A family of microporous carbons prepared via a simple metal salt carbonization route with high selectivity for exceptional gravimetric and volumetric post-combustion CO 2 capture
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Heating of a high carbon-containing metal salt under nitrogen generates microporous carbons with exceptional post-combustion CO2 capture ability. Depending on carbonisation temperature (600–1000 °C) and duration (0.5 to 4 h), microporous carbons with surface area and pore volume of 500–2100 m2 g−1 and 0.27–1.1 cm3 g−1, respectively, are generated. The proportion of microporosity is high; 96% of surface area and up to 92% of pore volume. The porosity is dominated by 6–7 Å pores, which is advantageous for CO2 uptake; the carbons capture up to 4.8 mmol g−1 at 1 bar and 25 °C. Under post-combustion flue gas stream conditions (0.15 bar CO2), the carbons capture up to 1.7 mmol g−1 of CO2, which is the highest so far reported for porous carbons. The carbons have excellent selectivity for CO2 over N2; a selectivity factor of 43 based on CO2 and N2 initial adsorption rates, an Ideal Adsorbed Solution Theory (IAST) selectivity factor of 50 and an equilibrium CO2/N2 adsorption ratio at 1 bar of 22.5 compared to typical values of 5–11 for carbons. Moreover, the carbons show excellent CO2 uptake for low pressure swing operations; working capacity of up to 5.4 and 3.4 mmol g−1 for pressure swing adsorption (PSA) from a pure CO2 stream (6 to 1 bar) and flue gas stream (1.2 to 0.2 bar), respectively. The working capacity for vacuum swing adsorption (VSA) is even more remarkable; reaching 5.1 and 2.4 mmol g−1 under pure CO2 (1.5 to 0.05 bar) and flue gas (0.3 to 0.01 bar) conditions, respectively. The carbons also have excellent working capacity for temperature swing adsorption (TSA). The carbons are readily densified to high packing density of up to 1.12 g cm−3 with no penalties on textural properties or gravimetric CO2 uptake. The densification and high gravimetric uptake, gives exceptional volumetric CO2 capture capacity of 71, 187 and 320 g l−1 at 0.15, 1 and 5 bar, respectively, and unrivalled working capacity for PSA, VSA and TSA processes. The simple synthesis route, optimal pore size and exceptional CO2 uptake means that the carbons offer an unprecedented combination of characteristics for post-combustion CO2 capture.