Tailoring Activation Strategies to Engineer Narrow Microporosity in Sugarcane Bagasse-Derived Activated Carbon for Efficient CO2 Capture

Abstract The continuous rise in atmospheric carbon dioxide (CO2) concentrations has emerged as a worldwide environmental concern, underscoring the urgent need for innovative capture technologies. Activated carbons derived from renewable biomass feedstocks offer a sustainable and low-cost route for developing efficient CO2 adsorbents. In this work, we report the synthesis of microporous activated carbons from waste sugarcane bagasse via ZnCl2-assisted activation, exploring three pathways: direct activation, precarbonization prior to activation, and hydrothermal carbonization followed by activation. Compared with conventional KOH activation, ZnCl2 enabled the development of well-defined microporosity at 600 °C, a substantially lower temperature than that typically required for KOH (800–900 °C). Systematic optimization of the synthesis conditions enabled precise tuning of the pore structure. The optimal sample obtained via hydrothermal carbonization–activation route exhibited a high specific surface area of 2714 m2/g, with micropores contributing about 85% of the total porosity. When evaluated for CO2 capture, the adsorbent showed CO2 adsorption capacities of 5.97 mmol/g at 0 °C and 3.47 mmol/g at 25 °C, both at 1 bar. Adsorption isotherms were best described by the Sips model, indicating heterogeneous surface interactions. Correlation analysis revealed that CO2 uptake is predominantly influenced by micropore characteristics, particularly the narrow micropore volume, which exhibited a strong linear relationship, with R2 > 0.9 at 0 °C. Further analysis of the temperature-dependent adsorption behavior revealed that narrow micropores (<1 nm) act as the dominant adsorption sites, with the effective pore width narrowing from 0.33–0.82 nm at 0 °C to 0.33–0.72 nm at 25 °C. This reduction in effective pore width with increasing temperature limits the number of accessible adsorption sites, accounting for the observed decline in CO2 uptake. Moderate isosteric heats of adsorption confirmed physisorption, enabling facile regeneration. Furthermore, the activated carbons exhibited high CO2 uptake over N2 and excellent cyclic stability, demonstrating strong potential as sustainable adsorbents for CO2 capture.