KCl as a sole green salt template for converting sodium lignosulfonate into ultramicroporous carbons for efficient CO2 capture

Developing carbon-based adsorbents that simultaneously deliver high CO 2 adsorption capacity and high selectivity is crucial for energy-efficient carbon capture technologies. To overcome the limitations of conventional activators, we employed potassium chloride (KCl) as a single green salt template and successfully converted sodium lignosulfonate into microporous carbon materials (MCMs) by tailoring the ultramicroporous structure through a molten-salt method. The sample prepared at a KCl-to-sodium lignosulfonate mass ratio of 3:1 exhibited a micropore specific surface area of 1452 m 2 /g, with ultramicropores concentrated in the range of 0.3–0.6 nm. At 0 ℃ and 25 ℃ under 1 bar, the maximum CO 2 adsorption capacities reached 5.04 mmol/g and 3.36 mmol/g, respectively. The CO 2 /N 2 selectivity, estimated by ideal adsorbed solution theory (IAST) under simulated flue gas conditions (15% CO 2 /85% N 2 ) at 25 ℃ and 1 bar, reached 20.2. This outstanding combination of high capacity and high selectivity originates from the substantial micropore volume and the enhanced CO 2 pore wall interactions within the narrow ultramicropores, as evidenced by the low-coverage isosteric heat of adsorption of approximately 29 kJ/mol. This study demonstrates that even with a single potassium chloride salt template, precise control over ultramicropore size distribution can be achieved, enabling both high adsorption capacity and high selectivity, and thus provides a promising candidate for post-combustion CO 2 capture and a sustainable route for the valorization of lignin industrial by-products.