Template-free and fast one-step synthesis from enzymatic hydrolysis lignin to hierarchical porous carbon for CO2 capture

Abstract Conventional two-step synthesis (carbonization followed by activation) of porous carbon required long heating duration at high temperature, leading to high energy consumption. To obtain carbon with hierarchical porous structure, a templating method was typically used. In view of the above drawbacks, this study provided a template-free and fast one-step synthesis from enzymatic hydrolysis lignin to hierarchical porous carbon by microwave heating in humidified nitrogen. It was shown that moisture in humidified nitrogen acts as a physical activating agent combined with KOH to achieve a hierarchical porous structure consisting of abundant micropores and considerable meso-/macropores. The results show high dependency of porosity, amorphous structure, and surface chemical properties of porous carbon samples on microwave heating duration. In particular, a porous carbon with a prominent hierarchical pore structure (65.3% mesopores), high specific surface area (2870 m2/g) and total pore volume (2.02 cm3/g) was synthesized using a microwave heating duration of 30 min. Under this condition, a high content of O-C=O group (10.0%) was also achieved, implying the presence of a hydrophilic carbon surface. These properties of prepared porous carbon synergistically enhance its CO2 uptake (up to 1.31 mmol/g measured by thermogravimetric analyzer at 30 °C). This time-saving and energy efficient synthesis of hierarchical porous carbon could be an effective approach to utilize industrial lignin waste to reduce greenhouse gas emission.