Valorization of orange peel waste to tunable heteroatom-doped hydrochar-derived microporous carbons for selective CO₂ adsorption and separation

Constrained by the extortionately expensive carbon sources, low carbon yields, inadequate adsorption capacities, and corrosive chemical activating agents, the commercialization of carbonaceous CO₂ adsorbents remains a challenging task. Herein, potassium oxalate (K₂C₂O₄), an activating agent with less corrosive properties, was used for the synthesis of activated carbons from inexhaustibly available “orange peel biowaste.” For the first time, a comprehensive report is presented on the effect of hydrothermal treatment, hydrochar/K₂C₂O₄ ratio, activation temperature, and melamine modification in tailoring the porosity and surface functionalization of activated carbons. The optimized sample, OPMK-900, exhibited large specific surface area ~2130 m²/g; micropore volume ~1.1166 cm³/g, and a high pyrrolic nitrogen content (~ 46.1 %). Notably, melamine played the dual role as a promoter to K₂C₂O₄ porosity generation and a nitrogen dopant, which synergistically led to an efficient CO₂ uptake of ~6.67 mmol/g at 273 K/ 1 bar via micropore-filling mechanism and Lewis acid–base interactions. Moreover, remarkably high IAST CO₂/N₂ selectivity (105 at 273 K and 96 at 298 K) surpasses most of the biomass-derived carbons. Furthermore, the moderately high isosteric heat of adsorption (∆H_ads ~ 38.9 kJ/mol) revealed the physisorption mechanism of adsorption with a limited energy requirement for the regeneration of the spent adsorbents.