for the N-doped carbon spheres, benefiting from the ultramicroporosity and one-pot synthesis which simultaneously realizing the carbonization and activation (Dassanayake and Jaroniec, 2017).”