Direct and mild non-hydroxide activation of biomass to carbons with enhanced CO2 storage capacity

Abstract

Potassium oxalate (PO) was trialled as a non-corrosive and less toxic activating agent for the direct activation of biomass (sawdust, SD). The PO + SD mixtures were activated either in powder form or after compaction into pellets. The resulting activated carbons are highly microporous with surface area in the range of 550 to 2100 m² g⁻¹ and pore volume between 0.3 and 1.0 cm³ g⁻¹. The porosity of the directly activated and compactivated carbons is similar to that of conventionally activated (via hydrothermal carbonisation) equivalents. In general, pelletized (i.e., compactivated) carbons achieved higher levels of porosity for any identical set (with respect to amount of PO and temperature) of preparation conditions. Unlike hydroxide activation, the amount of PO used, for PO/SD mass ratio between 2 and 4, does not have a significant effect on porosity. On the other hand, the activation temperature plays a critical role in determining the textural properties at any given PO/SD ratio. The porosity of the carbons is dominated by pores of size 6–8 Å, which are suitable for post-combustion (low pressure) CO₂ storage. At 25 °C, the carbons capture up to 1.6 and 4.3 mmol g⁻¹ of CO₂ at 0.15 bar and 1 bar, respectively. Our findings show that the use of potassium oxalate as a mild activating agent via direct activation succeeds in addressing the need for non-corrosive and less toxic activators and also negates the need for hydrothermal treatment or pyrolysis of biomass prior to activation. The present carbons are attractive as sustainable energy materials especially for post-combustion CO₂ capture and storage.