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/N-2 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 (delta H-ads -38.9 kJ/mol) revealed the physisorption mechanism of adsorption with a limited energy require-ment for the regeneration of the spent adsorbents.

Automated summary

This study used potassium oxalate as an activating agent to synthesize activated carbons from orange peel biowaste. The researchers found that hydrothermal treatment, hydrochar/K2C2O4 ratio, activation temperature, and melamine modification could tailor the porosity and surface functionalization of the activated carbons. The optimized sample exhibited large specific surface area, micropore volume, and a high pyrrolic nitrogen content, resulting in efficient CO2 uptake and selectivity.