Self-activated, urea modified microporous carbon cryogels for high-performance CO₂ capture and separation

Conventionally, porous carbons were synthesized using corrosive chemical activating agents. On contrary, we designed a single-step synthesis method for the preparation of a series of self-activated heteroatom (N)-doped carbon cryogels from potassium hydrogen phthalate (KHP) and urea at various temperatures (700–1000 °C), in the absence of any activating agent. To highlight the effectiveness of N-doping in porosity generation and CO₂ adsorption/separation, a comparative analysis was drawn with a series of undoped carbon cryogels. Notably, the N-doped cryogels exhibit a highly porous structure with remarkable specific surface area (SSA; 2084 m² g⁻¹), micropore volume (1.1806 cm³ g⁻¹), well-defined pore size distribution, and abundant narrow micropores (<1.04 nm). Additionally, a considerable mechanical strength (compressive strength = 0.621 MPa and compressive modulus = 8.86 MPa) was also observed. Moreover, the porous cryogel endowed with sufficient pyrrolic nitrogen content (≈51.4 at %) lead to high CO₂ adsorption (280.57 mg/g at 273 K and 171.31 mg/g at 298 K at 1 bar) and an excellent ideal adsorbed solution theory (IAST)-based CO₂/N₂ selectivity (≈115) at 273 K, surpassing the CO₂ adsorption/separation performance of previously reported N-doped carbons and resorcinol-based carbon gels. Furthermore, the moderate heat of adsorption (36.109 kJ mol⁻¹) and stable cyclic behavior of CO₂ adsorption-desorption under flue gas conditions (i.e., 15% CO₂/85% N₂) unveils the physisorption nature of adsorption which governs the energy-effective regeneration process. Summarizing, the present work demonstrates the successful fabrication of highly porous carbon cryogels as efficient CO₂ adsorbents, thereby opening new synthetic avenues for self-activating porous carbon formation.