Preparation of lignin-based porous carbon through thermochemical activation and nitrogen doping for CO2 selective adsorption

Preparing CO2 adsorbents from biomass feedstock has garnered considerable attention due to its potential for fostering sustainable, economic, and eco-friendly development. Introduction of nitrogen-containing functional groups can enhance the affinity for CO2 of biochar but also inevitably affect the pore structure. The challenge lies in ascertaining and regulating the effect of N-doping on the pore structure. In this work, lignin was activated using KOH, and then followed by urea N-doping through pyrolysis or hydrothermal methods. At small dosages of KOH (the mass ratio of KOH/lignin = 0.25), in addition to introducing nitrogen-containing functional groups, urea modification boosted pore volume of <1 nm favorable for CO2 adsorption through etching carbon skeleton, while the small dosage of KOH reduced its inherent corrosive hazards. Correlation analysis showed that the dominant influencing factor for CO2 adsorption at ambient temperature and pressure was the pore volume with pore size of 0.6–0.8 nm, whereas nitrogen content (especially N-5 content) was the main influencing factor at high temperatures (50 °C) and low pressures (0.15 bar). The peak enhancement of the C–N bond under the CO2 adsorption at 50 °C by in situ diffuse reflectance infrared Fourier transform test indicated that the nitrogen-containing functional groups have a chemisorption effect on CO2. The adsorbent PK-0.25-HU exhibited adsorption capacity of 4.46 mmol/g (25 °C, 1 bar), 2.97 mmol/g (50 °C, 1 bar). Dynamic separation coefficient of CO2/N2 reached 93.82 in a gas mixture of CO2/N2 = 15 %/85 %.